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Crosswell seismic data collected at BP’s test site near Devine, Texas show clear evidence of P-wave anisotropy (v,/u, up to 1.3) confined to shale layers in a shale/carbonate sequence. Tomograms made assuming isotropy are severely degraded, but could be mistakenly interpreted as indicating (nonexistent) low velocity zones within the carbonates. The effects of the anisotropy are directly evident in the raw data once headwaves and internally reflected waves are correctly identified. Inversion for a layered, transversely isotropic, anisotropic medium yields a model that fits the data to the accuracy of the time picking.

A method is presented for the analysis of the transmitted and reflected transient elastodynamic wavefield at a fracture that may be modeled as a linear slip interface. This interface model implies that the traction is continuous at the fracture, while the displacement may show a jump discontinuity that is proportional to this local traction. To start with, for both the SH and the P/SV wave systems, transform domain expressions are determined for the transmitted and reflected wavefields at the interface. Subsequently, an almost completely analytical transformation back to the space‐time domain is carried out. The method provides the exact time domain transmitted and reflected waves (including the body wave, interface wave, and head wave contributions) that are caused by a spatially curved, incident wavefield due to a point source. Various numerical results are given. For the SH case, the directionally dependent filter behavior of the fracture is shown. For the P/SV case, first the reflection, transmission,...

WKBJ waveform inversion of Expanding Spread Profile 5 from the North Atlantic Transect is presented. The inversion consists of a Monte Carlo search to determine the major features of the model, followed by a gradient descent to the global minimum. Nonlinear error analysis completes the analysis. Modifications to existing algorithms to allow the search for low velocity zones and interfaces are described. The results agree closely down to midcrustal depths with results from trial-and-error inversions. Previous interpretations have concluded that a low velocity zone exists in the lower crust. Our waveform analysis shows that this feature may exist but is not necessarily needed to satisfy the data.

Earthquakes from the Tonga and Samoa islands were used as a source for dT/dΔ determinations at distances of 80°–95°. The data were recorded by two University of Alberta arrays, Vasa and Peace, and three Canadian network seismic stations, EDM, FSJ, and MCC. Earthquakes from Japan, Asia, and South America have small travel time station anomalies and phase velocities that are in agreement with the Jeffreys-Bullen tables. However, events from Tonga and Samoa arrive with phase velocities that are up to 15% higher. The anomaly increases from 0 at 84° to 4 km/sec at 95° at both arrays. The results are not interpretable readily by using anomalous conditions in the downgoing plate in the source region or at the receivers. We conclude that the observations are due to a heterogeneous region of high velocity in the mantle at the core-mantle interface underneath the island of Hawaii.

In cross-hole tomography straight lines connecting the source and receiver are often used to approximate the true ray paths, thus linearizing the inverse problem. A recent paper by Bregman et al. (1989a) has shown that this approximation may be very poor in the regions of greatest interest where large velocity gradients cause significant ray curvature. The interpretation method presented by Bregman et al. (1989a) iteratively ray traces and updates the velocity model with model perturbations obtained by a damped least squares inversion of the travel time residuals. This paper presents a modification of the travel time tomography method so that it may be applied to the inversion of first motion amplitudes for inverse quality factor Q−1. After determining the velocity structure using travel time tomography the amplitudes are inverted using a similar scheme. No additional ray tracing is required for the amplitude inversions. The effect of the velocity structure previously determined through travel time tomography is taken into account so that amplitude variations due to the focusing and defocusing effects of the velocity structure are not interpreted as being due to attenuation structure. Both the travel time and amplitude tomography methods are applied here to field data from a cross-hole experiment in crystalline rock (Wong et al., 1983). The frequency range of the seismograms is 1–6.6 kHz, allowing resolution of velocity structure on a scale of several meters. The resulting velocity image shows good agreement with other geological and geophysical data. Reversing the model by placing the model boreholes on opposite sides relative to the original reconstruction yields almost identical results, indicating that the interpolation scheme used in the forward modeling has not biased the final model. Synthetic Maslov seismograms calculated for the derived velocity model agree well with the waveform data, further confirming the validity of the model. In addition, the amplitude tomography results place absorptive regions in the same location as the most pronounced low-velocity features which correlate with fractures in the boreholes. Highly transmissive regions also correlate well with high-velocity regions.