The measurement of the in situ compressional wave properties of marine sediments

Geoacoustic inversion requires a generic knowledge of the frequency-dependence of compressional wave properties in marine sediments, the nature of which is still under debate. The use of in situ probes to measure sediment acoustic properties introduces a number of experimental difficulties that must be overcome. To this end, a series of well-constrained in situ acoustic transmission experiments were undertaken on inter-tidal sediments using a purpose-built in situ device, the Sediment Probing Acoustic Detection Equipment. Compressional wave velocity and attenuation coefficient were measured from 16 to 100 kHz in medium to fine sands and coarse to medium silts. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing, as were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between probes and sediment. Over the experimental frequency range and source-to-receiver separations of 0.99 – 8.1 m, resulting velocities are accurate to between + 1.1 to + 4.5 % in sands and less than + 1.9 % in silts, while attenuation coefficients are accurate to between + 1 to + 7 dB•m-1 in both sands and silts. Preliminary results indicate no velocity dispersion and an attenuation coefficient which is proportional to frequency.