David M. Fromm

Demonstration at sea of the decomposition-of-the-time-reversal-operator techniquea)

This paper presents a derivation of the time reversal operator decomposition (DORT) using the sonar equation. DORT is inherently a frequency-domain technique, but the derivation is shown in the time-frequency domain to preserve range resolution. The magnitude of the singular values is related to sonar equation parameters. The time spreading of the time-domain back-propagation image is also related to the sonar equation. Noise-free, noise-only, and signal-plus-noise data are considered theoretically. Contamination of the echo singular component by noise is shown quantitatively to be very small at a signal-to-noise ratio of 0dB. Results are shown from the TREX-04 experiment during April 22 to May 4, 2004 in 94m deep, shallow water southwest of the Hudson Canyon. Rapid transmission of short, 500Hz wide linear frequency modulated beams with center frequencies of 750, 1250, 1750, 2250, 2750, and 3250Hz are used. Degradation caused by a lack of time invariance is found to be small at 750Hz and nearly complete a...

Comparison of a subrank to a full-rank time-reversal operator in a dynamic ocean

This paper investigates the application of time-reversal techniques to the detection and ensonification of a target of interest. The focusing method is based on a generalization of time-reversal operator techniques. A subrank time-reversal operator is derived and implemented using a discrete set of transmission beams to ensonify a region of interest. In a dynamic ocean simulation, target focusing using a subrank matrix is shown to be superior to using a full-rank matrix, specifically when the subrank matrix is captured in a period shorter than the coherence time of the modeled environment. Backscatter from the point target was propagated to a vertical 64-element source-receiver array and processed to form the sub-rank time-reversal operator matrix. The eigenvector corresponding to the strongest eigenvalue of the time-reversal operator was shown to focus energy on the target in simulation. Modeled results will be augmented by a limited at-sea experiment conducted on the New Jersey shelf in April-May 2004 measured low-frequency backscattered signal from an artificial target (echo repeater).

The Influence of the Sea Surface and Fish on Long‐Range Reverberation

Acoustic detection for active sonars involves identifying target signatures in the presence of environmental effects, such as acoustic scattering from the ocean boundaries and fish. The Naval Research Laboratory has recently developed 3D broadband models that provide physics‐based estimates of the dependence of scattering from the sea surface, bubble clouds and near‐boundary fish (including boundary‐interference effects) on the incident and scattered angles, and physical/biological descriptors of the environment. In this paper, these models and a surface‐loss model are used as kernels in reverberation models, which in turn are used to assess the sensitivity at 3.5 kHz of long‐range reverberation to environmental variables. It is shown that the acoustic field in shallow water waveguides could be quite sensitive to the values of sea surface (wind speed) and fish (density, size, depth) parameters, and that physics‐based models are needed for accurate field characterization.

The effect of coherence and noise on the decomposition of the time reversal operator

Active sonar in shallow water in shallow water is often reverberation-limited and the detectability is often limited by the presence of too many false alarms. The decomposition of the time reversal operator (DORT) is a method that potentially alleviates this problem by separating echoes from different depths in the water column. For example, DORT can separate a target in the water column from reverberation on the bottom. DORT requires a set of echoes recorded on a line array that result from a set of independent transmissions from a source array. A short derivation of DORT using the sonar equation is given. Because DORT is inherently a frequency-domain method, the time-frequency domain is derived to implement the algorithm on the data. Lastly, the similarity of DORT to adaptive beam forming is shown. In this paper, data taken on the Atlantic shelf, east of Cape May, NJ, during Geoclutter 03 and TREX-04 experiments, is processed and shown. The data was taken with a 64 element vertical line array of source-receiver elements. The target was an echo repeater using an XF4 source from 500 to 2500 Hz or an ITC 200 source from 2500 to 3500 Hz. The data cover six 500 Hz-bands from 500 Hz to 3500 Hz. The data are processed using DORT in the time-frequency domain. The analysis produces singular values in the time-frequency domain and in the time-delay domain. It also produces singular vectors that are used with a broad-band propagation model to form back-propagation images in the range, depth, frequency or range, depth, time domain. The analysis shows that the limiting factors in this data set arise from 1) motion that causes a lack of time-invariance, 2) additive noise and 3) the independent transmission scheme. The lack of time invariance is shown to spread the echo energy into several singular indices. Additive noise is shown to contaminate the singular values and back-propagation images. The particular transmission scheme used, time division multiplexed LFMs, is shown to create large side lobes in the time domain. Alternative transmission sequences, as well as alternative source and receiver orientations, are discussed

Analysis of melon‐headed whale aggregation in Hanalei Bay, July 2004

On 3 July 2004, an aggregation of ca. 150–200 melon‐headed whales (Peponocephala electra) appeared in the shallow waters of Hanalei Bay, Kauai and congregated there for over 27 h. Preceding the whales’ appearance and partially coincident with their time in the Bay, midrange (3.5–5 kHz) tactical sonars were intermittently deployed during the Rim of the Pacific 2004 (RIMPAC) joint military exercises being conducted in waters near Kauai by the U.S., Japan, and Australia Navies. An NOAA report (Southall et al., 2006) attributed the active sonar usage as a plausible, if not likely, contributing factor. A detailed timeline and reconstruction of the RIMPAC activities is presented showing the worst‐case estimates of the sonar sound levels in the waters surrounding Kauai. A re‐examination of available evidence combined with a new report of a simultaneous and similar aggregation in Sasanhaya Bay, Rota, Commonwealth of the Northern Mariana Islands, brings the plausibility conclusion into question. [This work was spo...

The Dependence of Long‐Range Reverberation on Bottom Roughness

At long‐range, shallow‐water reverberation can be driven by sub‐critical‐angle scattering, i.e. by rough interrace scattering. The Naval Research Laboratory has recently developed a small‐slope model for elastic seafloors that provides physics‐based estimates of the dependence of scattering on the incident and scattered angles, and physical descriptors of the environment. In this paper, this incoherent model is used as kernels in reverberation models, which in turn are used to assess the sensitivity at 3.5 kHz of long‐range monostatic reverberation to the roughness of the water‐sediment interface. It is shown that when sub‐critical‐angle scattering dominates, the acoustic field could be quite sensitive to the parameter values of the roughness, thus arguing for the need for regional in‐situ methods for its estimation.

Application of DORT to active sonar

Active sonar in shallow water is often reverberation-limited and the detectability is often limited by the presence of too many false alarms. The problem of improving detection, and classification, in shallow water is being worked on in several different ways. The time reversal operator decomposition (DORT) is a technique that has recently been applied to the problem of discriminating echoes in shallow water based on the different depths of the scatterers. DORT uses scattering data from a multiple source and multiple receiver sonar arrangement to separate scatterers that are resolvable by the source and receiver arrays. DORT is the application of the singular value decomposition of the frequency-domain data. This paper presents a derivation of DORT from the sonar equation. DORT is inherently a frequency-domain technique. In order to preserve range-resolution, the sonar equation is transformed into the time-frequency domain. With that representation, DORT can be applied to the frequency domain signal within a range resolution cell. Following the derivation of DORT from the sonar equation, numerical simulations are shown that demonstrate the depth resolution of a vertical line array of sources and receivers. Sufficient depth resolution is shown using few sources at frequencies near 500 Hz in water with a depth of 100 m. Problems that are encountered with the implementation of the technique are discussed. Target motion causes leakage of signal energy into several singular values. Motion of the source or receiver is shown to have little effect. The problems and constraints that arise from different multiplexing techniques, including frequency, code and time division, are shown. Results are shown with data taken on the Atlantic shelf, east of Cape May, NJ, during Geoclutter 03 and TREX-04 experiments

Range-Dependent, Normal-Mode Reverberation Model for Bistatic Geometries

An acoustic normal-mode reverberation model for bistatic source and receiver configurations in multi-layered, range-dependent underwater environments has been developed. Adiabatic normal-mode theory is used to propagate the signal field (with dispersion) to and from the scattering regions. The model is designed such that scattering from the interfaces and volume inhomogeneities may be easily included. Currently, scattering from the ocean bottom is treated by using a three-dimensional Lambert’s-law/facet reflection model. Sources and receivers may be either omnidirectional or vertical/horizontal line arrays. An explicit description of the coupling between incident and scattered modes and their contribution to reverberation is obtained.

A computationally efficient multistatic reverberation algorithm.

The multistatic reverberation algorithm (MURAL), which supports trainer development, is presented. The algorithm can be used with any propagation model that produces range‐sampled grids of transmission loss, travel time, launch, and grazing angles. The flexibility of MURAL with functions that calculate the propagation, scattering, and beam patterns is shown. A reverberation model should produce high‐fidelity accurate results in real‐time. In reality, the calculation almost always involves the trade‐off between computational speed and accuracy. The operation of MURAL couples the algorithm controls to the requested resolution of the prediction with the goal of self‐optimizing its performance for the requested resolution. Details are presented that address the technical issues of speed, accuracy, fidelity, and flexibility through several interrelated approaches including (1) the utilization of large amounts of memory available on today’s computers, 2) carefully tying the resolution of the look‐up tables to t...

Detection in shallow water using broadband—DORT

The decomposition of the time‐reversal operator (DORT) [Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)] has been extended into a coherent, broadband method. Broadband DORT has also been shown to isolate resolvable scatterers at various depths and ranges in a bistatic, active sonar in shallow water. Results are shown from the application of DORT to sea data taken in an area south of Hudson Canyon off the New Jersey coast during Geoclutter II. The vertical source/receiver array with 56 hydrophones spanning the water column was operated between 3.0 and 3.5 kHz. The elements were divided into four groups, with each group acting as a coherent, broadside source. Two methods were used for exciting the separate channels. One method was the use of subsequent LFMs and the other was the use of simultaneous transmission of four pseudorandom‐noise signals. The target was a midwater column echo‐repeater. Results are compared with modeling based on in situ environmental measurements during the experiment. [The a...