Computerized geophysical tomography

Abstract

Computerized tomography is used as an aid in geophysical exploration. With this method, detailed pictures of electromagnetic properties in the regions between pairs of boreholes can be reconstructed. The spatial distribution of attenuation or propagation velocity is calculated from line integrals along rays in the plane between boreholes, and displayed as a digital picture. In principle, the transmission of seismic data can also be analyzed by this method as long as it obeys the line integral model. Iterative solution techniques, similar to those used in medical X-ray tomography are applied to solve the large sets of linear equations relating the line integral data and the remote observables. A straight-line ray optics model was used for energy propagation between boreholes. The performance of the reconstruction algorithm is demonstrated using computer-generated data and it is then applied to experimental data collected by continuous-wave electromagnetic transmission probing. Experimental attenuation reconstructions are presented of a proposed underground urban mass-transit site. Both lateral and vertical variations are displayed using these methods.

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