Measurement and modeling of elliptical particle motion in the seabed

Abstract

As part of the SAX04 experiment conducted off the coast of Florida in the Gulf of Mexico, four vector sensors containing three-axis accelerometers and pressure sensors were buried in the seabed. These served as the receivers to measure sediment sound-speed dispersion using a variety of techniques. One technique involved the generation of spherical waves from a point source in water and transmitted into the seabed in the frequency band 800 to 3000 Hz. In this geometry, the contribution of inhomogeneous waves to the field results in elliptical—rather than longitudinal—particle motion. The orientation of the elliptical orbit varies with the source-receiver geometry, frequency, and the sediment sound speed, thereby allowing measurements of sediment sound-speed dispersion. However, the orientation of the ellipse major axis is not always aligned with the direction of wave propagation suggested by Snell’s law. Measurements were made at several angles approaching the nominal critical angle in order to address the trade-off between increased sensitivity of the measurement versus greater departure from Snell’s law. Measurements and modeling of the elliptical particle motion are compared to ensure that the sediment sound-speed estimates account for any bias created by the inhomogeneous waves. [Work partially supported by ONR.]