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1
KUWAHARA, Yasuto, and Hisao ITO. "Subsurface Exploration with Vertical Seismic Profiles." Journal of Geography (Chigaku Zasshi) 104, no. 7 (1995): 1008–18. http://dx.doi.org/10.5026/jgeography.104.7_1008.
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Payne, Michael A. "Looking ahead with vertical seismic profiles." GEOPHYSICS 59, no. 8 (August 1994): 1182–91. http://dx.doi.org/10.1190/1.1443676.
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Several operations enhance our ability to predict the subsurface below the bottom total depth (TD) of the well when applied to zero‐offset vertical seismic profiling (VSP) data. Other key issues regarding the use of VSP data in this fashion are resolution and look‐ahead distance. An impedance log is the most useful form for presenting VSP data to look ahead of the drill bit. The VSP composite trace must first tie reliably to the surface seismic section and to the well log synthetic seismogram. The impedance log is obtained by inverting this VSP composite trace. However, before performing the inversion, we need to (1) correct the composite trace for attenuation effects below TD and (2) input velocities to provide low‐frequency information. An exponential gain function applied to the VSP data below TD adequately compensates for the loss of amplitude caused by attenuation. A calibration of the seismically derived velocities with VSP velocities yields the necessary low‐frequency information. These concepts are illustrated using a field data set and its subset truncated above TD. The output of these operations on the VSP data are compared to well log data. The question of resolution with these data was determined with a model VSP data set based on the well log data. The investigations indicate that the resolution attainable from look‐ahead data is on the order of 50–75 ft (15–23 m). This is one‐quarter seismic wavelength for the frequencies present in these data. In addition, the maximum look‐ahead distance for these data is shown to be easily 2000 ft (600) m and, perhaps, 4000 ft (1200 m5). By way of illustration, the techniques described and investigated 6were applied to an offshore VSP data set to yield an impedance log. After calibrating this curve with the well log data, the base of the target sand was correctly identified below TD. This prediction successfully yielded the thickness of the sand. Individual zones within the sand unit were identified with less confidence.
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Haldorsen, Jakob B. U., Douglas E. Miller, and John J. Walsh. "Multichannel Wiener deconvolution of vertical seismic profiles." GEOPHYSICS 59, no. 10 (October 1994): 1500–1511. http://dx.doi.org/10.1190/1.1443540.
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We describe a technique for performing optimal, least‐squares deconvolution of vertical seismic profile (VSP) data. The method is a two‐step process that involves (1) estimating the source signature and (2) applying a least‐squares optimum deconvolution operator that minimizes the noise not coherent with the source signature estimate. The optimum inverse problem, formulated in the frequency domain, gives as a solution an operator that can be interpreted as a simple inverse to the estimated aligned signature multiplied by semblance across the array. An application to a zero‐offset VSP acquired with a dynamite source shows the effectiveness of the operator in attaining the two conflicting goals of adaptively spiking the effective source signature and minimizing the noise. Signature design for seismic surveys could benefit from observing that the optimum deconvolution operator gives a flat signal spectrum if and only if the seismic source has the same amplitude spectrum as the noise.
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Dodds, K., and P. Farmer. "3D Vertical Seismic Profiles: A Users' Guide." Journal of Petroleum Technology 50, no. 01 (January 1, 1998): 50–53. http://dx.doi.org/10.2118/0198-0050-jpt.
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Dietrich, Michel, and Michel Bouchon. "Synthetic vertical seismic profiles in elastic media." GEOPHYSICS 50, no. 2 (February 1985): 224–34. http://dx.doi.org/10.1190/1.1441912.
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Numerical simulations of vertical seismic profiles in flat‐layered elastic media using the discrete wavenumber method are presented. The effect of source‐borehole sep‐ aration on recorded wave types and amplitudes is studied. For nonzero source offsets, transverse and converted waves become very important and can be more energetic than the direct compressional arrivals. A systematic comparison of results from acoustic and elastic simulations shows that the acoustic approximation is quite valid for a zero source offset but becomes inadequate when the configuration of the source and vertical geo‐ phone array is two‐dimensional. Recording of both pressure and displacement allows a simple separation of transverse and compressional arrivals as long as the effect of the borehole on the incoming waves can be neglected.
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Hu, Liang‐Zie, and George A. McMechan. "Wave‐field transformations of vertical seismic profiles." GEOPHYSICS 52, no. 3 (March 1987): 307–21. http://dx.doi.org/10.1190/1.1442305.
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Vertical seismic profile (VSP) data may be partitioned in a variety of ways by application of wave‐field transformations. These transformations provide insights into the nature of the data and aid in the design of processing operations. Transformations are implemented in a reversible sequence that takes the observed VSP data from the depth‐time (z-t) domain through the slowness‐time intercept (p-τ) domain (by a slant stack), to the slowness‐frequency (p-ω) domain (by a 1-D Fourier transform over τ), to the wavenumber‐frequency (k-ω) domain (by resampling using the Fourier central‐slice theorem), and finally back to the z-t domain (by an inverse 2-D Fourier transform). Multidimensional wave‐field transformations, combined with k-ω, p-ω, and p-τ filtering, can be applied to wave‐field resampling, interpolation, and extrapolation; separation of P-waves and S-waves; separation of upgoing and downgoing waves; and wave‐field decomposition for isolation, identification, and analysis of arrivals.
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Grivelet, Pierre A. "Inversion of vertical seismic profiles by iterative modeling." GEOPHYSICS 50, no. 6 (June 1985): 924–30. http://dx.doi.org/10.1190/1.1441971.
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I present an application of iterative modeling to the inversion of vertical seismic profiles (VSPs). This method is derived from linear inversion which allows the extraction from VSP data of an impedance profile as a function of time and thus permits the prediction of impedance ahead of the drill bit. There are two steps in this process: first, detection of the major events on the seismogram which is achieved by a recursive detection algorithm; and second, an optimal estimate of the impedances carried out by a gradient algorithm. Seismic data are band‐limited, and consequently the solution of the inversion is nonunique. This nonuniqueness is handled by assuming a piecewise‐constant or blocked impedance model and by adding a priori constraints. Some synthetic examples are used to illustrate the method, and a field example shows a comparison between an impedance profile extracted from VSP data with this inversion method and an impedance profile from well logging data. In this example the accurate prediction of impedance values illustrates the usefulness of the method.
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Gaiser, James E., Terrance J. Fulp, Steve G. Petermann, and Gary M. Karner. "Vertical seismic profile sonde coupling." GEOPHYSICS 53, no. 2 (February 1988): 206–14. http://dx.doi.org/10.1190/1.1442456.
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P-wave and S-wave displacements occur at high angles of incidence in vertical seismic profiles (VSPs). Therefore, the coupling of a geophone sonde to the borehole wall must be rigid in all directions. A sonde that is well coupled should have no resonant frequency within the seismic band and should provide geophone outputs that accurately represent the earth’s ground motion. An in‐situ coupling response experiment conducted under normal VSP field conditions provides a measure of the sonde‐to‐borehole wall coupling. The sonde is locked in the borehole and a surface source is excited at different offsets and azimuths. An analysis of the P-wave direct arrivals enhances damped oscillations that represent an estimate of the coupling impulse response. This response is characterized by the viscoelastic behavior of a Kelvin model related to the complex compliance [Formula: see text], where κ is the elastic spring constant, η is the damping constant, and ω is the angular frequency. The complex modulus κ−iωη is proportional to the contact width of the sonde with the borehole wall. Increasing the width by a factor of 4.5 causes a similar increase in κ−iωη where the resonant frequency and initial amplitude of the coupling impulse response increase by a factor of two. Also, the initial amplitude of the coupling impulse response appears to be inversely proportional to the locking force of the sonde. For a constant contact width, increasing the locking force by a factor of 1.37 decreases the amplitude of the response by 3.5 dB.
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Aminzadeh, F., and J. M. Mendel. "Synthetic vertical seismic profiles for nonnormal incidence plane waves." GEOPHYSICS 50, no. 1 (January 1985): 127–41. http://dx.doi.org/10.1190/1.1441823.
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Vertical seismic profiles (VSPs) are, by definition, recordings of seismic signals (total upgoing and downgoing seismic wave fields) at different depth points, usually at equally spaced intervals [Formula: see text], i = 1, 2, …, I. In a nonnormal incidence (NNI) elastic model, where each layer is described by thickness, density, and P- and S-wave velocities, the mapping between time and depth needed to generate synthetic VSPs is not usually straightforward. In this paper we develop a relatively simple procedure for generating synthetic vertical and horizontal direction plane wave NNI VSPs. No spatial discretization is necessary. We (1) compute two surface seismograms, one vertical and the other horizontal, exactly as described in Aminzadeh and Mendel (1982); and (2) downward continue the surface seismograms to fixed VSP depth points. This paper demonstrates an algorithm for downward continuation of an elastic wave field using state‐space representation and gives simulations which illustrate both z- and x-direction primaries and complete VSPs for different geologic models and different incident angles.
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Boulfoul, M., and Doyle R. Watts. "Application of instantaneous rotations to S‐wave vertical seismic profiling." GEOPHYSICS 62, no. 5 (September 1997): 1365–68. http://dx.doi.org/10.1190/1.1444240.
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The petroleum exploration industry uses S‐wave vertical seismic profiling (VSP) to determine S‐wave velocities from downgoing direct arrivals, and S‐wave reflectivities from upgoing waves. Seismic models for quantitative calibration of amplitude variation with offset (AVO) data require S‐wave velocity profiles (Castagna et al., 1993). Vertical summations (Hardage, 1983) of the upgoing waves produce S‐wave composite traces and enable interpretation of S‐wave seismic profile sections. In the simplest application of amplitude anomalies, the coincidence of high amplitude P‐wave reflectivity and low amplitude S‐wave reflectivity is potentially a direct indicator of the presence of natural gas.
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Oristaglio, Michael L. "A guide to current uses of vertical seismic profiles." GEOPHYSICS 50, no. 12 (December 1985): 2473–79. http://dx.doi.org/10.1190/1.1441878.
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Vertical seismic profiles (VSPs) are small‐scale seismic surveys in which geophones are lowered into a well to record waves traveling both down into the earth (direct waves from the surface source and downgoing multiples) and back toward the surface (primary reflections and upgoing multiples). VSPs thus contain information about the reflection and transmission properties of the earth with a coverage that depends upon the geometry of the VSP experiment and the structure near the well. This article describes the uses of VSPs in seismic exploration that have been published in the last three years and is designed to complement the more detailed surveys by Hardage (1983) and Balch and Lee (1984). When the earth is horizontally layered, the well is vertical, and the source is close to the wellhead, upgoing and downgoing waves recorded by the VSP travel vertically, and the VSP can be used to calibrate surface seismic sections by providing the time‐to‐depth curve and allowing a detailed analysis of reflection and transmission properties of the earth at a given location. These applications rely heavily on signal processing to separate the upgoing and downgoing waves and to study their relationships to data recorded at the surface. When the earth varies laterally or when the source is offset from the well, the VSP can be used to complement surface surveys by providing high‐resolution images of structure near the well. Current work has concentrated on forming images from the reflected waves by the methods of common‐depth‐point (CDP) stacking and migration. Tomographic methods for inverting the traveltimes and amplitudes of transmitted waves are also being developed and will become important when downhole arrays and powerful downhole sources are available. The most significant advance in the next few years, however, will be the development of a reliable three‐axis tool with internal devices for determining both the orientation of the tool and the quality of its coupling to the borehole wall.
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Zimmerman, Linda J., and Sen T. Chen. "Comparison of vertical seismic profiling techniques." GEOPHYSICS 58, no. 1 (January 1993): 134–40. http://dx.doi.org/10.1190/1.1443343.
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To study the imaging characteristics of various vertical seismic profiling techniques, two vertical seismic profiles (VSP) and a reversed vertical seismic profile (RVSP), where source and receiver positions are interchanged, were collected in the Loudon Oil Field in Illinois. Both VSPs were collected using a line of dynamite charges on the surface as sources. One was collected with geophones and the other with hydrophones as downhole receivers. The RVSP was collected by detonating 25 gram explosive charges in a well and detecting the seismic response with geophones at the surface. Three subsurface images (VSP with geophones, VSP with hydrophones, and RVSP) were produced using VSP-CDP transforms. For comparison, a surface seismic profile was collected along the same line with dynamite sources and vertical geophone receivers. The RVSP and hydrophone VSP stacked sections both produced higher frequency images at shallower depths than did the geophone VSP stacked section. However, the lower frequency geophone VSP stacked section produced an interpretable subsurface image at much greater depths than either the RVSP or the hydrophone VSP sections. The differences are due in part to the more powerful surface sources that were used for the VSPs than the downhole sources used for the RVSP. Furthermore, tube‐wave noise was a more severe problem for both the RVSP and the hydrophone VSP than for the geophone VSP. The results of this experiment demonstrate that if tube‐wave noise could be suppressed, hydrophone VSPs would provide attractive alternatives to geophone VSPs, because it is much easier and cheaper to deploy multilevel hydrophones downhole than geophones. Also, if a high‐powered, nondestructive source is developed, RVSP could be a practical alternative to VSP since one can easily lay out numerous receivers on the surface to record multioffset or three‐dimensional (3-D) VSP data.
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Campbell, Allan, Andrew Fryer, and Suzanne Wakeman. "Vertical seismic profiles—more than just a corridor stack." Leading Edge 24, no. 7 (July 2005): 694–97. http://dx.doi.org/10.1190/1.1993259.
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Ramsden, C. R. T., A. R. James, and E. A. Howell. "Case History – Vertical Seismic Profiles At The Harriet Oilfield." Exploration Geophysics 18, no. 1-2 (March 1, 1987): 179–82. http://dx.doi.org/10.1071/eg987179.
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Dietrich, Michel, and Michel Bouchon. "Measurements of attenuation from vertical seismic profiles by iterative modeling." GEOPHYSICS 50, no. 6 (June 1985): 931–49. http://dx.doi.org/10.1190/1.1441972.
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We present a numerical simulation of vertical seismic profiles (VSP) using the discrete horizontal wavenumber representation of seismic wave fields. The theoretical seismograms are computed in the acoustic case for flat layered media, and they include the effects of absorption and velocity dispersion. A study using the synthetic seismograms was conducted to investigate the accuracy and resolution of attenuation measurements from VSP data. It is shown that in finely layered media estimates of the anelastic attenuation obtained by use of the reduced spectral ratio method are usually inaccurate when the attenuation is measured over a small vertical extent. An iterative method is presented which improves the resolution of the measurements of intrinsic dissipation. This method allows determination for synthetic data of the quality factor over depth intervals of about one wavelength of the dominant seismic frequency.
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IOANNIS, I. F. "Prospecting for voids with vertical radar profiling." Bulletin of the Geological Society of Greece 34, no. 4 (January 1, 2001): 1363. http://dx.doi.org/10.12681/bgsg.17229.
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Vertical Radar Profiling measurements were conducted to image subsurface cavities encountered in Akrotiri Archaeological Excavations area on Thera Island. The vertical radar profiling technique is able to explore much deeper than conventional surface GPR because it uses wells. The transmitting-receiving antenna unit was moved within the excavated well along six vertical profile lines in equally divided positions. A local electrical resistivity survey preceded the GPR profiles to investigate if the conductivity of the pyroclastic formation satisfies the presuppositions to conduct GPR measurements. The vertical GPR profiles revealed locations where cavities exist but they were unable to show their shape and extent. Cross-well seismic tomography images supported the vertical radar profiling results.
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Larsen, Anne Louise, Marit Ulvmoen, Henning Omre, and Arild Buland. "Bayesian lithology/fluid prediction and simulation on the basis of a Markov-chain prior model." GEOPHYSICS 71, no. 5 (September 2006): R69—R78. http://dx.doi.org/10.1190/1.2245469.
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A technique for lithology/fluid (LF) prediction and simulation from prestack seismic data is developed in a Bayesian framework. The objective is to determine the LF classes along 1D profiles through a reservoir target zone. A stationary Markov-chain prior model is used to model vertical continuity of LF classes along the profile. The likelihood relates the LF classes to the elastic properties and to the seismic data, and it introduces vertical correlation because the seismic data are band-limited. An approximation of the likelihood model provides an approximate posterior model that is a Markov chain. The approximate posterior can be assessed by an exact and efficient recursive algorithm. The LF inversion approach is evaluated on a synthetic 1D profile that is inspired by a North Sea sandstone reservoir. With a realistic wavelet-colored noise model and a S/N ratio of three in the seismic data, the results are reliable. The LF classes and the interfaces between zones are largely correct. The prediction uncertainty increases if the number of zones increases and zone thicknesses decreases. The study clearly demonstrates the impact of a vertically coupled prior Markov model for the LF classes.
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Vignoli, Giulio, Rita Deiana, and Giorgio Cassiani. "Focused inversion of vertical radar profile (VRP) traveltime data." GEOPHYSICS 77, no. 1 (January 2012): H9—H18. http://dx.doi.org/10.1190/geo2011-0147.1.
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The reconstruction of the GPR velocity vertical profile from vertical radar profile (VRP) traveltime data is a problem with a finite number of measurements and imprecise data, analogous to similar seismic techniques, such as the shallow down-hole test used for S-wave velocity profiling or the vertical seismic profiling (VSP) commonly used in deeper exploration. The uncertainty in data accuracy and the error amplification inherent in deriving velocity estimates from gradients of arrival times make this an example of an ill-posed inverse problem. In the framework of Tikhonov regularization theory, ill-posedness can be tackled by introducing a regularizing functional (stabilizer). The role of this functional is to stabilize the numerical solution by incorporating the appropriate a priori assumptions about the geometrical and/or physical properties of the solution. One of these assumptions could be the existence of sharp boundaries separating rocks with different physical properties. We apply a method based on the minimum support stabilizer to the VRP traveltime inverse problem. This stabilizer makes it possible to produce more accurate profiles of geological targets with compact structure. We compare more traditional inversion results with our proposed compact reconstructions. Using synthetic examples, we demonstrate that the minimum support stabilizer allows an improved recovery of the profile shape and velocity values of blocky targets. We also study the stabilizer behavior with respect to different noise levels and different choices of the reference model. The proposed approach is then applied to real cases where VPRs have been used to derive moisture content profiles as a function of depth. In these real cases, the derived sharper profiles are consistent with other evidence, such as GPR zero-offset profiles, GPR reflections and known locations of the water table.
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Christie, P. A. F., and J. A. Dangerfield. "Borehole seismic profiles in the Ekofisk Field." GEOPHYSICS 52, no. 10 (October 1987): 1328–45. http://dx.doi.org/10.1190/1.1442246.
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In October 1983 a major borehole seismic survey was carried out in the Ekofisk oil field in the Norwegian sector of the North Sea on behalf of the Phillips Petroleum Licence 018 group of companies. A vertical seismic profile with the source vertically above the geophone in the highly deviated wellbore and three multilevel walk‐away borehole profiles were acquired in an area showing very poor surface seismic returns owing to gas charging in the overlying sediments. The processing of the data through to a series of conventional common‐midpoint sections has permitted detailed interpretation of the top of the Ekofisk formation and the top of the Tor formation apart from well control. Both formations are producers separated by a tight zone. The Tor formation is the primary horizon to waterflood. Information as to its lateral continuity is important in the location of proposed waterflood injector wells. Prior to the survey, the field was considered effectively unfaulted. An apparent graben lying subparallel to the borehole was detected by the surveys. Reflections from below the reservoir formations are evident. A byproduct of the survey is strong evidence for the existence of lateral velocity gradients or apparent transverse velocity isotropy associated with the overlying gas‐charged sediments. Subsequent to the oral presentation of this paper, the graben was drilled and found to have a 41 m fault throw, in its western flank.
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Harlan, William S. "Separation of signal and noise applied to vertical seismic profiles." GEOPHYSICS 53, no. 7 (July 1988): 932–46. http://dx.doi.org/10.1190/1.1442530.
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Inversion of the band‐limited one‐dimensional VSP response is nonunique because impedance functions with very different statistics produce equivalent responses. Least‐squares methods of inversion linearly transform noise and tend to produce impedance functions with a Gaussian distribution of amplitudes. I modify a least‐squares inversion procedure to exclude nonzero impedance derivatives that are significantly influenced by noise. The resulting earth model shows homogeneous intervals unless the data have reliable information to the contrary. The data are modeled with a one‐dimensional wave equation and three invertible functions: acoustic impedance, a source wavelet, and the traces’ amplification. First, a linearized least‐squares inverse perturbs the source function to model the downgoing wave. A relinearized inverse finds perturbations of all three modeling functions to account for first‐order reflections. Further iterations explain higher order reflections. To estimate the reliability of impedance perturbations, each linearized inversion is repeated for pure noise that equals or exceeds the noise in the data. Amplitude histograms are used to estimate probability density functions for the amplitudes of the signal and of the noise in the perturbations. Nonzero impedance derivatives are accepted as reliable if, according to the probability functions, the perturbations contain, with a high probability, only a small amount of noise. For a set of VSP data provided by L’Institut Francais du Petrole, four iterations allowed only a few nonzero impedance derivatives and modeled a recorded VSP as well as did a least‐squares inversion that accepted all proposed perturbations. Estimated probability densities for the remaining signal and noise were used to extract a tube wave that contained little signal.
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McMechan, George A. "Synthetic finite‐offset vertical seismic profiles for laterally varying media." GEOPHYSICS 50, no. 4 (April 1985): 627–36. http://dx.doi.org/10.1190/1.1441938.
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The analysis of vertical seismic profile (VSP) data is generally directed toward determination of rock properties (such as velocity, impedance, attenuation, and anisotropy) as functions of depth (that is, in a one‐dimensional model). If VSPs are extended to include observations from sources at multiple, finite offsets, then lateral variation in structure near the drill hole can be studied. Synthetic offset VSPs are computed by an acoustic finite‐difference algorithm for two‐dimensional models that include the main types of structural traps. These show that diagnostic lateral variations can be detected and interpreted in VSPs. In a VSP, lateral structure variations may produce changes in the type and number of arrivals, in amplitudes, in time and phase shifts, in interference patterns, in curvature of arrival branches, and in the focusing and defocusing of energy. All of these effects are functions of the positions of the source(s) and receiver(s); numerical modeling is a potentially useful tool for interpretation of VSP data from laterally varying structure.
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Snyder, David, Gervais Perron, Karen Pflug, and Kevin Stevens. "New insights into the structure of the Sudbury Igneous Complex from downhole seismic studies." Canadian Journal of Earth Sciences 39, no. 6 (June 1, 2002): 943–51. http://dx.doi.org/10.1139/e02-013.
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New vertical seismic profiles from the northwest margin of the Sudbury impact structure provide details of structural geometries within the lower impact melt sheet (usually called the Sudbury Igneous Complex) and the sublayer norite layer. Vertical seismic profile sections and common depth point transformation images display several continuous reflections that correlate with faults and stratigraphic boundaries logged from drill cores. Of four possible mechanisms that explain repeated rock units, late-stage flow or normal faulting that occurred within the last layers to cool and crystallize might best explain the observations, especially the most prominent reflectors observed in the seismic data. These results reaffirm previously proposed two-stage cooling and deformation models for the impact melt sheet.
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Kazei, Vladimir, Oleg Ovcharenko, Pavel Plotnitskii, Daniel Peter, Xiangliang Zhang, and Tariq Alkhalifah. "Mapping full seismic waveforms to vertical velocity profiles by deep learning." GEOPHYSICS 86, no. 5 (August 31, 2021): R711—R721. http://dx.doi.org/10.1190/geo2019-0473.1.
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Building realistic and reliable models of the subsurface is the primary goal of seismic imaging. We have constructed an ensemble of convolutional neural networks (CNNs) to build velocity models directly from the data. Most other approaches attempt to map full data into 2D labels. We exploit the regularity of seismic acquisition and train CNNs to map gathers of neighboring common midpoints (CMPs) to vertical 1D velocity logs. This allows us to integrate well-log data into the inversion, simplify the mapping by using the 1D labels, and accommodate larger dips relative to using single CMP inputs. We dynamically generate the training data in parallel with training the CNNs, which reduces overfitting. Data generation and training of CNNs is more computationally expensive than conventional full-waveform inversion (FWI). However, once the network is trained, data sets with similar acquisition parameters can be inverted much faster than with FWI. The multiCMP CNN ensemble is tested on multiple realistic synthetic models, performs well, and was combined with FWI for even better performance.
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Malinverno, Alberto, and W. Scott Leaney. "Monte-Carlo Bayesian look-ahead inversion of walkaway vertical seismic profiles." Geophysical Prospecting 53, no. 5 (September 2005): 689–703. http://dx.doi.org/10.1111/j.1365-2478.2005.00496.x.
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Schaffner, Jack, Mike Reisinger, and Johnny W. Rutherford. "Offset vertical seismic profiles define shale boundaries in Morgan’s Bluff Field." Leading Edge 13, no. 11 (November 1994): 1095–100. http://dx.doi.org/10.1190/1.1436995.
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Bell, David W., and Vernon D. Cox. "Process for separating upgoing and downgoing events on vertical seismic profiles." Journal of the Acoustical Society of America 86, no. 4 (October 1989): 1629. http://dx.doi.org/10.1121/1.398646.
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Chiu, Stephen K. L., and Robert R. Stewart. "Tomographic determination of three‐dimensional seismic velocity structure using well logs, vertical seismic profiles, and surface seismic data." GEOPHYSICS 52, no. 8 (August 1987): 1085–98. http://dx.doi.org/10.1190/1.1442374.
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A tomographic technique (traveltime inversion) has been developed to obtain a two‐ or three‐dimensional velocity structure of the subsurface from well logs, vertical seismic profiles (VSP), and surface seismic measurements. The earth was modeled by continuous curved interfaces (polynomial or sinusoidal series), separating regions of constant velocity or transversely isotropic velocity. Ray tracing for each seismic source‐receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized Snell’s law. Surface‐to‐borehole and surface‐to‐surface rays were included. A damped least‐squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes. Synthetic results indicated the following conclusions. For noise‐free cases, the inversion converged closely from the initial guess to the true model for either surface or VSP data. Adding random noise to the observations and performing the inversion indicated that (1) using surface data alone allows reconstruction of the broad velocity structure but with some inaccuracy; (2) using VSP data alone gives a very accurate but laterally limited velocity structure; and (3) the integration of both data sets produces a more laterally extensive, accurate image of the subsurface. Finally, a field example illustrates the viability of the method to construct a velocity structure from real data.
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Hu, Liang-Zie, George A. McMechan, and Jerry M. Harris. "Elastic finite-difference modeling of cross-hole seismic data." Bulletin of the Seismological Society of America 78, no. 5 (October 1, 1988): 1796–806. http://dx.doi.org/10.1785/bssa0780051796.
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Abstract Cross-hole seismic data exhibit unique characteristics not seen in surface survey data or even in vertical seismic profile data. These are, to a large extent, due to the near-horizontal propagation involved. Transmitted, reflected, evanescent, guided, and converted waves are all prominent; these require an elastic algorithm for realistic simulation. Elastic finite-differences are used to synthesize responses (both fixed-time snapshots and seismogram profiles) for a series of two-dimensional models of increasing complexity. Special emphasis is given to guided waves in continuous and segmented low-velocity zones.
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Sheen, Katy L., Nicky White, C. P. Caulfield, and Richard W. Hobbs. "Estimating Geostrophic Shear from Seismic Images of Oceanic Structure*." Journal of Atmospheric and Oceanic Technology 28, no. 9 (September 1, 2011): 1149–54. http://dx.doi.org/10.1175/jtech-d-10-05012.1.
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Abstract It is shown that geostrophic vertical shear estimates can be recovered from seismic (i.e., acoustic) images of thermohaline structure. In the Southern Ocean, the Antarctic Circumpolar Current forms a loop within the Falkland Trough before it flows northward into the Argentine Basin. Seismic profiles that cross this loop show the detailed structure of different water masses with a horizontal resolution of O(10 m). Coherent seismic reflections are tilted in response to current flow around the Falkland Trough. Average slopes were measured on length scales that are large enough to ensure that the geostrophic approximation is valid (i.e., with a Rossby number <0.1). By combining shear estimates with satellite altimetric measurements and acoustic Doppler current profiles, geostrophic velocities can be calculated throughout the data volume. This technique for estimating geostrophic vertical shear from legacy seismic images yields useful information about the spatial and temporal variation of mesoscale circulation.
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Cai, Qi Peng, Yun Huang, and Fan Yan Meng. "Normalized Soil Deformation Induced by Underlying Bedrock Fault." Advanced Materials Research 790 (September 2013): 150–54. http://dx.doi.org/10.4028/www.scientific.net/amr.790.150.
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Prediction of subsurface ground deformation during bedrock faulting is important for structures located at potential seismic areas. In this paper, a theoretical approach was developed based on error function. Settlement profiles are found to be well represented using error function. Normalization issues of settlement profiles are discussed and it is found that the vertical displacements can be normalized with vertical displacement of the bedrock hanging wall h.
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Rickett, James. "Integrated estimation of interval-attenuation profiles." GEOPHYSICS 71, no. 4 (July 2006): A19—A23. http://dx.doi.org/10.1190/1.2209722.
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Quantitative estimates of seismic attenuation are useful for a variety of applications, ranging from seismic-acquisition design, to seismic processing, amplitude analysis, and reservoir characterization. I frame the estimation of interval attenuation from a set of seismic wavelets as a linear inversion of their log-amplitude spectra. The initial spectrum at the first depth location and a set of depth-varying amplitude scalers are estimated simultaneously with an effective-attenuation [Formula: see text] profile. The algorithm can be regarded as a tomographic extension of the spectral-ratio method that uses all the information available in the amplitude spectra, appropriately weighted so that estimates are not biased by noise. Constraints can be applied to ensure the [Formula: see text] values vary smoothly, and solving for log [Formula: see text] rather than [Formula: see text] ensures only positive attenuation values. Tests on synthetic and field data illustrate the trade-off between vertical resolution and sensitivity to noise. A covariance study indicates that improvements in interval-attenuation estimates over the traditional spectral-ratio method come from systematic-noise handling and the explicit constraints on [Formula: see text], rather than the fact that the inversion ties the log-spectral data together with a single estimate of the spectrum at the first depth location.
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Köhler, K., and M. Koenig. "Reconstruction of reflecting structures from vertical seismic profiles with a moving source." GEOPHYSICS 51, no. 10 (October 1986): 1923–38. http://dx.doi.org/10.1190/1.1442049.
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When a vertical seismic profile (VSP) is recorded, the illuminated part of a reflector depends upon the shape and position of the reflector itself as well as on the seismic velocities and the positions of sources and receivers. A preferable arrangement for the investigation of structures of reflectors is to fix the receiver(s) at constant depth(s) in the well and move the source horizontally along a line at the Earth’s surface, usually called a “multioffset VSP” (MSP) or “walkaway VSP.” As a test of the resolution power of this survey geometry, synthetic records were generated from a subsurface model by inverse Kirchhoff migration. Three different methods were applied for the reconstruction. Wavefront construction leads to the correct shape of the reflectors, thus assuring the validity of the modeling method applied. Reflection‐point mapping delivered a near similarity to the model, but without focusing fault edges. Kirchhoff migration resulted in a detailed image of the reflectors with fault edges focused. Application of reflection‐point mapping and Kirchhoff migration to a real survey delivered results consistent with results from a survey at the Earth’s surface.
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Carr, Bradley J., and Zoltan Hajnal. "P- and S-wave characterization of near‐surface reflectivity from glacial tills using vertical seismic profiles." GEOPHYSICS 64, no. 3 (May 1999): 970–80. http://dx.doi.org/10.1190/1.1444606.
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Fundamental reflectivity properties are established within the glacial deposits of central Saskatchewan, Canada. Multicomponent vertical seismic profile (VSP) data collected in three shallow boreholes are used to obtain detailed acoustic property information within the first 80 m of the near‐surface strata. The integration of both P- and S-wave VSP data, in conjunction with other borehole geophysics, provided a unique opportunity to obtain in‐situ seismic reflection response properties in layered clay and sand tills. P- and S-wave interval velocity profiles, in conjunction with P- and S-wave VSP reflectivities are analyzed to provide insight into seismic wavefield behavior within ∼80 m of the surface. In general, shear wave energy identifies more reflective intervals than the P-wave energy because of better vertical resolution for S-wave energy (0.75 m) compared to P-wave energy (2.3 m) based on quarter wavelength criterion. For these saturated, unconsolidated glacial deposits, more details about the lithologic constituents and in‐situ porosity are detectable from the S-wave reflectivity, but P-wave reflections provide a good technique for mapping the bulk changes. The principal cause of seismic reflectivity is the presence and/or amount of sand, and the degree of fluid‐filled porosity within the investigated formations.
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Bakulin, Andrey, Marta Woodward, Dave Nichols, Konstantin Osypov, and Olga Zdraveva. "Localized anisotropic tomography with well information in VTI media." GEOPHYSICS 75, no. 5 (September 2010): D37—D45. http://dx.doi.org/10.1190/1.3481702.
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We develop a concept of localized seismic grid tomography constrained by well information and apply it to building vertically transversely isotropic (VTI) velocity models in depth. The goal is to use a highly automated migration velocity analysis to build anisotropic models that combine optimal image focusing with accurate depth positioning in one step. We localize tomography to a limited volume around the well and jointly invert the surface seismic and well data. Well information is propagated into the local volume by using the method of preconditioning, whereby model updates are shaped to follow geologic layers with spatial smoothing constraints. We analyze our concept with a synthetic data example of anisotropic tomography applied to a 1D VTI model. We demonstrate four cases of introducing additionalinformation. In the first case, vertical velocity is assumed to be known, and the tomography inverts only for Thomsen’s [Formula: see text] and [Formula: see text] profiles using surface seismic data alone. In the second case, tomography simultaneously inverts for all three VTI parameters, including vertical velocity, using a joint data set that consists of surface seismic data and vertical check-shot traveltimes. In the third and fourth cases, sparse depth markers and walkaway vertical seismic profiling (VSP) are used, respectively, to supplement the seismic data. For all four examples, tomography reliably recovers the anisotropic velocity field up to a vertical resolution comparable to that of the well data. Even though walkaway VSP has the additional dimension of angle or offset, it offers no further increase in this resolution limit. Anisotropic tomography with well constraints has multiple advantages over other approaches and deserves a place in the portfolio of model-building tools.
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von Huene, Roland, Dirk Klaeschen, and Cord Papenberg. "Potential of 3-D vertical seismic profiles to characterize seismogenic fault zones." Geochemistry, Geophysics, Geosystems 9, no. 7 (July 2008): n/a. http://dx.doi.org/10.1029/2008gc002013.
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URSIN, B., and B. ARNTSEN. "COMPUTATION OF ZERO-OFFSET VERTICAL SEISMIC PROFILES INCLUDING GEOMETRICAL SPREADING AND ABSORPTION*." Geophysical Prospecting 33, no. 1 (February 1985): 72–96. http://dx.doi.org/10.1111/j.1365-2478.1985.tb00422.x.
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Esmersoy, Cengiz. "Inversion of P and SV waves from multicomponent offset vertical seismic profiles." GEOPHYSICS 55, no. 1 (January 1990): 39–50. http://dx.doi.org/10.1190/1.1442770.
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Downgoing waves in multicomponent VSP experiments are used to obtain seismic P- and S-wave velocities as a function of depth and angle of incidence. If P and SV waveforms do not overlap in time at the depth of interest, local velocities of the medium are obtained by separate analysis of these events. The apparent velocity of the event (P or SV) is computed from the moveout across several neighboring depth locations. The angle of incidence of the same event is computed from the particle‐motion hodogram within an appropriately chosen time window. Then, the local medium velocity (P wave or S wave depending on the chosen event) is given by the apparent velocity multiplied by the cosine of the angle of incidence. Layer interfaces with reasonably sharp velocity contrasts are efficient P-wave to SV-wave converters, even at moderate angles of incidence. In offset VSP experiments, converted SV waves are generated with varying strengths at practically all depths. Consequently, the converted SV waveforms partially overlap with the direct P waveforms, making the separate event analysis difficult and inaccurate. These overlapping waveforms can be handled properly by modeling the data in a given time window as a superposition of several events. In particular, the downgoing data at each depth level are modeled as a superposition of a P wave and an SV wave, with local P and S velocities, angles of incidence, and waveforms as model parameters. These parameters are then estimated by minimizing the squared error between the observed data and the model‐generated data. The unknown waveforms are eliminated from the minimization problem, leaving only four nonlinear parameters (velocities and angles) for estimation. Once these four parameters are found, least‐squares estimates of waveforms are obtained by evaluating a simple expression.
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van Ginkel, Janneke, Elmer Ruigrok, and Rien Herber. "Using horizontal-to-vertical spectral ratios to construct shear-wave velocity profiles." Solid Earth 11, no. 6 (November 9, 2020): 2015–30. http://dx.doi.org/10.5194/se-11-2015-2020.
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Abstract. For seismic hazard assessment and earthquake hypocentre localization, detailed shear-wave velocity profiles are an important input parameter. Here, we present a method to construct a shear-wave velocity profiles for a deep unconsolidated sedimentary layer by using strong teleseismic phases and the ambient noise field. Gas extraction in the Groningen field, in the northern part of the Netherlands, is causing low-magnitude, induced seismic events. This region forms an excellent case study due to the presence of a permanent borehole network and detailed subsurface knowledge. Instead of conventional horizontal-to-vertical spectral ratios (H∕V ratios) from amplitude spectra, we calculate power spectral densities and use those as input for H∕V calculations. The strong teleseisms provide resonance recordings at low frequencies, where the seismic noise field is too weak to be recorded well with the employed geophones and accelerometers. The H∕V ratios of the ambient noise field are compared with several forward modelling approaches to quality check the teleseism-based shear-wave velocity profiles. Using the well-constrained depth of the sedimentary basin, we invert the H∕V ratios for velocity profiles. A close relationship is observed between the H∕V spectral ratios from the ambient noise field, shear-wave resonance frequencies and Rayleigh-wave ellipticity. By processing only five teleseismic events, we are able to derive shear-wave velocities for the deeper sedimentary sequence with a 7 % bias in comparison with the existing detailed velocity model for the Cenozoic sediments overlying the Groningen gas field. Furthermore, a relation between resonance frequency and unconsolidated sediment thickness is derived, to be used in other areas in the Netherlands, where detailed depth maps are not available.
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Liao, Zonghu, Lin Zhang, Long Wen, and Lianbo Zeng. "Collapse columns in Permian and Carboniferous Formations of coal, Qinshui Basin, China." Interpretation 8, no. 4 (October 26, 2020): SR33—SR35. http://dx.doi.org/10.1190/int-2020-0031.1.
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Seismic survey data collected for coal gas exploration show that there are many collapse columns distributed in the subsurface of Qinshui Basin, China. The interesting features of the collapse columns are observed by the seismic attributes, including the circular discontinuous patches on the horizon of the Shanxi Formation and multiple parallel discontinuities in vertical profiles of amplitudes. We speculate that the wide presence of these collapse columns are point constraints for the migration and accumulation of coal gas on a large scale. Geological feature: Collapse columns within coal reservoirs Seismic appearance: The coherence illuminates circular/oval discontinuities on the horizon of the Shanxi Formation; the vertical amplitude profiles show cylindrical/funnel-shaped discontinuities. Alternative interpretations: Fault damage zones; velocity pulldown from the overburden Features with similar appearance: Fault-karst in carbonate reservoir; reef pinnacles Formation: Permian Shanxi Formation and Carboniferous Taiyuan Formation Age: Late Permian Location: Qinshui Basin in Shanxi, north-central China Seismic data: Provided by PetroChina Huabei Oilfield Company Contributors: Zonghu Liao, Lin Zhang, and Lianbo Zeng Analysis tools: The seismic amplitude and attribute of coherence from the seismic survey (prestack time migrated)
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Huo, Jianjian, Binzhong Zhou, Qing Zhao, Iain M. Mason, and Ying Rao. "Migration-based filtering: Applications to geophysical imaging data." GEOPHYSICS 84, no. 4 (July 1, 2019): S219—S228. http://dx.doi.org/10.1190/geo2018-0703.1.
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Migration is used to collapse “diffractions,” i.e., to focus hyperbolic events that appear in the space-time of a seismic profile — into spots of finite area in the image space. These usually represent scattering objects. However, there are situations in which some of the energy can be focused by migration, and muted without significantly damaging the remaining echoes. Demigration or forward modeling then restores the remaining data, and the removed signals can be rebuilt by subtracting these restored data from the original records. This process can be classified as migration-based filtering. It is demonstrated by synthetic and field data that this filter can be used for suppressing unwanted coherent signals or separating/extracting wavefields of interest: (1) the suppression of ground roll in seismic shot gathers, (2) the suppression of axially guided arrivals in borehole radar profiles, (3) suppressing the direct arrivals to enhance Stoneley-wave reflections in full-waveform sonic logging data, and (4) separating up- and downgoing waves in vertical seismic profiles.
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Freire, Sergio L. M., and Tad J. Ulrych. "Application of singular value decomposition to vertical seismic profiling." GEOPHYSICS 53, no. 6 (June 1988): 778–85. http://dx.doi.org/10.1190/1.1442513.
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An essential part of the interpretation of vertical seismic profiles (VSP) is the separation of the upgoing and downgoing waves. This paper presents a new approach which is based on the decomposition of time‐shifted VSP sections into eigenimages, using singular value decomposition (SVD). The first few eigenimages of the time‐shifted VSP section contain the contributions of the horizontally aligned downgoing waves. The last few eigenimages contain the contribution of uncorrelated noise components. The separated upgoing waves are recovered as a partial sum of the eigenimages. Important aspects of this approach are that regular sampling of the recording levels is not required, that the first‐break times need not be measured with extreme accuracy, that noise rejection may be automatically included in the processing, and that eigenimages or sums of eigenimages which may be computed as part of the approach can provide important additional information.
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Whitmore, N. D., and Larry R. Lines. "Vertical seismic profiling depth migration of a salt dome flank." GEOPHYSICS 51, no. 5 (May 1986): 1087–109. http://dx.doi.org/10.1190/1.1442164.
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Vertical seismic profiles (VSPs) can supply information about both velocity and subsurface interface locations. Properly designed VSPs can be used to map steeply dipping interfaces such as salt dome flanks. Mapping subsurface interfaces with VSP data requires careful survey design, appropriate data processing, interval velocity estimation, and reflector mapping. The first of these four ingredients is satisfied, in most cases, by preacquisition modeling. The second is accomplished by careful data processing. Initial velocity estimates are provided by seismic tomography. Velocity‐model refinement is accomplished by a combination of iterative modeling and iterative least‐squares inversion. Finally, the resultant interval velocities are used in depth migration of the processed VSP. These four ingredients have been combined to map a salt dome flank.
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Swift, Stephen A., D. Lizarralde, Ralph A. Stephen, and Hartley Hoskins. "Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles." Journal of Geophysical Research: Solid Earth 103, B7 (July 10, 1998): 15361–76. http://dx.doi.org/10.1029/98jb00766.
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Takam Takougang, Eric M., and Youcef Bouzidi. "Imaging high-resolution seismic velocity from walkaway vertical seismic profile data in a carbonate reservoir using acoustic waveform tomography." GEOPHYSICS 83, no. 3 (May 1, 2018): B77—B85. http://dx.doi.org/10.1190/geo2017-0180.1.
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High-resolution seismic velocity was obtained using acoustic full-waveform tomography of walkaway vertical seismic profile (VSP) data from an oil field dominated by carbonate rocks, offshore Abu Dhabi in the United Arab Emirates. The data were collected in a deviated borehole with receivers located from 521 to 2742 m depth. The inversion was performed in the frequency domain. The success of the inversion was determined by three important factors: the starting model, the preconditioning of the input data, and the inversion strategy, which included an appropriate selection of a damping term [Formula: see text] in the Laplace–Fourier transformation. The inversion was performed between the frequencies of 4 and 50 Hz, and a logarithmic data residual was used. The extracted 1D velocity profiles from the final high-resolution velocity model correlate well with the sonic log, and estimated vertical incidence VSP velocities. The predicted data obtained by the final velocity model indicate a generally good fit with the field data, thus confirming the success of the inversion. A reverse time migrated section derived by the final velocity model provides additional structural details. The velocity model indicates anomalous zones of low-velocity values that correlate with known locations of hydrocarbon reservoirs.
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Sun, Robert, and George A. McMechan. "Nonlinear reverse‐time inversion of elastic offset vertical seismic profile data." GEOPHYSICS 53, no. 10 (October 1988): 1295–302. http://dx.doi.org/10.1190/1.1442407.
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An iterative nonlinear inversion algorithm for two‐dimensional elastic media gives estimates of P-velocity and S-velocity distributions from synthetic offset vertical seismic profiles. The algorithm is a hybrid of inversion and principles borrowed from reverse‐time migration. Gradients of the misfit function are dynamically determined by crosscorrelations of the computed incident wave fields with the scattered compressional and shear wave fields. Model perturbations are defined in the steepest descent direction. In order to optimize the sensitivity of the inversion to both compressional and shear velocity distributions, two data collection experiments are required, one with a compressional wave source and the other with a shear wave source. Inversion iterations alternate between the compressional and shear source data sets. In test examples, the new algorithm converges successfully to the correct solution when the starting model’s compressional and shear velocities deviated by as much as 20 percent from the correct solution.
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Jarvis, Kevin D., and Rosemary Knight. "Near‐surface VSP surveys using the seismic cone penetrometer." GEOPHYSICS 65, no. 4 (July 2000): 1048–56. http://dx.doi.org/10.1190/1.1444798.
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We have found that high‐quality vertical seismic profile (VSP) data can be collected for near‐surface applications using the seismic cone penetrometer. Cone‐mounted accelerometers are used as the VSP receivers, and a sledgehammer against the cone truck baseplate is used as a source. This technique eliminates the need to drill a borehole, thereby reducing the cost of the survey, and results in a less invasive means of obtaining VSP data. Two SH-wave VSP surveys were acquired over a deltaic sand/silt sequence and compared to an SH-wave common‐depth‐point (CDP) reflection profile. The VSP data were processed using a combination of singular‐value‐decomposition filtering, deconvolution, and f-k filtering to produce the final VSP extracted traces. The VSP traces correlate well with cone geotechnical logs and the CDP surface‐seismic data. The first breaks from the VSP can be used to generate shear‐wave velocity profiles that are important for time‐to‐depth conversion and the velocity correction of the CDP surface data.
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THYBO, H. "AN ALGORITHM FOR FAST TIME-DOMAIN COMPUTATION OF ONE-DIMENSIONAL SYNTHETIC VERTICAL SEISMIC PROFILES*." Geophysical Prospecting 34, no. 6 (October 1986): 833–44. http://dx.doi.org/10.1111/j.1365-2478.1986.tb00496.x.
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Bahavar, Manochehr, Zack J. Spica, Francisco J. Sánchez-Sesma, Chad Trabant, Arash Zandieh, and Gabriel Toro. "Horizontal-to-Vertical Spectral Ratio (HVSR) IRIS Station Toolbox." Seismological Research Letters 91, no. 6 (August 19, 2020): 3539–49. http://dx.doi.org/10.1785/0220200047.
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Abstract The horizontal-to-vertical spectral ratio (HVSR) for seismic ambient noise is a popular method that can be used to estimate the predominant frequency at a given site. In this article, we introduce the Incorporated Research Institutions for Seismology (IRIS) Data Management Center’s (DMC’s) openly available HVSR station toolbox. These tools offer a variety of ways to compute the spectral ratio by providing different averaging routines. The options range from the simple average of spectral ratios to the ratio of spectral averages. Computations take advantage of the available power spectral density estimates of ambient noise for the seismic stations, and they can be used to estimate the predominant frequency of the many three-component seismic stations available from the IRIS DMC. Furthermore, to facilitate the identification of the peaks in HVSR profiles for the assessment of the predominant frequency of station sites, the toolbox can also process the results of HVSR analysis to detect and rank HVSR peaks. To highlight the toolbox capabilities, three different examples of possible use of this toolbox for routine site-effect analysis are discussed: (1) site effects related to thawing in Arctic regions, (2) ground-motion amplification in urban area, and (3) estimation of station VS30.
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Lizarralde, Daniel, and Steve Swift. "Smooth inversion of VSP traveltime data." GEOPHYSICS 64, no. 3 (May 1999): 659–61. http://dx.doi.org/10.1190/1.1444574.
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Vertical seismic profile (VSP) direct arrivals provide an insitu measurement of traveltime with depth into the earth. In this note, we describe a weighted, damped least‐squares inversion of VSP traveltimes for a smooth velocity/depth function that inherently reveals the resolution of the data. Smooth velocity/depth profiles of this type are suitable for migration or as a starting models for waveform inversion or tomography. The application of this inversion is particularly simple, requiring only the value of the damping parameter to be determined, and this value is determined from residual statistics.
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Gusmeroli, Alessio, Tavi Murray, Roger A. Clark, Bernd Kulessa, and Peter Jansson. "Vertical seismic profiling of glaciers: appraising multi-phase mixing models." Annals of Glaciology 54, no. 64 (2013): 115–23. http://dx.doi.org/10.3189/2013aog64a106.
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Abstract We have investigated the speed of compressional waves in a polythermal glacier by, first, predicting them from a simple three-phase (ice, air, water) model derived from a published ground-penetrating radar study, and then comparing them with field data from four orthogonally orientated walkaway vertical seismic profiles (VSPs) acquired in an 80 m deep borehole drilled in the ablation area of Storglaciären, northern Sweden. The model predicts that the P-wave speed increases gradually with depth from 3700ms–1 at the surface to 3760ms–1 at 80m depth, and this change is almost wholly caused by a reduction in air content from 3% at the surface to <0.5% at depth. Changes in P-wave speed due to water content variations are small (<10 ms–1); the model’s seismic cold–temperate transition surface (CTS) is characterized by a 0.3% decrease downwards in P-wave speed (about ten times smaller than the radar CTS). This lack of sensitivity, and the small contrast at the CTS, makes seismically derived water content estimation very challenging. Nevertheless, for down-going direct-wave first arrivals for zero- and near-offset VSP shots, we find that the model-predicted travel times and field observations agree to within 0.2 ms, i.e. less than the observational uncertainties.