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SUMMARY This paper presents implementation techniques for the coupled wavenumber integration approach that is occasionally used in the underwater acoustics community to address larger scale problems in seismo-acoustics. This numerically efficient method is suitable for detailed wavefield representation of the long, thin waveguide propagating geometries such as those within subducting plate. It therefore represents an attractive alternative to ray-based solutions and finite element methods when thin layers with strong velocity contrast are expected. This wavefield modelling technique is based upon range-dependent wavenumber integration combined with Kirchhoff approximation. Examples of the details of the propagating wavefield with a dipping slab-like structure and the effects of low-velocity layers (LVLs) are presented. These results include the propagation of the wavefield through the wedge and through very LVLs as a function of setups of different complexity as well as a function of horizontal range. By identifying the existence of a sediment-like layer, the presence of a source within and the effects of the changes in the receiver range location, we demonstrate the ability of the coupled wavenumber technique to capture the physics associated with LVLs configuration, and provide classification features for effective identification of these, often intricate, subducted slab structure characteristic.

One of the current Integrated Marine Mammal Monitoring and Protection System (IMPAS) directions is concentrated on the design and development of the active sonar modality representing just one component of the global system. The active sonar was designed, built, and tested during the Marine Mammal Active Sonar Test (MAST 04), producing whale detections and whale tracks. The experiment was conducted in January 2004 off the coast of California. One of the objectives of the current work is to distinguish whale backscattered responses from the ones generated by the environmental clutter in a waveguide. Furthermore, the work aims to identify and analyze the target signature features that are necessary for enhanced active sonar detection and classification of marine mammals. Over the years there have been very few documented attempts to capture and analyze the backscattering response of whales using an active sonar system. Nevertheless, whales, mostly owing to their size, their motion, and the aspect dependence of their backscattered field, possess desirable properties that help distinguish their scattered response from clutter and other environment related false alarms. As an initial step, data collected during the MAST 04 experiment are presented, and gray whale target strength measurements are obtained. Results are compared to the previously published whale target strengths. Additionally, an investigation is conducted in an effort to provide whale feature identification points suitable for automated detection and classification, as means of relating gray whale active acoustic signatures to their inherent characteristics and their motion.

For active sonar systems, transmission Loss (TL) and reverberation level (RL) are key parameters derived from the acoustic fields predicted by models and used to assess sonar performance. The existing propagation and scattering models may be appropriate for applications in the deep ocean or open littorals, but sonar operators are increasingly being asked to perform tasks including navigation or detection in significantly more confined waterways such as rivers or ports. Physics-based models are generally not available for predicting the acoustic field in such highly geometrically constrained and dynamic 3D environments often characterized by highly variable azimuthal boundaries such as irregularly shaped port geometries, piers, and breakwaters that may also have high large tidally driven depth variations over short time periods. They also may be populated with large scattering objects such as deep draft vessels and mooring dolphins. A virtual source research model capable of predicting the three-dimensiona...

Buried objects have been largely undetectable by traditional high-frequency sonars due to their insignificant bottom penetration. Further, even a high grazing angle sonar approach is vastly limited by the coverage rate dictated by the finite water depth, making the detection and classification of buried objects using low frequency, subcritical sonar an interesting alternative. On the other hand, such a concept would require classification clues different from the traditional high-resolution imaging and shadows to maintain low false alarm rates. A potential alternative, even for buried targets, is classification based on the acoustic signatures of man-made elastic targets. However, the elastic responses of buried and proud targets are significantly different. The objective of this work is to identify, analyze, and explain some of the effects of the sediment and the proximity of the seabed interface on the scattering of sound from completely and partially buried elastic shells. The analysis was performed using focused array processing of data from the GOATS98 experiment carried out jointly by MIT and SACLANTCEN, and a new hybrid modeling capability combining a virtual source—or wave-field superposition—approach with an exact spectral integral representation of the Green’s functions for a stratified ocean waveguide, incorporating all multiple scattering between the object and the seabed. Among the principal results is the demonstration of the significant role of structural circumferential waves in converting incident, evanescent waves into backscattered body waves, emanating to the receivers at supercritical grazing angles, in effect making the target appear closer to the sonar than predicted by traditional ray theory.

The analysis of hull vibrations has been a long-standing topic of interest in the US Navy for both surface and underwater vehicles. Understanding of the physics controlling acoustic scattering and radiation from coated, fluid-loaded structures is important as it can provide the required knowledge of the self-noise modeling of hull arrays and of the acoustic target strength of the submersibles. Currently, models are typically limited to low frequency regime of operation, not being able to consider a broad mid-high frequency range, commonly rich in physical phenomena that characterize sound fields in underwater vehicle environments. The goal of this effort is to provide a robust, innovative, and computationally efficient tool for analytical modeling of a fluid-loaded acoustic coating affixed to a rib-stiffened backing plate, capable of representing high frequency acoustic environments not suitable for conventional finite element approaches.The approach taken in this effort is based on the A.J. Hull’s deriva...

Complex subsurface structure makes seismo‐acoustic imaging difficult, raising the need for novel detection, classification, and imaging approaches. For the representation of range‐dependent seismo‐acoustic propagation, we employ a hybrid, coupled wavenumber integration approach to range‐dependent seismo‐acoustic modeling based on the OASES environmental modeling framework. In feature identification and classification, time series analysis frequently provides the sought‐after recognition clues. Nevertheless, the geometry and the structure of the subsurface features generate a variety of complexities to the wave field caused by different physical mechanisms, geometric constraints, and intrinsic properties. Based on the time series attributes such as the time of arrival, waveform amplitude, and the frequency content, a set of identification features is extracted and a set of corresponding classes is formed. Using the simulated data scenarios, general rules incorporating the identification features of subsurface structures are deduced. This classification methodology is then applied to the data recorded by an array of seismographs and hydrophones on and off the coast of Japan. The results create a basis for classification and description of the structures of interest such as the presence or absence of very low‐velocity range‐dependent layers below the water‐seabed interface.

It has been suggested that the tomographic approach that feeds the 8–11 kHz ocean acoustic measurements into an ocean model using data from bottom‐mounted hydrophones, suffers from a significant travel‐time variability due to the surface waves. In this work, the influence of the ocean surface motion on the acoustic travel times has been studied using both the experimental data collected at the Pacific missile range facility (PMRF) off Kauai, and an appropriate acoustic model. The ray‐based acoustic model developed is an adaptation of the range‐dependent version of Bellhop with the modified top surface characterizing ocean surface waves. The model‐generated time series suggests a direct relationship between the phase of the top surface wave and the time‐of arrival, which is further confirmed by the experimental data. Furthermore, an approach is developed that automatically decouples the effect of the top surface waves from the environment‐related phenomena, making it possible to assign the travel time variations produced by the physical phenomena independently of the surface geometry, in turn eliminating the surface related error. [Work supported by the ONR.]

With a broader objective of establishing a global solution for systematic assessment and protection of the marine environment, one of the central objectives of this work is to distinguish whale backscattered responses from the ones generated by the environmental clutter in a waveguide. Furthermore, it aims to identify and analyze the target signature features that are necessary for enhanced active sonar detection and classification of marine mammals. Over the years there have been very few documented attempts to capture and analyze the backscattering response of whales using an active sonar system. Nevertheless, whales, mostly owing to their size, their motion, and the aspect dependence of their backscattered field, possess desirable properties that help distinguish their scattered response from clutter and other environment related false alarms. As an initial step, data collected during the Marine Mammal Active Sonar Test (MAST04) is presented, and long‐range gray whale target strength measurements are o...