Effects of liquid pore water on acoustic wave propagation in snow as a Biot-type porous material

TL;DR

This paper presents a Biot theory-based method to estimate how liquid water in snow affects acoustic wave velocity and attenuation, with potential applications in snow water content measurement.

Contribution

It introduces a novel approach combining empirical relations and Biot's theory to quantify liquid water effects on acoustic waves in snow.

Findings

Liquid water reduces compressional wave velocity by ~300 m/s per 0.1 saturation increase.

Attenuation of waves increases with more liquid water.

Velocity changes depend on compaction model, attenuation mainly on water saturation.

Abstract

A method to estimate phase velocity and attenuation of acoustic waves in the presence of liquid water in a snowpack is presented. The method is based on Biot's theory of wave propagation in porous materials. Empirical relations and a priori information is used to characterize snow as a porous material as a function of porosity. Plane wave theory and an equivalent pore fluid are used to solve Biot's differential equations and to asses the impact of the air and water in the pore space. The liquid water in the pore space of a snow pack reduces the velocity of the first compressional wave by roughly 300 m/s for every 0.1 increase in liquid water saturation. Also the attenuation of the compressional waves is increased with increasing liquid water content. Two end member models for compaction are evaluated to asses the importance of an independent density measurement for an estimate of liquid pore water saturation in snow with acoustic waves. The two end members correspond to no compaction at all and to a melting sphere packing where the grains remain in contact. The change of velocity for the first compressional wave was found to strongly depend on the compaction model, while the velocity of the second compressional wave mainly depends on liquid water saturation. Also the attenuation for both compressional waves depends mainly on the liquid water saturation and only little on the compaction model. Finally, a simple field experiment illustrates the potential use of the method to estimate the liquid water content of snow with acoustic sensors.