Institute for Geophysics

Permanent URI for this communityhttps://hdl.handle.net/2152/65334

Since its founding in 1972, the University of Texas Institute for Geophysics (UTIG) is a world leader in expeditionary-scale geophysical research, conducting research in five broad themes: climate, energy, marine geosciences, seismology and tectonophysics, and planetary and polar geophysics. UTIG is home to more than 50 research scientists and postdocs — research entrepreneurs — providing a broadband of expertise that can do everything from conducting scientific ocean drilling to leading airborne radar studies of ice sheets. UTIG scientists supplement their fieldwork with computer analysis, modeling, and laboratory work. Whether collecting seismic data, responding to natural disasters, or searching space for signs of life, UTIG is there.

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3-D Traveltime Calculations
(Institute for Geophysics, 1994) Jervis, Michael; Sen, Mrinal K.; Stoffa, Paul L.
3D seismic surface multiple attenuation: algorithms and analysis
(2001-12) Alaslani, Abdulaziz Saleh, 1965-; Stoffa, Paul L.; Sen, Mrinal K.
The aim of seismic exploration is to provide a comprehensive description of subsurface geologic structure in terms of its reflectivity function at the boundaries between geological units. Seismic multiples are coherent noise that obscure primary events and considerably degrade the quality of seismic images in the target zones. In spite of the fact that many methods have been designed to suppress multiples, only a limited success has been achieved. I have developed two different approaches to address the problem of seismic multiples. The first approach attempts to suppress multiples in terms of decomposition of the measured seismic wavefields into its upgoing and downgoing waves. The separation process is accomplished by using some statistical characteristics of the data in the plane-wave p domain. The ratio of these two components yields the true reflectivity function free of multiples. Although encouraging results are obtained in the separation process, instability occurs during the wavefield division step. As a result, the effectiveness of this approach is limited. I have also investigated seismic multiples for 3D geology and proposed a new methodology in which 3D multiples are predicted and attenuated successfully. The departure of the predicted multiple arrival times from the observed multiple arrival times explains why demultiple algorithms that assume two-dimensional multiple reflections often fail. In this approach, I employed 3D ray tracing to predict the arrival times of the primary and its multiples in individual shot gathers generated from a three-dimensional reflector. A non-linear optimization method, called Very Fast Simulated Annealing (VFSA) is used to determine geometry of the subsurface reflector in 3D. This is achieved by applying a ray traced normal moveout (NMO) correction to seismic reflections with respect to the zero offset time. Based on the optimized NMO-corrected shot gathers, the autoconvolution of the seismic trace is employed to predict the multiple reflections, which are then scaled and subtracted from the original data. The application of this technique to real data demonstrates that the new method successfully suppresses many surface multiples, and is able to recover several deep primary events. This algorithm is robust and computationally very efficient.
A generalized model to estimate the elastic stiffness tensor of mudrocks based on the full strain tensor
(2021-05-07) Wiggs, David McLean; Flemings, Peter Barry, 1960-
I develop a three-step framework to model the anisotropic elastic properties of a mechanically compacted mudrock based on the full strain tensor. I model the microstructure as an effective medium representative of locally aligned domains of clay grains and fluid filled porosity with isolated quartz. Then I predict the orientation of these building blocks due to the application of any strain field. Finally, the previous two steps are combined to determine an effective medium model for the entire mudrock that predicts the elastic stiffness matrix. I focus on the relationship of deformation to porosity reduction and grain alignment in mudrocks. My results show that the application of axial loading leads to the development of elastic anisotropy with stiffnesses increasing more rapidly in the direction perpendicular to loading. These stiffness predictions closely match experimental data on a mudrock specimen from Eugene Island – Gulf of Mexico. I further apply my three-step framework to predict elastic stiffnesses in a synthetic salt basin based on the full strain tensor predicted by an evolutionary poromechanical model. This coupling allows us to predict elastic stiffnesses and anisotropy due to sediment deposition and non-uniaxial salt loading. Accurate estimation of elastic stiffnesses for mudrocks based on the full strain tensor holds immense potential to improve pressure prediction, seismic imaging in complex geologic environments, and prospect evaluation
A mixed forward/inverse modeling framework for earthquake deformation problems
(2023-08-11) Puel, Simone; Becker, Thorsten W.; Lavier, Luc; Ghattas, Omar; Hesse, Marc A; Johnson, Kaj M
Subduction is responsible for the most powerful earthquakes and dangerous volcanic eruptions, resulting in significant human casualties and economic losses. However, the prediction of these natural events remains challenging due to an incomplete understanding of the underlying physics that govern these phenomena. Key questions persist regarding stress accumulation and dissipation, rock behavior under extreme pressure and temperature, and the influence of fluids and melt in these processes. Recent advancements in space geodesy and seismic networks have enabled the measurement of seismic responses and surface displacements, revealing the complex dynamics of subduction zones. To enhance our comprehension of these processes, computational modeling that integrates various types of observations and constraints is crucial. This can be achieved through forward modeling, where model parameters are adjusted to better fit the observations, or through inverse modeling, which extracts critical parameters and the underlying physical mechanisms directly from the data. However, a comprehensive numerical physics-based modeling framework that combines both forward and inverse capabilities, using adjoints, within a unified infrastructure is currently lacking. The objective of this dissertation is to address this gap by developing an open-source, flexible, transparent, and easily extendable framework capable of handling multi-physics coupled problems. This framework will also drive the advancement of innovative techniques for analyzing earthquake systems. It introduces a novel implementation of fault discontinuity within the finite-element model, an improved fault slip inversion method that does not require Green’s function computations, and a novel approach to infer material structure solely from surface displacement data, eliminating the need for seismic velocity analysis. Furthermore, by incorporating these approaches, it offers a novel joint inversion of surface geodetic data, facilitating the simultaneous recovery of subduction zone structure and coseismic slip distribution. This provides valuable insights into the interplay between heterogeneous material structure and fault processes. As a demonstration, the framework successfully recovers the coseismic slip distribution and subduction zone structure by inverting the coseismic surface displacements recorded during the 2011 M9 Tohoku-oki earthquake in Japan. The results reveal weaker material beneath several volcanoes in the same region where local coseismic subsidence was reported during the earthquake. Accounting for heterogeneity in fault slip inversions is crucial for accurately matching the surface displacement data, as suggested by previous studies. Overall, the proposed framework represents a significant advancement in subduction zone modeling, providing a comprehensive tool for understanding and analyzing these complex phenomena, thereby paving the way for improved hazard assessment and risk mitigation strategies.
A modern distribution of foraminifera to reconstruct environmental change offshore Galveston Bay, Texas
(2022-08-11) Schilling, Solveig H.; Lowery, Christopher M.; Goff, John A.; Martindale, Rowan
Sand is an important resource for coastal engineering. In Texas, many offshore sand deposits are found buried in Holocene fluvial sediments or estuarine environments like tidal channels or bayhead, flood, and ebb tide deltas. Exploring for these resources requires a comprehensive understanding of the depositional system in which they are buried. The distribution of benthic foraminifera can be used to reconstruct the paleoenvironment of sand-bearing Holocene estuary deposits. The Gulf of Mexico foraminifera have long been tied to specific environments, and facies in which one genera of benthic foraminifera is most abundant (i.e., the predominance facies) represent a promising technique to reconstruct paleoenvironment in Holocene cores. Although predominance facies have been well documented in Galveston Bay, to date, there are no equivalent records directly offshore for comparison, leaving the inner shelf assemblage unconstrained. Additionally, no direct comparisons have been made between ancient estuary assemblages and the modern living assemblages in Galveston Bay. To address this knowledge gap, this project examines the environmental evolution of the Trinity River estuary using benthic foraminifera and core data. The foraminifera trends offshore Galveston Bay show a greater diversity compared to estuary samples, which is likely driven by increased salinity on the inner shelf compared to Galveston Bay. The increased diversity of inner shelf assemblages compared to those of the bay can be used for recognizing offshore assemblages as distinct from estuarine samples in core data. Additionally, the comparison of living and Holocene estuary populations shows ancient samples with much higher dominance of Elphidium than is observed in the modern bay. This non-analogue population suggests environmental conditions (likely salinity) in the Holocene Trinity River estuary varied significantly from modern conditions implying a recent environmental change in the bay.
A Slide Show for Global Plate Motions from Jurassic to Present Day (Paleoceanographic Mapping Project Report No. 54-0389)
(Institute for Geophysics, 1989) Mueller, R. Dietmar; Royer, Jean-Yves
A global model for Mesozoic and Cenozoic plate motions has been developed by the Paleoceanographic Mapping Project (POMP) during the last four years. It is based on a digital tectonic database that includes modern plate boundaries, marine magnetic anomaly data, fracture zone lineations, bathymetric data, Seasat and Geosat altimetry data, mapped ocean-continent boundaries and continental tectonic data. We used this global model to produce a set of 22 slides that display global and regional plate reconstructions, which illustrate the global plate tectonic development of the ocean basins from the breakup of Pangea in the Middle Jurassic to present day. Our reconstructions for the eastern Indian Ocean are consistent with the known seafloor spreading history of the Australian-Antarctic Basin and the Wharton Basin since the Late Cretaceous. In particular, they resolve the problem of overlap between the Kerguelen Plateau and Broken Ridge. An internally consistent plate model for the entire South Pacific region has been developed using a combination of satellite altimetry, magnetic anomaly and bathymetry data. Our closure of the South Atlantic that takes into account intracontinental deformation in Africa and South America does not assume any of the substantial gaps or overlaps that are inherent in rigid plate models. Reconstructions for the North Atlantic region are based on a new compilation of magnetic and fracture zone data.
Acoustic full-waveform to elastic pre-stack seismic inversion of the Yakutat Terrane, Gulf of Alaska
(2018-08-16) Alqatari, Hala; Sen, Mrinal K.
The Yakutat-North American collision in the Gulf of Alaska has developed a complex subduction zone followed by major deformations such as the Chugach-St Elias mountain range creation, intensified exhumation, fold and thrust-fault formation. I generate a compressional velocity model of the Yakutat microplate using two-dimensional acoustic and isotropic time-domain full waveform inversion (FWI) of marine seismic reflection and refraction data from the STEEP project (ST. Elias Erosion/tectonics Project). FWI is a non-linear data-fitting algorithm that aims to recover subsurface parameters from the recorded seismic wavefield. Seismic wave propagation along the Yakutat terrane is simulated using a staggered-grid finite difference modeling scheme. Drawbacks associated with FWI is cycle skipping during the minimization process, which results in converging to the wrong velocity model. Starting with a good initial model that contains the low-frequency information can help mitigate this issue. The starting velocity model input to FWI in this case is generated by a traveltime tomographic inversion of ocean-bottom seismometer and streamer seismic data. Data preconditioning includes muting, filtering, noise removal and amplitude rescaling of the field seismic data to match the corresponding amplitudes of the synthetic traces. The forward model is able to produce a good match between the observed and the modeled wavefield within half the propagated wavelength. I use the FWI result, which shows good correlation with the industry well, as an input to two additional seismic inversion methods: acoustic post-stack and elastic pre-stack seismic inversion in order to recover shear impedance and density models along the seismic line. Extending the problem to the elastic medium is important to support more advanced seismic interpretation. Both techniques were able to produce higher-resolution images of the Yakutat terrane that are well correlated with the well response. Structural complexities identified in the generated models include the northwest-dipping Pamplona fault system, the offshore folding zone, thickening of the Yakutat basement and, lastly, significantly lower velocities in the Poul Creek formation compared to the younger Yakataga formation, which may be attributed to high-fluid pressure within that formation.
Advanced methods for subsurface velocity estimation : trans-dimensional inversion and machine learning
(2019-12) Biswas, Reetam; Sen, Mrinal K.; Arnulf, Adrien F.; Spikes, Kyle T.; Grand, Stephen P.; Bennett, Nicholas
Inversion is a widely adopted tool to estimate the subsurface elastic properties of the Earth from seismic data. However, it faces several obstacles due to lack of adequate data coverage, and various assumptions made in forward modeling and inversion algorithms resulting often in sub-optimal results. One such assumption is the choice of parameterization of the model. In general, it is assumed to be known a priori and kept fixed. This can lead to either over or under parameterization, causing either overfitting or underfitting the data. In the first part of my thesis, I address the problem of model parameterization. Along with searching for models that fit the data, I also solve for the optimum number of model parameters required as dictated by the data. In a deterministic approach, I use the Basis Pursuit Inversion (BPI), which imposes sparsity in the model parameterization by adding a regularization term of L₁ norm of the model vector. The weight of the regularization term plays a dominant role, and I propose an approach for automatic calculation of this weighting factor. Alternately, I also develop a stochastic method, using Bayesian framework to solve my inverse problem in which the model parameters are treated as unknown. Unlike BPI, this method also provides us with estimates of uncertainty. Here, I make use of the Reversible Jump Markov Chain Monte Carlo (RJMCMC) framework, which allows changing the number of model parameters. However, the conventional RJMCMC is generally very slow as it attempts to sample a variable dimensional model space. To address this, I propose a new method called the Reversible Jump Hamiltonian Monte Carlo (RJHMC), which improves the efficiency by combining RJMCMC with a gradient-based Hamiltonian Monte Carlo (HMC). The gradient-based steps ensure quick convergence by allowing the sampling to take large steps guided by the gradient instead of complete random steps. I represent my model space using a layer-based earth model for the 1D problem and using an adaptive ensemble of nuclei along with Voronoi partition for a 2D problem. Subsequently, I use the method to solve the deconvolution problem in 1D, and tomography and Full Waveform Inversion problems in a 2D setting. It also provides estimates of the elastic parameters and marginal distribution of the number of model parameters. I use the 1D RJHMC to estimate density, along with P- and Swave velocities from a pre-stack angle gather. The region contains paleo-residual gas (PRG), which shows same signature as that of normal gas saturation, and can be better differentiated using density. Additionally, I applied trans-dimensional tomography to invert for P-wave velocity structure at an Axial Seamount, which is one of the most volcanically active regions in northeastern Pacific. In addition to BPI and RJMHC, I develop workflows, which take advantage of the hybrid schemes and Machine Learning (ML) algorithms. Solving an elastic FWI problem can be challenging, as it is very computationally expensive in comparison to the more commonly used acoustic formulation. I propose a hybrid scheme, where the initial P-wave velocity result from an acoustic FWI can be used to perform less expensive pre-stack Amplitude vs. Angle (AVA) inversion. This provides us with all three elastic parameters: P-wave velocity, S-wave velocity, and Density. Several inverse problems can be mapped into a neural network architecture, which can be solved using the currently developed deep learning algorithms. The last part of my dissertation describes two machine learning (ML) algorithms that I have developed for seismic inversion. I use a Convolutional Neural Network (CNN) to perform seismic inversion, in which instead of using the traditional way of using input-output pairs to train the network, I use the physics of the forward wave-propagation to guide the training. It circumvents the need for providing the label data during training and makes it unsupervised. In addition to this, I propose to use a Recurrent Neural Network (RNN) to estimate NMO velocities, which is a basic seismic processing technique. Generally, the NMO velocity is hand-picked and requires a lot of human intervention and computation time. Using this workflow with only 10% of data used as training, the network estimates NMO velocities almost instantly for the rest of the dataset
Age, Depth, and Residual Depth Anomalies in the North Pacific: Implications for Thermal Models of the Lithosphere and Upper Mantle
(Institute for Geophysics, 1986) Renkin, Miriam K.; Sclater, John G.
We present an empirical basement depth versus age relation for the North Pacific Ocean, based on the statistical treatment of an ocean-wide gridded data set. The SYNBAPS bathymetry was averaged into half-degree intervals and corrected for the effects of sediment loading. The resulting basement depths were plotted against ages determined from a revised isochron chart based on a recent compilation of magnetic lineations and various published plate reconstructions. On crust older than 80 Ma, the depths are skewed to the shallow side of the depth versus age distribution by large numbers of seamounts. Therefore the mean and standard deviations are not useful representations of the data. A more appropriate representation is the mode (or greatest concentration of points) and contours around the mode. The contours around the mode show that most ocean floor increases in depth with the square root of age out to crust of 80 Ma. Beyond this the majority of the data oscillates about a line that remains essentially constant as the age in-creases. Approximately 56% of all the data points lie within a + 300m band about the mode. If the sediment thickness data in the older basins of the western North Pacific is correct then the flattening of the depths favor a model in which extra heat is supplied to the base of the lithosphere on older ocean floor. Residual depth anomalies were calculated by removing the depths predicted by such a model. These anomalies correlate with bathymetric features and occur predominantly on crust of 120 and 160 Ma. They account for the rises in the mode at these two ages. The overall subsidence of the ocean floor can be accounted for by the cooling of a thermo-mechanical boundary layer. Correlations between geoid height and depth are evidence that many of the residual depth anomalies result from convective plumes which reset the thermal structure of the lithosphere. It is possible that this process observed at different times after the initial resetting of the isotherms may account for many of the depth anomalies in the western North Pacific.
An Integrated Approach to Seismic Event Location: 1. Evaluating How Method of Location Affects the Volume of Groups of Hypocenters
(Institute for Geophysics, 1992) Frohlich, Clifford A.
When seismic events occur in spatially compact clusters, the volume and geometric characteristics of these clusters often provides information about the relative effectiveness of different location methods, or about physical processes occurring within the hypocentral region. This report defines and explains how to determine the convex polyhedron of minimum volume (CPMV) surrounding a set of points. We evaluate both single-event and joint hypocenter determination (JHD) relocations for three rather different clusters of seismic events: 1) nuclear explosions from Mururoa relocated using P and PKP phases reported by the ISC, 2) intermediate depth earthquakes near Bucaramanga, Colombia, relocated using P and PKP phases reported by the ISC, and 3) shallow earthquakes near Vanuatu (formerly, the New Hebrides), relocated using P and S phases from a local station network. This analysis demonstrates that different location methods markedly affect the volume of the CPMV, however, volumes for JHD relations are �not always smaller than volumes for single-event relocations.
An Integrated Approach to Seismic Event Location: II. Sources of Location Uncertainty for Teleseismic and Local Network Data, (Period Covered - 1 July 1991 through 28 February 1994)
(Institute for Geophysics, 1994) Frohlich, Clifford A.
The locations of seismic events determined from travel times by independent agencies differ for at least three reasons, including differences in picking of or availability of the phases, effects produced by individual structural peculiarities of each station, and systematic mislocations caused by the choice of the travel time model used for relocation. The present study evaluates earthquake locations for events in three geographically diverse regions chosen to represent a broad spectrum of location problems. These are: Macquarie Ridge; Bucaramanga, Colombia; and Efate, Vanuatu. The evaluation methods include a comparison of locations reported by different agencies, an analysis of the sources of the variance of travel-time residuals, and a determination of the volume of groups of events thought to originate from a common source region. For the data analyzed it appears that station-dependent effects are a more significant source of location differences than are effects related to picking phase arrivals or choice of velocity model. Also, this study finds many situations where it seems inappropriate to utilize the strategy of minimizing travel time variance to improve location quality.
Analysis of GPU-based convolution for acoustic wave propagation modeling with finite differences: Fortran to CUDA-C step-by-step
(2014-05) Sadahiro, Makoto; Stoffa, Paul L., 1948-; Tatham, R. H. (Robert H.), 1943-
By projecting observed microseismic data backward in time to when fracturing occurred, it is possible to locate the fracture events in space, assuming a correct velocity model. In order to achieve this task in near real-time, a robust computational system to handle backward propagation, or Reverse Time Migration (RTM), is required. We can then test many different velocity models for each run of the RTM. We investigate the use of a Graphics Processing Unit (GPU) based system using Compute Unified Device Architecture for C (CUDA-C) as the programming language. Our preliminary results show a large improvement in run-time over conventional programming methods based on conventional Central Processing Unit (CPU) computing with Fortran. Considerable room for improvement still remains.
Analysis of hydrocarbon under-filled Miocene deep-water reservoirs, eastern Mexico offshore
(2019-09-17) Apango Perez, Fernando Agustin; Snedden, John W. (John William); Chuchla, Richard J. (Richard Julian)
Hydrocarbon exploration in the deep-water portion of the southwestern Gulf of Mexico commenced a decade ago, but discoveries have not been able to meet the optimistic expectations based on historical success in the northern Gulf of Mexico. Based on the existence of organic-rich source rocks and high-quality Miocene reservoir rocks, this research initially hypothesized that the lack of significant hydrocarbon accumulations in some prominent traps in the northern Catemaco fold belt (Veracruz Trough) is the result of an ineffective sealing and/or trapping mechanism. By integrating well core, 3D seismic, and other well data provided by the Mexican National Hydrocarbon Commission, this thesis attempts to determine the cause of hydrocarbon under-filled or dry Miocene traps within the Veracruz Trough using the structures drilled by Pemex-operated Kunah-1 and Yoka-1 wells as case studies. Three hypotheses are considered and analyzed: top seal leakage, the existence of unmapped shallow spill points, and late trap formation relative to the timing of hydrocarbon charge. Seal bed leakage analyses, which involved pressure data analysis and MICP laboratory measurements on cuttings, suggest that the evaluated intra reservoir and top seal rocks are at least moderately effective. In addition, 3D seismic structural interpretation around the Yoka-1 trap strongly suggests that the spill points are much deeper than fluid contacts. However, the trap formation assessment showed that by the time the traps began to develop, the oil generation phase was exhausted and gas generation was at a very late stage for the most prominent source rocks. Consequently, a more robust explanation for the Kunah-1 and Yoka-1 under-filled structures is poor timing between the generation and migration of hydrocarbons and trap volume evolution. A high-level economic assessment was made to determine if it would be commercially viable to produce some of the natural gas reserves that have already been found in the Catemaco fold belt. A notional gas development that would tie in production from the Kunah, Lakach, and Piklis fields into an onshore gas processing plant, was proposed and analyzed. The net present value calculated for this project suggests that the high costs of building the required infrastructure and the relatively low forecasted future prices of natural gas, make the profitability of a deep-water gas development project around the study area unlikely at the present time
Analysis of multicomponent seismic data from the Hydrate Ridge, offshore Oregon
(2005) Kumar, Dhananjay; Sen, Mrinal K.; Stoffa, Paul L.
Multicomponent seismic data can be used to derive P- and S-wave velocity structures of the subsurface, which can be used further to estimate rock and reservoir properties. Most seismic analysis methods and algorithms assume that the earth is isotropic. In many geologic situations, however, sedimentary rocks exhibit anisotropic behavior, and the isotropic assumption will introduce errors in the estimates of the elastic properties of the subsurface. With the goal of analyzing multicomponent seismic data from complex regions (which may show anisotropic behavior), I have developed new algorithms for 1) seismic modeling based on a ray-Born approximation and 2) traveltime computation in tilted transversely isotropic media based on Fermat’s principle. This new traveltime computation algorithm is tested on prestack depth migration of a physical model dataset. Such algorithms are essential for estimating subsurface rock properties in complex areas such as the Hydrate Ridge area, offshore Oregon. viii I participated in the acquisition of multicomponent seismic data (summer 2002), at the Hydrate Ridge of the Cascadia convergent margin. The primary goal of the experiment was to map the gas hydrates and free gas, and to understand the mechanism of fluid migration. Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules. Gas hydrates and free-gas are generally detectable with seismic methods because the seismic velocity increases in the presence of gas hydrates, and it decreases in the presence of free-gas. My analysis results in estimates of P- and S-wave interval velocities and anisotropic parameters with the final goal of relating these parameters to the presence and quantification of gas hydrate and free gas. I performed interval velocity analysis in the τ-p (intercept time - ray parameters) domain following three main steps: 1) P-wave velocity analysis, 2) P- to S-wave (converted PS-wave) event correlation, and 3) S-wave velocity analysis. P- to Swave event correlation is done using synthetic seismograms and traveltime tables. Seismic velocities are correlated to gas hydrate and free gas saturation using a Modified Wood equation. I find that Hydrate Ridge is heterogeneous and is weakly anisotropic (maximum of 10%) in some regions caused possibly by the hydrate veins. The P-wave velocity is more sensitive to the saturation of gas hydrates (maximum of 7% of rock volume) and free gas than the S-wave velocity. The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus, I conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region.
An analysis of subduction related tectonics offshore southern and eastern Taiwan
(2014-12) Eakin, Daniel Hoyt, Jr.; Lavier, Luc Louis; McIntosh, Kirk D.
Arc-continent collision is associated with vigorous mountain building and terrane accretion on relatively short (<10 Ma) geologic timescales. It is believed to be an important mechanism for the growth of continents. Taiwan represents one of the few active examples of this process. As such, is the perfect natural laboratory to investigate the nature of the continent ocean boundary and the uncertain behavior of the accretionary prism and extended, transitional rifted margin crust during the collision process. Taiwan also provides a unique opportunity to investigate structures in the backarc, yielding key insights into the still controversial tectonic conditions that were responsible for the unique subduction-collision system observed today. The obliquity of the collision between the North Luzon Arc and the Chinese rifted margin allows for examination of different temporal stages of collision at different locations. Recently acquired seismic reflection and wide-angle seismic refraction data, offshore Taiwan, document the crustal structure of the incipient mountain belt and of the Philippine Sea Plate in the backarc domain to the east. Geophysical profiles offshore southern Taiwan show evidence for a transition from the subduction of ocean crust to highly extended, transitional continental crust of the northern South China Sea distal margin. During oceanic subduction, accretion and underplating of thick sedimentary cover sequences create a large 13-15 km thick accretionary prism. Prior to the encroachment of the continental shelf, there is evidence for further underplating of transitional distal margin crust to the base of the prism. These findings support a multi-phase collisional model in which early growth of the mountain belt is driven by structural underplating of the previously sedimentary-only accretionary prism with blocks of transitional crust from the distal rifted margin. Geophysical profiles offshore eastern Taiwan show evidence for asymmetric crustal thickening, from 12-18 km, along the entire length of the Gagua Ridge suggesting the West Philippine Basin oceanic crust is underthust beneath that of the Huatung Basin. In this interpretation, the Gagua Ridge was the result of a failed subduction initiation event during the early Miocene that may have existed simultaneously and, for a short time, competed with the Manila subduction zone in accommodating convergence between the Eurasia and Philippine Sea plates.
Analysis of the Importance of Extension in Accounting for the Post-Carboniferous Subsidence of the North Sea Basin
(Institute for Geophysics, 1986) Sclater, John G.; Hellinger, Steven J.; Shorey, Mark
Andean deformation and basin evolution during changes in subduction zone geometry (29–33°S)
(2023-05-01) Mackaman-Lofland, Chelsea; Horton, Brian K., 1970-; Ketcham, Richard A; Stockli, Daniel F; Constenius, Kurt N; Folguera, Andres; Fosdick, Julie C
Understanding the surface, crustal, and tectonic processes controlling deformation and crustal evolution along subduction margins remains an outstanding challenge in Earth science, with implications for characterizing Earth systems, resource potential, and natural hazard assessment. The southern Central Andes of Chile and Argentina define type examples of deformation, arc magmatism, and basin evolution during long-lived subduction, and provide unique opportunities to study the overriding plate response to changing plate margin conditions and tectonic regimes. This dissertation presents new geo- and thermochronological assessments of sediment provenance, basin accumulation, and deformational timing at ~29–33°S that provide critical insight into retroarc deformational patterns and subsidence mechanisms over the past ~200 Myr. Mesozoic to mid-Cenozoic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback, followed by initial shortening and sediment derivation from western sources during the Late Cretaceous. Newly dated volcanic and sedimentary deposits in the retroarc hinterland provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension preceding Negoene growth of the modern Andes. Geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill indicate Andean shortening initiated with the inversion of the intra-arc basin system, followed by sequential eastward migration of the fold-thrust belt–foreland basin system along a regionally connected, hybrid thin- and thick-skinned decollement. Finally, new structural interpretations and flexural thermo-kinematic model results quantify the influence of inherited structures, precursor stratigraphic architectures, thrust belt critical taper, and non-flexural geodynamic processes during Neogene construction of the modern Andes
Anisotropic analysis and fracture characterization of the Haynesville Shale, Panola County, Texas
(2015-08) Barone, Anthony William; Sen, Mrinal K.; Spikes, Kyle T; Grand, Stephen P
In unconventional resources such as the Haynesville Shale, a proper understanding of natural fracture patterns is essential to enhancing the economic success of petroleum extraction. The spatial density of naturally occurring fracture sets affects drainage area and optimal drilling location(s), and the azimuth of the strike of the predominant fracture set affects the ideal orientation of wells. In the absence of data to directly determine these fracture characteristics, such as Formation Microimaging (FMI) logs, these natural fracture patterns can be analyzed by examining the seismic anisotropy present in the reservoir. Anisotropy introduced from aligned fracture sets creates predictable azimuthal variations in the seismic wavefield. This allows the reservoir anisotropy, and thus the fracturing present in the reservoir, to be studied indirectly through the azimuthal analysis of industry standard 3D seismic data. The work presented here outlines three distinct methodologies, which utilize azimuthal amplitude variations (AVAZ) present in 3D seismic data, to infer fracture characteristics without the need for substantial well log information. Two of these methods have been previously established and assume the reservoir to be characteristic of Horizontally Transverse Isotropic (HTI). The last method is novel and assumes orthorhombic anisotropy when inverting for fracture density and is able to unambiguously invert for fracture azimuth. All methodologies used in this work produced similar results, increasing confidence in the accuracy of these results through statistical repeatability. Fracture density inversion results indicate spatially varying fracture density throughout the area, with a distinct area of higher fracture density present in the Northwestern corner of the area analyzed. Spatially varying fracture density and localized pockets of fracturing is consistent with expectation from analyzing production data and FMI logs from other areas of the Haynesville. Fracture azimuth inversion results showed some variability; however, the novel method presented in this thesis indicates that the azimuth of the predominant fracture set is oriented at a compass bearing of approximately 82 degrees – rotated slightly counterclockwise from an east-west orientation. Fracture azimuth results agree well with expectations from a regional stress analysis and from examining comparable formations with known fracture patterns in the surrounding area.
Anisotropic seismic characterization of the Eagle Ford Shale: rock-physics modeling, stochastic seismic inversion, and geostatistics
(2016-05) Ren, Qi; Sen, Mrinal K.; Spikes, Kyle; Srinivasan, Sanjay; Fomel, Sergey
Quantitative reservoir characterization using integrated seismic data and well log data is important in sweet spot identification, well planning, and reservoir development. The process includes building up the relations between rock properties and elastic properties through rock physics modeling, inverting for elastic properties from seismic data, and inverting for rock properties from both seismic data and rock physics models. Many quantitative reservoir characterization techniques have been developed for conventional reservoirs. However, challenges remain when extending these methods to unconventional reservoirs because of their complexity, such as anisotropy, micro-scale fabric, and thin beds issues. This dissertation focuses on developing anisotropic rock physics modeling method and seismic inversion method that are appliable for unconventional reservoir characterization. The micro-scale fabric, including the complex composition, shape and alignment of clay minerals, pore space, and kerogen, significantly influences the anisotropic elastic properties. I developed a comprehensive three-step rock-physics approach to model the anisotropic elastic properties, accounting for the micro-scale fabric. In addition, my method accounts for the different pressure-dependent behaviors of P-waves and S-waves. The modeling provides anisotropic stiffnesses and pseudo logs of anisotropy parameters. The application of this method on the Upper Eagle Ford Shale shows that the clay content kerogen content and porosity decrease the rock stiffness. The anisotropy increases with kerogen content, but the influence of clay content is more complex. Comparing the anisotropy parameter pseudo logs with clay content shows that clay content increases the anisotropy at small concentrations; however, the anisotropy stays constant, or even slightly decreases, as clay content continues to increase. Thin beds and anisotropy are two important limitation of the application of seismic characterization on unconventional reservoirs. I introduced the geostatistics into stochastic seismic inversion. The geostatistical models, based on well log data, simulate small-scale vertical variations that are beyond seismic resolution. This additional information compensates the seismic data for its band-limited nature. I applied this method on the Eagle Ford Shale, using greedy annealing importance sampling as inversion algorithm. The thin Lower Eagle Ford Formation, which cannot be resolved by conventional inversion method, is clearly resolved in the inverted impedance volume using my method. In addition, because anisotropy is accounted for in the forward modeling, the accuracy of inverted S-impedance is significantly improved.
Antarctica: Gravity Anomaly and Infrared Satellite Image 1992.
(Institute for Geophysics, 2018-06-10) Sandwell, DavidT.; Lawver, Lawrence A.; Dalziel, Ian W.D.; Smith, Walter H.F.; Wiederspahn, Mark

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