Seismic Attenuation Problem

Nigam, Nilima (2004) Seismic Attenuation Problem. Canadian Industrial Problem Solving Workshops > 8th IPSW [Vancouver 17/5/2004 - 21/5/2004].

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Abstract/Summary

Seismic imaging, a technique in which the reflections of a source seismic wave are recorded as it passes through the earth, is a major tool for geophysical exploration. Seismic imaging can be used to reconstruct a profile of the material properties of the earth below the surface, and is thus widely used for locating hydrocarbons.

The problem presented by Husky Energy concerns seismic attenuation: the loss of energy as a seismic wave propagates through the earth. As an exploration tool, attenuation effects have only recently attracted attention. These effects can prove useful in two ways: as a means of correcting seismic data to enhance resolution of standard imaging techniques, and as a direct hydrocarbon indicator. Theoretically, a subsurface reservoir full of hydrocarbons will tend to be acoustically softer than a porous rock filled only with water; Kumar et al show that attenuation is highest in a partially fluid-saturated rock.

Many physical processes can lead to the attenuation of a seismic trace. In the present work, we ignore attenuation effects such as spherical divergence or scattering, and concentrate on intrinsic attenuation effects exclusively. The latter are caused by friction, particularly in porous rocks between fluid and solid particles.

The goal of the workshop was to find a means of computing seismic attenuation from relatively short windows of seismic imaging data, and particularly be able to identify regions of anomalous attenuation.

Problem Statement

The ability of a material to attenuate seismic waves is measured by a dimensionless quantity Q, called the attenuation factor, defined as the energy of the seismic wave divided by energy dissipated per cycle of wave. Typical values of Q range from 5-20 (dirt) through 100 (rock) to 10,000 (steel).

In what follows, we assume that this attenuation factor is independent of the frequency in the useful seismic bandwidth. The attenuation of the wave is directly linked to the different layers that compose the Earth, so that whenever changes in the composition of layers occur, the attenuation changes too. This is why we would like to be able to detect changes in attenuation, as it would enable us to identify and change material properties. The goal is therefore to estimate Q from given seismic data.

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