In unconsolidated sand at low saturations, water exists in pendular rings at grain contact points. As the grains are pressed together by a passing wave, the squirt flow will contribute to wave attenuation. In a model which consists of a sphere-pack framework, fluid flow equations are solved under certain approximations to calculate the viscous losses and, hence, attenuation. Attenuation is negligible for a sphere-pack composed of equal-sized spherical grains. The model is extended to include a spectrum of grain contact geometries, in particular a log-normal spectrum of aspect ratios. The attenuation is substantial and depends markedly upon a lower limit imposed on the aspect ratios. For contacts of Type B (water separating two grains) only, results at 1 Hz are consistent with those of Mavko and Nur (1979) based on two-dimensional (2-D) cracks. If contacts of Type A (two grains in contact) and Type B are present, the attenuation is greater by 2-3 orders of magnitude.The model with Type B contacts displays a dependence of attenuation on water saturation in the range 1-10 percent; the model with Type A and B contacts does not. When the results were fitted to the measurements of absolute attenuation and saturation dependence at kHz frequencies by Gardner et al (1964), only the model with Type B contacts with aspect ratios > or =10 (super -2) could fit the data.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.