Marcia J. Isakson

High-Frequency Seafloor Acoustics

This article reviews High-Frequency Seafloor Acoustics by Darrell R. Jackson, Michael D. Richardson , 2007. 616 pp.

A broadband model of sandy ocean sediments: Biot–Stoll with contact squirt flow and shear drag

139.00 (hardcover). ISBN 978-0-387-34154-5

A comparison of sediment reflection coefficient measurements to elastic and poro-elastic models

Unlike the application of the Biot model for fused glass beads, which was conclusively demonstrated by Berryman [Appl. Phys. Lett. 37(4), 382–384 (1980)] using the experimental measurements by Plona [Appl. Phys. Lett. 36, 259–261 (1980)], the model for unconsolidated water-saturated sand has been more elusive. The difficulty is in the grain to grain contact physics. Unlike the fused glass beads, the connection between the unconsolidated sand grains is not easily modeled. Measurements over a broad range of frequencies show that the sound speed dispersion is significantly greater than that predicted by the Biot–Stoll model with constant coefficients, and the observed sound attenuation does not seem to follow a consistent power law. The sound speed dispersion may be explainable in terms of the Biot plus squirt flow (BISQ) model of Dvorkin and Nur [Geophysics 58(4), 524–533 (1993)]. By using a similar approach that includes grain contact squirt flow and viscous drag (BICSQS), the observed diverse behavior of ...

Shear wave attenuation and micro-fluidics in water-saturated sand and glass beads.

This work compared three plane wave reflection coefficient models to laboratory sediment reflection measurements collected using a spherical source and receiver. Plane wave decomposition was used to modify the reflection coefficient models to include the inherent spherical effects such as lateral wave interference for direct comparison with the data. Sets of data at two transducer separation distances (1.27 and 0.25m) were collected to compare the range dependence of the spherical effects. Bandpass filtered linear frequency-modulated chirps from 30 to 160kHz were used to measure frequency dependence. Grazing angles from 5° to 75° were measured to compare angle dependence. Each set of data was collected along approximately 3m of smoothed sediment for spatial averaging. Unavoidable experimental effects including transducer response, beam pattern, and spherical spreading were accounted for in order to compare the reflection coefficient measurements with the modified models. Significant spherical wave effects...

High-frequency dispersion from viscous drag at the grain-grain contact in water-saturated sand

An improvement in the modeling of shear wave attenuation and speed in water-saturated sand and glass beads is introduced. Some dry and water-saturated materials are known to follow a constant-Q model in which the attenuation, expressed as Q(-1), is independent of frequency. The associated loss mechanism is thought to lie within the solid frame. A second loss mechanism in fluid-saturated porous materials is the viscous loss due to relative motion between pore fluid and solid frame predicted by the Biot-Stoll model. It contains a relaxation process that makes the Q(-1) change with frequency, reaching a peak at a characteristic frequency. Examination of the published measurements above 1 kHz, particularly those of Brunson (Ph.D. thesis, Oregon State University, Corvalis, 1983), shows another peak, which is explained in terms of a relaxation process associated with the squirt flow process at the grain-grain contact. In the process of deriving a model for this phenomenon, it is necessary to consider the micro-fluidic effects associated with the flow within a thin film of water confined in the gap at the grain-grain contact and the resulting increase in the effective viscosity of water. The result is an extended Biot model that is applicable over a broad band of frequencies.

Finite Element Modeling of Acoustic Scattering From Fluid and Elastic Rough Interfaces

Shear viscous drag within the thin fluid film at the contact between grains in water-saturated sand is an important loss mechanism for high-frequency sound in the Biot-Stoll plus contact squirt flow and shear viscous drag (BICSQS) model [J. Acoust. Soc. Am. 116, 2011-2022 (2004)]. Couette flow was assumed for the shear drag but it breaks down when inertial effects within the film become significant. Using Biots method, a correction is derived for the shear drag and inserted into the BICSQS model. The result is a prediction of negative sound speed dispersion, consistent with dynamic theories of fluid-filled poroelastic bodies.

Quantifying the effects of roughness scattering on reflection loss measurements

Quantifying acoustic scattering from rough interfaces is critical for reverberation modeling, acoustic sediment characterization, and propagation modeling. In this study, a finite element (FE) scattering model is developed. The model computes the plane wave scattering strength for an ensemble of rough power-law surfaces for ocean bottoms described as fluid and elastic. The FE model is compared with two models based on approximations to the Helmholtz-Kirchhoff integral: the Kirchhoff approximation (KA) and the perturbation theory (PT). In the case of a fluid-like bottom, the KA and FE models agree except at small grazing angles. The PT and FE models deviate near specular especially at small angles. For the elastic case, the PT predicts the FE results well except at the intromission angle of the shear wave. The KA deviates for angles that are below the critical angle of the compressional wave. At the shear wave intromission angle, the FE model shows a more plausible solution likely due to multiple scattering events that are not accounted for in PT for the modeled roughness.

Acoustic virtual mass of granular media

Seafloor reflection loss and roughness measurements were taken at the Experimental Validation of Acoustic Modeling Techniques experiment in 2006. The magnitude and phase of the reflection loss was measured at frequencies from 5 to 80 kHz and grazing angles from 7° to 77°. Approximately 1500 samples were taken for each angle. The roughness was measured with a laser profiler. Geoacoustic parameters such as water and sediment sound speed and density were measured concurrently. The reflection loss data were compared with three models: A flat interface elastic model based on geoacoustic measurements; a flat interface poro-elastic model based on the Biot/Stoll model; and a rough interface model based on the measured interface roughness power spectrum. The data were most consistent with the poro-elastic model including scattering. The elastic model consistently predicted values for the reflection loss which were higher than measured. The data exhibited more variability than the model due to layering and fluctuations in the propagating medium.

The calibration of a laser light line scan method for determining local interface roughness of the ocean floor

Mechanical coupling between grains in a randomly packed unconsolidated granular medium is shown to cause an increase in the effective inertia, hence, a reduction in sound and shear wave speeds, relative to predictions by the standard expressions for a uniform elastic solid. The effect may be represented as a virtual mass term, and directly related to the scintillation index of the grain-to-grain contact stiffness.

High frequency backscattering from sandy sediments: Single or multiple scattering

An accurate model of acoustic interaction with sandy sediments is crucial to the application of SONAR in shallow-water environments. Because acoustic scattering from interface roughness plays a major role in the reverberation from and penetration into sandy sediments, it is imperative to be able to accurately measure the roughness of the sediment/water interface. An interface roughness measurement system has been developed in which a laser light sheet is projected onto the ocean floor. A resulting image can then be analyzed to determine the interface roughness. The system has been shown to achieve a height measurement error of less than 0.9 mm over a spatial frequency range of 15 to 60 cycles/m with about 0.5 mm standard deviation. These spatial frequencies correspond to acoustic Bragg frequencies of 11 to 45 kHz for backscattering applications. The error in wavelength was less than 5 mm with a standard deviation of about 1.0 mm. The system is inexpensive, easily deployable and automated in terms of data extraction. This system could greatly aid in determining the local interface profile for in situ acoustic scattering experiments.