Mapping subsurface fractures using nonlinearity measurements

A method for determining the location and the orientation of the open natural fractures in an earth formation from the interaction of the two seismic signals, one signal transmitted into the formation from one wellbore and the second signal transmitted from the surface of the earth; the interaction of the two seismic waves as they are transmitted through the fractured space is recorded in the second wellbore. The two seismic waves have two selected discrete frequencies. The seismic signal or the seismic wave transmitted from the surface is a low frequency of large amplitude identified as "modulation' wave and the signal transmitted from the wellbore is a higher frequency by an order of one hundred times, identified as "carrier' wave. The interaction of the "carrier' wave during the compression and rarefaction cycles of the "modulation' wave is spectrally analyzed. The relative amplitudes of the fundamentals frequency of the "carrier' wave and its harmonics changes during compression and rarefaction cycles of the "modulation' wave, since the fractures are squeezed during compression and open during rarefaction cycles of the large amplitude low-frequency signal. This method includes the determination of the spectral changes in the "carrier' wave during the alternate half cycles of rarefaction and compression of the low-frequency signal. When the low-frequency wave arrives at or near right angles to the fractures the difference is maximized and when arrives parallel or near parallel to the fractures the difference is minimized. By using the surface source at various locations on the surface in a predetermined geometric pattern and analyzing the results of the interaction of the two waves from each location, the orientation and the location of the fractures can be determined.