Interaction of seismic background noise with oscillating pore fluids causes spectral modifications of passive seismic measurements at low frequencies
Published in
SEG/San Antonio 2007 Annual Meeting
Date of publication
01 November 2007
Abstract
Studies of passive seismic data in the frequency range below 20Hz have shown that the frequency content of the ever-present seismic background noise changes above hydrocarbon reservoirs. Different explanations for this observation have been proposed. In this study, the effect of oscillating pore fluids, i.e. oil, on the seismic background noise is investigated. A non-wetting fluid drop entrapped in a pore can oscillate with a characteristic eigenfrequency. Capillary forces act as the restoring force driving the oscillations. A 1D wave equation is coupled with a linear oscillator equation, which represents these pore fluid oscillations. The resulting linear system of equations is solved numerically with explicit finite differences. The most energetic part of the seismic background noise, i.e. frequencies around 0.1-0.3Hz, is used as the external source. This part is presumably related to seismic surface waves generated by ocean waves. It is shown that the resulting elastic wave initiates oscillations of the fluid drops. The oscillatory energy of the pore fluid is transferred continuously to the elastic rock matrix. In consequence, seismic waves in the elastic rock carry a second frequency, the eigenfrequency of the pore fluid oscillations on top of the applied external frequency. Both frequencies can be measured at the earth surface. The presented model is considered as a possible explanation for observed spectral modifications above hydrocarbon reservoirs. Time evolution of the pore fluid oscillations seems to be related
to the thickness of the hydrocarbon reservoir.
to the thickness of the hydrocarbon reservoir.
