Numerical Simulation Of Seismic Attenuation Due To Wave-Induced Fluid Flow.

Abstract

Seismic data from sedimentary rocks usually exhibits attenuation levels than can not be explained
by existing theoretical models. An important dissipation mechanism for waves in heterogeneous
poroelastic media is the effect of wave-induced fluid flow created by mesoscopic scale heterogeneities,
known as mesoscopic loss. Mesoscopic length scales are those larger than pore size but smaller than
wavelengths in the seismic range (1- 100 Hz). A typical mesoscopic heterogeneity has a size of tens of
centimeters. Mesoscopic heterogeneities can be due to local variations in lithological properties or to
patches of immiscible fluids. For example, a fast compressional wave traveling across a porous rock saturated
with water and patches of gas induces a greater fluid pressure in the gas patches than in the water
saturated parts of the material. This in turn generates fluid flow and slow Biot waves which diffuse away
from the gas-water interfaces generating significant losses in the seismic range. In this work an iterative
domain decomposition finite element procedure is presented and employed to solve Biot’s equations of
motion for saturated poroelastic materials. The domain decomposition procedure is naturally parallelizable,
which is a necessity in this type of simulations due to the large number of degrees of freedom
needed to accurately represent these attenuation effects. The numerical simulations, run on a parallel
computer, were designed to show the effects of the wave-induced fluid flow on the traveling waves in
the seismic range of frequencies. The simulated recorded traces show evidence of the mesoscopic loss
mechanism in this type of materials.