Seismic noise emission induced by seismic waves

The in situ rock undergoing different kinds of deformation accumulates elastic energy that can be released spontaneously o r under artificial effect. Triggering of energy can also be performed by means of a dynamic excitation caused by elastic waves. W e 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. T h e 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 t o 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.