Abstract: Data for use in producing a tomograph of subsoil structure between boreholes is derived by placing spaced geophones in one borehole, and if desired also on the earth surface, and by producing a sequence of shots at spaced apart locations in the other borehole. The signals, detected by each of the geophones from the various shots, are processed either on a time of arrival basis, or on the basis of signal amplitude, to provide information of the characteristics of a large number of incremental areas (pixels) between the boreholes. Such information is useable to produce a tomograph of the subsoil structure between the boreholes. By processing signals of relatively high frequencies, e.g., up to 1000 Hz, and by closely spacing the geophones, a high resolution tomograph can be produced.

Abstract: Bursts of signals at different frequencies are induced into substructure, adjacent a borehole. The return signals from each burst of signals are normalized to compensate for the attenuation, experienced by more distant return signals. The peak amplitudes of return signals, above a selected level, are cut off, and an average signal is produced from the normalized amplitude-limited return signals of each burst. The averaged signals of the return signals of all the signal bursts at the different frequencies are processed, to provide a combined signal, whose amplitude is related to the microfracture density of the substructure adjacent the borehole.