Subcritical acoustic scattering across a rough fluid–solid interface—laboratory experiment and perturbation theory model

Abstract

Several mechanisms have been proposed by which an incident acoustic wave could be coupled into the ocean floor at angles below the compressional critical angle. Recent observations of acoustic transmissions from water into sediment with arrival times and amplitudes inconsistent with the refractive compressional path have been interpreted as the excitation of a Biot slow wave in the sediment. Another hypothesis attributes the observed signals to scattering at the rough water-sediment interface. A third entails scattering of evanescent waves by volume inhomogeneities in the sediment. The existence of multiple hypotheses, each of which could account for the received energy, invited further investigation. A series of well-controlled laboratory measurements were made and compared with the results of a first-order perturbation theory model in order to evaluate the accuracy of the rough interface hypothesis. The experimental measurements of transmission through adjacent flat and rough interfaces show good agreement with the results of the numerical model, giving clear evidence of subcritical penetration in an environment incompatible with the requirements of the Biot slow wave and volume scattering hypotheses.

Special Award in Ocean Acoustics

“…Work was completed on tank measurements to show that the random rough surface can account for the energy that arrives in sediments below the critical angle (Mellema, 1999). This experiment makes the far more complicated hypotheses of a slow wave being responsible for the energy unlikely, and validates the rough surface scattering hypotheses.…”

Design, Calibration and Application of a Seafloor Laser Scanner

“…Creating a realistic rough surface with known topography is challenging and is rarely done. With CNC milling machines, several researchers have constructed rough surfaces for use in studying the scattering of acoustic (Mellema, 1999;Summers et al, 2005) and electromagnetic waves (Kuga & Phu, 1996) from rough surfaces. These surfaces typically implement an isotropic roughness with a power-law spectrum and a surface size that is compatible with the acoustic or electromagnetic wavelength of interest.…”

Experimental Study of Sound Waves in Sandy Sediment

“…At the critical angle, typically at grazing angles of about 20 to 30 degrees, the refracted compressional wave is parallel to the interface and no longer propagates into the sediment. There is significant interest in mechanisms by which a compressional wave in the water can be coupled into the sediment at angles below the critical angle with energy sufficient enough for finding buried objects [24,27,34]. It has been proposed that the slow Biot wave may provide such a mechanism [34].…”

“…by Boyle and Chotiros (1992), both in magnitude and in arrival time [28,40]. Mellema [27] demonstrated that significant energy is scattered across a rough fluid-solid interface at shallow grazing angles and that the received intensity can be accurately modeled using first order perturbation theory. For the roughened case, it is clear that the later time arrival is best described by an assumed ray path in the water to a point above the buried hydrophone, scattered from the roughened interface, and then propagation nearly vertically to the buried hydrophone at the speed of the ordinary compressional wave (1680 m/s).…”