Shallow-water Data Research Articles

HOTWIT: An unconstrained shallow-water reverberation estimation algorithm

A new algorithm/method for the inversion of activated towed array reverberation data in shallow water, known as the heading optional transformation of Wagstaff’s iterative technique (HOTWIT), has been developed. By taking advantage of the activation and motion of the towed array, the algorithm resolves the inherent left–right ambiguity of the towed array without the need for multiple headings and is an active analog to the passive 3-D WIT algorithm. The result is an unambiguous estimate of bottom target strength from which the reverberation density at a given receiver location can be inferred. The algorithm lends itself well to a rapid characterization of shallow-water reverberation in shallow-water areas. A description of the algorithm with a discussion of results from the algorithm with measured and modeled towed array data will be discussed.

Extracting modal structure from vertical array ambient noise data in shallow water

Measurements of the acoustic mode structure in shallow water can be inverted to determine ocean environmental acoustic parameters. This modal stucture is embedded in shallow-water ambient noise [Kuperman and Ingenito, J. Acoust. Soc. Am. 67, 1988–1996 (1980)]. Ambient noise measurements (SWellEx 1) using a vertical array spanning the lower duct of a summer sound-speed environment were used to extract the acoustic modal structure of a coastal water environment off southern California. The experimental results derived from eigenvector processing are consistent with simulations using the measured environmental parameters.

Reduced-complexity spatial and temporal processing of underwater acoustic communication signals

Multichannel processing of high-speed underwater acoustic communication signals requires computationally intensive receiver algorithms. The size of adaptive filters, determined by the extent of ocean multipath, increases with signaling rate and limits system performance through large noise enhancement and increased sensitivity of computationally efficient algorithms to numerical errors. To overcome these limitations, reduction in receiver complexity is achieved by exploiting the relationship between optimal diversity combining and beamforming. Under relatively simple conditions, two adaptive receivers, one based on diversity combining which does not rely on any spatial signal distribution, and the other based on optimal beamforming, are shown to achieve the same performance. The beamforming approach, however, leads to a receiver of lower complexity. Carrying these observations over to a general case of broadband transmission through an unknown channel, a fully adaptive receiver is developed which incorporates a multi-input multi-output, many-to-few combiner, and a reduced-complexity multichannel equalizer. Receiver operations are optimized jointly to ensure minimum mean-squared error performance of data detection. Results of processing experimental shallow water data demonstrate the capability to fully exploit spatial diversity of underwater multipath while keeping the complexity at an acceptable level.

Results on the sequential classification of spherical objects in shallow water

Based on the theoretical results reported in [ 1460 (1993)] and the simulated results reported in [ 3312(A) (1994)] an experiment to determine the performance of sequential classifying spherical objects in shallow water has been carried out. The shallow water data were obtained from experiments off the East Coast of the United States in 1993. Five spherical objects of different composition were embedded in shallow water data and the performance of a sequential classifier to correctly identify the objects was obtained. The performance results of these tests using standard measures of effectiveness will be reported.

Multiple‐frequency robust adaptive matched‐field processing (MFP) in shallow water

Application of robust adaptive algorithms to multiple‐frequency MFP in shallow water has been studied and the minimum‐detectable‐level (MDL) has been examined via simulations. The multiple‐frequency MFP output is obtained by incoherently averaging the MFP outputs of the discrete narrow‐band‐components. Performance of three different averaging techniques (arithmetic‐mean, geometric‐mean, and harmonic‐mean) have been compared. The arithmetic‐mean averages the amplitudes, the geometric‐mean averages the dBs, and the harmonic‐mean averages the reciprocals of the amplitudes. These algorithms have been applied to shallow water data from the Hudson Canyon experiment and have demonstrated excellent source localization results. simulations show a 10‐ to 20‐dB improvement in the MDL against the simulated near or distant surface noise.

Measurements of the azimuthal distributions of very low‐frequency noise with high resolution in a shallow water site in the Mediterranean

An extremely long horizontal towed array was used to perform high‐resolution noise measurements at very low frequencies. The directional measurements in one shallow site are compared with more extensive measurements in deep water. The very low‐frequency noise at all sites is highly anisotropic due to the high azimuthal resolution of the discrete merchant shipping sources that dominate underwater noise at these frequencies. The deep water measurements were performed at sites in three oceans, covering an extreme range of shipping density conditions, however, the shapes of the azimuthal distributions of the noise levels are relatively insensitive to shipping density. The shallow water noise exhibits azimuthal spreading not apparent in the deep water. From the loudest sources this spreading may partly obscure the spatially quiet regions that are otherwise present in directions between loud ships. Although they have different shape parameters, both the shallow and deep water data fall between Weibull and gamma...

Estimate of sea-bottom scattering in shallow water with a thermocline

Sea-bottom scattering coefficients for extremely low grazing angles have been estimated from sound propagation data in shallow water with a summer thermocline. Explosive charges were detonated under the thermocline and two hydrophones were located, respectively, above and below the thermocline. At moderate and long ranges the signals received by the upper hydrophone are composed of two parts: signals propagating from the source to the receiver and the signals scattered from the sea bottom induced by the sound propagating under the thermocline. The former arrived earlier and was much weaker than the latter. This offers one an opportunity of estimating the characteristics of sea-bottom scattering. The bottom scattering coefficients and their directivities for frequencies 0.8, 1, 2, and 4 kHz are estimated by adjusting the scattering coefficients so as to fit the calculated waveforms of scattered signals to the received ones. The incident field was estimated from the signals received by the lower hydrophone. The effective incident grazing angles are less than 2.5 deg. The bottom reflection losses due to bottom scattering are also estimated.

Integrated acoustic technologies form new shallow water survey system

Abstract Total bottom bathymetry, producing high‐resolution bottom and subbottom data in shallow water, has been accomplished through the integration of fanbeam, sidescan sonar, scanning sonar, and impedance profiling acoustics. The project called HI‐MAP (Hydrographic Inland Marine Acoustic Platform) set out in early 1990 to design a multipurpose high‐resolution, shallow water survey system. Today, the system is in full operation, simultaneously generating frequencies of 3.5, 33, 200, 500, 650, and 1000 kHz. HI‐MAP is able to survey large corridors of a river in one pass, acquiring bottom and subbottom bathymetry right up to water's edge. Included in this article is a discussion of the acquisition technologies used and the new surveying and data processing techniques developed to correlate and “thin” the data. Also presented is a description and the results of the HI‐MAP acceptance test performed by the Army Corps of Engineers.

Normal-mode matching localization in shallow water: Environmental and system effects

Matched-field processing is a passive range and depth source localization technique that has been extensively used in shallow-water environments. A vertical array of sensors is used to spatially sample the acoustic waveguide where the source signal embedded in additive ambient noise propagates. The array output is then matched with the signal replica field generated by a normal-mode model based on the environmental parameters that characterize the waveguide. Recent results obtained from real data show the feasibility of the technique and give evidence of its strong dependence both on the array aperture and on the knowledge of the environmental parameters used in the model. This paper describes a modified matched-field technique, called normal-mode matching, that is applied to real shallow-water data. Its performance is compared to that obtained by conventional matched-field processing using the same data set. Unlike conventional matched-field processing, the results indicate that unambiguous localizations can be obtained even for ‘‘short’’ arrays spanning only half of the water column.

Observations of the fractal dimension of deep- and shallow-water ocean surface gravity waves

The fractal dimension of field measurements of time series of sea surface elevation (ocean waves) was calculated. For a range of oceanic conditions, the fractal dimension estimates of deep water waves did not show low values. Although the mutual information content of waves approaching a beach increased with decreasing water depth, the corresponding fractal dimension was practically independent of depth. The observed dimensions are not statistically different from the dimension of a linear process with the same power spectrum as the field data, although the shallow water data are known to be nonlinear.

The performance of an eigenvalue‐based stabilization algorithm for high‐resolution beamformers: Experimental tests with shallow water data

This work investigates the performance of high‐resolution beamforming techniques as used with acoustic arrays in shallow water. The maximum likelihood method (MLM) is the high‐resolution technique of principal interest and the maximum entropy method (MEM) is addressed briefly. The topics dealt with are the performance of beamformers with computer‐simulated data, the performance degradation caused by system phase errors, and performance with actual field data. High resolution methods are found to suffer performance degradation in the form of instabilities in the beamform outputs. Results indicate that the proper pre‐processing method (namely, the elimination of the lowest‐lying eigenvalues from the cross‐spectral density matrix) can stabilize maximum likelihood beamformers so that they outperform conventional beamformers at the tasks of resolving targets with similar bearings and tracking them through bearing crossings. No stabilizing method was found for maximum entropy beamformers. [Work supported by ONT.]

Measurements of sediment transport on the sea bed southwest of England

Sand transport rates were measured via bedload traps that were inserted into the sediment surface on the continental shelf southwest of England. Analysis of the trapped mobile sediment shows this to be finer and better sorted than nearby Shipek grab samples. The transport rates are combined with simultaneous, near-bed flow measurements to assess a Bagnold type predictive equation,j = k1(u2100 -u2100cr)u100. The measured transport rates vary between 0.41 × 10−3 and 1.67 × 10−3 gm/cm s. The data yield a mean calibration coefficient (k1) of 0.4 × 10−6 which is slightly lower than values computed from flume and shallow water data.

Low‐frequency velocity structure of continental shelves

Broadly speaking, shelves are cut back into basement rocks by wave action and the erosion products are then transported to the seaward edge of the shelf. Seafloor sound velocities therefore range from more than 6.0 km/s in the “cutting” mode (SW Australian margin) to 1.5 km/s in the rapidly “filling” mode (northern Sunda Shelf). Intermediate seafloor sound velocities can result from the erosion of the original sediment cover (Barents Sea), from very slow sediment accumulation rates, and/or the addition of pelagic materials. Variable‐density analog recordings of Sonobuoy data (18 Hz) from different shelf environments reveal outstanding differences in the character of the traveltime graphs. Rapidly accumulating sediments commonly have low water/sediment velocity contrasts and produce curved‐path refracted arrivals. In this case, the seafloor reflections are weak but are augmented by strong curved path multiples at the minimum range, where the rays are focused at a velocity cusp. Records from areas with a high seafloor velocity yield late‐arriving, energetic bands developed from multiple critical reflection events, whereas first‐arrival refractions form straight lines. Curved path data in shallow water can be inverted to provide very accurate velocity/depth distributions. Because there are no masked refraction layers and no major sub‐bottom reflectors, spreading losses can be computed with rather more precision than is possible with deep water data (source and receiver at the surface).

Intensity–range relations for shallow‐water sound propagation

Convincing experimental evidence is presented to show that various ’’decay laws’’ for the decrease of sound intensity with increasing range can give high‐quality representation for shallow water data. However, while a sequence of decay laws may fit data well and are simple, the transition ranges for changeover from one law to another can show a complicated dependence on frequency and environmental parameters. Transition ranges are probably no easier to predict or model than is the transmission loss itself.