Academic literature on the topic 'Seismic refraction tomography'

It is often claimed that the first Fresnel zone associated with the dominant frequency represents the spatial resolution limit of traveltime tomography. We show, however, that the relevant Fresnel limit for tomographic resolution is the maximum, not the dominant frequency in the data. For physically realizable causal wavelets, the maximum frequency is infinite. In practice, noise lowers the effective possible maximum frequency. To demonstrate these points, synthetic seismic data were generated for traveltime picking and inversion for a single, small velocity anomaly embedded in a homogeneous background velocity. A variety of traveltime picking techniques were tested and compared for their ability to detect the presence of objects smaller than that Fresnel zone associated with the dominant frequency. All methods produced accurate ray-theoretical (infinite-frequency) picks from noise-free seismic data for objects smaller than the dominant-frequency Fresnel zone. For the lowest dominant frequencies with Fresnel zones many times larger than the object, picking methods that focus on features along the onset of the first arrival were the most accurate, while cross-correlation with a known wavelet preformed less accurately. First-onset picking methods perform better because they take advantage of the highest frequencies in the data, whereas the correlation wavelet is typically in line with the dominant frequency. All methods successfully detected the presence of objects smaller than a wavelength. The inversion of the traveltime picks from the different picking methods always recovered the position and shape of the object. Random noise at a range of signal-to-noise ratios was then added to the seismic data and the data were repicked. Pick times with different noise realizations are statistically centered on the noise-free pick, not the time that would be recorded in the absence of the object. Trace stacking prior to picking or the averaging of many picks improves the signal-to-noise ratio and can extract signal that is not detected on an individual pick. An averaging of traveltime picks also occurs during tomographic inversion. This inherent signal-to-noise improvement allows tomography to image objects that are undetectable in individual trace picks. The resolution of tomography is limited not by the Fresnel zone associated with the dominant frequency, but by the accuracy of the traveltime picks. Resolution is further improved by dense ray coverage.<br>Master of Science

High-resolution electrical-resistivity, seismic-refraction, and seismic-reflection surveys were performed at three locations in the Inner Bluegrass Region of Kentucky along coincident survey lines in order to correlate results and determine which method is most effective at locating karst features in this area. The first two survey locations at Slack’s Cave and the Kentucky Horse Park were chosen in order to investigate known karst features. High and low electrical-resistivity anomalies were correlated to air- and water-filled karst voids, respectively. Seismic velocity anomalies, including parabolic time suppressions, amplitude terminations, and surface-wave backscatters, were also observed and correlated to these karst voids. These findings were applied to a third location along Berea Road in order to investigate undiscovered karst voids. Three seismic targets were selected based on backscatter anomaly locations and were aligned in a northwest trend following the general bedrock dip, joint orientations, and suspected conduit orientation. Overall, the seismic-reflection method provided the highest resolution and least ambiguous results; however, integration of multiple methods was determined to help decrease ambiguities in interpretation created by the inherent non-uniqueness found in the results of each method.

The Chesapeake Bay impact structure is one of the largest and most well preserved impact structures on Earth. It has a unique morphology composed of an inner crater penetrating crystalline basement surrounded by a wider crater in the overlying sediments. In 2004, the U.S. Geological Survey conducted a seismic survey with the goals of constraining crater structure and in support of the drilling of a borehole into the deepest part of the crater. Travel-time and waveform inversion were applied to the data to produce a high-resolution velocity model of the crater. Low-fold reflection processing was also applied. Northeast of the crystalline crater, undeformed, eastward-sloping crystalline basement is ~1.5 km deep. The edge of the inner crater is at ~ 15 km radius and slopes gradually down to a depth of 1.5 - 1.8 km. A central peak of 4-5 km radius rises to a depth of ~0.8 km. Basement velocity in the crystalline crater is much lower than undeformed basement, which suggests ~10% fracturing of the crater floor, and up to 20% fracturing of the central uplift. A basement uplift and lateral change of velocity, interpreted as the edge of the transient crater, occurs at a radius of ~ 11 km. Assuming a 22 km diameter transient crater, scaling laws predict a ~30 km diameter crater and central peak diameter of 8-10 km. This indicates that post-impact collapse processes that created the ~ 30 km diameter crystalline crater were unaffected by the much weaker rheology of the overlying sediments.<br>Master of Science

The South Portuguese Zone (SPZ), which host world-class massive sulphide deposits, forms the southern fold-and-thrust belt of the Iberian Variscan orogeny. This thesis focuses on seismic-reflection and seismic-refraction processing efforts on a subset of the IBERSEIS deep seismic-reflection data set aiming at resolving the SPZ upper crust in high resolution. A comparison of different crooked-line seismic-reflection imaging schemes showed that a processing sequence involving dip-moveout corrections, a common-midpoint projection, and poststack time migration of common-offset gathers provided the most coherent images considering the crooked acquisition geometry. Correlation with surface-geological data allows four units of different reflection character to be identified: the ~0–2 km deep Upper Carboniferous Flysch group, the highly reflective ~2–4 km thick and up to ~5 km deep Volcano-Sedimentary Complex (VSC) group, and two deep Paleozoic metasedimentary units, with the shallower Phyllite-Quartzite group exposed in an antiform. Prominent diffracted energy was enhanced using a modified Kirchhoff imaging routine. High reflectivity and distinct diffractions mark extensive dike bands at 6–12 km depth, possibly related to the intense hydrothermal activity that led to the formation of the ore-bearing VSC group. Source-generated noise obscures potential signals from depths shallower than ~500m depth on the seismic-reflection sections. P- and SV-wave first-arrival traveltimes were inverted for velocity models imaging the shallowest crust. Overall, the velocity models correlate well with surface-geological data marking high (&gt;5.25 km/s) and uniform P-velocities for the Flysch unit in the southern SPZ. A prominent P-wave low-velocity body (~4.5 km/s) is resolved where the Phyllite-Quartzite unit forms the core of an antiform. P-velocities fluctuate the most in the northern SPZ with Flysch group units exhibiting high velocities (&gt;5.25 km/s) and VSC group bodies showing intermediate velocities (~5 km/s). Low VP/VS-ratios (~1.8) computed for the southern profile part are interpreted as less deformed Flysch-group units, whereas high VP/VS-ratios (~1.9) indicate fractured units.

The formation of oceanic lithosphere along ocean ridges, and the role that crustal magma chambers play in the accretionary process, continue to be fundamental issues in plate tectonics. To address these issues, a multi-receiver airgun/ocean bottom seismograph refraction line, designed to allow definition of lateral velocity and attenuation variations within the shallow crust, was shot across the Endeavour segment of the Juan de Fuca Ridge near 48° N, 129° W. A tomographic inversion procedure has been developed to invert the first arrival travel times and amplitudes from this profile for 2[sup D] velocity and attenuation structure. The inversion method is suited to multi-source, multi-receiver refraction profiles where source/receiver spacings are denser than for conventional profiles. The travel time-velocity inversion scheme is based on an iterative solution of the linearized problem and allows for determination of continuous velocity variations as well as geometry of subhorizontal interfaces. The iterative procedure requires a good initial estimate of the velocity model. In each iteration, two-point ray tracing is performed to construct a linear system relating travel time residuals to velocity perturbations. A damped least-squares algorithm is used to solve this system for a velocity perturbation which is used to update the current velocity estimate. Once the final velocity structure of the model has been determined, amplitudes can be inverted directly for attenuation. Tests to ascertain resolution of the method reveal horizontal smearing of the solution due to ray geometry, drop-off in resolution with depth, and the effects of source-receiver geometry and velocity structure on resolution. Parameter weighting is important in removing streaking effects (caused by inhomogeneous ray coverage) from the solution. For the purposes of ray tracing, the model is parameterized in terms of constant gradient (velocity and attenuation) cells, which allow use of analytic expressions for kinematic and dynamic ray properties, attenuation and inversion quantities. This parameterization causes scatter in the amplitudes calculated using zero-order asymptotic ray theory, a problem which is remedied by smoothing the velocity models before amplitude calculation. Application of this 2[sup D] tomographic inversion scheme to first arrival travel times and amplitudes for the cross-ridge refraction line produced a 4-layer model for the shallow crust. Layer 1 is 250 — 650 m thick, with v₁ = 2.5 km/s and [Nabla, sub z]v₁ = 0.5 s⁻¹. Layer 2 is ~800 m thick, v₂ = 4.8 km/s and [Nabla, sub z]v₂ — 1.0 s⁻¹. Layer 1 and layer 2 likely represent the sequence of extrusives whereas layer 3 (~800 m thick, v₃=5.8 km/s, [Nabla, sub z]v₃=0.5 s-1) and layer 4 (v₄=6.3 km/s, [Nabla, sub z]v₄=0.3 s⁻¹) are associated with the dike complex and massive gabbro sequence, respectively. An abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the pillow basalts. A low velocity-high attenuation anomaly (velocities decreased by < 0.4 km/s and Q ~20-100), which is interpreted as a zone of increased fracture porosity and/or permeability associated with axial hydrothermal circulation, exists beneath the ridge in layer 2 and upper layer 3. Smaller low velocity-attenuative zones in layer 2, located 8 km to either side of the ridge may be loci of off-axis hydrothermal circulation. No evidence is found for the existence of a crustal magma chamber in the depth range of 1.5 — 3.0 km below the seafloor. Tests indicate that a 1 X 1 km zone of partial melt represents the minimum dimension of such a feature that would be clearly detected by this refraction experiment. These results suggest that Endeavour Ridge may be experiencing a period of diminished magma supply with the magma chamber reduced or eliminated by hydrothermal circulation. Asymmetry of the velocity anomalies observed in layer 3 and layer 4 suggest that crustal temperatures are elevated by 125 — 200° C beneath the ridge and to the east relative to temperatures west of the ridge, indicating that a deep crustal or upper mantle melting anomaly may exist east of the ridge.<br>Science, Faculty of<br>Earth, Ocean and Atmospheric Sciences, Department of<br>Graduate

This dissertation is devoted to seismic tomography. I have implemented a new modelling tool for 3-D joint refraction and reflection travel-time tomography of wide-angle seismic data (TOMO3D). The reason behind this central objective is the evidence that the information based on 2-D seismic data does not allow to capture the structural complexity of many 3-D targets, and in particular that of the seismogenic zone in subduction margins. The scientific rationale for this statement, which justifies the central part of my thesis work, is based on the analysis of 2-D models obtained in the convergent margin of Nicaragua, a seismically active area where a textbook example of tsunami earthquake took place in 1992. In this application I modelled two perpendicular wide-angle seismic profiles for the characterisation of the overriding plate and the interplate fault. To do this, I applied TOMO2D, a state-of-the-art joint refraction and reflection 2-D travel-time tomography code. The inversion outcomes are two 2-D velocity models along both profiles, together with the 1-D geometry of the interplate boundary. In combination with other geophysical data measurements, namely coincident multichannel seismic profiles and gravity data, these models provide new constraints on the nature and structure of the margin, and in particular add new insights on the nucleation and propagation of the said earthquake and its tsunamigenic behaviour. Ultimately, this case study evidences the aforementioned limitations of 2-D modelling in the investigation of 3-D geological structures and phenomena. Following from this first application and with the idea of increasing the amount of data used in travel-time tomography, I focused on an a priori paradoxical phenomenon related to water-layer multiple phases, that under certain circumstances, is observed on wide-angle record sections. The interest of this study lies in the fact that this phenomenon can provide additional constraints on travel-time tomography models. First, I propose and corroborate the hypothesis explaining the apparent paradox, and then derive the most favourable geological conditions for the phenomenon to occur. Subsequently, the possibility to model this multiple-like phases is introduced in TOMO3D. The development of TOMO3D, which constitutes the core of my work, is founded on TOMO2D, from which it inherits the numerical methods for solving the forward and inverse problems. Source files have been rewritten, redefining and introducing the necessary variables and functions to handle 3-D data inversion. The tests made with the sequential version of the code emphasise the need of parallelisation for practicality reasons. Indeed, the increasing size of data sets along with the modelling of the additional spatial dimension results in computationally demanding inversions. Hence, I parallelised the forward modelling part of the code, which takes up to 90% of the computing time, with a combination of multiprocessing and message-passing interface extensions. Subsequently, the parallel version of TOMO3D is applied to a complex synthetic case simulating a subduction zone. This first 3-D application serves to evaluate the correctness of the code's programming, and as step-by-step description of the modelling procedure, with particular attention on the layer-stripping strategy used to successively model several reflectors. The outcomes demonstrate the ability of the code and the chosen inversion strategy to accurately recover the velocity distribution and the geometry of the two reflectors. Finally, TOMO3D is applied to a real 3-D wide-angle seismic data set acquired at the Pacific margin of Ecuador and Colombia to extract a 3-D velocity model of the overriding and incoming plates, which is then compared to previous results obtained with an extensively tested and used 3-D refraction travel-time tomography code (FAST). The comparison indicates that TOMO3D is more accurate than FAST but at the same time it is computationally more demanding. However, the parallelisation of TOMO3D allows using high-performance computing facilities, which is not the case of FAST or most of the existing codes.<br>Aquesta tesi està dedicada a la tomografia sísmica. Concretament, he implementat una eina de modelització 3D per a la tomografia conjunta de temps de trajecte de refraccions i reflexions (TOMO3D). La raó darrere d'aquest objectiu és l'evidència de que la informació basada en dades sísmiques 2D no permet copsar la complexitat de gran part dels cossos geològics, i en particular de la zona sismogènica en marges de subducció. El desenvolupament del TOMO3D es basa en el TOMO2D, un codi d'avantguarda per a la tomografia conjunta de refraccions i reflexions en 2D. Els arxius de codi han estat reescrits, redefinint i introduint les funcions necessàries per dur a terme la inversió 3D. Els testos fets amb la versió seqüencial del codi posen de manifest la necessitat de paral·lelització ja que l'increment de la mida dels conjunts de dades així com la modelització de la dimensió espacial afegida fan que les inversions siguin computacionalment exigents. La versió paral·lelitzada del TOMO3D ha sigut aplicada a un cas sintètic complex que simula una zona de subducció. Aquesta primera aplicació 3D serveix per avaluar la correcció de la programació del codi, i com a descripció pas a pas del procediment de modelització. Els resultats demostren la capacitat del codi per recuperar acuradament la distribució de velocitat i la geometria dels dos reflectors. Finalment, el TOMO3D és aplicat a un conjunt 3D de dades de sísmica de gran angle adquirit al marge pacífic d'Equador i Colòmbia per extreure'n un model 3D de la velocitat de les plaques cavalcant i subduïda, que és comparat amb el resultat obtingut amb un codi 3D de tomografia de temps de trajecte de refraccions (FAST). La comparació indica que el TOMO3D és més acurat que el FAST però al mateix temps és computacionalment més exigent. Tot i així, la paral·lelització del TOMO3D permet utilitzar plataformes de supercomputació, a diferència del que passa amb el FAST i la majoria de codis existents.

Sedimentary rock-hosted deposits are a major source of copper and cobalt, with the Neoproterozoic central African Copperbelt being among the largest Cu-Co provinces in the world, accounting for around 15% of its copper resource. The deposits occur primarily in the carbonates and siliciclastic sediments overlying the basement, and formed during early diagenesis (around 820 Ma) and late diagenesis/metamorphism during the Pan-African Orogeny (580-520 Ma). The northwest province of Zambia hosts three major copper deposits, amongst which is Kansanshi: the focus of this study. The deposit, which lies north of the Solwezi dome, is hosted within the Katangan Supergroup, particularly within the carbonaceous phyllites and porphyroblastic schists of the Mshwaya subgroup and lower Nguba Group and extends along the strike length of the North-West trending Kansanshi antiform. In this study, tomographic inversion is applied to first arrival refraction data collected at the Kansanshi Copper Mine with the aim of locating potential copper-bearing structures.  The survey was carried out using both dynamite and VIBSIST sources along 3 profiles; 2 trending North-East across the Kansanshi anticline and 1 trending north-west parallel to it. Seismic refraction tomography is an excellent tool for investigating the shallow subsurface, providing a velocity distribution. Unlike conventional refraction seismics, it allows for the velocity calculation of each cell in a non-homogeneous earth model, rather than just the average velocity of individual layers - allowing us to map structure and infer geological units and weathering profiles. The data highlights abundant faulting and varying depth to fresh bedrock. The various lithologies have also been interpreted.