Passive interferometry of deepwater ocean bottom seismic data

Microseismic monitoring can be either carried from the surface or borehole. While noise levels in the borehole are usually much lower than on the surface (typically by a factor of 10), we show that the signal amplitude at the earth’s surface is also larger due to a free surface boundary condition (simultaneous observation of both direct and reflected waves) and impedance contrasts at the low-impedance near surface layers. Thus a comparison of detectability of microseismic events between surface and downhole monitoring systems needs to take into consideration amplitude gains in near surface layers. We show a typical example of such gains on a vertical geophone array where the amplitude of the direct P-wave increases by more than a factor of 3 from a 150 m deep geophone to a geophone at the earth’s surface. Considering this amplitude gain one can calculate a theoretical detectability between surface and downhole monitoring arrays and we show that surface and downhole monitoring arrays can have approximately similar detectability at distances of a few hundred meters from a monitoring borehole, and that a surface array detects significantly more events at distances beyond 500 meters from a monitoring borehole.