Low Frequency Modifications of Seismic Background Noise Due to Interaction with Oscillating Fluids in Porous Rocks

Abstract

SUMMARY Low frequency spectral modifications of seismic background waves (noise) due to interaction with partially saturated porous rocks are investigated. Non-wetting fluid drops entrapped in pores can oscillate with a characteristic eigenfrequency. A 1D wave equation is coupled with a linear oscillator equation representing these oscillations. The resulting system of equations is solved numerically with explicit finite differences. The background noise is reduced to its dominant frequency (0.1-0.3Hz) which is presumably related to surface waves generated by ocean waves. This frequency is used as the external source. The resulting incident monochromatic wave excites the pore fluid which thereafter oscillates with its eigenfrequency. Oscillatory energy is transferred to the porous rock which leads to an amplitude decay of the fluid oscillation. The elastic matrix carries the eigenfrequency of the fluid oscillation in addition to the external frequency. Fourier spectra of the solid velocity therefore show two distinct peaks: the external frequency and the eigenfrequency of the fluid oscillation. Interestingly, such low frequency modifications of seismic noise are observed above hydrocarbon reservoirs and the presented model is considered as one possible explanation. Time evolution of the amplitude decay of the fluid oscillation seems to be related to the thickness of the porous rock.