High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Dynamic Measurement of Sediment Grain Compressibility
at Atmospheric Pressure: Acoustic Applications

Michael Richardson and Kevin Briggs
Marine Geosciences Division, Naval Research Laboratory,
Stennis Space Center, Mississippi, 39529
(228) 688-4621, -5752 (fax) Mike.Richardson@nrlssc.navy.mil

Kevin Williams and Eric Thorsos
Applied Physics Laboratory, University of Washington
Seattle, Washington 98105

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Under certain conditions, Wood’s equation can be used to predict sound speed in liquid/solid grain suspensions if the bulk moduli and densities of the grains and liquid are known. In this study that relationship is used to measure grain bulk modulus in suspensions where sound speed, fluid density, fluid bulk modulus, grain density, and particle concentrations are known or accurately measured. Estimates of grain bulk moduli made for polystyrene beads suspended in water and glass beads suspended in a heavy liquid provided controls which demonstrated the validity of the suspension technique to estimate values of particle bulk modulus. The measured bulk modulus of sand grains previously collected from the coastal sediments of the northeast Gulf of Mexico was slightly higher than handbook values for polycrystalline quartz (3.6 to 4.0 x 1010 Pa) but given the sensitivity of the techniques and the uncertainties in the measurements is not inconsistent with handbook values. The values, however, are inconsistent with recent high-strain laboratory measurements (0.7 x 1010 Pa) reported by Chotiros and co-workers. Further refinements in the measurement system hold potential to allow direct use of measurements made on individual sediment samples.

Samples of sand collected during the SAX99 experiments will be used to determine the variability of the bulk modulus of sand grains at that site. These data together with values of other sediment physical properties measured at that site will be used to test predictions and implications of the various sediment propagation models that are used to predict scattering from and penetration into seafloor sediments. Sample collection will have no impact on the already proposed sampling strategy.

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