Coupled acoustic and electromagnetic disturbances in a granular material saturated by a fluid electrolyte

The U.S. Navy has an ongoing need for a reliable model of acoustics in ocean sediments. Viscoelastic fluid and solid descriptions are commonly used, but are often unable to account for the variability exhibited by different types of sediments. Poroelasticity (also known as Biot theory) relates the seabed's observed behavior to sediment microstructure and pore-fluid motion explicitly. Traditional acoustical techniques have had difficulty distinguishing between Biot theory predictions and those based on fluid and solid models. Electrokinetic (EK) phenomena---the coupling of relative fluid motion and grain surface chemistry---are generated by wave propagation in electrolyte-saturated sediments. The coupled EK-Biot theory developed by Pride (1994) describes how acoustic waves generate electromagnetic fields, and simultaneously, how electromagnetic fields affect wave behavior.; We devised two reciprocal experiments to study these phenomena.{09}"EK transmission" occurs when an applied voltage creates an electro-acoustic wave; in practice, this leads to thermoelastic motion, as well as electrokinetics, so that we have had to account for both effects.{09}Conversely, "EK reception" occurs when a pressure wave generates a measurable voltage in electrolyte-saturated sediments. The EK reception apparatus made use of a submerged, acoustic transducer to insonify a water-sediment interface with short, 50 kHz sine-wave bursts and chirped pulses from 10--800 kHz. The resulting wave motion was monitored using Ag/AgCl electrodes fixed in a vertical array above and below the sediment interface. We measured the conductivity dependence of two kinds of EK behavior: (1) voltages generated within the samples that were localized around the transmitted "fast" waves, and (2) electromagnetic (EM) waves produced at the water-sediment interface. Fast-wave voltages were often greater than 500 muV, while the EM-wave potentials were usually 100 muV in magnitude. A model of plane-wave reflection from a water-EK-Biot interface leads to theoretical predictions that compare very well to experimental data for sand and glass microspheres. Both EM- and fast-wave voltages are caused by relative fluid motion in the sediment, a feature that is characteristic of poroelastic media---but not predicted by either fluid or solid models.