R-3 Repository :: Browsing by Author "Levander, Alan R."

Browsing by Author "Levander, Alan R."

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A New Approach to the High-Resolution Linear Radon Transform based on Compressive Sensing Theory: Application on Teleseismic Wavefields
(2013-04-19) Aharchaou, Mehdi; Levander, Alan R.; Niu, Fenglin; Baraniuk, Richard G.
The development of new tools for high-resolution seismic imaging has been for many years one of the key challenges faced by earthquake and exploration seismologists. In order to make data amenable to imaging analysis, pre-processing steps are of great importance. This thesis proposes a new method for pre-processing teleseismic data based on the linear radon transform implemented according to compressive sensing theory – a novel theory about acquiring and recovering the sparsest signals (with minimum significant coefficients) in the most efficient way possible with the help of incoherent measurements. The LRT works by mapping data into a sparsity-promoting domain (called the radon domain) where the desired signals can be easily isolated, classiﬁed, ﬁltered and enhanced; and where noise can be attenuated or completely removed. The performance of the LRT is enhanced in terms of both high-resolution and computational cost by formulating the problem as an inverse problem in the frequency domain. This work shows that, unlike the common wisdom, irregularity in spatial sampling of teleseismic wavefields can be beneficial because it provides the incoherency needed to solve the compressive sensing problem and therefore recover the sparsest solutions in the radon domain. The inverse problem formulation yields the added advantage of automatic spatial interpolation and phase isolation after data reconstruction, and enables to regularize the problem by imposing sparsity constraint (instead of smoothness, which is the constraint usually adopted). We discuss and investigate the resolving power and applicability of convex and non-convex types of regularizers inspired from compressive sensing theory, and we establish a lower bound on the number of measurements needed to resolve certain time dips related to signals of interest within the data. We finish by applying the method to synthetic and recorded datasets and show how we do signal extraction, noise removal and spatial interpolation on teleseismic wavefields.
A study of seismic wave propagation in heterogeneous crust
(1999) Akerberg, Peeter Michael; Levander, Alan R.
Three different aspects of estimating properties from seismic data are treated in this thesis: (1) Deterministic processing of a high resolution shallow seismic data set with good geologic control, (2) traveltime estimation from complicated models described statistically, and (3) estimation of a the vertical autocorrelation length of such models. The first part of this thesis is the processing and interpretation of a shallow seismic dataset collected in an open pit copper mine near Tyrone, New Mexico. The seismic image is compared with the outcrop in the open pit mine wall along which the seismic line was collected, and with drill data obtained from the mine operators. Specific features imaged by the experiment include the base of the overlaying sediment, the base of the leached capping, and fractures and shear zones that control local ground water flow. The features in the migrated section compare well with outcrop and drill data. The second part of the thesis studies the systematic bias of velocities estimated from first arrival travel times measured from a class of very complicated velocity models. Traveltimes were computed for statistically described velocity models with anisotropic von Karman correlation functions. The results of a finite difference eikonal solver, corresponding to very small wavelength experiments, are compared to results from picking first arrivals of full wavefield finite difference simulations. The eikonal solver results show the largest systematic bias, corresponding to the ray theoretical limit, and the results from the full wavefield experiments are smaller, but with very similar dependence on aspect ratio of the anisotropic correlation function. The third part defines two methods to obtain the vertical correlation length from seismic data approximated by the primary reflectivity series, which conventionally is used as the ideal result of seismic imaging. The first method is based on fitting a theoretical power spectrum based on the known source spectrum and fractal dimension to the average vertical power spectrum of the seismograms. This method works for a range of conditions where the correlation length is relatively small compared to the wavelength. Larger correlation lengths can be estimated by a second method based on deconvolving and integrating the seismogram to obtain an approximation of a vertical slice of the velocity model.
A unified approach to complex seismic imaging problems
(1991) Lafond, Claude F.; Levander, Alan R.
Two current challenges in seismic imaging are to obtain more detailed images of complex structures from reflection data and to constrain the regional structure of the Earth using wide-angle data. These are complex problems for which traditional methods fail because they are based on too many simplifying assumptions. I develop a unified approach which addresses these tasks by starting with a fundamental problem formulation, leading to a practical numerical solution which converges rapidly. It is based on pre-stack depth migration and cell-stripping tomography in heterogeneous media, which allow layer-stripping and retain all the information from the data, incorporates a depth focusing technique for improved image resolution and utilizes user-interaction and geologic input to guide and constrain the imaging process. I first describe a fast and accurate dynamic ray-tracing scheme in heterogeneous media which allows complex model definition and rapid two-point ray tracing. This ray tracing method is then used to compute Green functions in a layer-stripping pre-stack depth migration algorithm. The algorithm itself is based on a Kirchhoff integral in heterogeneous media using exact weighting factors and specialized to 2.5 D migration. I examine the migration results with a depth-focusing technique which analyzes common image panels for horizontal alignment, relating the degree of non alignment, or Migration Moveout (MMO) to corrections in the velocity model along the raypaths. Finally, I develop a cell-stripping tomography (CST) algorithm which distributes velocity residuals only to the relevant cells, allowing resolution of both horizontal and vertical discontinuities and providing starting models for migration. Although computer-intensive, this unified approach is successful both in synthetic tests and for obtaining local and regional images of the edge of the Santa Maria basin in central California. It is more faithful to the velocity and dip information contained in the data, allows more control over the imaging process and with available computing power promises to be routinely applicable.
Characterizing the southeast Caribbean-South American plate boundary at 64°W
(2007) Clark, Stephen Anthony; Levander, Alan R.
The crustal and lithospheric structure of the northern South America plate boundary with the southeast Caribbean has been the focus of many studies. In this region, westward subduction of (Atlantic) oceanic South America transitions to east-west transform between continental South America and the Caribbean plate. Previous models invoke a poorly-constrained component of north-south convergence between the Caribbean and continental South America, predicting that the westward subduction transitions to northwest-dipping subduction beneath the Serrania del Interior. These models predict that continental crust extends north of the Venezuela coast beneath the Leeward Antilles remnant arc islands, and that the Leeward Antilles are accreting onto South America. The results presented in this dissertation determine instead that the dextral strike-slip system along the Venezuelan coast cuts near-vertically through the crust and offsets the Moho. The strike-slip system fundamentally defines the plate boundary, deriving from a shear tear through the entire lithosphere that is actively propagating north of the Paria peninsula. This shear tear detaches subducting oceanic crust from buoyant continental crust along the weakened, former passive margin. Thrust faults flanking the strike-slip system to the north and south dip systematically toward the plate boundary. These faults have been previously interpreted as delineating a 300 km-wide diffuse plate boundary zone, caused by oblique convergence partitioned into orthogonal thrust and strike-slip displacements. Instead, these faults are driven largely by vertical rather than horizontal tectonics, and are the result of the geodynamic response to the shear tear.
Combining deterministic and stochastic velocity fields in the analysis of deep crustal seismic data
(1997) Larkin, Steven Paul; Levander, Alan R.
Standard crustal seismic modeling obtains deterministic velocity models which ignore the effects of wavelength-scale heterogeneity, known to exist within the Earth's crust. Stochastic velocity models are a means to include wavelength-scale heterogeneity in the modeling. These models are defined by statistical parameters obtained from geologic maps of exposed crystalline rock, and are thus tied to actual geologic structures. Combining both deterministic and stochastic velocity models into a single model allows a realistic full wavefield (2-D) to be computed. By comparing these simulations to recorded seismic data, the effects of wavelength-scale heterogeneity can be investigated. Combined deterministic and stochastic velocity models are created for two datasets, the 1992 RISC seismic experiment in southeastern California and the 1986 PASSCAL seismic experiment in northern Nevada. The RISC experiment was located in the transition zone between the Salton Trough and the southern Basin and Range province. A high-velocity body previously identified beneath the Salton Trough is constrained to pinch out beneath the Chocolate Mountains to the northeast. The lateral extent of this body is evidence for the ephemeral nature of rifting loci as a continent is initially rifted. Stochastic modeling of wavelength-scale structures above this body indicate that little more than 5% mafic intrusion into a more felsic continental crust is responsible for the observed reflectivity. Modeling of the wide-angle RISC data indicates that coda waves following PmP are initially dominated by diffusion of energy out of the near-surface basin as the wavefield reverberates within this low-velocity layer. At later times, this coda consists of scattered body waves and P to S conversions. Surface waves do not play a significant role in this coda. Modeling of the PASSCAL dataset indicates that a high-gradient crust-mantle transition zone or a rough Moho interface is necessary to reduce precritical PmP energy. Possibly related, inconsistencies in published velocity models are rectified by hypothesizing the existence of large, elongate, high-velocity bodies at the base of the crust oriented to and of similar scale as the basins and ranges at the surface. This structure would result in an anisotropic lower crust.
Crustal and upper mantle investigations of the Caribbean-South American plate boundary
(2010) Bezada, Maximiliano J.; Levander, Alan R.
The evolution of the Caribbean --- South America plate boundary has been a matter of vigorous debate for decades and many questions remain unresolved. In this work, and in the framework of the BOLIVAR project, we shed light on some aspects of the present state and the tectonic history of the margin by using different types of geophysical data sets and techniques. An analysis of controlled-source traveltime data collected along a boundary-normal profile at ∼65°W was used to build a 2D P-wave velocity model. The model shows that the Caribbean Large Igenous Province is present offshore eastern Venezuela and confirms the uniformity of the velocity structure along the Leeward Antilles volcanic belt. In contrast with neighboring profiles, at this longitude we see no change in velocity structure or crustal thickness across the San Sebastian - El Pilar fault system. A 2D gravity modeling methodology that uses seismically derived initial density models was developed as part of this research. The application of this new method to four of the BOLIVAR boundary-normal profiles suggests that the uppermost mantle is denser under the South American continental crust and the island arc terranes than under the Caribbean oceanic crust. Crustal rocks of the island arc and extended island arc terranes of the Leeward Antilles have a relatively low density, given their P-wave velocity. This may be caused by low iron content, relative to average magmatic arc rocks. Finally, an analysis of teleseismic traveltimes with frequency-dependent kernels produced a 3D P-wave velocity perturbation model. The model shows the structure of the mantle lithosphere under the study area and clearly images the subduction of the Atlantic slab and associated partial removal of the lower lithosphere under northern South America. We also image the subduction of a section of the Caribbean plate under South America with an east-southeast direction. Both the Atlantic and Caribbean subducting slabs penetrate the mantle transition zone, affecting the topography of the 410-km and 660-km discontinuities.
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.
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.
Crustal structure and tectonic evolution: Central California
(1989) Meltzer, Anne S.; Levander, Alan R.
Vertical incidence and wide angle seismic data suggest the central California transform margin is underlain at depth by a horizontal detachment surface above which strike-slip and compression occur. The boundary between the Pacific and North American plates is interpreted as a combination of horizontal and vertical surfaces distributed from the base of slope offshore eastward past the San Andreas fault rather than a single vertical boundary associated with a strike-slip fault extending through the lithosphere. Deep crustal reflection profiling shows the offshore margin is decoupled above landward dipping Pacific plate oceanic crust which underthrusts the margin. A 12 km thick wedge of material was accreted offshore during pre-Neogene convergence between North America and oceanic plates located westward. Neogene shortening offshore is minor, implying compression from oblique convergent motion between the plates is accommodated onshore. Structures in the Santa Maria basin exhibit lower to middle Miocene extension accompanied by strike-slip faulting followed by compression in the upper Miocene-lower Pliocene. Reflection and refraction profiles onshore also suggest shallow decoupling levels within the crust. Structural relief of anticlines along the western edge of the San Joaquin basin is attributed to thrusting in buried Cretaceous sediments. Thrust faults sole westward at 12 km depth, into a detachment surface. Crustal rocks beneath the decollement have velocities of 6.4-6.6 km/sec and extend eastward to the middle of the Central Valley where they shallow abruptly to 5 km depth. The Greenville fault near Livermore Valley, is a strike-slip fault only penetrating the upper 5 km of crust. West of the fault, Miocene sediment with high compressional velocity, 5.0 km/sec, are present to 5 km depth. Similar velocities have been associated with shallow Franciscan rocks calling into question the use of seismic velocities alone to distinguish between different Coast Range - Great Valley lithologies. The crustal structure and style of deformation observed at the western and eastern margins of the transform zone suggest that the entire zone is decoupled above depths ranging from 6-30 km, and that shallow subduction or underplating of oceanic crust is occurring beneath the margin.
Estimating crustal heterogeneity in the northern Basin and Range from COCORP and PASSCAL seismic data
(1999) LaFlame, Lisa Marie; Levander, Alan R.
This research investigates the different scales of crustal heterogeneity. A method for estimating the characteristic smaller scales of crustal heterogeneity directly from reflection seismic data is applied to the COCORP dataset. Smaller scales of crustal heterogeneity range between 250-800 m. The method is also applied to another reflection dataset that images extended crust similar to the Basin and Range and the results are compared. To determine the larger scale of heterogeneity, 2-D velocity models for the PASSCAL Basin and Range E-W refraction profile are derived using 2-D traveltime inversion. Combined stochastic and deterministic models that represent crustal velocity heterogeneity are generated using the results from the characteristic scale estimation and 2-D inversion. From these models, finite-difference synthetic seismograms are generated and a preferred model is chosen because the corresponding synthetic seismograms reproduce well the characteristic features of the seismic wavefield seen in the PASSCAL data.
Imaging lithospheric structure beneath the Colorado Plateau and its adjacent regions using Rayleigh wave tomography
(2010) Liu, Kaijian; Levander, Alan R.
This thesis presents a new 3-D shear velocity (Vs) model of the lithospheric structure beneath the Colorado Plateau and its surrounding regions to understand the complicated lithospheric modifications caused by major Cenozoic tectonic and magmatic activities. Prior to this work, lack of an overall velocity model had prevented a comprehensive understanding of the current encroachment patterns near the margins, and of the perplexing crust/mantle boundary beneath the plateau. Using the new USArray data, I have inverted the isotropic Vs model from the Rayleigh wave dispersion curves obtained by a modified two-plane wave method. The resulting Vs structures not only clearly image the sublithospheric low-velocity channels, but also identify the high velocity anomalies underlying the western plateau for the first time in surface wave tomography to support the recent delamination hypothesis. This work also provides a high-resolution velocity model for geodynamic modeling on lithosphere-asthenosphere interactions.
Kirchhoff Common Offset Migration Velocity Analysis via Differential Semblance
(2007) Li, Jintan; Symes, William W.; Levander, Alan R.; Zelt, Colin A.
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.
Layer-stripping reverse-time migration
(1990) Shih, Ruey-Chyuan; Levander, Alan R.
Reverse-time migration has proven to be successful for structures with steep dips and strong velocity contrasts. Applying this algorithm to a large scale seismic model requires significant computational expense, particularly if strong velocity contrasts are present in the model. Here I present a layer-stripping migration technique, in which I use the reverse-time method to migrate seismic sections through constant or smoothly varying velocity layers, one layer at a time. As part of the migration in a given layer, the bottom boundary of the layer is defined and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated layer by layer. The final migration result is composited from the individual layers images. The layer-stripping migration algorithm can be summarized as three steps: (1) model definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme posed in this way is similar to datuming with an imaging condition. The advantages of the layer-stripping method are: (1) it preserves the benefits of the reverse-time method, i.e., it handles strong velocity contrasts between layers and steeply dipping structures; (2) it eliminates artificial interlayer multiples; (3) it reduces computational expense in high velocity layers; and (4) it allows interpretational constraint during image formation. The method has been implemented with both an explicit 4th-order time, 10th-order space, finite-difference approximation to the scalar wave equation, and an implicit 2nd-order time, 4th-order space finite-difference scheme applied to the linearly transformed wave equation (Li, 1986). The capability of post-stack layer-stripping reverse-time migration is illustrated on a synthetic CMP data and a CMP data from a survey over a faulted anticline in a fold and thrust belt. For pre-stack layer-stripping reverse-time migration, I present migrations of two synthetic data sets and a field data example from part of the marine seismic reflection profile RU-3, crossing the Hosgri fault offshore southern central California. The Hosgri fault appears as a northeast dipping high angle fault with a thrust component.
Modeling wide-angle seismic data from the central California margin
(1988) Putzig, Nathaniel Edward; Levander, Alan R.
Rice conducted an onshore-offshore seismic survey across the central California transform margin. Refraction velocity modeling of shot and receiver gathers has generated two models with similar near surface features. Model synthetics produce excellent fits to first arrivals and good matches for later arrivals in the data. CMP stacking of nearer offset traces imaged an event interpreted as reflections from the top of a dipping lower crustal layer. Model raytracing indicates layer flattening landward and seaward of the coast. High amplitude late arrivals seen at long offset on receiver gathers are modeled by imbricating the lower crust beneath the coast. This feature is interpreted as a result of overthrusting of the continental and Farallon plates onto the Pacific plate. The models differ in the middle crust, where one includes a laterally discontinuous low velocity zone. No direct indication of this LVZ exists in the data or in offshore reflection profiles.
Modeling, inversion and imaging of seismic data in viscous media
(1995) Blanch, Joakim Oscar; Levander, Alan R.
Real Earth media is anelastic, which affects both kinematics and dynamics of propagating waves. Waves are attenuated and dispersed. If anelastic effects are neglected, inversion and migration can yield erroneous results. The anelastic effects, on propagating waves, in real rocks can be well described by a viscoelastic model. Hence, viscoelastic wave propagation simulation is a well suited base for a realistic inversion algorithm derived through the adjoint state technique. We have developed a finite-difference simulator to model wave propagation in viscoelastic media. The viscoelastic scheme is only slightly more expensive than analogous elastic schemes. This thesis also presents a method for modeling of constant Q as a function of frequency based on an explicit closed formula for calculation of the parameter fields. The $\tau$-p (intercept time-slowness) domain permits economical modeling and inversion of 3-D wave propagation in layered media.
Near-surface seismic imaging at a porphyry copper mine
(2001) Dana, Diana; Levander, Alan R.
In June 1996, a 2-D high-resolution seismic survey was conducted at an open pit porphyry copper mine in southwest New Mexico. The local geology consists of a Miocene conglomerate, overlying Precambrian to Eocene basement rock containing unaltered igneous rock and mineralogically altered rock that hosts the sulfide deposits. The survey employed reflection and refraction techniques to image the top of the ore body. A variety of filtering techniques was used to suppress strong coherent source generated noise, and high amplitude ambient noise from mine operations. The resulting seismic sections compare well with borehole data and photographs of the mine walls; showing lithology and structural features within the initial 300 m of the subsurface. Imaging differences in lithology, contrasts due to sulfide deposition, and structures has potential importance for mine operations for efficiency of extraction and for safety. Locating structural features, such as those controlling groundwater flow, also has environmental applications.
Near-surface seismic reflection investigations at a groundwater contamination site
(2004) Dana, Diana; Levander, Alan R.
A 3-D seismic reflection survey was conducted at a groundwater contamination site to adequately image the near surface (<20m) at a level of detail to enhance ongoing remediation activities at the site. The site, Operable Unit 2 (OU2), located at Hill Air Force Base in Ogden, Utah has been the subject of a continuing effort to remove dense nonaqueous phase liquids (DNAPLs), solvents and degreasers, which had been dumped into trenches at the site for eight years. Subsurface maps of the near surface geology, based solely on data from the over 200 monitoring wells drilled at the site, do not provide sufficient detail of subsurface features that trap these contaminants to conduct site remediation in a cost-effective manner. The target geologic feature for remediation, a paleochannel complex, under a surface layer of unconsolidated sands, silts and gravels, cut into a thick layer of clay. The paleochannel traps both groundwater and contaminants. The paleochannel was the focus of an earlier 2-D survey, demonstrating the viability of the site for seismic methods. Successfully imaging the channel profile in three places, results from the 2-D survey served as a testbed for planning for the 3-D survey. The 3-D reflection survey covered an area of 37 x 95m centered over the paleochannel. After data processing, the resulting seismic volume shows a paleochannel complex with two channels from the north merging into a single channel that spreads out across the southern portion of the survey. Intersecting the channel are two displacement surfaces, origins unknown, disrupting channel flow and altering the pattern of erosion and deposition. The resulting map shows the channel complex with depth to base clay varying from 6 to 18.33m. Vertical resolution of the depth-to-clay map is 1.25m (lambda/2). Stratigraphic analysis of the sedimentary package above the interpreted clay surface confirms the seismic interpretation. Comparison with results from a tomographic study (Azaria, 2003) and a VSP study (Gao et al., 2003) at the site confirm the reflection survey results.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).
Scattering of acoustic energy from rough deep ocean seafloor: A numerical modeling approach
(1995) Robertsson, Johan Olof Anders; Levander, Alan R.
The highly heterogeneous and anelastic nature of deep ocean seafloor results in complex reverberation as acoustic energy incident from the overlaying water column interacts and scatters from it. To gain a deeper understanding of the mechanisms causing the reverberation in sonar and seafloor scattering experiments, we have developed numerical simulation techniques that are capable of modeling the principal physical properties of complex seafloor structures. A new viscoelastic finite-difference technique for modeling anelastic wave propagation in 2-D and 3-D heterogeneous media, as well as a computationally optimally efficient method for quantifying the anelastic properties in terms of viscoelastic mechanics are presented. A method for reducing numerical dispersion using a Galerkin-wavelet formulation that enables large computational savings is also presented. The widely different regimes of wave propagation occurring in ocean acoustic problems motivate the use of hybrid simulation techniques. HARVEST (Hybrid Adaptive Regime Visco-Elastic Simulation Technique) combines solutions from Gaussian beams, viscoelastic finite-differences, and Kirchhoff extrapolation, to simulate large offset scattering problems. Several scattering hypotheses based on finite-difference simulations of short-range acoustic scattering from realistic seafloor models are presented. Anelastic sediments on the seafloor are found to have a significant impact on the backscattered field from low grazing angle scattering experiments. In addition, small perturbations in the sediment compressional velocity can also dramatically alter the backscattered field due to transitions between pre- and post-critical reflection regimes. The hybrid techniques are employed to simulate deep ocean acoustic reverberation data collected in the vicinity of the northern mid-Atlantic ridge. In general, the simulated data compare well to the real data. Noise partly due to side-lobes in the beam-pattern of the receiver-array is the principal source of reverberation at lower levels. Overall, the employed seafloor models were found to model the real seafloor well. Inaccurately predicted events may partly be attributed to the intrinsic uncertainty in the stochastic seafloor models. For optimal comparison between real and HARVEST simulated data the experimental geometry should be chosen so that 3-D effects may be ignored, and to yield a cross-range resolution in the beam-formed acoustic data that is small relative to the lineation of the seafloor.
Seismic imaging and wave scattering in zones of random heterogeneity
(1988) Gibson, Bruce Sanderson; Levander, Alan R.
Most current interpretations of lower crustal seismic reflectivity suggest the existence of fine-scale ($\approx$100 m thick) layered structures at depth. Typical common-midpoint (CMP) stacked images of deep structures are, however, noisy and show discontinuous reflections characterized by numerous subhorizontal segments. Present interpretation techniques cannot definitively resolve whether such a reflection response is attributable to heterogeneity at the target or whether the seismic image is distorted by propagation effects and contaminated by noise. The quantitative assessment of lateral heterogeneity in the deep crust is fundamental to understanding mechanisms of crustal formation and evolution. Here, crustal heterogeneity is represented by velocity structures that vary randomly in two dimensions, with a correlation distance comparable to the dominant wavelength of the seismic source. Synthetic CMP seismic data are computed for various models using a fourth-order finite-difference solution to the acoustic wave equation. A conventional data processing sequence produces CMP stacked sections with greater lateral continuity than is present at the target and an overall appearance comparable to that of field-recorded data. Lateral coherence is quantified using the spectral coefficient of coherence, applied to trace pairs having various spatial separations. Increased continuity in the CMP stack is attributable to the dip-filtering action of stacking and can be compensated by the application of migration before stack or equivalent processes. The reflection response observed in common-shot trace gathers shows amplitude and lateral coherence increasing with offset, an effect attributable to the source-receiver geometry. Observed wavefield coherence is related to the correlation properties of the target through a convolutional expression. A field data example from the Black Forest, Germany shows that enhanced coherence can be expected in wide-angle field experiments and that a model having two-dimensional heterogeneity matches field data previously interpreted in terms of extreme fine-scale layering. A densely-sampled field data set from the Basin and Range Province, Nevada shows increased coherence for P$\sb{\rm m}$P at large offsets. Observed lateral coherence values are lower than predicted for scattering purely in the Moho transition zone; coherence levels can be modeled by including scattered energy from inhomogeneities above the target zone.
Seismic imaging of the Upper Mantle structure and dynamics beneath the Southern Caribbean plate boundary and Venezuela
(2013-11-01) Masy, Jeniffer; Niu, Fenglin; Levander, Alan R.; Alexander, David; Morgan, Julia K.
The Caribbean-South America plate boundary has a complicated tectonic history that has been matter of debate and the focus of many studies for decades, yet many questions remain unanswered. The aim of this work, developed within the framework of the BOLIVAR (USA) and GEODINOS (Venezuela) projects, is to use different seismological techniques to study the lithospheric structure under the southern Caribbean and Venezuela, in order to understand some aspects of the present structure and its tectonic evolution. A shear wave splitting analysis in northwestern Venezuela revealed three areas with different deformation mechanisms: (1) Islands and coastal regions have large splitting times (~2-3 s) and a fast polarization direction parallel to the direction of the relative plate motion of the Caribbean plate respect to South America, which can be explained by a strong eastward flow confined at the CAR-SA plate boundary. (2) The stable South America plate showed weak seismic anisotropy with an origin likely in the asthenosphere. (3) Large splitting times and a ~NE-SW fast direction are observed at stations deployed along the Mérida Andes range, suggesting that the subcontinental mantle is also deformed beneath the range. It is likely the lithospheric mantle played a major, if not dominant, role in the formation of the Mérida Andes. The upper mantle structure of the area was obtained by combining three types of seismic data: Ps and Sp receiver functions and Rayleigh wave tomography. Results reveal the presence of the Moho of the subducting Caribbean Plate beneath the northwestern part of the Maracaibo Block. Tomographic images indicate that the subducting Atlantic slab appears to be attached to the continental South American lithosphere, pulling it down and removing the continental lithospheric mantle beneath the Serrania del Interior. A lithospheric thickness map was also obtained. The lithosphere asthenosphere boundary shows significant variations and seems to correlate well with major tectonic provinces in the region. Finally ambient noise cross-correlations between station pairs yields to Empirical Greens Function as waveform data input for the adjoint tomography based on spectral element methods. The adjoint tomography utilizes a more accurate full wave finite-frequency theory compared to the previous ray theory, and will iteratively refine the initial smooth 3D model to achieve more detailed high-resolution images of the upper most mantle structure of eastern Venezuela. Low velocity anomalies correspond to the major sedimentary basins and high velocity anomalies correspond to the stable craton.
Seismic waveform tomography with multicomponent data at a groundwater contamination site
(2009) Chambers, Geoffrey; Levander, Alan R.
This thesis develops an SH-wave version of frequency-domain, full waveform tomography, and applies it, together with traditional acoustic waveform tomography, to a multicomponent seismic data set acquired over a shallow contaminated aquifer at Hill Air Force Base, Utah. The study combines the high resolution provided by waveform tomography with inherent advantages of SH-wave imaging, such as reduced seismic velocity and independence of pore fluid content. Presented are synthetic tests of the method, its application to the field data, and interpretation of the resulting P- and S-wave velocity models. Synthetic tests reveal fundamental differences between acoustic and SH waveform tomography, and demonstrate, together with the field data inversions, improved resolution for SH-wave imaging due to smaller velocities. High-resolution velocity models from inversion of the field data are interpreted in terms of lithology and water saturation, which are better constrained by the availability of both P- and S-wave velocity.

Browsing by Author "Levander, Alan R."

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