SEISMIC NOISE EMISSION INDUCED BY SEISMIC-WAVES

The in situ rock undergoing different kinds of deformation accumulates elastic energy that can be released spontaneously or under artificial effect. Triggering of energy can also be performed by means of a dyna mic excitation caused by elastic waves. We carried out special field i nvestigations with the use of seismic vibrators (Vibroseis-like source s) to study the effect of the energy release under the influence of se ismic waves. In the first experiment we conducted borehole observation s of high-frequency seismic noise in two narrow frequency bands around 500 Hz and 1 kHz. The induced noise emission occurred in both frequen cy windows when the medium had been irradiated with the low-frequency seismic vibrations. Intensity of emission was proportional to the mech anical power of a vibrator. In the second experiment we carried out do wnhole measurements of a broad-band seismic noise. They showed a gradu al increase of the noise energy in the frequency range 100-200 Hz duri ng a series of monochromatic vibrations with frequency of 67 Hz. Facts observed give evidence that the elastic energy which accumulated duri ng the rock deformation can be triggered by a relatively weak seismic excitation.