C. A. Zelt

Three‐dimensional seismic refraction tomography: A comparison of two methods applied to data from the Faeroe Basin

This paper presents a comparison of two tomographic methods for determining three-dimensional (3-D) velocity structure from first-arrival travel time data. The first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays. The second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors. Both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration. Travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing. Both inverse methods are applied to 3-D ocean bottom seismometer (OBS) data collected in 1993 over the Faeroe Basin, consisting of 53,479 travel times recorded at 29 OBSs. This is one of the most densely spaced, large-scale, 3-D seismic refraction experiments to date. Different starting models and values for the free parameters of each tomographic method are tested. A new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion, but the latter method provides a simpler model for little additional computational expense when applied to the Faeroe Basin data. Bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints. An inversion of synthetic data corresponding to 100% data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3-D refraction experiments to provide well-resolved, long-wavelength velocity models. The similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion.

Modeling wide-angle seismic data for crustal structure: Southeastern Grenville Province

A modeling methodology for obtaining two-dimensional (2-D) crustal structure from wide-angle seismic data is applied to data from the southeastern Grenville Province. Pre-modeling steps include (1) assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal-to-noise ratio, (2) using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity, and (3) identifying data unsuitable for 2-D modeling. The goal of the travel time inversion-amplitude modeling approach is to obtain a minimum-structure and minimum-parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide-angle data. Each step of a layer-stripping procedure involves a series of inversions in which a one-dimensional or simple starting model is improved with additional velocity and/or interface nodes until a satisfactory trade-off between travel time fit, parameter resolution and complete ray coverage of all source-receiver pairs is achieved. Using zero vertical-velocity gradient layers and head waves during preliminary first-arrival inversion can (1) decrease the number of intermediate models, (2) allow greater lateral heterogeneity to be imaged, and (3) simplify incorporation of amplitude modeling constraints into the final model. Using amplitude-distance curves allows quantitative modeling of the relative amplitude and offset variations of phases. Discrepancies between observed and calculated reflection amplitudes are used to infer fundamental, non step-like velocity changes at layer boundaries. Later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that “float” within the model without an associated velocity structure. These reflectors provide a spatial image like that obtained from vertical-incidence reflection data, as opposed to a velocity image. The model of Grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data. A crustal-scale zone of wide-angle reflectors with an average easterly apparent dip of 13° defines a major Grenvillian terrane boundary.

SEISMIC INVESTIGATION OF THE CONTINENTAL MARGIN OFF- AND ONSHORE VALPARAISO, CHILE

Abstract At the latitude of Valparaiso, Chile, a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Three legs of the German R/V Sonne (cruises SO101, SO103 and SO104) surveyed the continental margin and oceanic plate offshore Valparaiso, aiming at studying the crustal structure and investigating possible causes for the change in slab configuration. Sonne cruise SO101 investigated the tectonic setting with swath-mapping bathymetry, magnetics and high-resolution seismics. Following these investigations cruise SO103 collected land-sea wide-angle seismic data, and coincident deep seismic reflection data were acquired during cruise SO104. Coincident near-vertical and wide-angle seismic measurements were made along two profiles. Profile 1, located at the south of the study area, away from the influence of the subducting ridge, crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed. Profile 2, located in the north, runs from the Juan Fernandez Ridge to the Chilean coast. The crustal velocity models obtained for the two profiles show that the continental crust extends to the middle-lower slope boundary, which is also reflected in morphology. In addition, they show that the crustal structure of the oceanic plate is rather similar, but the plate seems to be slightly more inclined along the northern profile (13° versus 10° in the south). The two profiles are only about 70 km apart but their structures differ significantly. No straightforward correlation exists between the two profiles that can be attributed to ridge collision. The data support that the 1985 central Chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate.

3D seismic refraction traveltime tomography at a groundwater contamination site

We have applied traveltime tomography to 3D seismic refraction data collected at HillAir Force Base, Utah, in an approximately 95 40-m area over a shallow 20 m groundwater contamination site.The purpose of this study is to test the ability of 3D first-arrival-time data to characterize the shallow environment and aid remediation efforts. The aquifer is bounded below by a clay aquiclude, into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquidDNAPLcontaminants.Aregularizednonlineartomographicapproachwasappliedto187,877 first-arrival traveltimes to obtain the smoothest minimumstructure 3D velocity model. The resulting velocity model contains a velocity increase from less than 300 to 1500 m/s in the upper 15 m. The model also contains a north-southtrendinglow-velocityfeatureinterpretedtobethepaleochannel, based on more than 100 wells in the area. Checkerboard testsshow7.5‐10 mlateralresolutionthroughoutmostofthe model. The preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach,includingthestartingmodel.Thefinalvelocitymodel compares favorably with a 3D poststack depth migration and 2D waveform inversion of coincident reflection data. While the long-wavelength features of the model reveal the primary target of the survey, the paleochannel, the velocity model is likely a very smooth characterization of the true velocity structure, particularly in the vertical direction, given thesizeofthefirstFresnelzoneforthesedata.

Crustal structure and tectonics of the southeastern Canadian Cordillera

The crustal structure of the Omineca (OB) and Foreland (FB) belts of the southeastern Canadian Cordillera are interpreted from the inversion of seismic refraction/reflection travel times and amplitudes, and modeling of the Bouguer gravity data from a 350-km east-west wide-angle profile. The main features of the resultant velocity and density models include (1) a low average crustal velocity of 6.2 km/s, (2) variable upper crustal velocities (5.6–6.3 km/s) within the FB and Purcell Anticlinorium as compared to farther west in the OB (6.1–6.2 km/s), (3) a midcrustal (∼20 km depth) 0.4–0.5 km/s velocity increase within the OB, (4) eastward crustal thickening from 35 to 42 km over 80 km distance beneath the OB-FB boundary, (5) the Slocan Lake fault (SLF) dipping east at 15–20° to at least 35 km depth, (6) decreased lower crustal velocities (6.7–6.4 km/s) across the SLF from the OB into the FB, (7) increased uppermost mantle velocities (7.9–8.0 km/s) from the OB into the FB, and (8) an average crustal density increase of 40 kg/m3 from the OB into the FB. Lower velocities, higher conductances, and increased densities within the upper crust (0–10 km depth) of the Purcell Anticlinorium appear to be associated with Middle and Upper Proterozoic rift-related metasediments and gabbroic dikes (35–55% by volume) in contrast to the mainly Mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the OB. Elevated crustal temperatures within the OB (a high heat flow province) are responsible for low-velocity gradients to 20 km depth. A more mafic lower crust beneath the OB (inferred from higher velocities) and the westward decrease in crustal thickness are interpreted as resulting primarily from Middle and/or Late Proterozoic extension and rifting of the craton. Penetration of the SLF into the lower crust may be controlled by lateral strength contrasts at the edge of the craton. Farther west, a midcrustal strength contrast (a velocity boundary at 20–25 km depth) may have acted as a regional detachment zone during compressional and extensional tectonic episodes.

Waveform tomography at a groundwater contamination site: Surface reflection data

Wehaveappliedacoustic-waveformtomographyto452D seismic profiles to image the 3D geometry of a buried paleochannel at a groundwater-contamination site at Hill Air Force Base in Utah. The paleochannel, which is incised into an alluvium-covered clay aquitard, acts as a trap for dense nonaqueous-phase liquids DNAPLs that contaminate the shallowest groundwater system in the study area. The 2D profiles were extracted from a 3D surface reflection data set. First-arrival traveltime tomography provided initial velocity models for the waveform tomography. We inverted for six frequency components in the band 30‐90 Hz of the direct and refracted waves to produce 45 2D velocity models. The flanks and bottom of a channel with a maximum depth of about 15 m were well modeled in most of the 45 parallel 2D slices,whichallowedustoconstructa3Dimageofthechannel by combining and interpolating between the 45 image slices. The 3D model of the channel will be useful for siting extraction wells within the site remediation program.The alluvium that fills the channel showed marked vertical and lateral velocity heterogeneity. Traveltime tomography and waveform tomography can be complementary approaches. Used together, they can provide high-resolution images of complicatedshallowstructures.

Waveform tomography at a groundwater contamination site: VSP-surface data set

Application of 2D frequency-domain waveform tomography to a data set from a high-resolution vertical seismic profiling (VSP) experiment at a groundwater contamination site in Hill Air Force Base (HAFB), Utah, reveals a surprisingly complicated shallow substructure with a resolution of approximately 1.5 m. Variance in the waveform misfit function is reduced 69.4% by using an initial velocity model from first-arrival traveltime tomography. The waveform tomography model suggests (1) a low-velocity layer at 1 to 4 m depth, (2) a high-vertical-velocity gradient of 80 m/s/m on average, and (3) severe lateral variations — velocity contrasts as large as about 200 m/s occur in a distance as short as 1.5 m. The model is well correlated with lithologic logs and is interpreted geologically. A Q-value of 20 is estimated for the target area. The extreme lateral and vertical variations of the subsurface compromise many standard seismic processing methods.

Evolution of the Southern Caribbean Plate Boundary

It is generally accepted that the cores of the continents, called cratons, formed by the accretion of island arcs into proto-continents and then by proto-continental agglomeration to form the large continental masses. Mantle-wedge processes, combined with higher melting temperatures during the Archean (2.5–3.8 billion years ago) and possibly thrust stacking of highly depleted Archean oceanic lithosphere, produced a strong, buoyant, upper mantle chemical boundary layer. This stabilizing mantle layer, known as the tectosphere, has shielded the Archean cratons from most subsequent tectonic disruption and is highly depleted in iron, providing the positive buoyancy that is required to ‘float’ the continents more than four kilometers above the surrounding ocean basins.

3D simultaneous seismic refraction and reflection tomography of wide‐angle data from the Central Chilean Margin

We present an application of three-dimensional (3D) simultaneous seismic refraction and reflection tomography for velocity and interface structure. The inversion technique and method for developing the starting model are specifically designed for relatively sparse wide-angle data acquired across strongly-varying structure. The data were recorded in a region of seamount subduction on the Chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90×90 km area providing constraint to 25 km depth. The tomographic method and the final model are assessed through a comparison with the large-scale geologic features of the margin and a resolution test. The 3D model shows the Valparaiso forearc basin, the accretionary wedge, the subducting plate, and possibly a subducted seamount. Our results show the potential of relatively sparse 3D wide-angle data.

Prestack depth migration of dense wide-angle seismic data

Abstract Prestack depth migration of wide-angle seismic data represents an extension of traditional imaging with near-vertical incidence data because it includes a larger component of the recorded wave field. To date, however, studies that have employed wide-angle migration have suffered because only widely spaced data were available and because only very simple synthetic tests were performed. Although wide-angle migration has the potential to increase our ability to image deep-crustal structures, particularly when closely spaced data are collected, a thorough study of this technique has been lacking. To address this, we present a case study of prestack depth migration of relatively dense synthetic wide-angle marine data. The objectives are to identify potential benefits and limitations of this approach and answer such fundamental questions as how close the receiver spacing must be for a typical survey to image effectively with wide-angle data. This will facilitate the design of better seismic experiments. Our study employs Kirchhoff prestack depth migration of variably spaced full wave-field synthetic wide-angle ocean-bottom hydrophone (OBH) data generated using a realistic velocity model based on the passive eastern margin of the United States. We show how an increase in OBH density improves the migration by increasing the lateral resolution and signal-to-noise ratio. We also investigate the contribution of various offset ranges to the migrated image and show how the wider-angle components contribute primarily to the deepest parts of the image with relatively low spatial frequency compared to the near-vertical incidence components. To investigate how errors in the velocity model affect imaging as a function of offset range we migrate the data using a velocity model derived from refraction and reflection traveltime inversion. This example demonstrates the need to obtain an increasingly accurate model as increasingly wider-angle data are migrated. To effectively image structures in our 200 × 40 km synthetic velocity model, an OBH spacing of approximately 2 km is required.