Matched-field replica models based on an inaccurate knowledge of geoacoustic parameters such as bottom attenuation, shear, and interfacial sound-speed discontinuities, can predict an incorrect number of propagating modes for a shallow-water channel. The resulting degradation in the matched-field ambiguity surface can be substantially reduced by obtaining optimal replica models via modal-sum-limit optimization or bottom-property inversion. The use of these techniques for multi-tone (70, 95, 145, and 195 Hz) source-tow data recorded near San Diego during the first Shallow-Water Evaluation Cell Experiment (SWellEX-1) significantly increased matched-field correlation levels and improved source localization relative to results obtained with a previous nonoptimized model. The predicted number of propagating modes was also reduced substantially. The inversion for bottom properties (attenuation, interfacial sound-speed discontinuities, no shear) provided sediment attenuation estimates which agree well with Hamilton’s models and were an order-of-magnitude greater than that used in the nonoptimized model, which accounts for the reduction in the number of modes. A simulated modal decomposition using the inverted optimal replica model verifies the number of modes predicted by the modal-sum-limit optimization.

Alerted detection of low-level broad-band signals using adaptive matched-field processing (MFP) is illustrated in results from a shallow-water experiment carried out 12 km west of Point Loma, CA, in 200-m water of complex bathymetry. A 118-m vertical line array was deployed next to an identical line array tilted at 45 degrees. Array gain and signal excess for each of the arrays with linear and adaptive broad-band MFP is measured and compared using a low-level (118 dB//1 /spl mu/Pa/sup 2//Hz) broad-hand signal from a towed source. Surface/submerged classification was achieved at the minimum detectable level due to the depth resolution obtained with MFP. The results are compared qualitatively with adaptive plane-wave beamforming on a horizontal line array deployed nearby.

Data on a tilted line a array (TLA) from the 1996 Shallow Water Cell Experiment (SWellEx-96), which was performed in 200 meters of water over a relatively flat bottom, are used to quantitatively evaluate the performance of processors used in matched field processing (MFP). The MVDR processor, the dominant-mode rejection processor and the partially-adaptive reduced-rank processor have been evaluated using data on a tilted line array (TLA). According to this evaluation, the MVDR processor with white noise gian constraint (WNGC) has the best performance. followed by the dominant-mode rejection processor, the partially-adaptive reduced-rank processor and the linear processor.

The depth discrimination capability of matched-field processing provides the possibility of separating surface from submerged sources of acoustic energy. Data collected in 200-m water during SWellEx-96 will be used to illustrate the achieved depth resolution in a downward refracting, shallow-water environment. During the event analyzed, two sources were towed simultaneously. The nominal depths of the deep and shallow sources were 60 and 10 m, respectively. Both projected unique, multitone transmissions with the deep source covering the 50–400 Hz band and the shallow source covering the 100–400 Hz band. In addition, the source ship herself (R/V SPROUL) had a detectable radiated acoustic signature across this same frequency region. The data were received by a 120-m aperture, 64-element vertical array deployed from the R/P FLIP. Broadband adaptive matched-field processing has been carried out on the data. The depth resolution characteristics of each of the three sources of acoustic energy will be presented. [Work supported by ONR, Code 321US.]

An intercomparison is presented of multi-tone, low-level signal detectability in a shallow water environment using various array configurations and processing approaches. The data were collected west of Point Loma in 200-m-deep water during SWellEx-96 which was carried out in May 1996. The multi-tone transmissions covered the 50-400 Hz band and consisted of 13 sets of tonals each having a pilot tone and 4 lower-level tonals. These were broadcast from a source towed at 60-m depth. Two 120-m aperture, 64-element arrays were deployed from the R/P FLIP—a vertical array and a tilted vertical array (tilted westerly at 45 deg). In addition, a 240-m aperture, 32-element horizontal array (slightly bowed) was deployed on the seafloor approximately 2-km south of FLIP and oriented SW to NE. Broadband adaptive spatial processing has been carried out on the data; matched-field processing in the case of the two vertical arrays and both matched-field processing and plane-wave beamforming in the case of the horizontal array. The low-level signal detectability results for the various array configurations and processing approaches will be intercompared. [Work supported by ONR, Code 321(US).]

Accurate knowledge of array shape is essential for carrying out full wavefield (matched-field) processing. Direct approaches to array element localization (AEL) include both nonacoustic (tilt-heading sensors) and acoustic (high-frequency, transponder-based navigation) methods. The low-frequency signature emitted from a distant source also can be used in an inversion approach to determine array shape. The focus of this paper is on a comparison of the array shape results from these three different methods using data from a 120-m aperture vertical array deployed during SWellEx-3 (Shallow Water evaluation cell Experiment 3). Located 2 m above the shallowest array element was a self-recording package equipped with depth, tilt, and direction-of-tilt sensors, thereby permitting AEL to be performed non-acoustically. Direct AEL also was performed acoustically by making use of transponder pings (in the vicinity of 12 kHz) received by high-frequency hydrophones spaced every 7.5 m along the vertical array. In addition to these direct approaches, AEL was carried out using an inversion technique where matched-field processing was performed on a multitone (50-200 Hz), acoustic source at various ranges and azimuths from the array. As shown, the time-evolving array shape estimates generated by all three AEL methods provide a consistent picture of array motion throughout the 6-h period analyzed.

Acoustic source localization using matched-field processing is presented for multitone signals from the Shallow Water Evaluation cell Experiment #3 (SWellEX-3), The experiment was carried out in July 1991 west of Point Loma, CA, in 200 m of water of complex bathymetry, The multitone signal (ten tones between 50 and 200 Hz) was transmitted from an acoustic source towed at various depths over tracks which produced complex propagation paths to a vertical line array receiver, Broad-band and narrow-band processing, localization, and tracking results are compared with each other and with independent estimates of source position, With narrow-band processing, mismatch between the data and the predicted signal replica of similar to 1 dB reduced the mainlobe to levels equal to or below the sidelobes, Incoherently averaging the processing output over the multiple tones reduced range/depth sidelobe levels, allowing accurate source localization and tracking.

Detailed seafloor environmental modeling of a moderately complex shallow water area, coupled with multitone matched-field processing, produced reliable and unambiguous source detection and tracking. Frequency averaging of the matched-field output sufficiently reduced sidelobe ambiguities. The seabed geologic model is a gridded database containing water depth, sediment grain size, sediment thickness, and acoustic basement type. Grid cells are squares of side length equal to 2 arc s. Software, separate from the geologic model, computes a geoacoustic model for any desired grid square.

Broadband matched field processing (MFP)-derived estimates of 3D source location using data from hydrophone line arrays deployed in various geometries, i.e., vertical, horizontal, and tilted 45° from vertical, are presented in this paper. These data were collected in two shallow water (100–200 m) experiments off the coast of San Diego. Results show that estimates of source range and depth remain surprisingly coherent in the presence of large mismatch in bathymetry, but are offset from the true position by as much as 100%. The offsets are independent of array geometry. In contrast, for estimates of source azimuth, bathymetry mismatch typically causes a degradation in MFP correlation rather than an appreciable offset. However, errors in the assumed tilt from vertical of an array can lead to large offsets in the estimated source azimuth, particularly as the nominal angle of the array from vertical becomes smaller. Predictions from a simple analytical model based on adiabatic normal modes in ideal waveguides provide good fits to the broadband MFP results.

SWellEx-3 (Shallow Water evaluation cell Experiment 3) was conducted in July 1994 west of Point Loma in 200 m water. During the experiment, a MPL 64-element, 120-m aperture, vertical array was deployed on the ocean bottom from the R/P FLIP. Located 2-m above the shallowest array element was a self-recording package equipped with depth, tilt, and direction-of-tilt sensors, thereby permitting array element localization (AEL) to be performed nonacoustically. In addition, AEL was performed acoustically using two different approaches. The first approach made use of transponder pings (in the vicinity of 12 kHz) received by high frequency hydrophones spaced every 7.5 m along the vertical array. The second approach was based on a self-cohering technique where matched-field processing was performed on a low frequency, multi-tone (50-200 Hz) sound source being towed at various ranges and azimuths from the array. The focus of this paper is on a comparison of the time-evolving array shape estimates generated by these three different methods. As shown, all three provide a consistent picture of array motion throughout the 6 hour period analyzed.