Abstract

Traveltime tomography is an appropriate method for estimating seismic velocity structure from arrival times. However, tomography fails to resolve discontinuities in the velocities. Wave-equation techniques provide images using the full wave field that complement the results of traveltime tomography. We use the velocity estimates from tomography as a reference model for a numerical propagation of the time reversed data. These 'backpropagated' wave fields are used to provide images of the discontinuities in the velocity field.The combined use of traveltime tomography and wave-equation imaging is particularly suitable for forming high-resolution geologic images from multiple-source/multiple-receiver data acquired in borehole-to-borehole seismic surveying. In the context of crosshole imaging, an effective implementation of wave-equation imaging is obtained by transforming the data and the algorithms into the frequency domain. This transformation allows the use of efficient frequency-domain numerical propagation methods. Experiments with computer-generated data demonstrate the quality of the images that can be obtained from only a single frequency component of the data. Images of both compressional (V p ) and shear wave (V s ) velocity anomalies can be obtained by applying acoustic wave-equation imaging in two passes. An imaging technique derived from the full elastic wave-equation method yields superior images of both anomalies in a single pass.To demonstrate the combined use of traveltime tomography and wave-equation imaging, a scale model experiment was carried out to simulate a crosshole seismic survey in the presence of strong velocity contrasts. Following the application of traveltime tomography, wave-equation methods were used to form images from single frequency components of the data. The images were further enhanced by summing the results from several frequency components. The elastic wave-equation method provided slightly better images of the V p discontinuities than the acoustic wave-equation method. Errors in picking shear-wave arrivals and uncertainties in the source radiation pattern prevented us from obtaining satisfactory images of the V s discontinuities.

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