Discrete Element Models of Seismic Wave Propagation in Sediments
Abstract
The propagation of seismic waves through heterogeneous sediments is a topic of major importance , particularly in reservoir exploration. In order to develop a better understanding of wave propagation through sediments, we employ a numerical model based on the discrete element method (DEM) used in modeling granular materials. An advantage of this method is that one can readily explore a large range of heterogeneity in grain properties, such as size and bulk modulus, as well as a variety of spatial distributions of those properties. Discrete element models (e.g. Cundall and Strack, 1979) have been used to study the deformation of granular materials under a wide variety of conditions. In this method, a small (103 to 104) set of simple (2D circular) model grains is defined. The pore space between the grains is taken to have zero bulk modulus, so these models represent drained sediments. Grain interact with specified elastic and frictional forces. The elastic forces can be both repulsive and attractive, allowing simulation of both non-cohesive and cohesive sediments. The position, velocity and acceleration of each grain is tracked through time, in response to external loads. One side of the sample is subjected to a single time-dependent pulse in normal force, sending a compression wave into the sample. The subsequent accelerations of the opposing side give the acoustic wave speed in the granular material as well the attenuation of the wave form. Initial experiments were conducted with non-cohesive grains in a system with a limited range of grain sizes (factor of 3). In such a system, the 2-D porosity is 18-19%. This system shows significant attenuation of the input wave and yeild a velocity that is about 15% slower than the acoustic velocity within a grain. We will examine the effect of pressure on seismic velocity and compare this with theoretical models. We will also explore the relationship between applied non-hydrostatic stresses and anisotropy.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T32E0925S
- Keywords:
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- 5102 Acoustic properties;
- 7203 Body wave propagation;
- 7260 Theory and modeling