Anatoliy N. Ivakin

A unified approach to volume and roughness scattering

A unified approach is proposed to study scattering from a fluid medium with irregularities of different types: volume inhomogeneities (spatial fluctuations of the compressibility and density) and roughness of the interfaces. The approach considers the roughness as a volume perturbation of a specific kind near flat (unperturbed) interfaces. It permits a description of the scattering problem on the basis of a unique integral equation with a kernel including both types of irregularities. In the case of small perturbations, the first-order solution of this equation is used to obtain the scattering amplitude and scattering cross section for a stratified randomly inhomogeneous fluid medium with an arbitrary number of rough interfaces. Expressions for the roughness and volume scattering cross section are obtained involving cross-correlation matrixes between the roughness of different interfaces and between the volume fluctuations of different parameters. Also, an example is considered where roughness–volume corr...

Scattering from Elastic Sea Beds: First-Order Theory.

A perturbation model for high-frequency sound scattering from an irregular elastic sea bed is considered. The sea bed is assumed homogeneous on the average and two kinds of irregularities are assumed to cause scattering: roughness of the water-sea bed interface and volume inhomogeneities of the sediment mass density and the speeds of compressional and shear waves. The first-order small perturbation approximation is used to obtain expressions for the scattering amplitude and bistatic scattering strength. The angular dependence of the scattering strength is calculated for sedimentary rock and the influence of shear elasticity is examined by comparison with the case of a fluid bottom. Shear effects are shown to be strong and complicated.

A geoacoustic bottom interaction model (GABIM)

The geoacoustic bottom interaction model (GABIM) has been developed for application over the low-frequency and midfrequency range (100 Hz to 10 kHz). It yields values for bottom backscattering strength and bottom loss for stratified seafloors. The model input parameters are first defined, after which the zeroth-order, nonrandom problem is discussed. Standard codes are used to obtain bottom loss, uncorrected for scattering, and as the first step in computation of scattering. The kernel for interface scattering employs a combination of the Kirchhoff approximation, first-order perturbation theory, and an empirical expression for very rough seafloors. The kernel for sediment volume scattering can be chosen as empirical or physical, the latter based on first-order perturbation theory. Examples are provided to illustrate the various scattering kernels and to show the behavior predicted by the full model for layered seafloors. Suggestions are made for improvements and generalizations of the model.

High frequency broad band scattering from water-saturated granular sediments: Scaling effects

Sound backscattering from water-saturated granular sediments at frequencies from 150 kHz to 8 MHz at oblique incidence was studied in controlled laboratory conditions. Two kinds of sediments, medium and coarse sands, were degassed, and their surface was flattened. In these conditions, the sediment granular structure can be considered as a controlling mechanism of backscattering. Comparison of frequency dependencies of backscatter for the two sediments with different mean grain size shows the existence of a persistent scaling effect that allows description of the backscattering strength as a function of one parameter, the mean grain size/wavelength ratio.

A model of narrow‐band normal‐mode reverberation in shallow water

A unified model of reverberation in a shallow‐water waveguide caused by the volume heterogeneity and rough interfaces is proposed. Normal modes are used to describe propagation of a narrow‐band signal from a point source to the scattering volume (a vertical column of the waveguide including both water and sediment) and from the scattering volume to the receiver. A local scattering matrix describes a process of reradiation from one normal mode to another within the scattering volume. The case of statistically axial symmetry, where the source and receiver are separated only in the vertical direction and the medium is statistically homogeneous and isotropic in the horizontal plane, is considered in more detail. A simple relationship of the temporal dependence of the reverberation intensity with the scattering matrix, attenuation, and group velocities of the normal modes is obtained. Contributions of different components of reverberation due to rough air‐water and water‐sediment interfaces, volume heterogenei...

Laboratory study of high‐frequency scattering from water‐saturated granular sediments

Sound backscattering and reflection from water‐saturated granular sediments at frequencies from 200 kHz to 7 MHz were studied in controlled laboratory conditions. Two kinds of well‐sorted sandy sediments, fine and coarse sands, and two kinds of glass beads with corresponding sizes were chosen for the study. The two types of sand had narrow log‐normal size distributions of particles with the mean diameters 0.25 and 1.5 mm for fine and coarse sand, respectively. The sediments were degassed and their surface was flattened carefully. In these conditions, the grain‐scattering mechanism can be considered as a dominating factor controlling incoherent component of the field scattered from the sediment. Frequency dependencies for the backscattering strength at various grazing angles and the reflection coefficient at normal incidence were measured. The effects related to the sediment grain size are analyzed and their possible applications to remote sensing of marine sediments are discussed. [Work supported by ONR a...

Multiple scattering in marine sediments with volume inhomogeneities

Scattering by sediment inhomogeneities is recognized as one of major contributors to sea bed scattering and reverberation. In practical modeling, various single‐scattering assumptions are normally used. While these assumptions may be reasonable in many cases, it is usually untested. Here, it is shown that conditions of validity for conventional single‐scattering approximations can fail at realistic values of sediment parameters. In particular, the field can be strongly fluctuating and randomized in significant part of insonofied sediment volume, which is in essential contradiction with the main assumption of single‐scattering theories. In certain cases, this difficulty can be overcome in the frame of the so‐called cumulative forward‐scatter single back‐scatter (CFSB) approximation developed for the problem of wave scattering in the atmosphere [de Wolf, 1971]. Examples of application of this theory to sea bed scattering are presented and multiple scattering effects are considered. Also, problems specific f...

Physics-based inversion of multibeam sonar data for seafloor characterization

As part of a continuing effort to develop a physics-based seafloor inversion technique, both acoustic and environmental data were collected during the Target and Reverberation Experiment 2013 (TREX13). The data were collected along a 350 m long survey track that sampled several sediment types including sand, silty-sand, and mud. A RESON 7125 multibeam sonar was modified to collect data along the track from 150-450 kHz in 50 kHz intervals. Ground-truth data on seafloor properties were acquired along this track, including measurements of roughness, sound speed, attenuation, and both discrete and continuous volume heterogeneity. A model was used to generate echo intensity time series including scattering by both seafloor roughness and volume heterogeneity. Model-data fits were used to provide estimates of acoustic attenuation, volume scattering strength, and roughness spectral parameters. Volume scattering is treated using an empirical model, while roughness scattering is treated using the small-slope approx...

Volume scattering and reverberation in shallow water: A simplified modeling approach

A simplified physics-based approach is described that allows significantly faster yet reasonably accurate estimations of volume reverberation in complex shallow water environments. An integral expression is presented for scattering intensity with a factorized integrand comprised of two kernels, the double propagator and local volume scattering coefficient. The propagator describes the local intensity and can be calculated using available models, such as PE, normal modes, or ray approximations. The scattering kernel can be specified using available volume scattering models for continuous or discrete heterogeneity of sea-water column and seabed caused by spatial fluctuations of compressibility and density, or randomly distributed discrete targets, such as bubbles, fish, shells, and others. The approach is more general than and can be used for verification of existing reverberation models. For instance, calculation of bottom reverberation is not based on using the equivalent surface scattering strength (alth...

Geoacoustic modeling based on sediment particle analysis.

Geological underwater processes are affected by physical properties of sediment particles, their size, shape, and spatial variability. In geological modeling, these parameters are normally provided by analysis of sediment cores and documented in certain geologically relevant terms. What is the set of parameters most relevant from acoustics standpoint, or in “acoustically relevant” terms, is an open question. It is addressed using an acoustic scattering model directly based on sediment particle analysis, geo‐acoustic model of bottom interaction taking into account the sediment discrete heterogeneity. The particle size distribution in this model is comprised of central and coarse parts. The central part describes the sediment matrix and its large scale variability, or continuous heterogeneity, critical for modeling of acoustic propagation in the sediment. To account for discrete component of heterogeneity, the coarse part of size/shape distributions is attributed to “inclusions” in the sediment matrix and p...