This paper is concerned with the development of a system for the classification of military munitions and unexploded ordnance (UXO) in shallow underwater environments. A Matched Subspace Classifier (MSC) is used in conjunction with Acoustic Color (AC) features generated from the raw sonar returns for munition characterization. Our classification hypothesis is that spectral content of the sonar backscatter display unique acoustic signatures providing good discrimination between different classes of detected contacts. The system is exclusively trained using synthetic sonar data and then tested using real data sets collected from a side-looking sonar system. These data sets were collected using underwater objects in relatively controlled and clutter-free environments. Classification results are presented using standard performance metrics such as probability of correct classification (P_CC), probability of false alarm (P_FA) in Receiver Operating Characteristic (ROC) curves, and confusion matrices.

In March 2010, a series of measurements were conducted to collect synthetic aperture sonar (SAS) data from objects placed on a water-sediment interface. The processed data were compared to two models that included the scattering of an acoustic field from an object on a water-sediment interface. In one model, finite-element (FE) methods were used to predict the scattered pressure near the outer surface of the target, and then this local target response was propagated via a Helmholtz integral to distant observation points. Due to the computational burden of the FE model and Helmholtz integral, a second model utilizing a fast ray model for propagation was developed to track time-of-flight wave packets, which propagate to and subsequently scatter from an object. Rays were associated with image sources and receivers, which account for interactions with the water-sediment interface. Within the ray model, target scattering is reduced to a convolution of a free-field scattering amplitude and an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicated target can be reduced to a (complex) scattering amplitude, and this amplitude then can be used within the ray model via interpolation. The ray model permits the rapid generation of realistic pings suitable for SAS processing and the analysis of acoustic color templates. Results from FE/Helmholtz calculations and FE/ray model calculations are compared to measurements, where the target is a solid aluminum replica of an inert 100-mm unexploded ordnance (UXO).

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.

During the Sediment Acoustics Experiment 2004 (SAX04), a synthetic aperture sonar (SAS) was used to detect simple targets that were either proud or buried below a water-sediment interface, where the nominal grazing angle of incidence from the SAS to the point above a buried target was well below the critical grazing angle. SAS images from other measurements below the critical angle have also produced target detections of buried spheres and finite cylinders. Models and numerical simulations are developed to investigate these proud and buried target detections. For buried targets, the simulations include estimates of reverberation from the rough seafloor, the subcritical penetration through the seafloor, scattering from a target, and propagation back to the SAS. For proud targets, the simulations include the scattering from the target where interaction with the seafloor is included through simple ray models. The simulations used environmental and material parameters measured during SAX04. The environmental measurements include profiles of small-scale surface roughness and superimposed ripple structure. The SAS simulations and model/measurement comparisons over a frequency range of 10-50 kHz further support scattering from sediment ripple structure as the dominant mechanism for subcritical penetration in this range.

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.

Because tumors are much larger in size compared with the beam width of high-intensity focused ultrasound (HIFU), raster scanning throughout the entire target is conventionally performed for HIFU thermal ablation. Thermal diffusion affects the temperature elevation and the consequent lesion formation. As a result, the lesion will grow continuously over the course of HIFU therapy. The purpose of this study was to investigate the influence of scanning pathways on the overall thermal lesion. Two new scanning pathways, spiral scanning from the center to the outside and spiral scanning from the outside to the center, were proposed with the same HIFU parameters (power and exposure time) for each treatment spot. The lesions produced in the gel phantom and bovine liver were compared with those using raster scanning. Although more uniform lesions can be achieved using the new scanning pathways, the produced lesion areas (27.5 plus/minus 12.3 mm^2 and 65.2 plus/minus 9.6 mm^2, respectively) in the gel phantom are significantly smaller (p < 0.05) than those using raster scanning (92.9 plus/minus 11.8 mm^2). Furthermore, the lesion patterns in the gel phantom and bovine liver were similar to the simulations using temperature and thermal dose-threshold models, respectively. Thermal diffusion, the scanning pathway and the biophysical aspects of the target all play important roles in HIFU lesion production. By selecting the appropriate scanning pathway and varying the parameters as ablation progresses, HIFU therapy can achieve uniform lesions while minimizing the total delivered energy and treatment time.

Details of the surrounding environment, for examples, sediment conditions and nearby clutter, influence the ability to successfully detect and classify proud or buried targets. These issues were investigated during experiments conducted in March 2010 in a fresh water pond, during which targets were placed at varying distances from each other, in proud and buried configurations within a sand sediment. This paper will focus on a subset of these experiments involving the acoustic scattering from unexploded ordnances (UXOs) in proud and fully buried configurations in which the incident grazing angle of the sonar onto the water-sediment interface is above the critical angle. Monostatic synthetic aperture sonar (SAS) data will be presented for the case of a single, isolated UXO, as well as the situation where multiple UXOs are in close proximity to each other, hence simulating a cluttered environment. To supplement the data, finite element models have been developed for the UXO with varying levels of complexity in both target specifications (shape and material composition) and general experimental setup. These simulations reveal the level of fidelity required to achieve good data-model agreement.

Monostatic and bistatic scattering measurements were conducted on a set of targets near a fresh water-sand sediment interface. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10). Monostatic synthetic aperture sonar (SAS) data were collected on a rail system with a mobile tower, while a stationary sonar tower simultaneously collected bistatic SAS data. Each tower is instrumented with receivers while the sources are located only on the mobile tower. For PondEx10, 11 targets, including 6 underwater munitions, were deployed at 2 ranges from the mobile tower system. Initially, the data were processed using standard SAS techniques, and then, the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Results of the data processing from proud targets are presented. Finite element model (FEM) predictions of the scattering from an ordnance in the free field and proud on the interface are also discussed. A processing technique that separates an individual target's response from nearby targets is also briefly discussed.

A series of monostatic and bistatic acoustic scattering measurements were conducted to investigate discrimination and classification capabilities based on the acoustic response of targets for underwater unexploded ordnance (UXO) applications. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10), where the fresh water pond contained a sand sediment. The measurements utilized a rail system with a mobile tower and a stationary sonar tower. Each tower is instrumented with receivers while the sources are located on the mobile tower. For PondEx10, eleven targets were deployed at two distinct ground ranges from the mobile tower system.

Acoustic data were initially processed using synthetic aperture sonar (SAS) techniques, and the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Preliminary results of the processing of data collected from proud targets are presented. Also presented are the results associated with a processing technique that permits isolation of the response of an individual target, which is in close proximity to other targets.

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.

Understanding acoustic scattering from objects placed on the interface between two media requires incorporation of scattering off the interface. Here, this class of problems is studied in the particular context of a 61 cm long, 30.5 cm diameter solid aluminum cylinder placed on a flattened sand interface. Experimental results are presented for the monostatic scattering from this cylinder for azimuthal scattering angles from 0 to 90 degrees and frequencies from 1 to 30 kHz. In addition, synthetic aperture sonar (SAS) processing is carried out. Next, details seen within these experimental results are explained using insight derived from physical acoustics.

Subsequently, target strength results are compared to finite-element (FE) calculations. The simplest calculation assumes that the source and receiver are at infinity and uses the FE result for the cylinder in free space along with image cylinders for approximating the target/interface interaction. Then the effect of finite geometries and inclusion of a more complete Green's function for the target/interface interaction is examined. These first two calculations use the axial symmetry of the cylinder in carrying out the analysis. Finally, the results from a three dimensional FE analysis are presented and compared to both the experiment and the axially symmetric calculations.

High intensity focused ultrasound (HIFU) is emerging as a modality for treatment of solid tumors. The temperature at the focus can reach over 65°C denaturing cellular proteins resulting in coagulative necrosis. Typically, HIFU parameters are the same for each treated spot in most HIFU control systems. Because of thermal diffusion from nearby spots, the size of lesions will gradually become larger as the HIFU therapy progresses, which may cause insufficient treatment of initial spots, and over-treatment of later ones. It is found that the produced lesion pattern also depends on the scanning pathway. From the viewpoint of the physician creating uniform lesions and minimizing energy exposure are preferred in tumor ablation. An algorithm has been developed to adaptively determine the treatment parameters for every spot in a theoretical model in order to maintain similar lesion size throughout the HIFU therapy. In addition, the exposure energy needed using the traditional raster scanning is compared with those of two other scanning pathways, spiral scanning from the center to the outside and from the outside to the center. The theoretical prediction and proposed algorithm were further evaluated using transparent gel phantoms as a target. Digital images of the lesions were obtained, quantified, and then compared with each other. Altogether, dynamically changing treatment parameters can improve the efficacy and safety of HIFU ablation.

Cylindrical targets of finite length can be used as reference targets not only for calibrating an existing SAS system, but more importantly, for testing new classification and identification algorithms. With only a few well-characterized measurements available for proud and buried cylindrical targets, numerical simulations of the acoustic response of these targets offer the potential to realize an unlimited set of target orientations with respect to the source and receiver locations.

This paper discusses recent progress with our acoustic scattering models and the generation of a set of pings suitable for SAS processing. SAS images generated from numerical simulations are compared to SAS images generated from data collected during the recent pond experiment 2009 (Pondex09) at NSWC-PCD's facility 383. The target is a solid aluminum cylinder with a 0.3 m diam and length of 0.61 m.

Acoustic signatures of elastic targets located near sediment interfaces include effects due to energy interacting with the sediment. Therefore, modeling target response also requires models of scattering from, penetration into and propagation within ocean sediments. We first describe at-sea and test pond measurements carried out on "proud" (target resting on the sediment) and buried targets at frequencies in the range of 2 to 50 kHz. The results from some of these measurements are then compared to models incorporating various levels of sophistication relative to both the target and the sediment physics.

The modeling hierarchy includes the following: (1) simple sonar equation estimates that treat the target physics via a frequency dependent target strength and use formally averaged results for sediment scattering, (2) realization level modeling that allows calculation of sediment and target scattering for individual pings with sufficient fidelity to carry out synthetic aperture processing (for a proud target only its geometrical scattering is considered while the elastic response can be included for a buried target), (3) T-matrix and finite element modeling in which the target elastic response is included but sediment scattering is treated using formal averages and/or flat surface approximations.

High Intensity Focused Ultrasound (HIFU) is rapidly gaining widespread clinical use in China, and is undergoing regulatory evaluation in Europe and the US for many target diseases. Nevertheless, tools for therapy planning, monitoring, and assessment remain at a rudimentary level. In particular, measurement of thermal dose in tissues exposed with HIFU has not been sufficiently quantitative to make detailed comparisons with numerical simulations, required for validation of therapy planning models. Indeed, model validation is complicated by high sensitivity of the results to small changes in parameter values and by the general difficulty of performing geometrical registration with sufficient precision to meaningfully compare millimeter scale features typical of HIFU lesions. Our work uses photographic measurement of visible tissue discoloration so that it can be used to accurately and rapidly quantify HIFU-induced bioeffects at scales of several centimeters for comparison with the prior therapy plan. Precise comparison between nonlinear acoustic simulation and macroscopic lesion data indicates that a newly defined "blanching index" is nearly linearly proportional to the logarithm of predicted thermal dose over a very wide range of exposure, including well below the 240 minute (at 43 degrees) necrotic threshold up to about 10,000 minutes.

Synthetic aperture sonar (SAS) is used often to detect targets that are either proud or buried below a sandy sediment interface where the nominal grazing angle of incidence from the SAS to the point above a buried target is below the critical grazing angle. A numerical model for scattering from simple targets in a shallow water environment will be described, and can be used to generate pings suitable for SAS processing. For buried targets, the model includes reverberation from the rough seafloor, penetration through the interface, target scattering, and propagation back to the SAS. The reverberation and penetration components are derived from first order perturbation theory where small-scale roughness and superimposed ripple can be accommodated. For proud targets, the simulations include the scattering from the target where interaction with the seafloor is included through simple acoustic ray models. The interaction of the target with an incident field is based on a free field scattering model. Simulations will be compared to both benchmark problems and measurements over a frequency range of 10–30 kHz. These comparisons further support sediment ripple structure as the dominant mechanism for subcritical penetration in this frequency range.

High-power, short-exposure time, High Intensity Focused Ultrasound (HIFU) treatment protocols are under development that offer the potential to increase procedure throughput and optimize individual therapies. Histological examination and optical image analysis of tissues following dynamic HIFU exposure in ex vivo bovine liver have revealed that cells undergo a fundamentally different form of cell death. The rapid temperature rise due to the HIFU exposure leaves the cells structurally intact but no longer viable, similar to the cell "fixation" induced by snap-freezing. These results suggest that careful choice of both staining technique and metric for determining cell death are important in quantifying type and morphology of cell ablation, and more broadly, safety and efficacy of treatment. This finding is similar to those obtained and under discussion in the laser and RF ablation communities. Specifically, the NADH staining technique is superior to H&E for assessing cell viability, and an alternative measure of cell death may be preferable to the binary thermal dose threshold currently the standard for HIFU treatment.

There have been several recent reports that active sonar systems can lead to serious bioeffects in marine mammals, particularly beaked whales, resulting in strandings, and in some cases, to their deaths. We have devised a series of experiments to determine the potential role of low-frequency acoustic sources as a means to induce bubble nucleation and growth in supersaturated ex vivo bovine liver and kidney tissues, and blood. Bubble detection was achieved with a diagnostic ultrasound scanner. Under the conditions of this experiment, supersaturated tissues and blood led to extensive bubble production when exposed to short pulses of low frequency sound.

The use of moving high-intensity focused ultrasound (HIFU) treatment protocols is of interest in achieving efficient formation of large-volume thermal lesions in tissue. Judicious protocol design is critical in order to avoid collateral damage to healthy tissues outside the treatment zone. A KZK–BHTE model, extended to simulate multiple, moving scans in tissue, is used to investigate protocol design considerations. Prediction and experimental observations are presented which 1) validate the model, 2) illustrate how to assess the effects of acoustic nonlinearity, and 3) demonstrate how to assess and control collateral damage such as prefocal lesion formation and lesion formation resulting from thermal conduction without direct HIFU exposure. Experimental data consist of linear and circular scan protocols delivered over a range of exposure regimes in ex vivo bovine liver.

Over the last several years, many researchers have compared model predictions of isolated thermal lesions, caused by acoustic fields generated from (spherically) focused transducers, to thermal lesions created in tissue phantoms, in vitro soft tissue samples, and in vivo soft tissue samples. The models typically couple a nonlinear acoustic field from a focused transducer to the bio-heat transfer equation (BHTE). Recent experiments in polyacrylamide gel phantoms and excised bovine liver samples have demonstrated the possible deposition of thermal lesions in a scanning mode. An initial modeling effort to predict scanned thermal lesions will be discussed. The nonlinear acoustic field from a spherically focused transducer is predicted by a time-domain solution of the Khokhlov–Zabolotskaya–Kuznetsov equation. This field is then used as a heat source in the BHTE. The importance of scan rate, acoustic frequency, and time-averaged intensity will be investigated.

Instrumentation and protocols for creating scanned HIFU lesions in freshly excised bovine liver were developed in order to study the in vitro HIFU dose response and validate models. Computer control of the HIFU transducer and 3-axis positioning system provided precise spatial placement of the thermal lesions. Scan speeds were selected in the range of 1 to 8 mm/s, and the applied electrical power was varied from 20 to 60 W. These parameters were chosen to hold the thermal dose constant. A total of six valid scans of 15 mm length were created in each sample; a 3.5 MHz single-element, spherically focused transducer was used. Treated samples were frozen, then sliced in 1.27 mm increments. Digital photographs of slices were downloaded to computer for image processing and analysis. Lesion characteristics, including the depth within the tissue, axial length, and radial width, were computed. Results were compared with those generated from modified KZK and BHTE models, and include a comparison of the statistical variation in the across-scan lesion radial width.

Therapeutic ultrasound is an emerging field with many medical applications. High intensity focused ultrasound (HIFU) provides the ability to localize the deposition of acoustic energy within the body, which can cause tissue necrosis and hemostasis. Similarly, shock waves from a lithotripter penetrate the body to comminute kidney stones, and transcutaneous ultrasound enhances the transport of chemotherapy agents. New medical applications have required advances in transducer design and advances in numerical and experimental studies of the interaction of sound with biological tissues and fluids. The primary physical mechanism in HIFU is the conversion of acoustic energy into heat, which is often enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. Other mechanical effects from ultrasound appear to stimulate an immune response, and bubble dynamics play an important role in lithotripsy and ultrasound-enhanced drug delivery. A dramatic shift to understand and exploit these nonlinear and mechanical mechanisms has occurred over the last few years. Specific challenges remain, such as treatment protocol planning and real-time treatment monitoring. An improved understanding of the physical mechanisms is essential to meet these challenges and to further advance therapeutic ultrasound.

During the sediment acoustics experiment, SAX99, a hydrophone array was deployed in sandy sediment near Fort Walton Beach, Florida, in a water depth of 18 m. Acoustic methods were used to determine array element positions with an accuracy of about 0.5 cm, permitting coherent beamforming at frequencies in the range 11–50 kHz. Comparing data and simulations, it has been concluded that the primary cause of subcritical acoustic penetration was diffraction by sand ripples that were dominant at this site. These ripples had a wavelength of approximately 50 cm and RMS relief of about 1 cm. The level and angular dependence of the sound field in the sediment agree within experimental uncertainties with predictions made using small-roughness perturbation theory.

Linear wave propagation through a bubbly liquid has seen a resurgence of interest because of proposed "corrections" to the lowest-order approximation of an effective wave number obtained from Foldy's exact multiple scattering theory [Foldy, Phys. Rev. 67, 107 (1945)]. An alternative approach to wave propagation through a bubbly liquid reduces the governing equations for a two-phase medium to an effective medium. Based on this approach, Commander and Prosperetti [J. Acoust. Soc. Am. 85, 732 (1989)] derive an expression for the lowest-order approximation to an effective wave number. At this level of approximation the bubbles interact with only the mean acoustic field without higher-order rescattering. That is, the field scattered from a bubble may interact with one or more new bubbles in the distribution, but a portion of that scattered field may not be scattered back to any previous bubble. The current article shows that modifications to the results of Commander and Prosperetti lead to a new expression for the effective wave number, which properly accounts for all higher orders of multiple scattering.

High-intensity focused ultrasound (HIFU) is becoming a widely accepted and "clean" modality to induce noninvasive coagulative necrosis of biological tissue for both cancer treatment and hemostasis. Theoretical predictions via the medusa (medical ultrasound algorithm) computer model of ultrasonic fields, temperature responses, and lesion dynamics are simulated for turkey breast. The model accounts for nonlinear sound propagation in inhomogeneous media, arbitrary frequency power law for acoustic attenuation, and temperature and lesion time histories. Generation of gas bubbles within the tissue may also be considered. Results are presented in terms of a comparison study with in vitro experiments on common turkey breast. Attention is mainly focused on temperature and lesion evolutions; in particular, induced lesion boundaries and collateral damage to surrounding areas.

Proposed corrections to the lowest-order approximation of an effective wave number obtained from Foldy's exact multiple scattering theory [Foldy, Phys. Rev. 67, 107 (1945)] has sparked renewed interest in linear wave propagation through bubbly liquids. An alternative approach to wave propagation through a bubbly liquid reduces the governing equations for a two-phase medium to an effective medium. Commander and Prosperetti [J. Acoust. Soc. Am. 85, 732 (1989)], based on this method, derive an expression for the lowest%u2010order approximation to an effective wave number. At this level of approximation the bubbles interact with the mean acoustic field without higher-order rescattering. That is, the field scattered from a bubble may interact with one or more new bubbles in the distribution, but a portion of that scattered field may not be scattered back to any previous bubble. Simple modifications to the results of Commander and Prosperetti lead to a new expression for the effective wave number, which properly accounts for all higher orders of multiple scattering.

Ultrasound has been used for decades as a means for noninvasive treatment of diseases. Low-intensity ultrasound is routinely applied in physical therapy for muscular and neurological related illnesses. In contrast, high-intensity focused ultrasound (HIFU) is used to induce coagulative necrosis of tissue for cancer treatment or hemostasis. Our efforts concern the latter. Predictions of ultrasound fields, temperature response, and lesion dynamics are obtained by a model which accounts for nonlinear sound propagation in inhomogeneous media, an arbitrary frequency power law for acoustic attenuation, and temperature time history [J. Acoust. Soc. Am. 107, No. 5, Pt. 2 (2000)]. The model is expanded from its previous version to include attenuation and sound speed dependence on temperature levels and also to consider generation of gas bubbles within the tissue. Results are presented in terms of treatment strategies that provide maximum energy transfer for coagulating the targeted tissue while minimizing damage to the surrounding area.

The linear wave equation in a lossless medium is time reversible, i.e., every solution p(x,t) has a temporal mirror solution p(x,–t). Analysis shows that time reversal also holds for the lossless nonlinear wave equation. In both cases, time-reversal invariance is violated when losses are present. For nonlinear propagation loses cannot normally be ignored; they are necessary to prevent the occurrence of multivalued waveforms. Further analysis of the nonlinear wave equation shows that amplification of a time-reversed pulse at the array elements also leads to a violation of time reversal even for lossless nonlinear acoustics. Numerical simulations are used to illustrate the effect of nonlinearity on the ability of a time-reversal system to effectively focus on a target in an absorbing fluid medium. We consider both the amplitude and arrival time of retrodirected pulses. The numerical results confirm that both shock generation (with the accompanying absorption) and amplification at the array, adversely affect the ability of a time-reversal system to form strong retrodirective sound fields.