DEEP‐HOLE GEOPHONE STUDIES

Abstract

Seismic surveys have been made in eight widely separated wells with two types of wall‐coupled geophones. These geophones accurately detected and reproduced elastic waves traveling in the earth. Both direct (initial) pulses from shots and reflection events were studied. The complexity introduced into the initial pulses by secondary reflections varied from well to well. Amplitude decayed as the negative 2.4 power of travel time. Pulse‐broadening caused by selective absorption of high frequencies was found. Different wells showed amounts of broadening ranging from nearly complete to that indicating little absorption of high seismic frequencies. Reflections from interfaces below the geophone were traced to their origin in the earth. At three wells, the same reflections were found on surface seismograms giving an identification of surface‐detected reflections and of reflector depth. Multiple reflections were distinguished from direct reflections and were found to mask the latter at four wells. In one case, multiple reflections were identified with events on surface records. Reflection coefficients found for direct reflections averaged 0.36. Accurate velocity surveys of the wells resulted from this work.