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 dynamic excitation caused by elastic waves. We carried out special field investigations with the use of seismic vibrators (Vibroseis-like sources) to study the effect of the energy release under the influence of seismic waves. In the first experiment we conducted borehole observations of high-frequency seismic noise in two narrow frequency bands around 500 Hz and 1 kHz. The induced noise emission occurred in both frequency windows when the medium had been irradiated with the low-frequency seismic vibrations. Intensity of emission was proportional to the mechanical power of a vibrator. In the second experiment we carried out downhole measurements of a broad-band seismic noise. They showed a gradual increase of the noise energy in the frequency range 100-200 Hz during a series of monochromatic vibrations with frequency of 67 Hz. Facts observed give evidence that the elastic energy which accumulated during the rock deformation can be triggered by a relatively weak seismic excitation.