WRRCTR No.11 Some Seismic Techniques for Mapping Small Scale Shallow Structures

Abstract

Seismic recording and playback techniques and equipment are being developed for use in mapping small scale geologic structures of significance in controlling the movement of ground water. High-frequency seismic waves produced by weight-dropping, or thumping, are used to provide the necessary resolution to detect these small structures. A vibrator has been constructed as another source of high frequency seismic energy. A DC motor is the transducer with storage batteries providing the power. A four-channel tape recorder is used to obtain seismic records on a l0-foot spacing. This spacing gives good correlation from record to record. A storage oscilloscope gives visual displays of seismic data as wiggly line playbacks which are stored on the screen and then photographed. Also being used is variable intensity playback of reflection records. This allows signal averaging to be done optically and gives an easily interpreted display. A seismic survey done in the Waimanalo area on Oahu, Hawaii indicates that the thumper can produce sufficient amounts of high frequency energy to permit shallow reflection prospecting.