The association of low frequency passive seismic anomalies with hydrocarbon reservoirs is an empirical result reported by a number of independent companies and academic institutions. Spectraseis has devoted over 40 man-years to researching this phenomenon through academic partnerships and internal research. Our current working hypotheses for the observations are based on an extension to far field elastic theory incorporating phenomena associated with multi-phase fluids in poroelastic media.

Hydrocarbon reservoirs are unique in the subsurface because they contain multiple fluid phases on the pore size scale. The different compressibility of two fluid phases gives rise to pressure differentials that can be stimulated by a passing seismic disturbance. At low frequencies, seismic waves can act more like pressure gradient disturbances rather than transient stimulations. As such, fluid flow is induced between connecting but differing regions in a porous medium. Restoring forces for this state of disequilibrium can include gravity, pressure, or surface tension between immiscible fluids.

These pore level phenomena may give rise to intrinsic attenuation, energy absorption/ re-emission, and even nonlinear effects that can be used to describe how energy is redirected from the earth’s energy field toward the surface so that it can be measured by broadband seismometers located on the surface. Statistical analysis of spectral attributes should then be used to discriminate this effect from near surface site effects and seismic noise.

A preliminary model that explains the origin of hydrocarbon-indicating tremors and that is consistent with the spectral attributes (i.e., PSDIZ and V/H signal).

Anomalous behavior of low frequency seismic wavefields is not unique to Spectraseis technology. Similar observations have been made in active seismic surveys. The motivation for spectral decomposition of subsurface images and extending the seismic bandwidth to low frequencies with improved acquisition technology (source and receivers) is fueled, in part, by case studies that show increased seismic reflectivity of hydrocarbon reservoirs at low frequencies.