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Item A Nonlinear Differential Semblance Algorithm for Waveform Inversion(2013-07-24) Sun, Dong; Symes, William W.; Heinkenschloss, Matthias; Zhang, Yin; Zelt, Colin A.Show more This thesis proposes a nonlinear differential semblance approach to full waveform inversion as an alternative to standard least squares inversion, which cannot guarantee a reliable solution, because of the existence of many spurious local minima of the objective function for typical data that lacks low-frequency energy. Nonlinear differential semblance optimization combines the ability of full waveform inversion to account for nonlinear physical effects, such as multiple reflections, with the tendency of differential semblance migration velocity analysis to avoid local minima. It borrows the gather-flattening concept from migration velocity analysis, and updates the velocity by flattening primaries-only gathers obtained via nonlinear inversion. I describe a general formulation of this algorithm, its main components and implementation. Numerical experiments show for simple layered models, standard least squares inversion fails, whereas nonlinear differential semblance succeeds in constructing a kinematically correct model and fitting the data rather precisely.Show more Item Application of Frequency-dependent Traveltime Tomography and Full Waveform Inversion to Realistic Near-surface Seismic Refraction Data(Society of Exploration Geophysicists, 2016) Chen, Jianxiong; Zelt, Colin A.Show more We present a synthetic test that uses a workflow consisting of a new frequency-dependent traveltime tomography (FDTT) method to provide a starting model for full waveform inversion (FWI) for near-surface seismic velocity estimation from refraction data. Commonly used ray-theory-based traveltime tomography methods may not be valid in the near surface given the likelihood of relatively large seismic wavelengths compared to the length scales of heterogeneities that are possible in the near surface. FDTT makes use of the frequency content in the seismic waves in both the forward and inverse modeling steps. In this application to a near-surface benchmark model, the results show that FDTT can better recover the magnitude of velocity anomalies than infinite frequency (ray-theory) traveltime tomography (IFTT). FWI can fail by converging to a local minimum when there is an absence of sufficiently low frequency data and an accurate starting model, either of which, if present, can provide long-wavelength constraints on the inverted velocity model. Both IFTT and FDTT models can serve as adequate starting models for FWI. However, FWI produces significantly better results starting from the FDTT model as compared to the IFTT model when low frequency data are not available. The final FWI models provide wavelength-scale structures allowing for direct geologic interpretation from the velocity model itself, demonstrating the effectiveness of FDTT and FWI in near-surface studies given the modest experiment and data requirements of refraction surveys.Show more Item Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes(Society of Exploration Geophysicists, 2013-09) Zelt, Colin A.; Haines, Seth; Powers, Michael H.; Sheehan, Jacob; Rohdewald, Siegfried; Link, Curtis; Hayashi, Koichi; Zhao, Don; Zhou, Hua-wei; Burton, Bethany L.; Petersen, Uni K.; Bonal, Nedra D.; Doll, William E.Show more Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the data’s frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply dipping low-velocity fault zone. The estimated models do not contain a subtle low-velocity zone and other fine-scale features, in accord with conventional wisdom. Together, the results support confidence in the reliability and robustness of modern refraction inversion and tomographic Methods.Show more Item Comparison of Full Wavefield Synthetics with Frequency-Dependent Traveltimes Calculated Using Wavelength-Dependent Velocity Smoothing(Society of Exploration Geophysicists, 2017) Chen, Jianxiong; Zelt, Colin A.Show more Ray theory-based traveltime calculation that assumes infinitely high frequency wave propagation is likely to be invalid in the near-surface (upper tens of meters) due to the relatively large seismic wavelength compared with the total travel path lengths and the scale of the near-surface velocity heterogeneities. The wavelength-dependent velocity smoothing (WDVS) algorithm calculates a frequency-dependent, first-arrival traveltime by assuming that using a wavelength-smoothed velocity model and conventional ray theory is equivalent to using the original unsmoothed model and a frequency-dependent calculation. This paper presents comparisons of WDVS-calculated traveltimes with band-limited full wavefield synthetics including the results from 1) different velocity models, 2) different frequency spectra, 3) different values of a free parameter in the WDVS algorithm, and 4) different levels of added noise to the synthetics. The results show that WDVS calculates frequency-dependent traveltimes that are generally consistent with the first arrivals from band-limited full wavefield synthetics. Compared to infinite-frequency traveltimes calculated using conventional ray theory, the WDVS frequency-dependent traveltimes are more consistent with the first arrivals picked from full wavefield synthetics in terms of absolute time and trace-to-trace variation. The results support the use of WDVS as the forward modeling component of a tomographic inversion method, or any seismic method that involves modeling first-arrival traveltimes.Show more Item Crustal structure across the Caribbean-South American plate boundary at 70W: Results from seismic refraction and reflection data(2007) Guedez, Maria C.; Levander, Alan R.; Zelt, Colin A.Show more The Caribbean-South America diffuse plate boundary is characterized by tectonic transpression with oblique convergence. In northwestern Venezuela, the underthrusting of the Caribbean Plate beneath northwestern South America, and the tectonic escape of the Maracaibo block complicate the boundary. The BOLIVAR project acquired onshore-offshore refraction and marine reflection data along the 450 km profile 70W, which extends from the Venezuela Basin, on the Caribbean plate, to the Maracaibo block, in the diffuse boundary zone. A 2-D velocity model was generated from wide-angle data, and it shows good correlation with the reflection data analyzed. We present evidence consistent with the underthrusting of the 15 km Caribbean plateau; however, we propose that the landward extent and depth of the oceanic crust are substantially less than previously suggested. The model also indicates the presence of an 8 km crustal thinning located to the north of the Oca-Ancon Fault and the inverted Falcon Basin.Show more Item Estimating subsurface structure through gravity and gravity gradiometry inversion(1999) Condi, Francis J.; Sawyer, Dale S.; Zelt, Colin A.Show more We use gravity to estimate rifted margin deep structure with an inversion method that links parameters in the shallow parts of the model to those in the deep parts through an isostatic, uniform extension model. The method provides for variable weighting of prior information, estimates densities and shapes simultaneously, and can be used in the presence and absence of deep seismic data. Synthetic tests of sensitivity to noise indicate that the isostatic extension constraint promotes the recovery of the short wavelength Moho topography, eliminates spatial undulations in deep structure due to noise in the data, and increases the range of acceptable recovered models over no isostatic extension constraint. In application to real data from the Carolina trough, the method recovers models that exhibit anomalously high density in the hinge zone area, apparently anomalously thick crust, and anticorrelation of subcrustal lithospheric densities with crustal densities. The first two features are observed in deep seismic studies. The latter is consistent with melting model predictions. We then present a unified view of the traditional gradiometric observables---differential curvature, horizontal gradient of vertical gravity, and vertical gradient of vertical gravity, in terms of invariants of the full gradient tensor, and examine their ability to recover subsurface structure through an efficient inversion method. Results of synthetic tests performed on selected complex bodies and noise free data indicate differential curvature and the horizontal gradient of vertical gravity do as well as the full tensor in recovering subsurface structure. In the presence of noise, we find that a mass constraint promotes recovery of smooth models and may be more appropriate than finite difference smoothing. Differential curvature appears to be a useful observable when inverted alone and as an early search technique in full tensor inversion.Show more Item Improved 3-D Models of Seismic Velocity and Density for the Island of Hawaii: Implications for Volcano-tectonics(2008) Park, Jaewoo; Morgan, Julia K.; Zelt, Colin A.Show more Improved 3-D models of P-wave velocity and density are presented for better understanding of volcano-tectonic processes around the Island of Hawaii. The summit and upper rift zones of Kilauea are underlain by high-velocity and positive-density anomalies, indicative of magma intrusives dominated by dikes and melt-rich olivine cumulates. Seismicity is clustered at the seaward edge of this body, indicating that the cumulate body pushes the flank outward above a frictional decollement. The intrusive rocks along Kilauea's and Mauna Loa's rift zones are not continuous along their lengths, suggesting that eruptions along the lower rift zones could be fed vertically from the mantle, rather than downrift from the summit reservoirs. Mauna Loa's southeast flank is underlain by an anomalously large volume of intrusive materials that lacks the distinctive positive density anomaly observed above active rift zones. Therefore, this cumulate body is probably now cold and solidified, representing an ancient rift zone. Similar to Kilauea, earthquakes are concentrated along the boundary of this body, but here accommodate seaward motion of the adjacent flank rather than the cumulate body. Mauna Loa also appears to have a buried northwest rift zone, overlying the older flanks of Hualalai and Mauna Kea. Both Hualalai and Mauna Kea show south trending high-velocity and density features, also indicative of buried rift zones. High- and low-velocity anomalies beneath Loihi seamount are interpreted to indicate the presence of intrusive cumulates within the volcanic edifice and oceanic crust, and partial melt within the upper mantle, respectively. Low velocities beneath the Hilina and Kao'iki fault zones are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks. In contrast, the submarine outer bench of Kilauea is marked by anomalously high-velocity materials, possibly evidence for a buried seamount that may impede outward spreading of the flank today.Show more Item Kirchhoff Common Offset Migration Velocity Analysis via Differential Semblance(2007) Li, Jintan; Symes, William W.; Levander, Alan R.; Zelt, Colin A.Show more This work gives an efficient and fast approach to obtain an accurate velocity model for seismic imaging. An accurate velocity model is prerequisite for obtaining the image that represents the true underground geological structure: major evidence to explore for potential reservoirs and indicate hydrocarbons. This method optimizes a differential measure of the image gather flatness by comparing the residual moveout between neighboring traces. It stands out as a more accurate and automated inversion method among a good number of existing approaches. In this thesis, two versions of differential semblance velocity analysis are introduced: NMO-based and Kirchhoffbased. For each version, both the efficiency and effectiveness are discussed. For NMObased differential semblance, the application is illustrated on both synthetic data set and field data set to invert both ID and 2D velocity models. For the Kirchhoff-based differential semblance velocity analysis, the application is applied on the synthetic data set to invert a constant velocity model, providing the preliminary evidence of the effectiveness and insight for inverting depth varying or 2D velocity models for further research. Overall, this automated inversion method for velocity analysis is a potential technique for the velocity inversion for exploration.Show more Item Marine Gas Hydrate: response to change of seafloor temperature, ocean sulfate concentration, and compositional effect(2013-10-30) Gu, Guangsheng; Hirasaki, George J.; Chapman, Walter G.; Zelt, Colin A.; Biswal, Sibani LisaShow more The global inventory of carbon in gas hydrate at present day is comparable to that in oil & coal reserve, therefore, gas hydrate could have played an important role in earth carbon cycle, e.g., during the Paleocene Eocene Thermal Maximum (PETM) event. However, ocean floor temperatures were ~6°C higher than today, so the hydrate abundance under warmer conditions was a question to be clarified. By using numeric simulations, this work showed that gas hydrate abundance is not only affected by ocean floor temperature, but, more essentially, greatly dominated by the organic carbon buried into sediment. During PETM, higher organic carbon contents due to less dissolved oxygen at seafloor and increased methanogenesis rates, both resulted from higher ocean temperatures, enhanced hydrate accumulation. Therefore, though hydrate stability zone would be thinner and shallower than present-day, depending on water depth and sedimentation rate, gas hydrate abundance could be still higher in some marine sediment columns than present-day value. The quantity of carbon stored in marine gas hydrates during PETM may have been similar to that of present-day. The ocean sulfate concentration is as another factor affecting hydrate abundance. From seafloor to sulfate-methane transition (SMT) zone, sulfate consumes a certain portion of organic carbon. Via numerical models, this work proposed and demonstrated that the organic carbon remaining at SMT, should be regarded as the real organic carbon content available for methanogenesis, which contributes to gas hydrate inventory. This work also revealed that lower ocean sulfate is favorable for higher gas hydrate inventory because it consumes less organic carbon in a shallow zone of sediment from seafloor to SMT. By using an example mixed gas system, this work showed that a transition zone which contains both solid hydrates and free gas can span over a thick zone (~300m). The gradual change of seismic impedance across the transition zone diminishes the strength of the Bottom Simulating Reflector (BSR). The results provide a possible mechanism for enigmatic weak-to-absent BSR in prolific hydrocarbon basins across the world.Show more Item Methods for concurrent generation of velocity models and depth images from seismic data(2014-12-02) Jaiswal, Priyank; Zelt, Colin A.; Rice University; United States Patent and Trademark OfficeShow more In various embodiments, the present disclosure describes methods for processing seismic data to concurrently produce a velocity model and a depth image. Various embodiments of the methods include: a) acquiring seismic data; b) generating a shallow velocity model from the seismic data; c) generating a stacking velocity model using the shallow velocity model as a guide; d) generating an initial interval velocity model from the stacking velocity model; and e) generating an initial depth image using the initial interval velocity model. The methods also include iterative improvement of the initial depth image and the initial interval velocity model to produce improved depth images and improved interval velocity models. Improvement of the depth images and the interval velocity models is evaluated by using a congruency test.Show more Item Receiver Function Analysis and Acoustic Waveform Modeling for Imaging Earth’s Crust: New Techniques and Their Applications(2013-09-16) Liu, Huafeng; Niu, Fenglin; Gordon, Richard G.; Zelt, Colin A.; Symes, William W.Show more The crust is the outer-most layer of the earth with thickness up to 80 km. Massive seismic waveform data have enabled imaging fine crustal structures with the aid of new imaging techniques. In this thesis, I develop seismic imaging techniques to take fullShow more Item Seismic characterization of a gas hydrate system in the Gulf of Mexico: A novel approach for evaluating high-resolution wide-aperture data(2002) Jaiswal, Priyank; Zelt, Colin A.Show more Gas hydrates were discovered in a mud diapir in the leased block Mississippi Canyon 798 - Gulf of Mexico, through piston coring. Subsequently, a seismic experiment was set up to investigate the dynamics behind the hydrate formation. Wide aperture seismic traveltime data obtained from the experiment have been inverted to estimate 2D P-wave velocity models of the five shot lines. The results from modeling indicate the presence of free gas in regions that show up as zones of high reflectivity on the reflection profiles. The topography of the study area suggests presence of active salt bodies, which in turn, makes it plausible for the gas in the Mississippi Canyon 798 to have deeper sources.Show more Item Seismic imaging with traveltime and waveform inversion: Naga Thrust and Fold Belt, India(2009) Jaiswal, Priyan; Zelt, Colin A.; Sawyer, Dale S.; Gordon, Richard G.; Hirasaki, George J.Show more I demonstrate the utility of traveltime and waveform inversion in depth imaging of seismic data with the help of two 2-D multichannel seismic lines, Reg-07 and PO-03, shot perpendicular to the trend of the Naga Thrust and Fold Belt (NTFB), India. I use both lines to demonstrate that a) a velocity model from traveltime inversion is suitable for pre-stack depth migration (PSDM) and can effectively serve as a starting model for frequency-domain full-waveform inversion; b) both PSDM and waveform inversion yield interpretable depth images but a combination of the two best describes the geology; and c) there still exists untested and unexploited hydrocarbon potential in the NTFB. Using the Reg-07 data I establish a working methodology for combining traveltime inversion with waveform inversion and PSDM. The results are validated using a nearby well. Using the PO-03 data I first demonstrate the advantages of combining traveltime inversion with PSDM for depth imaging; the combined method is referred to as unified imaging. Unified Imaging simultaneously yields a velocity model and a depth image that are consistent with each other. The velocity model from unified imaging is validated using an in-line well. The depth image from unified imaging reveals the presence of a triangle zone along PO-03 that was previously unknown and could be promising for exploration. Waveform inversion using the velocity model from unified imaging reveals the presence of a complex conjugate fault system in the supra-thrust along PO-03 which was also previously unknown and could also be promising for exploration. Traveltime inversion estimates a velocity model that is representative of the large scale features of the subsurface. This makes traveltime inversion a necessary first step regardless of the final choice of imaging--PSDM or waveform inversion. While PSDM uses the traveltime model as a whole for estimating a reflectivity image, waveform inversion enhances the resolution of a part of the traveltime model to yield a detailed acoustic property map. Even though the data and model requirements for PSDM are less stringent than for waveform inversion, waveform inversion appears to be resolving structural features that are imaged inadequately by PSDM.Show more Item Seismic traveltime inversion of wide-angle data for strongly-varying structure: Central Chilean margin and the subducting Juan Fernandez Ridge(2001) Naumenko, Julia Valentinovna; Zelt, Colin A.Show more This thesis presents the results of traveltime inversion of seismic wide-angle reflection/refraction data for strongly varying media. The two-dimensional velocity structure along two lines across the central Chilean margin (near Valparaiso) has been obtained and assessed in terms of resolution, uncertainties, and non-uniqueness. The traveltime inversion method was used to model the data and assess the model reliability. A tomographic approach was used to assess the objectivity of the structures in the final models. The final models include slope sediments, the Valparaiso forearc basin, subducting sediments, an accretionary wedge, upper and lower continental crust, a two-layer oceanic crust, and uppermost mantle. The thesis results were compared with the results of forward modeling of the same dataset and with results of analogous seismic surveys across convergent margins worldwide.Show more Item Three-dimensional seismic structure of the Leeward Antilles arc from seismic refraction and refraction and reflection tomography(2006) Arogunmati, Adeyemi T.; Zelt, Colin A.; Levander, AlanShow more We have developed new velocity and Moho interface models for the Dutch Antilles region of the Leeward Antilles arc from independent and simultaneous 3-D inversion of first arrival and PmP traveltimes. The first arrival and PmP traveltimes were picked from wide-angle offshore-onshore seismic data acquired in the Leeward Antilles arc region using 70 offshore and onshore instruments as part of the Broadband Onshore-Offshore Lithosphere Investigation of Venezuela and the Antilles are region (BOLIVAR) experiment. The resulting velocity models show evidence for fault bounded basins and features associated with the under-thrusting of the Caribbean plate beneath South America. Velocities beneath the arc are higher than velocities elsewhere in the study region at all depths and they suggest that the arc may be made of intermediate granulites. Moho interface models vary in depth from 19 km to 37 km in the study area. Beneath the arc, shallow basement topography lies above deep Moho topography and vice versa. In other words, the Moho structure mirrors the basement topography. This observation indicates the possibility of displacement partitioning during the formation of the arc. We compare an average 1-D velocity model from the Leeward Antilles arc to models from other island and continental arcs and find that the Leeward Antilles profile has velocities lower than mafic oceanic island arcs such as the Aleutians, Tonga and the Bonin arc, but has higher velocities than continental arcs such as the Sierra Nevada. Its velocities are however, similar to those of the Honshu island arc. The range of velocities in the Leeward Antilles arc profile is similar to that in the average continental crustal profile. Key words. Leeward Antilles arc, 3D crustal structure, seismic velocity, seismic tomography.Show more Item Three-dimensional traveltime tomography at a shallow groundwater contamination site(2003) Azaria, Aron; Zelt, Colin A.Show more I performed traveltime tomography on a 3-D seismic refraction dateset collected at Hill Air Force Base, Utah in 2000. The survey covers a 95m by 36m area over a contaminated aquifer that is bounded below by a clay aquiclude, in which a paleochannel acts as a trap for the contaminants. Presented are results using an iterative, nonlinear, traveltime tomographic approach applied to 187,877 traveltimes. The resulting velocity model shows that the near-surface velocity increases by roughly a factor of 5 in the upper 15m, from about 300m/s to 1500m/s. Cross-sections through the model show a north-south trending low-velocity feature interpreted to be the channel structure. Checkerboard tests applied to the velocity model establishes a 7.5m lateral resolution throughout most of the depth range of interest. While the long wavelength features of the model reveal the paleo-channel, the velocity model is likely a broad and smooth characterization of the true velocity structure.Show more Item Transfer-of-approximation Approaches for Subgrid Modeling(2013-07-24) Wang, Xin; Symes, William W.; Warburton, Tim; Riviere, Beatrice M.; Zelt, Colin A.Show more I propose two Galerkin methods based on the transfer-of-approximation property for static and dynamic acoustic boundary value problems in seismic applications. For problems with heterogeneous coefficients, the polynomial finite element spaces are no longer optimal unless special meshing techniques are employed. The transfer-of-approximation property provides a general framework to construct the optimal approximation subspace on regular grids. The transfer-of-approximation finite element method is theoretically attractive for that it works for both scalar and vectorial elliptic problems. However the numerical cost is prohibitive. To compute each transfer-of-approximation finite element basis, a problem as hard as the original one has to be solved. Furthermore due to the difficulty of basis localization, the resulting stiffness and mass matrices are dense. The 2D harmonic coordinate finite element method (HCFEM) achieves optimal second-order convergence for static and dynamic acoustic boundary value problems with variable coefficients at the cost of solving two auxiliary elliptic boundary value problems. Unlike the conventional FEM, no special domain partitions, adapted to discontinuity surfaces in coe cients, are required in HCFEM to obtain the optimal convergence rate. The resulting sti ness and mass matrices are constructed in a systematic procedure, and have the same sparsity pattern as those in the standard finite element method. Mass-lumping in HCFEM maintains the optimal order of convergence, due to the smoothness property of acoustic solutions in harmonic coordinates, and overcomes the numerical obstacle of inverting the mass matrix every time update, results in an efficient, explicit time step.Show more