Matched Beam Processing Research Articles

Target depth estimation by frequency interference matching for a deep vertical array

A vertical line array can be deployed in deep water below the critical depth, i.e., the depth where the sound speed equals the sound speed at the surface, to take advantage of the lower ambient noise level, relative to above the critical depth for target detection. The depth of a source can be estimated exploring the interference between the direct and reflected signals described by Lloyd’s mirror theory. Existing methods for source-depth estimation assume either a source transmitting a pulsed signal or a source traveling a straight line at a constant depth. For the latter, the source depth can be estimated after collecting observations over a sufficiently large domain of ranges. A method is proposed here that can estimate the source depth using a short snapshot of the random broad band signal, and can thus estimate the source depth semi-instantaneously, even as the source changes course and/or depth. An incoherent matched beam processing method is used matching the peak beam power measured from data as a function of frequency with a replica peak beam power spectrum for a hypothesized source depth. The performance of the proposed method is evaluated using simulated data and validated with experimental data from the South China Sea.

A simple method for moving source depth estimation applied to the SWellEx96 data.

Existing methods for source localization in an ocean waveguide generally require simultaneous range and depth estimation as they are intrinsically coupled in the acoustic field. Using beam intensities averaged over ranges longer than the mode cycle distances, it is shown that the range-averaged vertical beam distribution is insensitive to the source range and sensitive to the source depth. An incoherent matched beam processing method is presented and applied to the SWellEx96 moving source data to estimate source depth based on the 127 and 130 Hz signals with good results.

Matched beam-intensity processing for a deep vertical line array

A vertical line array can be deployed in deep water below the critical depth, the depth where the sound speed equals the sound speed at the surface, to take advantage of the lower ambient noise level (compared with above the critical depth) for target detection. To differentiate a submerged source from a surface source, a Fourier transform based method [McCargar and Zurk, J. Acoust. Soc. Am. 133, EL320-325 (2013)] was proposed for a narrowband signal that exploits the depth-related harmonic (oscillation) feature of the beam power time series associated with the target arrival. In this paper, incoherent matched beam processing is used to estimate the target depth. Where the replica (calculated) beam intensity or amplitude time series best matches that of the data is used to estimate the source depth. This method is shown, based on simulated data, to provide a better depth resolution in general and better ability to estimate the depth of a very shallow source (say at 10 m) and can be used to complement the Fourier transform based method. It can be extended to process (random) broadband signals and to environments where the Lloyd's mirror theory is not valid.

Range-dependent geoacoustic inversion of vertical line array data using matched beam processing

This paper describes the results of range-dependent geoacoustic inversion using vertical line array data obtained from the 4th Matched Acoustic Properties and Localization Experiment conducted in the East Sea of Korea. The narrowband multitone continuous-wave signal from the towed source was analyzed to estimate the range-dependent geoacoustic properties along the radial track. The primary approach is based on the sectorwise inversion scheme. The inversion region up to 7.5 km from the vertical line array was divided into several segments, and the subinversions for each segment were performed sequentially. To reduce the dominance of low-angle arrivals, which bears little information for the bottom segment in question, matched beam processing with beam filtering was used for the cost function. The performance of proposed algorithm was tested using simulated data for an environment representative of the experimental site. The inversion results for the experimental data were consistent with the geophysical database and were validated from matched-field source localization using frequencies different from those used in the inversion.

Geoacoustic inversion of broadband data by matched beam processing

This paper describes results of geoacoustic inversion using broadband signals from an experiment carried out at a site near the South Florida Ocean Measurement Centre in the Florida Straits. M-sequence coded pulse trains at different center carrier frequencies from 100to3200Hz were recorded at a vertical line array at a distance around 10km. Geoacoustic inversion was carried out to determine the feasibility of inverting the environmental parameters from this long-range propagation experiment. The received signal at lower frequencies below 400Hz consisted of a dominant water column signal and a secondary arrival delayed by 0.4s. The secondary signal was spatially filtered by beamforming the array data, and the beam data were inverted by matched beam processing in the time domain, combined with an adaptive simplex simulated annealing algorithm. The estimated values of compressional wave speed and density were in good agreement with ground truth values from sediment cores. The inverted shear wave speed appears to be a sensitive parameter and consistent with compressional wave speed. As a cross check, range and water depth were also included as inversion parameters, and the inversion results were close to the known values within small uncertainties.

Geoacoustic inversion of broadband data by matched beam processing

This paper describes results of geoacoustic inversion using broadband signals from an experiment carried out at a site near the South Florida Ocean Measurement Centre in the Florida Straits. M-sequence coded pulse trains at different center carrier frequencies from 100 to 3200 Hz were transmitted in the acoustic experiment, and received by a tri-axial array at a distance around 10 km. Geoacoustic inversion was carried out to determine the possibility of inverting the environmental parameters from this long-range propagation experiment. The received signal at 100 Hz consisted of a dominant water column signal and a secondary arrival delayed by 0.4 s. The secondary signal was spatially filtered by beamforming the array data, and the beam data were inverted by matched beam processing combined with an adaptive simplex simulated annealing algorithm. The estimated values of compressional wave speed and density were in good agreement with ground truth values from sediment cores. The inverted shear wave speed appears to be a sensitive parameter and consistent with compressional wave speed. Range and water depth were also included as inversion parameters, and the inversion results were close to the known values within small uncertainties. [Work supported by ONR.]

Broadband source localization in shallow water in the presence of internal waves

Source localization is a challenging problem in a time-varying random shallow-water environment in the presence of internal waves. The short correlation time of the acoustic field (a couple of minutes) makes it difficult to adjust the replica field in response to the environmental changes. Also, mode couplings induced by the internal waves are random and not predictable. In this paper, the effect of mode coupling on source localization in the presence of Garrett–Munk and solitary internal waves is investigated. It is found that mode coupling affects practically all modes in shallow water. Random-mode coupling reduces the matched-field correlation. To improve source localization, a broadband matched-beam processing algorithm is proposed to suppress the noncoherent coupled-mode contributions. It is shown that this technique significantly improves the localization performance over conventional matched-field processing.

Adaptive matched beam processing for source localization

Matched beam processing (MBP) has been previously applied to a horizontal line array for localizing a moving source in bearing and range, and for differentiating a submerged target from a surface clutter [T. C. Yang and T. Yates, J. Acoust. Soc. Am. 104 (1998)]. MBP is matched field processing in the beam domain. Using the Bartlett processor, bearing and depth resolutions are often inadequate for rejecting surface clutter and other noise sources. High-resolution processors may be required. We note that in situations where many clutters exist, adaptive (plane-wave) beamforming has effectively rejected clutter by either high-angular resolution or by steering a null to the interference source. Motivated by this approach, an adaptive MBP will be investigated in this paper. The new processor will apply adaptive beamforming to the data and replica fields such that interference has already been removed before the matched field correlation. This approach differs from the adaptive matched field processing where interference remains in the data field. The latter approach is known to be sensitive to environmental mismatch, as a result the correlation between the replica and the interference fields may be present as substantial clutter. Initial numerical results will be presented using simulated data with interference sources. [Work supported by the Office of Naval Research.]

Improved vertical array performance in shallow water with a directional noise field

The detection and localization performance of a vertical array against a submerged target in shallow water can be significantly improved if the noise field is directional and the signal arrives in the null (notch) of the noise field. This often happens in a summer environment with a downward refractive sound speed profile in which the surface generated noise field exhibits a notch in the noise vertical directionality distribution at mid (e.g., 500 Hz) frequencies. Using conventional beamforming, by steering the beams to the shallow arrival angle of the target signal, the (undesired) noise which arrives at steep angles can be suppressed. However, conventional beamforming often suffers from significant signal gain degradation due to multipaths in the shallow water environment. Matched-field processing yields a theoretical signal gain by coherently processing the signal, but its ability to reject the directional noise is limited. Matched-beam processing, which is matched-field processing in the beam domain, incorporates the advantages of matched-field processing for coherent signal integration, and conventional beamforming for noise rejection. For the summer environment studied, matched-beam processing yields the highest array gain, almost a twice longer detection range, and a superior capability in detecting a deep target compared with the other two processors. This is demonstrated using simulated signal and noise fields with a full water column vertical array.

Matched-beam processing: Range tracking with vertical arrays in mismatched environments

This paper addresses the problem of range tracking using one or more vertical arrays in certain mismatched shallow-water environments. The objective is to obtain continuous tracking of a target thereby allowing the target to be distinguished from false alarms. It is shown that bottom mismatch can cause large range errors at discrete source ranges as observed in previous experiments. Also, significant range errors (offsets) can result from unknown array tilt. Discrete range errors impede the ability to detect a target by integrating along the target track (track-before-detect) or by a probability based target detection and tracking algorithm. Large range errors impede the ability to localize and track the target from multiple vertical arrays. These problems are addressed using matched-beam processing whereby certain bottom mismatch is minimized by a beam filter and unknown array tilt is estimated by matching the measured beam pattern against the calculated one. Beam filtering and beam matching are simple and convenient methods to implement in practical systems. Comments are also given on other applications using beam domain processing.

Matched-beam processing: Application to a horizontal line array in shallow water

Matched-beam processing of a horizontal line array in a multipath shallow-water environment is presented. With conventional beamforming, the signal is split into several beams when the target is away from the broadside direction. This can result in signal gain degradation and severe bearing bias. Matched-beam processing is matched-field processing applied in the beam domain. It offers an efficient and robust approach for correcting bearing errors and signal gain degradation of horizontal line arrays in shallow water. It is based on conventional beams but extends conventional processing to incorporate full-field processing. It is applicable to existing systems where the conventional beam outputs are the only data readily available. Many of the signal processing algorithms for horizontal arrays, such as adaptive beamforming for nulling strong interferences, can be incorporated directly into matched-beam processing. Target bearing tracking and depth discrimination are illustrated using matched-beam processing for a horizontal line array.

Vertical array performance in shallow water with a directional noise field

The performance of a vertical array for source localization and for target detection (array gain) is studied in shallow water under a directional noise field. The acoustic environment consists of a downward refractive sound-speed profile. At mid (e.g., 500 Hz) frequencies the vertical directionality of the surface generated noise exhibits a notch near the signal arrival direction, i.e., which is close to horizontal for a submerged source. The ability of the beam filter in rejecting directional noise has been previously demonstrated using conventional beamforming. The same capability is carried over into full field processing by using matched-beam processing which is an equivalent of matched-field processing conducted in the beam domain. It is shown with simulated data that matched-beam processing incorporating a beam filter of 10 deg enhances the array output signal-to-noise ratio (array gain) by more than 5 dB compared with conventional beamforming and matched-field processing. For white noise, matched-field processing yields a higher peak-to-sidelobe ratio than matched-beam processing with a beam filter. For directional noise, matched-beam processing with a beam filter yields better results than matched beam processing using a minimum variance correlator. The noise field in the background ambiguity surface (the sidelobes) has been suppressed by the beam filter.

Matched-beam processing: A synthesis of conventional beamforming and matched-field processing

Conventional beamforming, based on the concept of delay and sum of plane-wave signals, has been the cornerstone for radar and underwater acoustic array processing for many years. Matched-field processing, which has received much attention in recent years, represents a departure from conventional beamforming in that the signal propagation is an integral part of the processor. In this paper a framework is presented (i.e., matched beam processing) in which conventional beamforming and matched-field processing can be unified and treated on an equal footing. Matched-beam processing is matched-field processing in the beam domain. The resulting benefits are (1) the extension of full field processing to conventional beams, (2) the ability to do spatial filtering based on the angle of arrival, and (3) the ease of integration of the (matched-beam) processor into existing systems. For a horizontal array in shallow water, conventional beamforming would be the processor to use, but it suffers from signal gain degradation and bearing bias when the signal energy is split over many beams. Matched-field processing can be used to estimate the bearing accurately, but it must simultaneously estimate the target range, depth, and bearing. Using matched-beam processing, bearing can be estimated accurately while maintaining some tolerance to environmental mismatch.

Acoustic inversion of bottom reflectivity and bottom sound‐speed profile in shallow water.

This paper presents a full field technique to invert bottom sound profile and bottom reflectivity from acoustic data collected in shallow water. Bottom sound‐speed profile and bottom reflectivity have been traditionally estimated using seismic reflection/refraction when acoustic ray paths and travel time can be identified and measured from the data. Due to the shallow depth in shallow water, the many multipaths due to bottom reflection/refraction make such identification and measurement rather difficult. A full field inversion technique is presented which uses a broadband source and an easily deployable array for bottom sound speed and reflectivity inversion. The technique is a modified matched‐field inversion technique referred to as matched beam processing. Matched beam processing uses conventional beamforming processing to transform the field data into the beam domain and correlate that with the replica field also in the beam domain. This allows the analysis to track the acoustic field as a function of incidence/reflected angle and minimize contamination or mismatch due to sidelobe leakage. This technique is illustrated with simulated data for a vertical array and can be generalized to other array configurations, such as a towed array.

Acoustic inversion of bottom reflectivity and bottom sound-speed profile

This paper applies a full-field technique to invert bottom sound profile and bottom reflectivity from simulated acoustic data in a shallow water environment. Bottom sound-speed profile and bottom reflectivity have been traditionally estimated using seismic reflection/refraction techniques when acoustic ray paths and travel time can be identified and measured from the data. However, in shallow water, the many multipaths due to bottom reflection/refraction make such identification and measurement rather difficult. A full-field inversion technique is presented here that uses a broad-band source and a vertical array for bottom sound-speed and reflectivity inversion. The technique is a modified matched field inversion technique referred to as matched beam processing. Matched beam processing uses conventional beamforming processing to transform the field data into the beam domain and correlate that with the replica field also in the beam domain. This allows the analysis to track the acoustic field as a function of incident/reflected angle and minimize contamination or mismatch due to sidelobe leakage.

Matched beam processing for geoacoustic inversion of bottom sound-speed profiles

A broadband signal (shot) received on a vertical array was processed for geoacoustic inversion of bottom sound-speed profile. The data were beamformed to show the distribution of waterborne and bottom interacting signals. By correlating the (complex) beams containing the bottom interacting signals with the equivalently formed beams for the replica field (matched beam processing), one evaluates the ambiguity function maximum at the source location and searches for the bottom sound-speed profile which yields the highest correlation for the matched beam processing. Different frequency bands of the signal are investigated to evaluate the sensitivity to bottom sound-speed variations. The inverted sound-speed profiles are found consistent (within the resolution of 50–100 m/s) with the sound-speed profiles estimated from (1) core samples and other geoacoustic data at the site, and (2) the arrival time of the Head waves [J. Wolf, J. Acoust. Soc. Am. 94, 1769 (1993)].