Using Snell’s Law to Measure Sound Speed Dispersion

A technique has been developed to measure the speed of sound in marine sediments at discrete frequencies from 0.6 to 10 kHz by transmitting pulses from acoustic projectors within the water column and measuring the angle to which they are refracted on directional receivers buried in the seabed. The receivers are vectors sensors that measure three-axis acceleration and pressure allowing acoustic intensity, impedance, and arrival angle to be computed. A special jig was designed to bury four vector sensors into the seabed in two holes and control their orientation and depth of burial in the process. Following sensor burial, the jig was fully extracted to leave only the sensors and their associated cables in the seabed. The exact orientation of the sensors was confirmed by transmissions from three orthogonal directions, using two acoustic projectors also buried in the seabed by the jig and by an acoustic projector in the water column directly above the buried receivers. For the sound speed dispersion measurements, the seabed was ensonified at several grazing angles, concentrated above and below the anticipated critical angle. In this paper, the experimental approach and methods of data analysis are presented along with initial results from the SAX04 experiment in the Gulf of Mexico.