A fast ray model for propagation in a homogenous water column tracks time-of-flight wavepackets from sources to targets and then to receivers. The model uses image sources and receivers to account for interactions with the water column boundaries, where the layer of water lies between an upper semi-infinite halfspace of air and a lower semi-infinite halfspace of a homogenous sediment. The sediment can be either an attenuating fluid with a frequency-independent loss parameter or a fluid consistent with an effective density fluid model (i.e., a fluid limit to Biot's model for a fluid-saturated poroelastic medium). The target scattering process is computed via convolution of a free-field scattering form function with the spectrum of an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicate target can be reduced to a scattering form function, and this form function then can be used within model via interpolation. The fast ray-based model permits the generation of sets of realistic pings suitable for synthetic aperture sonar processing for proud and partially buried target. Results from simulations are compared to measurements where the targets are an inert unexploded ordnance and aluminum cylinder.

The scattering from elastic targets in the low- to mid-frequency regime is affected by the environment surrounding the target. For axisymmetric targets with the axis of symmetry parallel to the water-sediment boundary, previous work has dealt with the change in the target strength as a function of frequency and aspect angle in relation to the burial depth in the sediment. The present work deals with the extension of a finite element model, based on the decomposition of the acoustic and elastic fields into azimuthal Fourier modes, to the case of a target buried at a tilt angle. The interaction between the target and the sediment is represented by the model up to the first order of the scattering series, which means that the scattering of the incident field and of the target reflected field is taken into account, but the rescattering of the boundary reflected echo from the target is neglected. Model results up to 30 kHz are compared to experimental data for a 2 foot long aluminum cylinder of 1 foot diameter buried in sand at a tilt angle.

Low frequency sound is a viable means for the detection of elastic targets in contact with the ocean floor. The incoming sound, with wavelengths on the order of the target dimensions, can excite resonant modes of the target leading to enhancements in the scattered field. A hybrid model has been developed to predict the acoustic scattering from cylinders, pipes and unexploded ordnance (UXO) in proud or buried configurations in the ocean sediment. The model exploits the symmetry by decomposing the 3-D problem into a sum of 2-D independent Fourier modal sub-problems. This hybrid modeling technique has been shown to agree well with experimental measurements conducted in a pond [A.L. España et al., J. Acoust. Soc. Am. 130, 2330 (2011)]. Presently, these hybrid model results are used to examine the target response on a mode-by-mode basis. A modal map is generated by keeping track of the number of dominant modes contributing to the bright features observed in the acoustic template. For features that are predominantly due to one or two modes, simple analytical models can be used to predict their evolution as a function of target/sensor geometry within the ocean waveguide.

The environment and the location of the target within that environment affect the scattering from elastic targets in ocean waveguides. Computational power is now realizable to compute the target scattering, in-situ, via finite elements. However, these calculations still require high cost computer facilities and in the end do not offer physical insight into processes involved. Here we compare two models, with different levels of simplification, to data acquired from an Aluminum target machined to replicate an Unexploded Ordnance (UXO). The first model treats the scattering using two-fluid Green's function propagators in combination with finite element calculations of the target scattering as placed within the waveguide. The second model uses free field, plane wave incidence, finite element results for the target scattering in conjunction with simple ray based propagation to account for the waveguide environment. The data/model comparisons are discussed in light of the physical insight they can help provide, the speed of the calculation and the level of fidelity they achieve.

Recent experiments conducted in a fresh water pond investigated the monostatic scattering from an aluminum pipe (length-to-diameter ratio of 2) in the free field, as well as in a proud configuration on a flattened sand sediment. Synthetic aperture sonar (SAS) techniques are used to process the data. Absolute target strength is calculated over various spatial filter boundaries of the SAS images in order to isolate the specular and elastic responses of the pipe. A finite element (FE) model has been developed for the aluminum pipe in the free field, making use of the exact geometry associated with the pond experiment. The absolute target strength from these FE calculations is plotted in a similar manner to the experimental data, whereby the specular and elastic contributions are identified and compared to the data.