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Direct Measurement Systems
Although many techniques have been developed to determine the geoacoustic properties of ma- rine sediments, this discussion focuses on direct measurements for which wave speed and at- tenuation are calculated from measurements of travel time and the amplitude of a known signal. Although indirect measurements of sediment properties (including statistical inference tech- niques, for which geoacoustic properties are in- ferred from measurements of transmission loss, acoustic time series, reflection coefficient, and other parameters) are invaluable to the under- water acoustics community, these approaches are out of the scope of this article. The advantages of concentrating on direct measurements in the context of the current discussion are that (1) they have historically been more closely linked to measured physical properties and (2) parameter uncertainty is primarily attributed to the mea- surement system rather than to model mismatch.
Figure 3. Frequency-dependent compressional sound speed ratio (a), shear-wave speed (b), compressional-wave attenuation (c), and shear-wave attenuation mea- sured in situ at two sites in Currituck Sound in North Carolina (d). Black hexa- gons: first site, first deployment; blue diamonds: first site, second deployment; red triangles: second site (further out in the Sound at greater water depth). The acoustic behavior from the two deployments at the first site is typical for fine-grained sandy sediment; however, it is markedly different at the second site even though the sedi- ment grain sizes are similar. Adapted from Lee et al. (2016a).
Edwin L. Hamilton was an early pioneer of techniques based on acoustic probes inserted into the sediment, measuring compressional wave speed and attenuation in a variety of ocean bottom locations (Hamilton et al., 1956). Early in situ measurements of compressional-wave speed and attenua- tion in mud were also reported by Wood and Weston (1964). Numerous devices have been designed in recent years for in situ measurements of sediment compressional-wave prop- erties, and they have been used to characterize the surficial sediment of the shallow ocean at a number of sites.
The in situ sound speed and attenuation probe (ISSAP) was designed to rapidly sample a large area by inserting a set of probes 15 cm into the seabed to obtain estimates of the com- pressional-wave speed and attenuation and then hopping to subsequent locations (Mayer et al., 2002). The sediment acoustic speed measurement system (SAMS), which uses a vibracore for a penetration depth of up to 3 m with an ar- bitrary step size, provides estimates of the compressional- wave speed and attenuation and has been used in offshore locations in support of ocean acoustic-propagation experi- ments (Yang et al., 2008). More recently, compact, manually deployed systems have been used to characterize compres- sional-wave properties of a variety of intertidal (Robb et al., 2006) and near-shore (Demoulin et al., 2015) sediments. A system capable of measuring shear-wave properties as well as compressional-wave properties has also been developed
(Barbagelata et al., 1991). For deeper penetration into the seabed, measurement systems involving down-hole meth- ods (Muir et al., 1991) or acoustic probes attached to the end of sediment-coring devices (Ballard et al., 2016) have been developed.
Examples of direct measurements collected 30 cm below the water-sediment interface in Currituck Sound in North Carolina are shown in Figure 3 (Lee et al., 2016a). The measurements were collected at two field locations hav- ing distinctly different sediment types: a shallow site (ap- proximately 1 m water depth a few hundred meters from the shore) with medium- to fine-grained sand and a deeper site (about 3 m water depth and out in the main channel of the Sound) with fine-grained sand with approximately 10% mud content. Two deployments of a measurement sys- tem consisting of a set of one compressional-wave source and three receivers and a set of one shear-wave source and three receivers were conducted a few meters apart at the shallow site and one measurement system deployment was performed at the deeper site. The measured wave properties at the two shallow-site deployments are statistically similar, as indicated by the overlapping error bars in Figure 3a. The compressional-wave speed at the deeper site was approxi- mately 200 m/s slower compared with that at the shallower site, and the shear-wave speed was approximately 40 m/s slower. The shear-wave attenuation was also much greater at the deeper site, which limited the shear-wave measurements
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