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1
Paterson, Norman R., and Colin V. Reeves. "Applications of gravity and magnetic surveys: The state‐of‐the‐art in 1985." GEOPHYSICS 50, no. 12 (December 1985): 2558–94. http://dx.doi.org/10.1190/1.1441884.
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There is a continuous large demand for gravity and magnetic surveys all over the world for a variety of exploration applications, all of which require the geophysicist to provide some new insight into the geology of an area at scales ranging from very large to very small. To achieve this objective, (a) surveys must be carried out accurately, and (b) their results must be interpreted in sympathy with what is already known of the geology. The methodology for acquiring and compiling data appears to be keeping pace with modern technology. Methods of quantitatively interpreting anomalies in terms of models of causative bodies are adapting rapidly to the burgeoning availability of computing power, from large, powerful machines to inexpensive and field‐portable microcomputers. Geologic interpretation, or the identification of physical property distributions in terms of realistic geologic models and processes, is still relatively neglected—in practice and, regretably, in the geophysical literature. Research into the relationships between physical rock properties—particularly magnetite distribution—and geology is gaining momentum, but research still lags behind the requirements of the conscientious geophysical interpreter.
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Broome, H. John. "Generation and interpretation of geophysical images with examples from the Rae Province, northwestern Canada shield." GEOPHYSICS 55, no. 8 (August 1990): 977–97. http://dx.doi.org/10.1190/1.1442927.
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Different types of images generated from gravity, magnetic, and gamma ray spectrometry data from the Rae Province of the Canadian shield were compared with each other and geologic maps to evaluate their effectiveness for displaying the geologically relevant content of the data sets. Shading methods were useful for enhancing weak directional anomalies in the aeromagnetic data. Multi‐directional, shaded‐relief images produced by overlaying three colored, shaded‐relief images are useful for analysis of anomalies associated with structure. Vertical gravity derivative images display a continuous gravity feature linking the Wager Bay and Amer Lake shear zones that is obscured on the Bouguer gravity intensity image. Detailed vertical magnetic derivative images of the shear zone clearly displayed anomalies associated with the internal structure. Composite images generated using three different geophysical parameters show correlations between the magnetic, gravity, and radiometric data which can be related to the geology. Subtle variations in uranium, thorium, and potassium concentrations determined by gamma ray spectrometry can be effectively displayed using ternary radioelement images
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Featherstone, William E., Mike Dentith, and Jonathan F. Kirby. "The determination and application of vector gravity anomalies." Exploration Geophysics 31, no. 1-2 (March 2000): 109–13. http://dx.doi.org/10.1071/eg00109.
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Clark, D. A., S. J. Saul, and D. W. Emerson. "Magnetic and gravity anomalies of a triaxial ellipsoid." Exploration Geophysics 17, no. 4 (December 1986): 189–200. http://dx.doi.org/10.1071/eg986189.
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Cooper, G. R. J. "An improved terracing algorithm for potential-field data." GEOPHYSICS 85, no. 5 (September 1, 2020): G109—G113. http://dx.doi.org/10.1190/geo2019-0129.1.
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Although the boundaries between geologic units with different physical properties are usually quite distinct, the potential-field anomalies associated with them are relatively smooth, particularly for deeper bodies. The terracing filter has been introduced to sharpen anomaly edges and to produce regions of constant amplitude between them, mimicking geologic units on a geologic map. The boundaries between the pseudogeologic units are defined by the zero contour of the Laplacian function. Unfortunately, this can result in the domains of terraced anomalies extending far from the original location of the causative body, producing an image that poorly represents the geology. I have determined that the use of the mathematical shape index of the anomalies, rather than their Laplacian, produces a much more geologically realistic result. The effect can be controlled as desired using a threshold parameter. I evaluate the benefits of the method on gravity and magnetic data from southern Africa.
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Chakravarthi, V. "Automatic gravity optimization of 2.5D strike listric fault sources with analytically defined fault planes and depth-dependent density." GEOPHYSICS 76, no. 2 (March 2011): I21—I31. http://dx.doi.org/10.1190/1.3541957.
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An automatic gravity inversion technique in the space domain simultaneously estimates the parameters of strike-limited listric fault sources and regional gravity background from a set of observed Bouguer gravity anomalies. The fault profile and regional effect are described by unknown polynomial functions of arbitrary but prescribed degree. Furthermore, the density contrast within the fault structure is presumed to be known, according to a prescribed parameterized nonlinear function of depth, in geologic settings where the detached downthrown block consists of a series of sedimentary beds whose density increases with depth. The inversion is automatic in that it initializes and determines polynomial coefficients for the fault boundary and regional gravity background from a set of observed Bouguer gravity anomalies and improves them iteratively until the modeled gravity anomalies mimic the observed anomalies. An analysis of a set of gravity anomalies attributable to a synthetic model of a listric fault structure in the presence of pseudorandom noise with and without regional background has disclosed that the algorithm yields reliable interpretations with modest error in model geometry, even in the presence of pseudorandom noise. In the presence of regional gravity background and pseudorandom noise, the estimated parameters of the structure deviate marginally from the true ones. The derived density-depth model of the Jharia coal basin in India, a pull-apart basin, has been used to analyze the observed Bouguer gravity anomalies of a boundary fault. The interpretation has yielded information consistent with drilling results and geologic setting of the basin.
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Barrows, Larry, and John D. Fett. "A high‐precision gravity survey in the Delaware Basin of southeastern New Mexico." GEOPHYSICS 50, no. 5 (May 1985): 825–33. http://dx.doi.org/10.1190/1.1441957.
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Since 1974, the Department of Energy has been studying bedded salt deposits in southeastern New Mexico as a possible location for disposing of defense‐generated transuranic and low‐level radioactive wastes. The program, known as the Waste Isolation Pilot Plant, includes intensive geologic characterization of about [Formula: see text] and construction of an underground test facility. The gravity survey reported here is part of the geologic site characterization. The gravity survey was conducted to delineate structural features near and at the proposed site. However, during the survey the gravity field was found to be dominated by effects of lateral density variations within relatively flat‐lying strata. Particularly distinctive is a pattern of elongate negative anomalies about one‐half mGal in amplitude. Boreholes in the anomalies encountered normal stratigraphy and no unusual geologic structures. However, borehole densilogs showed lower densities and uphole velocity surveys showed lower acoustic velocities than are measured outside of the anomalies. The low densities adequately account for the observed gravity anomalies. The regional stratigraphy contains water‐soluble minerals (halite, polyhalite, anhydrite‐gypsum, carbonates). Much of this material has dissolved and the region has been identified as a karstland. At the site, dissolution is slowly affecting the Rustler formation overlying the main salt‐bearing units. The low rock densities, associated with the negative gravity anomalies, are interpreted as due to alteration in the vicinity of solution conduits within the Rustler formation. This interpretation is supported by (1) partial coincidence between the negative gravity anomalies and closed topographic depressions (alluvial dolines); (2) greater anhydrite‐to‐gypsum conversion detected in boreholes within the anomalies; and (3) solution conduits encountered in one of the boreholes.
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Phelps, Geoff, Celine Scheidt, and Jef Caers. "Exploring viable geologic interpretations of gravity models using distance-based global sensitivity analysis and kernel methods." GEOPHYSICS 83, no. 5 (September 1, 2018): G79—G92. http://dx.doi.org/10.1190/geo2017-0742.1.
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ABSTRACT We have explored ways to integrate alternative geologic interpretations into the modeling of gravity data. These methods are applied to the Vaca Fault east of Fairfield, California, USA, where the structure across the fault is in question, and the Vaca Fault is used as a case study to demonstrate the method. The Vaca Fault is modeled using gravity data collected along a 10 km line perpendicular to the strike of the fault. Of particular interest is how the gravity data might inform on the dip of the Vaca Fault and thickness of the nonmarine section and whether spatial autocorrelation of density internal to the geologic units significantly influences the resulting gravity anomaly. We approach these questions by creating a suite of structural geologic models, which we then populate with geostatistically generated densities and from which the respective synthetic gravity anomalies are calculated. We perform distance-based generalized sensitivity analysis to identify which model inputs most leverage the calculated gravity anomaly. We then use multidimensional scaling to transform the gravity anomalies into a metric space and estimate the posterior probabilities of each structural geologic model using a Bayesian approach. We find that the gravity anomalies are particularly sensitive to zones of autocorrelated density values generated from geostatistical modeling. The structural geologic models most likely to produce gravity anomalies that match the observed data are the moderately dipping normal faults, 45° and 60°, although the probability that the fault dips more steeply, including in a strike slip or reverse fault orientation, is approximately 30%. The probability of a thicker nonmarine unit is 67%, more probable than a thinner nonmarine unit. This suggests that the Vaca Fault dips moderately to the east and truncates a thicker nonmarine unit, but that any further process modeling should include alternatives of the geologic structures.
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Dentith, M. C., A. Trench, and B. J. Bluck. "Geophysical constraints on the nature of the Highland Boundary Fault Zone in western Scotland." Geological Magazine 129, no. 4 (July 1992): 411–19. http://dx.doi.org/10.1017/s0016756800019506.
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AbstractPreviously published models of gravity anomalies across the Highland Boundary Fault in western Scotland interpret this structure as a high-angle reverse fault. These gravity anomalies have been re-interpreted in the light of more extensive gravity data now available, and new density data from the Highland Border Complex. The new data suggest that earlier interpretations have overestimated the fault anomaly and used over-simplified density models. New gravity models of the Highland Boundary Fault Zone are presented which show that the interface between the Dalradian and Highland Border Complex dips to the northwest at an angle of about 20°. We interpret the contact between these two formations as a thrust fault. The interface between the Highland Border Complex and the Lower Old Red Sandstone is shown to be vertical as suggested by surface geology, with the latter rocks a few hundred metres thick.
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Phelps, Geoff. "Forward modeling of gravity data using geostatistically generated subsurface density variations." GEOPHYSICS 81, no. 5 (September 2016): G81—G94. http://dx.doi.org/10.1190/geo2015-0663.1.
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Using geostatistical models of density variations in the subsurface, constrained by geologic data, forward models of gravity anomalies can be generated by discretizing the subsurface and calculating the cumulative effect of each cell (pixel). The results of such stochastically generated forward gravity anomalies can be compared with the observed gravity anomalies to find density models that match the observed data. These models have an advantage over forward gravity anomalies generated using polygonal bodies of homogeneous density because generating numerous realizations explores a larger region of the solution space. The stochastic modeling can be thought of as dividing the forward model into two components: that due to the shape of each geologic unit and that due to the heterogeneous distribution of density within each geologic unit. The modeling demonstrates that the internally heterogeneous distribution of density within each geologic unit can contribute significantly to the resulting calculated forward gravity anomaly. Furthermore, the stochastic models match observed statistical properties of geologic units, the solution space is more broadly explored by producing a suite of successful models, and the likelihood of a particular conceptual geologic model can be compared. The Vaca Fault near Travis Air Force Base, California, can be successfully modeled as a normal or strike-slip fault, with the normal fault model being slightly more probable. It can also be modeled as a reverse fault, although this structural geologic configuration is highly unlikely given the realizations we explored.
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Wellman, Peter. "Structure of the Mount Isa Region Inferred from Gravity and Magnetic Anomalies." Exploration Geophysics 23, no. 1-2 (March 1992): 423–27. http://dx.doi.org/10.1071/eg992417.
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Williams, J. P., and V. J. S. Grauch. "Comparison of magnetic and gravity terrain models." Exploration Geophysics 20, no. 2 (1989): 201. http://dx.doi.org/10.1071/eg989201.
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Modelling of magnetic terrain and comparison with actual data is an efficient method for assessing large sets when residual anomalies are important. The technique of Blakely (1981) which utilises a rapidly converging series of Fast Fourier Transforms is an efficient and sufficiently accurate method for this assessment.The technique has been applied to a data set at Kilkivan, south eastern Queensland. Here the magnetic sources are near horizontal Triassic volcanic flows unconformably overlying a non- magnetic Palaeozoic basement.Geological control is good so that it is possible to model the bottom of the flow. It is postulated that the difference between the calculated and actual data represents paleochannels in the basement. Similar techniques applied to gravity data have not been as successful.
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Stampolidis, A., G. Tsokas, A. Kiratzi, and S. Pavlides. "Major tectonic structures in northeastern Greece deduced from geophysical and seismological data." Bulletin of the Geological Society of Greece 40, no. 3 (June 5, 2018): 1279. http://dx.doi.org/10.12681/bgsg.16880.
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We apply lineaments analysis on the gravity and magnetic data of NE Greece, and combine seismological and geophysical data in order to delineate the major structural features. These methods are frequently used for extracting the dimensional and physical parameters of the buried structures that stimulate gravity and magnetic fields. These estimates concern the location, local depth, strike, dip and physical quantity contrast, of potential field contacts. We used results from previous studies in order to correct the Bouguer data for the gravity effect of the crust. The isostatic residual gravity anomalies, produced from the subtraction of the effect of the crust, are related to near-surface features. Noise suppression was achieved by slightly upward continuing the data by one cell size. Geologic significance of detected lineaments is confirmed by comparisons with the known geology, active tectonics and seismicity as well as with topographic lineaments
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LaFehr, T. R. "Standardization in gravity reduction." GEOPHYSICS 56, no. 8 (August 1991): 1170–78. http://dx.doi.org/10.1190/1.1443137.
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Gravity reduction standards are needed to improve anomaly quality for interpretation and to facilitate the joining together of different data sets. To the extent possible, data reduction should be quantitative, objective, and comprehensive, leaving ambiguity only to the interpretation process that involves qualitative, subjective, and geological decisions. The term (Bouguer anomaly) describes a field intended to be free of all nongeologic effects—not modified by a partial geologic interpretation. Measured vertical gradients of gravity demonstrate considerable variation but do not suggest, as often reported, that the normal free‐air gradient is in error or needs to be locally adjusted. Such gradients are strongly influenced by terrain and, to a lesser extent, by the same geologic sources which produce Bouguer anomalies. A substantial body of existing literature facilitates the comprehensive treatment of terrain effects, which may be rigorously implemented with current computer technology. Although variations in topographic rock density are a major source of Bouguer anomalies, a constant density appropriate to the area under investigation is normally adopted as a data reduction standard, leaving a treatment of the density variations to the interpretation. A field example from British Columbia illustrates both the variations in vertical gravity gradients which can be encountered and the conclusion that the classical approach to data reduction is practically always suitable to account for the observed effects. Standard data reduction procedures do not (and should not) include reduction‐to‐datum. The interpreter must be aware, however, that otherwise “smooth” regional Bouguer anomalies caused by regional sources do contain high‐frequency components in areas of rugged topography.
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BUSBY, J. P., and N. J. P. SMITH. "The nature of the Variscan basement in southeast England: evidence from integrated potential field modelling." Geological Magazine 138, no. 6 (November 2001): 669–85. http://dx.doi.org/10.1017/s0016756801005751.
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The Variscides of southeast England are buried beneath post-Carboniferous cover. Interpretations of the basement are based mainly on deep boreholes. Geophysical signatures from the basement are contained within the regional gravity and magnetic data. A gravity stripping exercise has been undertaken to remove the gravitational effect of the post-Variscan cover to generate a residual gravity map. This map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre-Permian subcrop map. The magnetic evidence suggests that Precambrian magnetic basement of the Midlands Microcraton has been buried southwards by north-vergent Variscan thusting over the foreland. North of the Variscan Front, short-wavelength anomalies superimposed upon this deep Precambrian source are due to shallower Silurian and Carboniferous volcanic rocks. Many residual gravity lows within the Rhenohercynian zone may be related to thick, low-density Devonian basins. In the English Channel a change in geophysical signature occurs north of the Portland–Wight Fault, coinciding with phyllites in the basement. Models are presented in which the English Channel magnetic anomalies originate within the pre-Permian basement. Comparisons with anomalies in the Southwestern Approaches suggest that the Portland–Wight Thrust is a terrane boundary, possibly a subduction-related suture, implying southerly directed Variscan subduction.
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Alarifi, Saad S., James N. Kellogg, and Elkhedr Ibrahim. "Geophysical Study of Gold Mineralized Zones in the Carolina Terrane of South Carolina." Economic Geology 116, no. 6 (September 1, 2021): 1309–27. http://dx.doi.org/10.5382/econgeo.4834.
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Abstract Large pyrite-dominated gold deposits are hosted in hydrothermally altered metamorphic rocks in the Carolina slate belt of South Carolina and are partly covered by Coastal Plain sedimentary rocks. In this study, we investigate the utility of geophysical data including aeromagnetic, electromagnetic (EM), and land gravity surveys as exploration tools. Observed geophysical anomalies are correlated with rock properties such as resistivity, susceptibility, and density. Mineral concentrations were measured for 40 samples from 16 new drill holes, as well as densities and pyrite concentrations for 49,183 samples from 448 drill holes in the Haile ore zone. Regional positive gravity anomalies are observed over the Haile, Ridgeway, and Barite Hill mine areas, and residual high-pass filtered positive gravity anomalies are observed over all mine areas. New measurements for drill cores in the Haile mine area directly confirm high rock densities and high pyrite concentrations in the ore zone. In the Haile and Brewer mine areas, high-conductivity EM anomalies are observed over the ore zones as well as over nearby metasedimentary rocks. The high concentration of pyrite in the metasedimentary units and in the ore zones may explain the high conductivities observed. All gold deposits in the Carolina slate belt are hosted in similar geologic settings near the contact between the Persimmon Fork metamorphosed volcaniclastic rocks and Richtex metamorphosed sedimentary rocks. Density and conductivity contrasts between the Persimmon Fork and Richtex Formation rocks may permit mapping of the contact zone between the two units. The magnetic anomalies do not correlate with the mineralized zones but rather with granite and gabbro plutons and diabase dikes. A prominent east-northeast linear magnetic anomaly correlates with the Modoc shear zone that separates low-grade metamorphic rocks of the Carolina terrane from higher-grade metamorphic rocks of the Kiokee belt. We use the Modoc linear magnetic anomaly to predict the southeastern boundary of the Carolina slate belt where it is covered by Coastal Plain sedimentary rocks.
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Lindsay, Mark D., Sandra Occhipinti, Crystal Laflamme, Alan Aitken, and Lara Ramos. "Mapping undercover: integrated geoscientific interpretation and 3D modelling of a Proterozoic basin." Solid Earth 11, no. 3 (June 24, 2020): 1053–77. http://dx.doi.org/10.5194/se-11-1053-2020.
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Abstract. Gravity and 3D modelling combined with geochemical analysis examine the subsurface within and below the poorly exposed Palaeoproterozoic Yerrida Basin in central Western Australia. Understanding the structure of a region is important as key features indicating past geodynamic processes and tectonic activity can be revealed. However, in stable, post-depositional tectonic settings only the younger sedimentary units tend to be widely exposed, rendering direct observation of basement and intrusive rocks impossible. Geophysical imaging and modelling can reveal the structure of a region undercover. High-magnitude density anomalies around the basin cannot be reconciled with current geological knowledge in the case presented here. The gravity anomalies infer an abundance of buried and high-density material not indicated by the surface geology. A hypothetical causative source for the high-magnitude gravity anomalies is mafic rocks that were intruded and extruded during basin rifting. The simplest and plausible stratigraphic attribution of these interpreted mafic rocks is to the Killara Formation within the Mooloogool Group. However, geochemistry reveals that the Killara Formation is not the only host to mafic rocks within the region. The mafic rocks present in the Juderina Formation are largely ignored in descriptions of Yerrida Basin magmatism, and results indicate that they may be far more substantial than once thought. Sulfur isotopic data indicate no Archean signature to these mafic rocks, a somewhat surprising result given the basement to the basin is the Archean Yilgarn Craton. We propose the source of mafic rocks is vents located to the north along the Goodin Fault or under the Bryah sub-basin and Padbury Basin. The conclusion is that the formation of the Yerrida Basin involves a geodynamic history more complex than previously thought. This result highlights the value in geophysics and geochemistry in revealing the complexity of the earlier geodynamic evolution of the basin that may be indiscernible from surface geology but may have high importance for the tectonic development of the region and its mineral resources.
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Dobretsov, N. L., D. V. Metelkin, and A. N. Vasilevskiy. "Typical Characteristics of the Earth’s Magnetic and Gravity Fields Related to Global and Regional Tectonics." Russian Geology and Geophysics 62, no. 1 (January 1, 2021): 6–24. http://dx.doi.org/10.2113/rgg20204261.
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Abstract —We present a summary and analysis of current views on the magnetic and gravity fields of the Earth as a reflection of global and regional tectonic processes. The discussion concerns the probable interconnection between the distribution of the geomagnetic field characteristics, gravity anomalies and the manifestations of mantle plume magmatism as the most remarkable geologic indicator of deep geodynamics. We demonstrate that the distribution of the characteristics of the main geomagnetic field has a qualitative similarity to anomalies of the gravity field. Brief variations of the geomagnetic field are due to high-frequency oscillations in the ionosphere, do not affect the general state of the field, and are useless when considering issues of global tectonics. On the contrary, variations with long periodicities, first of all geomagnetic reversals, can be among the main indicators of the evolution of the geodynamo – the heat mechanism controlling the entire series of global tectonic processes. The frequency of reversals is determined by the intensity of mantle plumes that cause the cooling of the core, increase the convection rate in the asthenosphere, and respectively, the periodic changes in the tectonosphere. We assume the existence of three modes of behavior for this system. The first one corresponds to steady convection, in which reversals are extremely rare or do not happen at all. These episodes – superchrons – compose no more than 20% of the duration of the Phanerozoic. The second mode occurs significantly more often in the geologic history and is characterized by active convection with frequent reversals happening at least once every 5 Myr. Finally, the third mode, which is rare for the Phanerozoic but was probably more prevalent in the early Precambrian, corresponds to hyperactive turbulent convection, when the frequency of reversals reached 20 and possibly more during one million years. Although the demonstrated qualitative similarity in the position of extreme values of the main geomagnetic field, the centers of free air gravity anomalies, and manifestations of large igneous provinces does not yet have a credible explanation, we consider it to be fundamental and requiring special study and detailed elaboration.
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Dobretsov, N. L., M. M. Buslov, A. N. Vasilevskiy, S. M. Zhmodik, and A. V. Kotlyarov. "First Results and Prospects of a New Approach to the Study of Active Geologic Processes by Space and Ground Instrumental Measurements (by the Example of Kamchatka and the Central Asian Orogenic Belt)." Russian Geology and Geophysics 62, no. 1 (January 1, 2021): 44–67. http://dx.doi.org/10.2113/rgg20204227.
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Abstract ––The use of satellite-geological information permits generalization of studies of various active geologic processes in a new way. As reference examples, we consider geologic regions extensively covered by research with our contribution. The joint use of satellite images, maps of gravity anomalies, and seismic-tomography data for Kamchatka made it possible to construct 3D models of surficial and deep-seated (depths from 10–50 to 650 km) volcanic structures. For young volcanosedimentary structures of Kamchatka, it is possible to trace the interaction of various processes, from crystallization of magmas in magma chambers to ore and oil formation in calderas. Ancient tectonic structures and superposed Cenozoic deformations in the Tien Shan, Altai, and Baikal regions are clearly displayed in satellite images and on maps of gravity anomalies. The long-range impact of the Indo-Eurasian collision on the Tien Shan, Altai, and Baikal regions was expressed as shearing, which resulted in the most contrasting structures in the zones of junction of regional faults and along the framing of cratonal structures. The active structures of Gorny Altai contain numerous travertines, whose abundance is correlated with seismic activity. The mass formation of methane and gas hydrates in Lake Baikal might be related to mantle plume fluids.
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Mantlík, František, and Miloš Karous. "Use of gravimetry in detailed geophysical prospection of potential geothermal energy exploitation sites: Case studies in the Czech Republic." GEOPHYSICS 78, no. 5 (September 1, 2013): B305—B315. http://dx.doi.org/10.1190/geo2013-0027.1.
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Methodological aspects of gravity data processing and interpretation for assessing the possibility of exploiting geothermal energy for electricity production are presented. Gravimetry is one of the most economical and most effective geophysical methods for exploration of deep geologic structures. Unfortunately the gravity inversion ambiguity problem introduces the necessity of constraining gravity models by other independent data. The procedures interpreting 2D gravity models constrained by the results of an integration of other suitable geophysical methods are described in three case studies in the Czech Republic. Gravity, reflection and refraction seismics, magnetometry, resistivity, and vertical electric sounding data were collected along profiles at each locality. Two-dimensional gravity models based on complete Bouguer anomalies were constructed taking into account the results of processing and interpretation of other geophysical data. Relevant factors influencing the potential for geothermal energy exploitation were evaluated from the interpreted geophysical models. Promising zones suitable for future exploration were established at each locality. The most important structures for locating geothermal production and injection drills are deep tectonic fault zones which can freely support fluid flow. The presented examples show that the gravity anomalies caused by fracture zones can be significant enough to unambiguously interpret and evaluate their importance in terms of hydrological permeability. In addition, the presented case histories demonstrate that two-dimensional geophysical surveys can effectively be used to reduce exploration costs in geothermal projects.
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Abdelrahman, E. M., A. I. Bayoumi, and H. M. El‐Araby. "A least‐squares minimization approach to invert gravity data." GEOPHYSICS 56, no. 1 (January 1991): 115–18. http://dx.doi.org/10.1190/1.1442946.
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The interpretation of gravity data often involves initial steps to eliminate or attenuate unwanted field components in order to isolate the desired anomaly (e.g., residual‐regional separations). These initial filtering operations include, for example, the radial weights methods (Griffin, 1949; Elkins, 1951; Abdelrahman et al., 1990), the fast Fourier transform methods (Bhattacharyya, 1965; Clarke, 1969; Meskó, 1969, 1984, Botezatu, 1970), the rational approximation techniques (Agarwal and Lal, 1971) and recursion filters (Bhattacharyya, 1976), and the bicubic spline approximation techniques (Bhattacharyya, 1969; Inoue, 1986). The derived local gravity anomalies are then geologically interpreted to derive depth estimates, often without properly accounting for the uncertainties introduced by the filtering process. When filters are applied to observed data, the filters often cause serious distortions in the shape of the gravity anomalies (Hammer, 1977). Thus the filtered gravity anomalies generally yield unreliable geologic interpretations (Rao and Radhakrishnamurthy, 1965; Hammer, 1977; Abdelrahman et al., 1985, 1989.
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Oksum, Erdinc, Mustafa Nuri Dolmaz, and Luan Thanh Pham. "Inverting gravity anomalies over the Burdur sedimentary basin, SW Turkey." Acta Geodaetica et Geophysica 54, no. 4 (November 6, 2019): 445–60. http://dx.doi.org/10.1007/s40328-019-00273-5.
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Wellman, P. "Structure of the New England Orogen/Clarence-Moreton Basin area from Gravity and Magnetic Anomalies." Exploration Geophysics 18, no. 1-2 (March 1, 1987): 222–23. http://dx.doi.org/10.1071/eg987222.
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Negi, J. G., P. K. Agrawal, and N. K. Thakur. "Inversion of regional gravity anomalies and main features of the deep crustal geology of India." Tectonophysics 165, no. 1-4 (August 1989): 155–65. http://dx.doi.org/10.1016/0040-1951(89)90044-9.
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Rao, B. Bhaskara, and M. J. Prakash. "Interpretation of Gravity Anomalies Over an Inclined Fault of Finite Strike Length With Quadratic Density Function." Exploration Geophysics 21, no. 3-4 (September 1, 1990): 169–73. http://dx.doi.org/10.1071/eg990169.
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Roshan, Ravi, and Upendra Kumar Singh. "Inversion of residual gravity anomalies using tuned PSO." Geoscientific Instrumentation, Methods and Data Systems 6, no. 1 (February 6, 2017): 71–79. http://dx.doi.org/10.5194/gi-6-71-2017.
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Abstract. Many kinds of particle swarm optimization (PSO) techniques are now available and various efforts have been made to solve linear and non-linear problems as well as one-dimensional and multi-dimensional problems of geophysical data. Particle swarm optimization is a metaheuristic optimization method that requires intelligent guesswork and a suitable selection of controlling parameters (i.e. inertia weight and acceleration coefficient) for better convergence at global minima. The proposed technique, tuned PSO, is an improved technique of PSO, in which efforts have been made to choose the controlling parameters, and these parameters have been selected after analysing the responses of various possible exercises using synthetic gravity anomalies over various geological sources. The applicability and efficacy of the proposed method is tested and validated using synthetic gravity anomalies over various source geometries. Finally, tuned PSO is applied over field residual gravity anomalies of two different geological terrains to find the model parameters, namely amplitude coefficient factor (A), shape factor (q) and depth (z). The analysed results have been compared with published results obtained by different methods that show a significantly excellent agreement with real model parameters. The results also show that the proposed approach is not only superior to the other methods but also that the strategy has enhanced the exploration capability of the proposed method. Thus tuned PSO is an efficient and more robust technique to achieve an optimal solution with minimal error.
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Abtahi, Sayyed Mohammad, Laust Börsting Pedersen, Jochen Kamm, and Thomas Kalscheuer. "Consistency investigation, vertical gravity estimation, and inversion of airborne gravity gradient data — A case study from northern Sweden." GEOPHYSICS 81, no. 3 (May 2016): B65—B76. http://dx.doi.org/10.1190/geo2014-0428.1.
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For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.
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Chandler, Val W., and Kelley Carlson Malek. "Moving‐window Poisson analysis of gravity and magnetic data from the Penokean orogen, east‐central Minnesota." GEOPHYSICS 56, no. 1 (January 1991): 123–32. http://dx.doi.org/10.1190/1.1442948.
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Analytical correlation of gravity and magnetic data through moving‐window application of Poisson's theorem is useful in studying the complex Precambrian geology of central Minnesota. Linear regression between the two data sets at each window position yields correlation, intercept, and slope parameters that quantitatively describe the relationship between the gravity and magnetic data and, in the case of the slope parameter, are often accurate estimates of magnetizatons‐to‐density ratios (MDR) of anomalous sources. In this study, gridded gravity and magnetic data from a 217.6 × 217.6 km area in central Minnesota were analyzed using a 8.5 × 8.5 km window. The study area includes part of the Early Proterozoic Penokean orogen and an Archean greenstone‐granite terrane of the Superior Province. The parameters derived by the moving‐window analysis show striking relationships to many geologic features, and many of the MDR estimates agree with rock property data. Inversely related gravity and magnetic anomalies are a characteristic trait of the Superior Province, but moving‐window analysis reveals that direct relationships occur locally. In the Penokean fold‐and‐thrust belt, gravity and magnetic highs over the Cuyuna range produce a prominent belt of large MDR estimates, which reflect highly deformed troughs of iron‐formation and other supracrustal rocks. This belt can be traced northeastward to sources that are buried by 3–5 km of Early Proterozoic strata in the Animikie basin. This configuration, in conjunction with recent geologic studies, indicates that the Animikie strata, which may represent foreland basin deposits associated with the Penokean orogen, unconformably overlie parts of the fold‐and‐thrust belt, and that earlier stratigraphic correlations between Cuyuna and Animikie strata are wrong. The results of this study indicate that moving‐window Poisson analysis is useful in the study of Precambrian terranes.
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Hang, Nguyen Thi Thu, Erdinc Oksum, Le Huy Minh, and Do Duc Thanh. "An improved space domain algorithm for determining the 3-D structure of the magnetic basement." VIETNAM JOURNAL OF EARTH SCIENCES 41, no. 1 (January 8, 2019): 69–80. http://dx.doi.org/10.15625/0866-7187/41/1/13550.
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The paper presents an improved algorithm based on Bhaskara Rao and Ramesh Babu’s algorithm to invert magnetic anomalies of three-dimensional basement structures. The magnetic basement is approximated by an ensemble of juxtaposed vertical prisms whose bottom surface coincides with Curie surface with the known depth. The computer program operating with the proposed algorithm is built in Matlab environment. Test applications show that the proposed method can perform computations with fast and stable convergence rate where the results also coincide well with the actual model structure. The effectiveness of the method is demonstrated by inverting magnetic anomalies of the southeast part of Vietnam continental shelf. The calculated magnetic basement relief of the study area provides useful additional information for studies in the aim of dealing with the geological structure of the area.References Beiki M., 2010. Analytic signals of gravity gradient tensor and their application to estimate source location, Geophysics, 75(6), i59–i74.Bui C.Q. (chief author), Le T., Tran T. D., Nguyen T. H., Phi T.T., 2007. Map of deep structure of the Earth’s crust, Atlas of the characteristics of natural conditions and environment in Vietnam’s waters and adjacent region. Publisher of Science and Technology, Ha Noi. Do D.T., Nguyen T.T.H., 2011. Atempt the improvement of inversion of magnetic anomalies of two dimensional polygonal cross sections to determine the depth of magnetic basement in some data profile of middle off shelf of Vietnam. Journal of Science and Technology, Vietnam Academy of Science and Technology, 49(2), 125–132.Do D.T., 2013. Study for application of 3D magnetic and gravity method to determine density contribution of basement rock and depth of magnetic basement on Vietnam’s shelf for oil research and prospecting Vietnam National University, Hanoi, Project code QG-11-04. Keating P. and Pilkington M., 2000, Euler deconvolution of the analytic signal, 62nd Annual International Meeting, EAGE, Session P0193.Keating P., Zerbo L., 1996. An improved technique for reduction to the pole at low latitudes, Geophysics, 61, 131–137.Le H.M., Luu V.H., 2003. Preliminary interpretation of the magnetic anomalies of the Eastern Vietnam sea and adiacent regions. J. Sci. of the Earth, 25(2), 173–181. Mai T.T., Pham V.T., Dang V.B., Le D.B., Nguyen B., Le V.D., 2011. Characteristics of Pliocene - Quaternary geology and Geoengineering in the Center and Southeast parts of Continental Shelf of Vietnam. J. Sci. of the Earth, 33(2), 109-118.Mushayandebvu M.F., Lesur V., Reid A.B., Fairhead J.D., 2004. Grid Euler deconvolution with constraints for 2D structures, Geophysics, 69, 489–496.Nguyen N.T., Bui V.N., Nguyen T.T.H., Than D.L., 2014a. Application of power density spectrum of magnetic anomaly to estimate the structure of magnetic layer of the earth crust in the Bac Bo gulf. Journal of Marine Science and Technology, 14(4A), 137–148.Nguyen N.T., Bui V.N., Nguyen T.T.H., 2014b. Determining the depth to the magnetic basementand fault systems in Tu Chinh - Vung May area by magnetic data interpretation. Journal of Marine Science and Technology, 14(4A), 16–25.Nguyen T.T.H., Pham T.L., Do D.T., Le H.M., 2018. Improving algorithm of determining the coordinates of the vertices of the polygon to invert magnetic anomalies of two-dimensional basement structures in space domain, Journal of Marine Science and Technology (preparing to print).Parker R.L., 1973. The rapid calculation of potential anomalies, Geophys. J. Roy. Astron. Soc, 31, 447–455. Pilkington M., Gregotski M.E., Todoeschuck J.P., 1994. Using fractal crustal magnetization models in magnetic interpretation, Geophysical Prospecting, 42, 677–692.Pilkington M., 2006. Joint inversion of gravity and magnetic data for two-layer models, Geophysics, 71, L35–L42.Rao D.B., Babu N.R., 1993. A fortran 77 computer program for three dimensional inversion of magnetic anomalies resulting from multiple prismatic bodies, Computer & Geosciences, 19(8), 781–801.Tanaka A., Okubo Y., Matsubayashi O., 1999. Curie point depth based on spectrum analysis of the magnetic anomaly data in East and Southeast Asia, Tectonic Pphysics, 306, 461–470.Thompson D.T., 1982. EULDTH – A new technique for marking computer-assisted depth estimates from magnetic data, Geophysics, 47, 31–37.Vo T.S., Le H.M., Luu V.H., 2005. Determining the horizontal position and depth of the density discontinuties in Red River Delta by using the vertical derivative and Euler deconvolution for the gravity anomaly data, Vietnam. Journal of Geology, Series A, 287(3–4), 39–52. Werner S., 1955. Interpretation of magnetic anomalies of sheet-like bodies, Sveriges Geologiska Undersokning, Series C, Arsbok, 43, 6.Xu S.Z., 2006. The integral-iteration method for continuation of potential fields, Chinese journal of geophysics (in Chinese), 49(4), 1176–1182.Zhang C., Huang D.N., Zhang K., Pu Y.T., Yu P., 2016. Magnetic interface forward and inversion method based on Padé approximation, Applied Geophysics, 13(4), 712–720.CCOP, 1996. Magnetic anomaly map of East Asia, scale 1:4.000.000, Geological survey of Japan and Committee for co-ordination of joint prospecting for mineral resources in asian offshore areas.
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Sun, Yanyun, Wencai Yang, Xiangzhi Zeng, and Zhiyong Zhang. "Edge enhancement of potential field data using spectral moments." GEOPHYSICS 81, no. 1 (January 1, 2016): G1—G11. http://dx.doi.org/10.1190/geo2014-0430.1.
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Edge enhancement in potential-field data helps geologic interpretation, where the lineaments on the potential-field frequently indicate subsurface faults, contacts, and other tectonic features. Therefore, a variety of edge-enhancement methods have been proposed for locating edges, most of which are based on the horizontal or vertical derivatives of the field. However, these methods have several limitations, including thick detected boundaries, blurred response to low-amplitude anomalies, and sensitivity to noise. We have developed the spectral-moment method for detecting edges in potential-field anomalies based on the second spectral moment and its statistically invariable quantities. We evaluated the spectral-moment method using synthetic gravity data, EGM-2008 gravity data, and the total magnetic field reduced to the pole. Compared with other edge-enhancing filters, such as the total horizontal derivative (TDX), profile curvature, curvature of the total horizontal gradient amplitude, enhancement of the TDX using the tilt angle, theta map, and normalized standard deviation, this spectral-moment method was more effective in balancing the edges of different-amplitude anomalies, and the detected lineaments were sharper and more continuous. In addition, the method was also less sensitive to noise than were the other filters. Compared with geologic maps, the edges extracted by the spectral-moment method from gravity and the magnetic data corresponded well with the geologic structures.
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Abdelrahman, El-Sayed M., and Khalid S. Essa. "A new approach to semi-infinite thin slab depth determination from second moving average residual gravity anomalies." Exploration Geophysics 44, no. 3 (September 2013): 185–91. http://dx.doi.org/10.1071/eg12045.
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von Frese, Ralph R. B., Michael B. Jones, Jeong Woo Kim, and Wen Sheng Li. "Spectral correlation of magnetic and gravity anomalies of Ohio." GEOPHYSICS 62, no. 1 (January 1997): 365–80. http://dx.doi.org/10.1190/1.1444139.
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Geologic interpretation of Ohio's magnetic or gravity anomalies is hindered by the effects of anomaly superposition and source ambiguity inherent to potential field analysis. A common approach to minimizing interpretational ambiguities is to consider analyses of anomaly correlations. A spectral procedure is adapted which correlates anomaly fields in the frequency domain to produce filters separating positively and negatively correlated, as well as null correlated features. The correlation filter passes or rejects wavenumbers between coregistered fields based on the correlation coefficient between common wavenumbers as given by the cosine of their phase difference. This procedure is applied to reduced‐to‐pole magnetic and first vertical derivative gravity anomalies of Ohio for mapping correlative magnetization and density contrasts within the basement rocks. The analysis reveals predominantly positive correlations between anomaly maxima and minima. Correlative anomaly maxima may be generally modeled as mafic bodies of the upper crust. They map out a possible dike complex in northwestern Ohio, a batholith as a possible source of volcanic rocks in southwestern Ohio, and numerous mafic bodies related presumably to Keweenawan rifting and Grenville tectonics. Correlative anomaly minima include several isolated features that may define felsic terranes of the upper crust, and ringed features around some of the larger mafic bodies which also may contain significant edge‐effect components. A large circular feature in south‐central Ohio involves correlative minima of a possible anorthosite body that is ringed by an inversely correlative zone of positive density and negative magnetization contrasts. Another prominent negative correlation involves an extensive area of possible extrusive rocks with positive magnetization and negative density contrasts just north of the batholith in southwestern Ohio.
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Wiszniewska, Janina, Ewa Krzemińska, Olga Polechońska, Zdzisław Petecki, Michał Ruszkowski, and Sylwester Salwa. "NEW RESULTS OF POLYMETALLIC, PGE AND REE MINERALIZATION RESEARCH IN THE SUWAŁKI ANORTHOSITE MASSIF (NE POLAND)." Biuletyn Państwowego Instytutu Geologicznego 472, no. 472 (November 20, 2018): 271–84. http://dx.doi.org/10.5604/01.3001.0012.6933.
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Suwałki Anortosite Massif (SAM) occurs in the crystalline basement of NE Poland within 200 km of the magmatic, Mesoproterozoic AMCG (anorthosite–mangerite–charnockite–granite) rock suite terrane called the Mazury Complex. SAM was discovered as a result of the drilling research of the prominent negative magnetic and gravimetric anomalies. There is an extensive negative anomaly of both potential fields related to the anorthosite massif. Gravimetric anomaly is surrounded by the bands of positive anomalies caused by rocks with elevated densities, such as granitoids, monzondiorites and granodiorites. A negative magnetic anomaly is surrounded by the bands of positive anomalies with significant amplitudes, particularly strongly marked from the south, west and north. Positive magnetic anomalies are associated with the presence of rocks with proven strong magnetic susceptibility due to the content of ferrolites (ilmenite-magnetite rocks) with accompanying Fe-Cu-Ni-Co sulphide mineralization. Fe-Ti-(V) ore deposits in the SAM were discovered in the early 1960s, in the region of Krzemianka and Udryn, but also Jeleniewo and Jezioro Okrągłe, under a thick overburden of Phanerozoic sedimentary rocks within small positive magnetic anomalies. These deposits were documented in about 100 deep boreholes to a depth of 2300 m, and the resources in C1 + C2 category were estimated for about 1.5 billion tons of titanium-magnetite ores with vanadium, mainly in the Krzemianka and Udryn ore fields. The model age obtained by the Re-Os NTIMS method for Fe-Ti-V ores and sulphides from the Krzemianka and Jezioro Okrągłe ore deposits was 1559 ±37 Ma with an initial ratio of 187Os/188Os = 1.16 ±0.06. This age was recognized as the age of the entire Suwałki Massif. Despite many years of research, the deep structure and the form of the massif has not been fully recognized. At present, geophysical and geological 3D modelling of borehole data is carried out using the OasisMontaj (Geosoft) software package. The 3D model is generated in the GeoModeller 3D application (Intrepid Geophysics) in order to recognize the geological correctness and interpretation of magnetic-gravity anomalies of the whole massif and its cover.
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Abdelrahman, El‐Sayed Mohamed, and Sharafeldin Mahmoud Sharafeldin. "A least‐squares minimization approach to depth determination from numerical horizontal gravity gradients." GEOPHYSICS 60, no. 4 (July 1995): 1259–60. http://dx.doi.org/10.1190/1.1443857.
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The sphere and the horizontal cylinder models can be very useful in quantitative interpretation of gravity data measured in a small area over buried structures. Several graphical and numerical methods have been developed by many workers for interpreting the residual gravity anomalies caused by these models to find the depth of most geologic structures. Excellent reviews are given in Saxov and Nygaard (1953) and Bowin et al. (1986). The numerical approaches (Odegard and Berg, 1965; Gupta, 1983; Sharma and Geldart, 1968; Lines and Treitel, 1984; and Shaw and Agarwal, 1990) may have advantages in theory and practice over graphical depth estimation techniques (Pick et al., 1973: Nettleton, 1976; Telford et al., 1976). However, effective quantitative interpretation procedures using the least‐squares method based on the analytical expression of simple numerical horizontal gravity gradient anomalies are yet to be developed.
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Vasanthi, A., and K. Mallick. "Bouguer gravity anomalies and occurrence patterns of kimberlite pipes in Narayanpet-Maddur Regions, Andhra Pradesh, India." GEOPHYSICS 70, no. 1 (January 2005): J13—J24. http://dx.doi.org/10.1190/1.1852778.
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The Narayanpet Kimberlite field, that lies southwest of Hyderabad, the capital city of Andhra Pradesh, India, hosts a number of kimberlite pipes. These pipes appear to be randomly positioned. However, based on regional geologic structures revealed by Bouguer gravity anomalies, especially in a regional gravity map, their locations form a definite pattern. In the Narayanpet-Maddur region, regional Bouguer gravity contours exhibit some features of geologic interest: (1) the eastward convex regional contours show an increase in convexity from the Maddur and Kotakonda area on the east to Narayanpet on the west, (2) convexity is maximum in the vicinity of Narayanpet, where a large number of Kimberlite pipes occur nearly parallel to the regional contour, and (3) between Narayanpet and the Maddur-Kotakonda region, kimberlite pipes occur at intersections of three eastward, convex concentric zones with four lineaments, one trending northeast-southwest and the other three nearly east-west. These linear trends are believed to be radial, extensional, deep-fracture zones, through which kimberlite magma erupted about 1100 Ma. Modeling the residual gravity anomaly over one of the four profiles shows fairly good agreement between observed and computed fields. Based on analysis of Bouguer gravity anomalies and modeling of the residual gravity field, likely locations for kimberlite pipes are the contact zones between granite plutons and the country rocks that coincide with the northeast-southwest–trending radial faults that pass through Narayanpet and Kotakonda to the south and through Kazipur to the north.
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Featherstone, W. E. "Improvement to long-wavelength Australian gravity anomalies expected from the CHAMP, GRACE and GOCE dedicated satellite gravimetry missions." Exploration Geophysics 34, no. 1-2 (March 2003): 69–76. http://dx.doi.org/10.1071/eg03069.
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Lemenkova, Polina. "Gobi Altai, Khangai and Khentii Mountains mapped by a mixed-method cartographic approach for comparative geophysical analysis." Mongolian Geoscientist 26, no. 52 (June 23, 2021): 62–79. http://dx.doi.org/10.5564/mgs.v26i52.1512.
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Geologic and geophysical mapping has been so far limited to the traditional single-method GIS-based mapping. A new approach combining integrated analysis of data on geology, gravity, topography and geomorphology is presented for regional characterization of the geophysical setting in Mongolia: the Gobi Altai Mountains, the Khangai Mountains and Khentii Mountains with surrounding areas. Nine new maps have been produced from the high-resolution datasets: GEBCO, gravity raster, USGS geological data and SRTM-90 DEM geomorphological grid. Methodology includes three tools for cartographic data visualization: i) Generic Mapping Tools (GMT), ii) R programming language (‘raster’ and ‘tmap’ libraries); iii) QGIS. The results demonstrated strong agreement between the estimated values in gravity and topography grids, distribution of geological units and provinces over the country and geomorphological landforms with respect to the mountain ranges: Altai, Khangai and Khentii Mountains. The highest values in the gravity anomalies correspond to the mountain ranges in the Altai Mountains and Khangai Mountains (<80 mGal); high values correspond to the Khentii Mountains (20–60 mGal). Contrariwise, the basins of the Uvs Nuur and Khyargas Nuur show negative values (<-80 mGal). The NE- to NNE-oriented faulting and rift basins are clearly visible in the geophysical grids and geologic maps. The geomorphometric analysis performed based on the SRTM-90 DEM using R scripting demonstrated (1) slope, (2) aspect, (3) hillshade and (4) elevation models of Mongolia supported by histograms of data distribution and frequency. The study contributed to the cartographic methods and regional geological studies of Mongolia.
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Khesin, Boris, Yevgeny Vapnik, and Sonya Itkis. "Geophysical evidence of deep hydrocarbon flow in Mottled Zone areas, Dead Sea Transform zone." GEOPHYSICS 75, no. 3 (May 2010): B91—B101. http://dx.doi.org/10.1190/1.3375236.
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The origin of unusual magnetic initially sedimentary rocks of the Mottled Zone (MZ) in Israel and Jordan remained enigmatic for several decades until integrated characterization of the MZ area was achieved by ground magnetic measurements and geologic observations along representative profiles in parallel with reprocessing and reinterpretation of available aeromagnetic and gravity data. Micromagnetic profiling was combined with gamma-radioactivity measurements, and representative samples were selected to determine rock physical properties. Results of field measurements, reduction and transformation of geophysical fields, anomaly inversion, and forward modeling accompanied by geologic analysis suggest that two types of magnetic anomalies and local gravity minima in the MZ areas are related to the same event, i.e., deep hydrocarbon flow associated with fossil mud volcanism. Physicochemical interaction of deep hydrocarbon flow and surrounding sedimentary rocks caused widespread weak magnetization and corresponding aeromagnetic anomalies. Other scattered heterogeneous magnetization and linked ground magnetic anomalies are common for surface/near-surface local sources and are caused by the burning of combustible gases ejected by mud volcanoes; such origin of the magnetization is confirmed by magnetic measurements of burned rocks in mud volcano areas of the Caucasus. Increased radioactivity of the lower part of the MZ likely indicates mud ejection from a deep uranium-enriched source. Locations of MZ outcrops commonly coincide with residual gravity minima, showing zones of disintegration and possible hydrocarbon accumulations. Seismic prospecting data support the existence of disintegration zones at depth. Results show the need for additional geophysical studies and a potential for revealing hydrocarbon accumulations in the MZ areas, particularly at the Halamish, Nevatim, Ma’ale Adumim, and Nabi Musa sites.
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Lee, M. K., T. C. Pharaoh, J. P. Williamson, C. A. Green, and W. De Vos. "Evidence on the deep structure of the Anglo-Brabant Massif from gravity and magnetic data." Geological Magazine 130, no. 5 (September 1993): 575–82. http://dx.doi.org/10.1017/s0016756800020872.
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AbstractGravity and aeromagnetic data from Britain, Belgium and the southern North Sea have been compiled to provide coverage of the greater part of the Anglo-Brabant Massif. Colour pseudo-relief maps of the gravity and magnetic fields highlight important anomalies and trends which provide new information on the structure of the massif and its margins. Within the massif, prominent SSE-trending geophysical lineaments define the margins of distinctive blocks within the upper crust. These are cross-cut on the northeastern margin of the massif by prominent ESE- and SE-trending magnetic and gravity lineaments. The possible history and origin of the more prominent geophysical anomalies and lineaments are considered. Integrated modelling of the potential field data has been carried out along the BIRPS MOBIL-7 seismic reflection line to provide an interpretation of crustal structure across the northeast margin of the massif constrained by all three datasets. The principal features of the model are a non-reflective, non-magnetic upper crust, interpreted as the Caledonian fold–thrust belt, overlying a heterogeneous middle–lower crust with laterally varying reflectivity, magnetization and density. ESE-trending magnetic anomalies along the northeast edge of the massif are explained in terms of an irregular mid-crustal magnetic layer with a susceptibility comparable to that of the Tubize Group in the Brabant Massif. The top of this body is coincident with prominent dipping mid-crustal reflectors observed on the seismic reflection profile and its overall geometry is compatible with mid-crustal imbrication inferred from the seismic data.
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Carvalho, Diogo Luiz Órphão de, Roberta Mary Vidotti, José Oswaldo de Araújo Filho, and Paulo Roberto Meneses. "GEOLOGY, AIRBORNE GEOPHYSICS AND GROUND GRAVITY OF THE CENTRAL GRABEN OF ÁGUA BONITA, BRAZIL." Revista Brasileira de Geofísica 30, no. 4 (December 1, 2012): 483. http://dx.doi.org/10.22564/rbgf.v30i4.235.
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Há mais de meio século a utilização de dados geofísicos representa importante ferramenta auxiliar nas pesquisas em geociências. Dentre outras características, tais produtos facilitam a discriminação de feições estruturais, ajudam a caracterização de corpos geológicos em subsuperfície e permitem a delimitação de domínios geofísicos com correspondentes geológicos. O Graben de Água Bonita, localizado nos estados de Goiás e Tocantins, foi definido na metade da década de 60 do século XX como uma estrutura de 80 km por 7 km, com bordas paralelas e retilíneas de direção N35-40◦E. O conhecimento desta estrutura restringe-se ao mapeamento geológico, inexistindo dados em profundidade. O presente trabalho trata da interpretação de dados aerogeofísicos do Levantamento Geofísico Brasil-Canadá (PGBC); aquisição e interpretação de dados gravimétricos terrestres visando contribuir ao conhecimento geológico-estrutural do Graben de Água Bonita, situado no contexto do Lineamento Transbrasiliano. A interpretação qualitativa e quantitativa dos dados geofísicos e geológicos originada de modelagem 2D, sugere que o Graben de Água Bonita inclina-se para oeste, devido às baixas anomalias e ao mergulho das sequências sedimentares detríticas. Esta inclinação evidencia que o depocentro da Formação Água Bonita localiza-se a aproximadamente 4 km de profundidade a oeste do limite superficial desta Formação. ABSTRACT: For over half a century geophysical data is an important auxiliary tool used in geosciences research. Among other features, these products facilit at ethe discrimination of structural features, aid the characterization of subsurface geological bodies and allow the delimitation of areas with corresponding geophysica land geological data. The Água Bonita Graben (GAB), which straddles the border of Goiás and Tocantins states, was defined in the mid-60s as an 80-km long by 7-km wide structure, with parallel, straight borders towards N35-40◦E. This paper presents an interpretation of the geophysical data obtained by the the Geophysical Survey Brazil-Canada (Programa de Levantamentos Geofísicos Brasil-Canadá, PGBC), as well as acquisition and interpretation of ground gravity data aiming at improving the geological knowledge of the Água Bonita Graben, situated in the Transbrasilian Lineament. The PGBC geophysical data allowed the extraction of lineaments and geophysical delineation of the GAB limits. The 2D model suggests that the Água Bonita Graben leans to the west, due to the observed low gravity anomaly values and inclined detrital sedimentary sequences dipping 8◦-12◦, in the direction 300◦ to 330◦, suggesting that the depocenter of the Água Bonita Formation is near by the western edge of the graben.Keywords: ground gravity, Água Bonita Graben, Transbrasilian Lineament.
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Luan, Pham Thanh, Le Huy Minh, Erdinc Oksum, and Do Duc Thanh. "Determination of maximum tilt angle from analytic signal amplitude of magnetic data by the curvature-based method." VIETNAM JOURNAL OF EARTH SCIENCES 40, no. 4 (September 18, 2018): 354–66. http://dx.doi.org/10.15625/0866-7187/40/4/13106.
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Imaging buried geological boundaries is one of a major objective during the interpretation of magnetic field data in Geophysics. Therefore, edge detection and edge enhancement techniques assist a crucial role on this aim. Most of the existing edge detector methods require to obtain special points such as in general the maxima of the resulting image. One of the useful tools in estimating edges from magnetic data is the tilt angle of the analytical signal amplitude due to its value slightly dependence on the direction of magnetization. In this study, the maxima of the tilt angle of analytical signal amplitudes of the magnetic data was determined by a curvature-based method. The technique is based on fitting a quadratic surface over a 3×3 windows of the grid for locating any appropriate critical point that is near the centre of the window. The algorithm is built in Matlab environment. The feasibility of the algorithm is demonstrated in two cases of synthetic data as well as on real magnetic data from Tu Chinh-Vung May area. The source code is available from the authors on request.ReferencesAkpınar Z., Gürsoy H., Tatar O., Büyüksaraç A., Koçbulut F., Piper, JDA., 2016. Geophysical analysis of fault geometry and volcanic activity in the Erzincan Basin, Central Turkey, Complex evolution of a mature pull-apart basin. Journal of Asian Earth Sciences, 116, 97-114. Beiki M., 2010. Analytic signals of gravity gradient tensor and their application to estimate source location, Geophysics, 75(6), 159-174.Blakely R. J., and Simpson R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies, Geophysics, 51, 1494-1498.Chen An-Guo, Zhou Tao-Fa, Liu Dong-Jia, Zhang Shu, 2017. Application of an enhanced theta-based filter for potential field edge detection: a case study of the LUZONG ORE DISTRICT, Chinese Journal of Geophysics, 60(2), 203-218.Cooper G.RJ., 2014. Reducing the dependence of the analytic signal amplitude of aeromagnetic data on the source vector direction, Geophysics, 79, 55-60.Cordell L., 1979. Gravimetric Expression of Graben Faulting in Santa Fe Country and theEspanola Basin, New Mexico. In Ingersoll, R.V., Ed., Guidebook to Santa Fe Country, New Mexico Geological Society, Socorro, 59-64.Cordell L and Grauch V.J.S., 1985. Mapping Basement Magnetization Zones from Aeromagnetic Data in the San Juan Basin, New Mexico, The Utility of Regional Gravity and Magnetic Anomaly Maps, Society of Exploration Geophysicists, Tulsa, 181-197.Hsu S.K., Coppense D., Shyu C.T., 1996. High- resolution detection of geologic boundaries from potential field anomalies: An enhanced analytic signal technique, Geophysics, 61, 1947-1957.Le D.C., Application of seismic exploration methods to identify geological structural characteristics supporting for hydrocarbon potential assessment in TuChinh - Vung May basin, Ph.D. Thesis, Hanoi University of Mining and Geology.Li X., 2006. Understanding 3D analytic signal amplitude: Geophysics, 71(2), 13-16.Miller H.G. and Singh V., 1994. Potential Field Tilt a New Concept for Location of Potential Field Sources, Journal of Applied Geophysics, 32, 213-217.Nabighian M.N., 1972. The analytic signal of two-dimensional magnetic bodies with polygonal cross-section: Its properties and use of automated anomaly interpretation, Geophysics, 37, 507-517.Nguyen N.T., Bui V.N., Nguyen T.T.H., 2014. Determining the depth to the magnetic basement and fault systems in Tu Chinh - Vung May area by magnetic data interpretation, Journal of Marine Science and Technology, 14(4a), 16-25.Nguyen X.H, San T.N, Bae W., Hoang M.C, 2014. Formation mechanism and petroleum system of tertiary sedimentary basins, offshore Vietnam, Energy Sources, Part A, 36, 1634-1649.Phillips J.D., Hansen R.O. and Blakely R.J., 2007. The use of curvature in potential-field interpretation, Exploration Geophysics, 38(2), 111-119.Rao D.B., and Babu N.R., 1991. A rapid method for three-dimensional modeling of magnetic anomalies, Geophysics, 56(11), 1729-1737.Roest W.R., Verhoef J., and Pilkington M., 1992. Magnetic interpretation using the 3-D analytic signal, Geophysics, 57, 116-125.Tran N., 2017. Sediment geology of Vietnam, VNU Press.Tran T.D., Tran N., Nguyen T.H., Dinh X.T., Pham B.N., Nguyen T.T., Tran T.T.T.N., Nguyen T.H.T., 2018. The Miocenedepositional geological evolution of Phu Khanh, Nam Con Son and Tu Chinh - Vung May basins in Vietnam continental shelf, VNU Journal of Science: Earth and Environmental Sciences, 34(1), 112-135.Vo T.S., Le H.M., Luu V.H., 2005. Three-dimensional analytic signal method and its application in interpretation of aeromagnetic anomaly maps in the Tuan Giao region, Proceedings of the 4th geophysical scientific and technical conference of Vietnam, Publisher of Science and Engineering 2005.Wijns C, Perez C and Kowalczyk P, 2005, Theta map: Edge detection in magnetic data, Geophysics, 70, 39-43.
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Silva, João B. C., and Valéria C. F. Barbosa. "Interactive gravity inversion." GEOPHYSICS 71, no. 1 (January 2006): J1—J9. http://dx.doi.org/10.1190/1.2168010.
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We have developed a new approach for estimating the location and geometry of several density anomalies that give rise to a complex, interfering gravity field. The user interactively defines the assumed outline of the true gravity sources in terms of points and line segments, and the method estimates sources closest to the specified outline to achieve a match between the predicted and observed gravity fields. Each gravity source is assumed to be a homogeneous body with a known density contrast; different density contrasts may be assigned to each source. Tests with synthetic data show that the method can be of use in estimating (1) multiple laterally adjacent and closely situated gravity sources, (2) single gravity sources consisting of several homogeneous compartments with different density contrasts, and (3) two gravity sources with different density contrasts of the same sign, one totally enclosed by the other. The method is also applied to three different sets of field data where the gravity sources belong to the same categories established in the tests with synthetic data. The method produces solutions consistent with the known geologic attributes of the gravity sources, illustrating its potential practicality.
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Awad, Ghareeb M. "A geophysical study on the Abu Gharadig basin, Egypt." GEOPHYSICS 50, no. 1 (January 1985): 5–15. http://dx.doi.org/10.1190/1.1441837.
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The area of study comprises one of the most hydrocarbon‐potential basins of the Egyptian Western Desert, the Abu Gharadig basin. Major marine transgression and regression cycles dominated the territory during different geologic times. Those depositional cycles, together with at least three tectonic cycles—the end of the Paleozoic Hercynean, the close of the Jurassic until the Late Cretaceous and, the close of the Cretaceous until Mid‐Teritary—resulted in a highly deformed, thick sedimentary cover. A study of the geophysical anomalies of the basin, including those indicated by aeromagnetic, gravity, and seismic data as well as the study of about 60 deep wells drilled within and around the Abu Gharadig basin, has revealed that the major tectonic disturbances of the area were caused by basement complex block faulting. These major tectonic disturbances have produced great variations in the thickness and distribution of the various geologic units throughout the region. Aeromagnetic anomalies and the wells which reached the basement indicate great variations in the depths and type of the basement complex and the presence of major intrusions in the region. The major fracturing is indicated to be mainly along an east‐west, west‐northwest and east‐northeast directions. The Bouguer gravity anomalies indicate major basement fracturing as well as variations in sedimentary patterns, erosions, and subsequent tectonic disturbances. The most obvious anomalous trends on the gravity map, based on frequency and amplitude, are the north‐east to east‐northeast, the east‐west and the west‐northwest. The main Abu Gharadig depositional center does not show sharp variations because of the homogeneity of the near‐surface rocks and the great basement depth (20 000-40 000 ft). Seismic interpretation has confirmed the presence of all these fracturing trends. It also identified some major structural trends. These are closely related to the depositional centers, and represent potential drilling locations, especially those associated with Late Cretaceous‐Tertiary active contemporaneous faulting.
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Eshaghi, Esmaeil, Anya M. Reading, Michael Roach, Mark Duffett, Daniel Bombardieri, Matthew J. Cracknell, John L. Everard, Grace Cumming, and Stephen Kuhn. "Inverse modeling constrained by potential field data, petrophysics, and improved geologic mapping: A case study from prospective northwest Tasmania." GEOPHYSICS 85, no. 5 (July 28, 2020): K13—K26. http://dx.doi.org/10.1190/geo2019-0636.1.
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The Heazlewood-Luina-Waratah area is a prospective region for minerals in northwest Tasmania, Australia, associated with historically important ore deposits related to the emplacement of granite intrusions and/or ultramafic complexes. The geology of the area is poorly understood due to the difficult terrain and dense vegetation. We have constructed an initial high-resolution 3D geologic model of this area using constraints from geologic maps and geologic and geophysical cross sections. This initial model is improved upon by integrating results from 3D geometry and physical property inversion of potential field (gravity and magnetic) data, petrophysical measurements, and updated field mapping. Geometry inversion reveals that the Devonian granites in the south are thicker than previously thought, possibly connecting to deep sources of mineralization. In addition, we identified gravity anomalies to the northeast that could be caused by near-surface granite cupolas. A newly discovered ultramafic complex linking the Heazlewood and Mount Stewart Ultramafic Complexes in the southwest also has been modeled. This implies a greater volume of ultramafic material in the Cambrian successions and points to a larger obducted component than previously thought. The newly inferred granite cupolas and ultramafic complexes are targets for future mineral exploration. Petrophysical property inversion reveals a high degree of variation in these properties within the ultramafic complexes indicating a variable degree of serpentinization. Sensitivity tests suggest maximum depths of 2–3 km for the contact aureole that surrounds major granitic intrusions in the southeast, whereas the Heazlewood River complex is likely to have a deeper source up to 4 km. We have demonstrated the value of adding geologic and petrophysical constraints to 3D modeling for the purpose of guiding mineral exploration. This is particularly important for the refinement of geologic structures in tectonically complex areas that have lithology units with contrasting magnetic and density characteristics.
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Whitaker, A. "A geophysical model of the Precambrian of the Albany 1:1M sheet, Western Australia, and its relevance to economic geology." Exploration Geophysics 20, no. 2 (1989): 195. http://dx.doi.org/10.1071/eg989195.
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In the Albany 1:1M sheet, the 10-50 km wavelength gravity and aeromagnetic anomalies define major boundaries and subdivisions of the Precambrian blocks/provinces and large bodies of granite, while the short wavelength magnetic anomalies define lithological banding and lineaments. The Yilgarn Block in the sheet area is readily subdivided into two major north-northwest to north trending zones of low magnetization separated by a 30 km wide zone of high magnetization. The eastern zone is considered to be due to granite-greenstone terrane, the western boundary of which is located 100 km west of that currently recognised from outcrop geology. The western zone is considered to be due to granite-geiss terrane while the 30 km wide zone between coincides with strongly magnetised granulites. The Albany Province is composed of two structurally distinct east-west trending zones. The southern zone of relatively low magnetization and density coincides with acid gneiss and granites, whereas the highly magnetised, relatively dense zone to the north and west, correlates with highly metamorphosed acid and mafic granulites. Thrusting of the Albany Province during the Mid-Proterozoic has demagnetised and or deformed the margin of the southern Yilgarn Block to at least 50 km north of the block boundary. Throughout the region, significant mineral deposits of Au, Ni, Sn, Ti, and Fe are located within greenstone and high grade metamorphic belts. These belts have characteristic signatures which contrast with extensive areas of relatively homogeneous, low economic mineral potential, granite-gneiss terrane.
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Hinze, William J., and Thomas G. Hildenbrand. "The Utility of Geopotential Field Data in Seismotectonic Studies in the Eastern United States." Seismological Research Letters 59, no. 4 (October 1, 1988): 289–97. http://dx.doi.org/10.1785/gssrl.59.4.289.
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Abstract The deterministic approach to seismic hazard evaluation utilizes all available geologic/geophysical information to map the structure and nature of the crust in three dimensions that may relate to earthquake activity. However, information on the crystalline crust of the eastern United States from direct observations, drilling and sparse crustal seismic studies is limited. In contrast, regional gravity and magnetic anomaly data exist over the entire eastern United States and are available in a digital grid to facilitate processing and analysis. Although these data have serious limitations for detailed interpretation, they can be used to estimate the strength of the crust and the lithosphere and to map and characterize (1) zones of weakness such as paleorifts, sutures, and faults; (2) regions of potential stress amplifications such as plutons and irregularities in fault zones; and (3) basement terranes of generally consistent structural pattern that may delimit coherent regional seismic zones. Free-air, Bouguer, and isostatic gravity anomalies have different applications in the characterization of the crust for seismogenic purposes and complement magnetic anomaly maps which focus on upper crustal features. In concert, these data have provided the insight to interpret the host structures that together with related seismic and geoscience data, suggest causative mechanisms of the New Madrid seismic zone and other seismogenic regions of the eastern United States. As a result, we conclude that interpretations of geopotential anomalies are an essential ingredient in seismotectonic studies in the eastern United States, but they are only one of several tools required in the concerted effort of assessing seismic hazards. The presence of anomalies with a particular set of attributes neither confirms nor denies the possible spatial relationship to seismicity.
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Huang, Jian-Liang, Chong-Yang Shen, and Hui Li. "Robust inversion analysis of local gravity anomalies caused by geological dislocation model of faults." Acta Seismologica Sinica 11, no. 1 (January 1998): 103–13. http://dx.doi.org/10.1007/bf02650460.
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Korte, Monika, and Mioara Mandea. "Geopotential field anomalies and regional tectonic features – two case studies: southern Africa and Germany." Solid Earth 7, no. 3 (May 9, 2016): 751–68. http://dx.doi.org/10.5194/se-7-751-2016.
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Abstract. Maps of magnetic and gravity field anomalies provide information about physical properties of the Earth's crust and upper mantle, helpful in understanding geological conditions and tectonic structures. Depending on data availability, whether from the ground, airborne, or from satellites, potential field anomaly maps contain information on different ranges of spatial wavelengths, roughly corresponding to sources at different depths. Focussing on magnetic data, we compare amplitudes and characteristics of anomalies from maps based on various available data and as measured at geomagnetic repeat stations. Two cases are investigated: southern Africa, characterized by geologically old cratons and strong magnetic anomalies, and the smaller region of Germany with much younger crust and weaker anomalies. Estimating lithospheric magnetic anomaly values from the ground stations' time series (repeat station crustal biases) reveals magnetospheric field contributions causing time-varying offsets of several nT in the results. Similar influences might be one source of discrepancy when merging anomaly maps from different epochs. Moreover, we take advantage of recently developed satellite potential field models and compare magnetic and gravity gradient anomalies of ∼ 200 km resolution. Density and magnetization represent independent rock properties and thus provide complementary information on compositional and structural changes. Comparing short- and long-wavelength anomalies and the correlation of rather large-scale magnetic and gravity anomalies, and relating them to known lithospheric structures, we generally find a better agreement in the southern African region than the German region. This probably indicates stronger concordance between near-surface (down to at most a few km) and deeper (several kilometres down to Curie depth) structures in the former area, which can be seen to agree with a thicker lithosphere and a lower heat flux reported in the literature for the southern African region.
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Kafka, Alan L. "Earthquakes, Geology and Crustal Features in Southern New England." Seismological Research Letters 59, no. 4 (October 1, 1988): 173–81. http://dx.doi.org/10.1785/gssrl.59.4.173.
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Abstract Within southern New england (SNE) and adjacent areas lies a variety of tectonic regions extending from the ancient North American craton to the Avalonian Terrane. This region, which is part of the northern Appalachians, has had a moderate level of seismic activity throughout its recorded history. Although the geology and geophysics of SNE have been studied extensively, it has been difficult to correlate the seismic activity in this region with geologic structures mapped on the earth’s surface. The distribution of earthquakes located by regional seismic networks in SNE generally resembles that of historical earthquakes in this region. In both the network and historical records, a cluster of earthquakes occurs near Moodus, CT and more diffuse seismicity occurs in the eastern coastal regions. Both data sets show earthquake activity in the vicinity of the 1755 earthquake that occurred on the coast of Cape Ann, MA, as well as seismicity near the boundary between southwestern CT and NY State. Aside from these very general similarities, the location uncertainties of the historical earthquakes make it difficult to compare the two records of seismicity in any greater detail. Information about the lateral variation of seismic velocities in the shallow crust beneath SNE has been obtained from studies of dispersion of short-period, fundamental-mode Rayleigh waves (Rg). Rg waves with periods between about 0.2 and 2.5 sec have been studied, and dispersive properties of Rg in that period range are sensitive to lateral variation in the upper few kilometers of the crust. Based on Rg dispersion studies, it appears that SNE can be divided into regions of distinct shallow crustal structure. The pattern of lateral variation is Rg dispersion is similar to the pattern of teleseismic residuals and gravity anomalies, suggesting that the lateral variation revealed by the Rg studies extends deeper into the crust. These results, however, do not reveal any obvious correlations between crustal features and seismicity.
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Pawlowski, Robert S. "Green’s equivalent‐layer concept in gravity band‐pass filter design." GEOPHYSICS 59, no. 1 (January 1994): 69–76. http://dx.doi.org/10.1190/1.1443535.
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Green’s concept of an equivalent‐source layer is invoked to construct a data adaptive, zero‐phase, Wiener band‐pass filter for regional‐residual gravity anomaly separation. The observed gravity field’s Fourier power spectrum is modeled with two Green’s equivalent‐source layers, one equivalent layer for the shallower (residual field) geologic sources, and a second equivalent layer for the deeper (regional field) geologic sources. The depths and average physical property contrasts of the two equivalent layers are determined by fitting the observed gravity field’s Fourier power spectrum with a two‐layer spectral model. Each equivalent layer is simulated by a horizontal thin sheet with randomly varying and randomly distributed point density sources spread throughout it. Adopting such a theoretical model for the Fourier power spectrum yields a stable and well‐behaved filter transfer function. Like all band‐pass filtering though, the method is ineffective in the case of insufficient vertical separation between the shallow and deep geologic sources whose gravity anomalies it is desired to separate. The filter design process is simple and easily automated, being well‐suited for modular implementation in a “filter tool kit” applications program running on either a workstation or personal computer. Equally important, the method yields repeatable, interpreter‐independent results.