Performance analysis of matched field processing localization with various line array configurations based on normal mode decomposition

This In order to reduce the difficulty of the practical application of the time reversal method (TRM), this paper proposed a new method of virtual TRM with a short vertical line array. The measurement field received by the short array is used to reconstruct an acoustic field of a virtual long vertical array. The method is mainly divided into three steps: Initially, using the modal decomposition to decompose the sound field received by the short vertical array into a modal function matrix and a modal coefficient vector. Furthermore, we obtain the estimated values of the modal coefficients of each order. In this paper, an iterative algorithm was used to obtain the modal coefficients of each order, and then the measured field of the long vertical line array is extended through the obtained modal coefficients. Finally, we use the extended vertical line array measurement field for time reversal processing. The results from the final computer simulation indicated that, due to the measurement field of the long vertical line after continuation contains more environmental information, the virtual time reverse processing method with the extended measurement field has superior focus positioning performance.

During the New Jersey Shelf RAGS03 experiment, acoustic signals emitted by ships of opportunity (merchant ships) are simultaneously recorded on three vertical line arrays (VLAs) and a horizontal line array (HLA). Recorded broadband (50–750 Hz) acoustic data sets are used to demonstrate the source localization capability as well as geoacoustic inversion capability. Waveguide invariant theory applied to beamforming by two vertical arrays that provided a range ratio of the source to the receivers. Beamforming by a horizontal array provided time‐evolving spectrum for a particular look direction (LOFARgram). The trajectories of the striations observed both in vertical array beamformer output and LOFARgrams are used to estimate the source speed, range, and azimuthal direction as well as bottom geoacoustic parameters. Trajectories of striations are identified by the Hough transform that converts a difficult global detection problem in the image domain into a simpler local peak detection problem. Analysis of both...

Source depth estimation is always a problem in underwater acoustic area, because depth estimation is a nonlinear problem. Traditional depth estimation methods use a vertical line array, which has disadvantage of poor mobility due to the size of sensor array. In order to estimate source depth with a horizontal line array, we propose a matched-mode depth estimation method based on autoregressive (AR) wavenumber estimation for a moving source in shallow water waveguides. First, we estimate the mode wavenumbers using the improved AR modal wavenumber spectrum. Second, according to the mode wavenumber estimation, we estimate the mode amplitudes by the wavenumber spectrum, which is obtained by generalized Hankel transform. Finally, we estimate source depth estimation by the peak of source depth function wherein the data mode best matches the replica mode that is calculated using a propagation model. Compared with synthetic aperture beamforming, the proposed method exhibits a better performance in source depth estimation under low signal-to-noise ratio or the small range span. The robustness of the proposed method is illustrated by simulating the performance in mismatched environment.

During the Target and Reverberation Experiment in 2013, a bottom-mounted, mid-frequency source and horizontal line array were deployed on the seafloor to measure reverberation over the course of the experiment. Extending from the source and parallel to shore was the “main reverberation track,” a 7-km long and 1.5 km wide region where a majority of the acoustic and environmental measurements were made. Along this track, two vertical line arrays were deployed to continuously collect propagation data. The combination of the HLA and VLAs made it possible to make contemporaneous measurements of both transmission loss (TL) and reverberation level (RL) over a 3-week timespan. During this time, a storm passed through the site and the variability in the TL is found to be driven by different mechanisms. Prior to the storm, the variability was primarily driven by changes in the sound speed profile; after the storm, the water was well-mixed and the variability was a steady decay of TL while the rough sea surface decayed. This talk examines the variability in TL during these time periods and the associated changes in the measured RL. [Work supported by ONR.]

Several studies have been directed towards the development of both numerical and analytical expressions of ambient noise directionality. In this paper, the results of a simplified ambient noise model are employed to examine the response of sonar arrays to directional noise fields. Expressions are found for the array gain against wind noise for near-surface arrays, employing vertical and horizontal line arrays as examples. The results are compared in each case with array gain against isotropic noise (i.e., directivity index). Array response is found as a function of array orientation, frequency (or size), and steering angle. For the broadside steered line array, a simplified beam response pattern is used to find array gain for these simple analytical noise models. Results compare very well with array gains computed using more precise models of the ambient noise field and exact array response patterns. The implications on sonar design and performance studies are discussed, as well as similar estimates for the response of more complex array configurations. [Study supported by NAVSHIPS, PMS 302-4.]

A method is developed in this paper to calculate the spatial gain of a vertical line array when the plane-wave assumption is not applicable and when the oceanic ambient noise is correlated. The proposed optimal array gain (OAG), which can evaluate the array’s performance and effectively guide its deployment, can be given by an equation in which the noise gain (NG) is subtracted from the signal gain (SG); hence, a high SG and a negative NG can enhance the performance of the array. OAGs and SGs with different array locations are simulated and analyzed based on the sound propagation properties of the direct-arrival zone (DAZ) and the reliable acoustic path (RAP) using ray theory. SG and NG are related to the correlation coefficients of the signals and noise, respectively, and the vertical correlation is determined by the structures of the multipath arrivals. The SG in the DAZ is always high because there is little difference between the multipath waves, while the SG in the RAP changes with the source-receiver range because of the variety of structure in the multiple arrivals. The SG under different conditions is simulated in this work. The “dual peak” structure can often be observed in the vertical directionality pattern of the noise because of the presence of bottom reflection and deep sound channel. When the directions of the signal and noise are close, the conventional beamformer will enhance the correlation of not only the signals but also the noise; thus, the directivity of the signals and noise are analyzed. Under the condition of having a typical sound speed profile, the OAG in some areas of the DAZ and RAP can achieve high values and even exceed the ideal gain of horizontal line array 10 logN dB, while, in some other areas, it will be lowered because of the influence of the NG. The proposed method of gain analysis can provide analysis methods for vertical arrays in the deep ocean under many conditions with references. The theory and simulation are tested by experimental data.

Large, transient increases in underwater noise intensity (noise bursts) were repeatedly observed during the ASIAEX experiment on a 48-hydrophone array consisting of vertical and horizontal line arrays. The arrays were located in the South China Sea in the area known for exceptionally strong nonlinear internal waves (NIWs). The ASIAEX experiment was intended to study sound propagation in the presence of NIWs and featured extensive, concurrent acoustic and oceanographic observations. The NIWs propagating past the acoustic array site were characterized using water temperature measurements with a nearby thermistor chain. Remarkable correlation is found between noise intensity increases on the vertical array hydrophones and the NIW presence. Intensity of the noise bursts strongly depends on the hydrophone depth. The low acoustic frequencies below 30–40 Hz are primarily responsible for the noise enhancement during the NIW passage. Analysis of the spectral properties and the depth dependence of the noise intensity suggests the flow noise due to NIW-induced currents as the physical mechanism of the noise bursts in the South China Sea. [Work was supported, part, by ISF award 946/20.]

Shallow water bottom properties can be very difficult to estimate and are usually done so by means of vertical line arrays over a range of frequencies. This work will examine the behavior of the minimization broadband processor at low frequencies (25–100 Hz) for a variety of horizontal line arrays at endfire in simulated Shallow Water 2006 environments. Do we ever have enough sensitivity and resolution to estimate any of the bottom properties when using a horizontal array?

Low frequency volume scatter and distant reverberation measurements have been made in the Icelandic Basin, Gulf of Alaska and the Ionian Sea. Volume scattering data showing effects of fish swimbladder resonance along with spatial and temporal dependencies have been obtained as a function of depth to 600 m and frequency from 200–1500 Hz using explosives and a vertical line array receiver. Distant reverberation results, obtained using controlled waveforms from a vertical line array of projectors and a horizontal line array receiver (i.e., a near-monostatic configuration), also show effects of biological scattering. Both ray and mode based distant reverberation model predictions, utilizing in situ volume scatter data, agree well with measurements.

Compared with electromagnetic wave, acoustic wave has great advantages in underwater sensing, which attracts great attention on how it propagates in underwater environment. A variety of types of underwater acoustic propagation models (APMs) have been developed, where the closest to actual underwater acoustic propagation should be selected as in most sensing cases, accurate APM are needed, like matched field processing, geoacoustic inversion, etc. However, selecting by experience is not reliable enough and matching the measured field with the modeled one is time-consuming. In this paper, a convolutional neural network (CNN) classifier is proposed to select appropriate underwater acoustic propagation model (APM) according to sound field collected by a horizontal line array (HLA). The simulated data are generated by five different APMs and the classification accuracy on testing set reaches 95.3%. The correlation and difference between the performance of this CNN-based classifier and the correlation coefficient (CC) of the classified data are discussed, which demonstrate that the CNN-based classifier outperforms the correlation-based classifier. We also validate the performance of the CNN classifier by the data measured in shallow water environment.

In this paper the bias table models for the wafer scale SmartNIL™ technology are addressed and validated using complete Scanning Electron Microscopy (SEM) characterizations and polynomial interpolation functions. Like the other nanoimprint lithography (NIL) technics, this replication technology is known to induce Critical Dimension (CD) variations between the master and the imprint, due to polymer shrinkage, soft stamp deformation or thermal expansion. The bias between the former and final object follows peculiar rules which are specific to this process. To emphasis these singularities, Critical Dimension (CD) uniformity analyses were analyzed onto 200 mm wafers imprinted with the HERCULES® NIL equipment platform. Dedicated masters were manufactured to capture the process signatures: horizontal and vertical line arrays, local densities ranging from 0.1 to 0.9 and minimum CD of 250 nm. The silicon masters were manufactured with 248 optical lithography and dry etching and treated with an anti-sticking layer from Arkema. CD measurements were made for the master and the replicates on 48 well selected features to build interpolations. The bias table, modelled by polynomial functions with a degree of 5 for the density and a degree of 3 for the CD, are compared between horizontal and vertical features, and between the center and the edge of the wafers. Finally the focus is made on the validation of the interpolations by comparing the computed bias and the experimental data.

Version III of the research ambient noise directionality (RANDI‐III) noise model is described and illustrated. The objective of the model is to make predictions of the ambient noise at low to mid frequencies (10 to several hundred Hz) for highly variable environmental conditions typical of shallow water and coastal areas. The receiver can be either a horizontal or vertical line array. Shipping and local wind are assumed to be the dominant sources of noise. Ships are treated as discrete sources of noise. The ship noise is propagated to the elements of the receiver using a wide‐angle finite element parabolic equation. The contributions from all ships are added coherently. The environmental and shipping information are automatically extracted from several databases. [Research supported by The Office of Naval Research, Program Element 62435N with technical management provided by the Naval Research Laboratory, Stennis Space Center, MS.]

Results from two field experiments aimed at investigating the detection and classification of buried targets are presented. In both experiments a 2-16-kHz parametric source was used. In the first experiment, the source was used in combination with a 12-m horizontal line array and in the second with a 1.4-m vertical line array which was displaced horizontally along an underwater rail to form a 10 m /spl times/ 1.4 m two-dimensional synthetic aperture sonar (SAS). To increase the SAS integration time, the parametric source was electronically scanned in azimuth during the displacement along the rail, as in spotlight mode. It is shown that both arrays allow important signal-to-reverberation gains, enhancing the detection of sub-bottom echoes. A new, environmentally adaptive, matched filter which further improves the signal to reverberation ratio while allowing discrimination between proud and buried targets is presented and validated experimentally. The use of resonant scattering for target classification of buried objects is discussed, in the particular case of spherical shells.

Matched field processing (MFP) is frequently used to localize underwater sources, in range and depth, using vertical arrays recording ultra low frequency waves (/spl les/100Hz). In this paper, we use matched-field techniques, guided waves propagation and signal processing tools to estimate source depth in a shallow water environment with a horizontal array. MFP is performed in the frequency-wavenumber domain: frequency-wavenumber transform (f-k) is the square modulus of the 2D Fourier transform in time and radial distance. This f-k representation provides much information on guided propagation of Ultra Low Frequency waves in shallow water: on this representation, modes can be separate. Besides, modes excitation depends on the source depth. As a result, it is possible to estimate the source depth using MFP based on modes excitation factors.

Summary Microseismic data acquired during hydraulic stimulations are a key tool in determining well spacing, stimulation parameters, frac barriers, and natural and hydraulic fracturing relationships. In addition to hypocenter locations of the fracturing mechanism, attributes associated with the microseismic event including magnitude and P-S amplitude ratios provide valuable insight into the stimulation. These attributes can be used as weights in the calculation of microseismic derived stimulated rock volumes to determine effective draining areas. High velocity layers associated with carbonate debris flows are known to distort the seismic wave field and may have an effect on associated microseismic derived attributes. In this study we use a 3D elastic finite difference algorithm to model double couple and open tensile source mechanisms at different levels in the Wolfcamp shale of West Texas to determine the sensitivity of microseismic attributes and location to the geology and array configuration. Magnitude, P-S ratio, and event location from a horizontal and vertical array are examined. 3D snapshot movies of the different mechanisms show the complexity of the developing wave field and the location of nodal planes which could interfere with the position or observation of microseismic events. The modeling showed that attributes derived from a vertical array are more robust than attributes from a horizontal array. Attributes from events that occurred near high velocity layers such as carbonate debris flows may be less accurate than events that occurred in more consistent lower velocity layers. Event location and microseismic event attributes recorded from a vertical array are more accurate than events recorded from a horizontal array.