Penetration of acoustic waves into rippled sandy seafloors

Abstract

The Helmholtz-Kirchhoff integral and the Kirchhoff approximation are applied to model the penetration of sound waves into rough sandy seafloors at grazing angles above and below the critical angle. As the seafloor of interest is anisotropic, emphasis is placed on simulating the response from a two-dimensional interface. The analytical development of the method is first presented, followed by numerical examples. Simulations and data acquired at sea are in very good agreement in the 2-15 kHz band [Maguer et al., J. Acoust. Soc. Am. 107, 1215-1225 (2000)]. The model predicts, in agreement with the 2-15 kHz acoustic data, the contributions due to roughness effects that dominate the evanescent wave component over most of this frequency band. Secondary effects such as coherent (Bragg) influence patterns and the loss of signal coherence with grazing angle or depth are correctly predicted. The model simulations strongly suggest that roughness of the sediment interface is most likely the cause of anomalous sound penetration into the seabed.

An overview of SAX99: acoustic measurements

“…Maguer et al [5] made measurements in the 2-15-kHz range, and argued, based on modeling studies, that the subcritical penetration they observed below 5-7 kHz is due to the evanescent wave, while above 5-7 kHz it is due to rough surface scattering. These conclusions were strengthened by very recent analyses [15], [16]. Though arguments are accumulating on the importance of seafloor roughness for subcritical acoustic penetration, seafloor characterization has not been extensive in previous acoustic measurements.…”

“…It went into a four-point moor on Oct. 16 and came out on Nov. 11. During this period, the R/V Seward Johnson supported a variety of diver-intensive acoustic and environmental measurements, and as many as 20 cables were deployed from ship to equipment on the bottom.…”

Subcritical penetration of acoustic waves into inhomogenous seafloors

“…[16][17][18][19][20] One of the mechanisms of enhanced penetration into the seafloor is the scattering by rough surfaces, which was modeled in references. [21][22][23] In addition, in two experiments, Sax99 (Ref. 24) and Sax04, 8 the primary cause of subcritical acoustic penetration was shown to be diffraction by sand ripples.…”

Subcritical penetration into rough seafloors due to Bragg scattering

“…From this research, mechanisms of subcritical penetration into seafloors have been explained as being due to the scattering from surface roughness of sea bottom, 1-6 the generation of Biot's slow waves in the poroelastic bottom, 7,8 and the scattering of evanescent waves induced by volume inhomogeneities. 9,10 Among them, scattering from interface roughness has been regarded as a primary cause, 11,12 and has attracted a broad interest. In explaining their experimental result 11 (SAX99), Jackson et al provided the evidence that the primary cause for the observed subcritical acoustic penetration was the diffraction by sand ripples via Bragg scattering.…”

“…They gave a diffraction model using first order perturbation theory for sinusoidal ripples and presented a high cutoff frequency of the minus first order Bragg wave. Pouliquen et al 12 investigated the scattering from sand ripples by the Helmholtz-Kirchhoff integral and correctly predicted the coherent (Bragg) interference patterns. To test the existence of Bragg scattering waves, Lim et al 13 performed a set of tank experiments by making an artificial "rough" interface that consists of polystyrene beads floating on the boundary between two immiscible fluids.…”