John C. Osler

High-resolution geoacoustic inversion in shallow water: A joint time- and frequency-domain technique

High-resolution geoacoustic data are required for accurate predictions of acoustic propagation and scattering in shallow water. Since direct measurement of geoacoustic data is difficult, time-consuming, and expensive, inversion of acoustic data is a promising alternative. However, the main problem encountered in geoacoustic inversion is the problem of uniqueness, i.e., many diverse geoacoustic models can be made to fit the same data set. A key, and perhaps unique, aspect of this approach is the combination of data analysis in both the space-time and the space-frequency domains. This combination attempts to ameliorate the uniqueness problem by exploiting as much independent data as possible. In order to meet the stringent requirements of high spatial resolution and uniqueness, an entire method has been developed including a new measurement technique, processing/analysis technique, and inversion strategy. These techniques are described and then illustrated with a shallow-water data set. Sound-speed gradients in the upper few meters of the sub-bottom appear to be much higher (one order of magnitude) than generally assumed. And, although often ignored, a large density gradient was observed in the top layer that played an acoustically significant role.

Bayesian evidence computation for model selection in non-linear geoacoustic inference problems

This paper applies a general Bayesian inference approach, based on Bayesian evidence computation, to geoacoustic inversion of interface-wave dispersion data. Quantitative model selection is carried out by computing the evidence (normalizing constants) for several model parameterizations using annealed importance sampling. The resulting posterior probability density estimate is compared to estimates obtained from Metropolis-Hastings sampling to ensure consistent results. The approach is applied to invert interface-wave dispersion data collected on the Scotian Shelf, off the east coast of Canada for the sediment shear-wave velocity profile. Results are consistent with previous work on these data but extend the analysis to a rigorous approach including model selection and uncertainty analysis. The results are also consistent with core samples and seismic reflection measurements carried out in the area.

Normal incidence reflection loss from a sandy sediment

Acoustic reflection loss at normal incidence from a sandy sediment, in the Biodola Gulf on the north side of the island of Elba, Italy, was measured in the frequency band 8-17 kHz, using a self-calibrating method. The water depth was approximately 11 m. The mean and standard deviation of the sand grain diameter were 2.25 (0.21 mm) and 0.6 phi, respectively. The reflection loss was measured using an acoustic intensity integral method, which is insensitive to roughness effects within the selected frequency band. The measured value of reflection loss was 11 dB, +/- 2 dB. The result is consistent with previous measurements in the published literature. The computed reflection loss for a flat interface between water and a uniform fluid or visco-elastic medium with the same properties is 8 dB, +/- 1 dB. The theoretical and experimental values do not significantly overlap, which leads to the conclusion that the visco-elastic model is inappropriate. The Biot model is suggested as a better alternative but more work is needed to ascertain the appropriate parameter values.

Boundary characterization experiment series overview

Ocean acoustic propagation and reverberation in continental shelf regions is often controlled by the seabed and sea surface boundaries. A series of three multi-national and multi-disciplinary experiments was conducted between 2000-2002 to identify and measure key ocean boundary characteristics. The frequency range of interest was nominally 500-5000 Hz with the main focus on the seabed, which is generally considered as the boundary of greatest importance and least understood. Two of the experiments were conducted in the Mediterranean in the Strait of Sicily and one experiment in the North Atlantic with sites on the outer New Jersey Shelf (STRATAFORM area) and on the Scotian Shelf. Measurements included seabed reflection, seabed, surface, and biologic scattering, propagation, reverberation, and ambient noise along with supporting oceanographic, geologic, and geophysical data. This paper is primarily intended to provide an overview of the experiments and the strategies that linked the various measurements together, with detailed experiment results contained in various papers in this volume and other sources

Quantifying the interaction of an ocean bottom seismometer with the seabed

A theory is presented for the coupling between an ocean bottom seismometer (OBS), the sediments upon which it rests, and the surrounding water. Assuming that rotational and tilt effects are negligible (or have been made negligible through instrument design), the response of the OBS to forced harmonic motion is considered in both horizontal and vertical directions. Under these conditions it is concluded that the measured ratio of velocities when the OBS is on the seabed and when it is freely suspended in water (for an identical force) completely characterizes the OBS/seabed interaction. This enables the velocity transfer functions to be directly calculated without recourse to a detailed model of sediment/structure interaction. An OBS was designed and constructed with good coupling to seabed motion and reduction of rocking effects as principal design criteria, including an onboard shaker to conduct in situ coupling experiments. The amplitude and phase of coupling data collected on a clay seabed provide transfer functions due to horizontal and vertical seabed motion and horizontal water motion. In addition, a simple mass-spring-dashpot model of OBS/seabed interaction permits the analysis of amplitude-only coupling data. Good vertical coupling can be achieved over a wide bandwidth by designing the sensor package to have a large hydrodynamic added mass in the vertical. Good coupling to horizontal seabed motion is more difficult to achieve but is possible within a limited bandwidth, even on very soft seabeds. Finally, an example of seismoacoustic noise is presented. Hydrophone signals are compared with horizontal geophone signals received from sources within the ocean and within the seabed, and the differences are explained in terms of the coupling transfer functions.

Acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 kHz

Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North Americas Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric arrays narrow beamwidth makes the system well-suited to this task.

Sensitivity of acoustic propagation to uncertainties in the marine environment as characterized by various rapid environmental assessment methods

Accurate sonar performance prediction modelling depends on a good knowledge of the local environment, including bathymetry, oceanography and seabed properties. The function of rapid environmental assessment (REA) is to obtain relevant environmental data in a tactically relevant time frame, with REA methods categorized by the nature and immediacy of their application, from historical databases through remotely sensed data to in situ acquisition. However, each REA approach is subject to its own set of uncertainties, which are in turn transferred to uncertainty in sonar performance prediction. An approach to quantify and manage this uncertainty has been developed through the definition of sensitivity metrics and Monte Carlo simulations of acoustic propagation using multiple realizations of the marine environment. This approach can be simplified by using a linearized two-point sensitivity measure based on the statistics of the environmental parameters used by acoustic propagation models. The statistical properties of the environmental parameters may be obtained from compilations of historical data, forecast conditions or in situ measurements. During a field trial off the coast of Nova Scotia, a set of environmental data, including oceanographic and geoacoustic parameters, were collected together with acoustic transmission loss data. At the same time, several numerical models to forecast the oceanographic conditions were run for the area, including 5- and 1-day forecasts as well as nowcasts. Data from the model runs are compared to each other and to in situ environmental sampling, and estimates of the environmental uncertainties are calculated. The forecast and in situ data are used with historical geoacoustic databases and geoacoustic parameters collected using REA techniques, respectively, to perform acoustic transmission loss predictions, which are then compared to measured transmission loss. The progression of uncertainties in the marine environment, within and between different REA categories, and the consequences on acoustic propagation are examined.

Nonlinear Inversion of Acoustic Scalar and Vector Field Transfer Functions

A study to investigate the use of the acoustic vector field, separately or in combination with the scalar field, to invert for geoacoustic properties of the seafloor was conducted. The analysis was performed in the context of the 2004 Sediment Acoustics Experiment (SAX04) conducted in the Northern Gulf of Mexico (GOM) where a small number of acoustic vector sensors were deployed in close proximity to the seafloor. The acoustic vector sensors were located both above and beneath the seafloor interface where they measured the acoustic pressure and the acoustic particle acceleration. A variety of acoustic waveforms were transmitted into the seafloor at normal incidence. Motion data provided by the buried vector sensors were affected by a suspension response that was sensitive to the mass properties of the sensor, the sediment density, and shear wave speed. The suspension response for the buried vector sensors included a resonance within the analysis band of 0.4-2.0 kHz. The response was sufficiently sensitive to the local geoacoustic properties, that it was integrated into the inverse methods developed for this study. Inversions of real and synthetic data sets showed that information about sediment shear wave speed was carried by the suspension response of the buried sensors, as opposed to being contained inherently within the vector acoustic field.

Acoustic and In-Situ Techniques for Measuring the Spatial Variability of Seabed Geoacoustic Parameters in Littoral Environments

Geoacoustic properties of the seabed are required for accurate modeling of acoustic propagation, and hence sonar performance prediction. Characterizing acoustic interaction with the seabed is particularly important in shallower water environments as the propagation typically involves extensive interaction with the sea surface and seabed. DRDC Atlantic is developing acoustic and in-situ techniques for seabed classification and measuring geoacoustic parameters. The acoustic technique uses normal incidence acoustic returns, in the 1 to 10 kHz band, from the seabed and sub-bottom. The transition from interface to volume scattering depends upon frequency and sediment type and can be used to distinguish the composition of near surface marine sediments. An experimental methodology has been developed using a vertical line array of receivers and a downward-looking superdirective projector array. The technique is also being adapted for use with commercial normal incidence sub-bottom profilers. In-situ measurements are being made using the DRDC Atlantic free fall cone penetrometer probe (FFCPT). It has been developed to measure acceleration and dynamic sediment porewater pressure as a function of depth of penetration into the seafloor. It also records hydrostatic pressure and optical backscatter for detection of the mudline. This combination of sensors permits the direct application of geotechnical analysis methods and parametric-based correlations already long established in engineering practice. The FFCPT provides two independent means of calculating the undrained shear strength, as well as other engineering variables that are used to identify the sediment grain size characteristics. The probe has a modular design allowing additional sensor payloads to be integrated, the first of which uses resistivity as a means to determine sediment bulk density. Experimental results, using the acoustic and in-situ techniques, will be presented from two joint US-SACLANTCEN-CAN sea-trials in 2001 at the New Jersey Strataform area and the Scotian Shelf.

On the use of acoustic particle motion in geoacoustic inversion.

Geoacoustic inversion estimates sediment properties based on one or more parameters of an observed acoustic field via inverse mathematical methods. The observed acoustic parameter is usually derived from the acoustic pressure measured at one or more locations. Recent advances in acoustic sensor technology have enabled the simultaneous measurement of the acoustic pressure and particle motion in three dimensions. The additional information provided by such acoustic vector sensors can be used to improve the performance of existing and novel geoacoustic inversion techniques. Current research seeks to use the additional information that is provided by the acoustic vector sensor to pose new inverse problems for the estimation of seabed sediment properties. In particular, data collected during the Sediment Acoustics Experiment 2004 (SAX04) included acoustic pressure and particle acceleration from a small number of acoustic vector sensors arranged in a vertical line, spanning the water‐sediment interface. A varie...