The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments

Abstract

To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured group velocities at specific locations for the silty sites. For both silts and sands predicted dispersions are comparable to the intrinsic errors in group velocity and hence undetectable. The attenuation coefficients predicted by the Grain Shearing model adequately describe the measured attenuation coefficients, within the observed variability.

Preliminary mapping of void fractions and sound speeds in gassy marine sediments from subbottom profiles

“…Nowadays it is commonplace to examine cores from regions of saturated sediment to determine density from gamma ray attenuation and compressional wave velocity from ultrasonic propagation at 500 kHz (which for these saturated sediments is assumed to provide a sound speed relevant to the frequencies used in this paper). 25 The final parameter required for the calculation is the pressure p 0 . This was computed for the central portion of each layer using p 0 = p A + w gh w + s gh / 2, where p A is the atmospheric pressure on the survey date ͑103 kPa͒, g is the acceleration due to gravity, w is the density of water ͑1000 kg m −3 ͒, h w is the depth of the water ͑15.5 m͒, and h is the depth of the sediment layer below the seabed (5.1 m for layer 1, and 7.4 m for layer 2).…”

Absolute calibration of hydrophones immersed in sandy sediment

“…This assumption is generally valid for kilohertz frequencies ͑for example, absorption at 200 kHz is 8.6ϫ 10 −3 Np m −1 or less 20 ͒ while the homogeneous nature of water allows correction factors to be determined for higher frequencies. 19 In contrast to water, the attenuation of acoustic waves in the range 16-100 kHz in saturated sediment, which consists of absorption and scattering losses, can reach values of 2.9 Np m −1 in muds and 9.5 Np m −1 in sand ͑predictions from the grain-shearing theory 5 for typical sediment properties 21,22 ͒. If this attenuation could be accurately predicted, suitable correction factors could be obtained.…”

“…For example, a compilation of attenuation data from marine sediment displays a scatter of Ϯ31% for a unique mean grain size, 6 while attenuation coefficients measured in sandy sediment varies by up to 2.8 Np m −1 for sediments with similar physical properties lying within a 100 m distance of one another. 22 This variability makes it extremely difficult to predict, and to account for, the attenuation losses in sediment. It is therefore preferable to devise a calibration method which does not critically depend on the need to correct for attenuation losses.…”

Innovation to Impact in a Time of Recession

“…The first IPR protection was filed in 2005, 119 but by when approval was given in 2009 to grant the patent 119 there were insufficient funds to meet the >£10 k annual cost of maintaining the patent. The work linked with two other projects (the first on seabed acoustics, [132][133][134][135][136][137] and the second on the ultrasonic assessment 138-140 of bone health and osteoporosis) to generate proposed methods for monitoring the populations of climatologically significant methane bubbles in the seabed. Such bubbles can also make the seabed unsuitable for civil engineering works.…”