DIRECT-S WAVE MODES PRODUCED BY VERTICAL AND HORIZONTAL VIBRATORS

摘要

Horizontal vibrators are considered to be the optimal source for generating direct-S modes, which are S modes produced directly at a source station, not by mode conversion at interfaces remote from a source station. We show some results of a field test in which we investigated direct-S modes produced by vertical-force seismic sources. The test data were acquired by positioning horizontal vibrators and various vertical-force sources (vertical vibrator, vertical impact, shot-hole explosive) at the same source stations and recording their downgoing illuminating wavefields with a vertical array of 3-component geophones. In this paper, we compare only direct-S modes produced by vertical vibrators and horizontal vibrators. We show that there are strong similarities between the direct-S modes produced by these two sources. This investigation has considerable economic value because if it can be established that direct-S modes produced by vertical-force sources can be substituted for direct-S modes produced by horizontal vibrators, a large number of direct-S sources become available that can be used in any environment where P-wave seismic data are acquired.