Rough Seabed Research Articles

Acoustic backscatter at very high frequencies from rough seabeds

Acoustic backscatter data were gathered from a variety of seabed types, both in the laboratory tank and in coastal waters. Data were gathered from sandy sediments in the tank, with a variety of characterized rough surfaces, using narrow-band pulses at frequencies between 100 and 950 kHz. Wideband data gathered at sea were obtained at frequencies between 80 and 650 kHz. Data gathered at sea from the Acoustic Range at QinetiQ Bincleaves included backscatter from the natural sandy seabed, but also scatter from several artificial sediments (sand, gravel, and pebbles) placed in a rotator. The latter equipment allowed acoustic interrogation of the same patch of seabed from multiple angles. Experimental data are compared with fluid, poroelastic, and discrete scatterer models, with a view to recommendations for the modeling of seabed backscatter in the frequency band 100 kHz to 1 MHz.

Effects of surface waves on a turbulent current over a smooth or rough seabed

Laboratory experiments by earlier authors have shown that the near-surface velocity of an otherwise uniform current is reduced by following waves, but is increased by opposing waves. By a boundary-layer analysis with depth-dependent eddy viscosities, we show analytically that this is the consequence of second-order effects of wave-current interaction. Physical effects of waves on the current profile due to the moving free surface, wave attenuation and convective inertia are discussed. Comparisons with available experiments for smooth and rough seabeds are discussed. New predictions of the longitudinal variations along the current are made.

Coherent reflection of acoustic plane wave from a rough seabed, with a random sediment layer overlying an elastic basement

The problem of coherent reflection of an acoustic plane wave from a rough seabed with a randomly inhomogeneous sediment layer overlying a uniform elastic basement is considered in this analysis. The randomness of the sound field is attributable to the roughness of the seabed and the sound-speed perturbation in the sediment layer, resulting in a joint rough surface and volume scattering problem. An approach based upon perturbation theory, combined with a derived Green's function for a slab bounded above and below by a fluid and an elastic half-space, respectively, is employed to obtain an analytic solution for the coherent field in the sediment layer. Furthermore, a boundary perturbation theory developed by Kuperman and Schmidt (1989) is applied to treat the problem of rough surface scattering. A linear system is then established to facilitate the computation of the coherent reflection field. The coherent reflection coefficients for various surface roughness, sediment randomness, frequency, sediment thickness, and basement elasticity have been generated numerically and analyzed. It was found that the higher/larger size of surface and/or medium randomness, frequency, thickness, and shear-wave speed, the lower the coherent reflection. Physical interpretations of the various results are provided.

The Helmholtz–Kirchhoff approach to modeling penetration of acoustic waves into rough seabeds

The Helmholtz–Kirchhoff integral is applied to model the penetration of sound waves into sandy seafloors at grazing angles above and below the critical angle. Although the conditions for the validity of the Kirchhoff approximation can be limiting, this approximation should be valid at high frequency for gently undulating seafloor surfaces even at moderate to low grazing angles, providing that the self-shadowing effect is carefully removed. The analytical development of the method is first presented, followed by numerical examples and comparisons with the experimental data of Maguer et al. [SACLANTCEN report, SR-287, April 1998]. The model predicts, in agreement with the 2- to 15-kHz acoustic data, the frequency where the contributions due to roughness effects begin to dominate those due to the evanescent wave. Secondary effects such as Bragg interference 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.

Bottom shear stresses and velocity profiles in stratified tidal planetary boundary layer flow from similarity theory

The paper presents the bottom shear stresses and velocity profiles in stratified tidal planetary boundary layer flow by using similarity theory. For a given seabed roughness length, free stream current velocity components, frequency of tidal oscillation, Coriolis parameter and stratification parameter, the maximum bottom shear stress is determined for flow conditions in the rough, smooth and transitional smooth-to-rough turbulent regime. Further, the direction of the bottom shear stresses and the velocity profiles are given. Comparison is made with data from field measurements of time-independent as well as tidal planetary boundary layer flow for neutral conditions, and the agreement between the predictions and the data is generally good. Further, an example of application for stable stratification is given, and qualitatively the predictions show, as expected, that the bottom shear stress and the thickness of the boundary layer become smaller for stable than for neutral stratification. Other features of the tidal planetary boundary layer flow are also discussed.

The spatial correlation of rough seabed scattering of surface-generated ambient noise

This article considers the effect of rough seabed scattering on the spatial correlation of low-frequency ambient noise. Extending the study presented by Liu et al. [J. Acoust. Soc. Am. 93, 753–769 (1993)], it is shown here that seabed roughness may alter the horizontal spatial correlation of the noise field primarily through the excitation of an interface wave along the bottom. The horizontal coherence and directionality of the noise field are shown to be a function of the correlation length and angular dependence of the roughness. Since the interface wave component decays rapidly away from the bottom, the vertical noise field correlation is minimumly affected by the seabed roughness. An examination of the noise field under two different statistical descriptions of seabed roughness, the Goff–Jordan and Gaussian models, shows that detailed differences in the roughness spectral shape produce only minor changes in the noise spatial correlation at infrasonic frequencies.

Gas-related sedimentary features from the eastern Canadian continental shelf

The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.

Combined wave and current boundary layer model for fixed rough seabeds

A theoretical model which describes the motion in combined wave and current boundary layers near fixed rough seabeds is presented. A three-layer time invariant eddy viscosity model is employed to find an analytical solution. Comparisons are made with small and large scale laboratory experiments as well as field measurements, and the model predicts the essential features of the experimental results reasonably well. A graphical representation of the model prediction results is also given, which can be used to derive a defined “velocity load” on a pipeline resting on a sea bottom in a combined wave-current environment.

Prediction of the Current Structure Under Drifting Pack Ice

Abstract A simple analytical theory which describes the motion in a turbulent planetary boundary layer near a rough sea bed by using a two-layer eddy viscosity model is presented. An inverted boundary layer similar to that at the sea bed is applicable under the pack ice. The vertical structure of the current in the boundary layer is presented, and comparisons are made with data from McPhee and Smith [1] obtained from measurements of the turbulent boundary layer under drifting Arctic pack ice. The present model predictions for the drag coefficient at the ice surface and the direction of the surface shear stress are also compared with corresponding formulas given in Davenport [2], which are obtained as fits to observations in atmospheric boundary layers.

Comparison of a simple planetary boundary layer model with measurements of a turbulent boundary layer under pack ice

A simple analytical theory which describes the motion in a turbulent planetary boundary layer near a rough sea bed by using a two-layer eddy viscosity model is presented. The vertical structure of the current in the boundary layer is presented, and comparisons are made with data from McPhee and Smith (1976, Journal of Physical Oceanography, 6, 696–711) obtained from measurements of the turbulent boundary layer under drifting Arctic ice.