Geoacoustic Inversion Results Research Articles

Transdimensional Inversion on the New England Mud Patch Using High-Order Modes

This article presents geoacoustic inversion results for modal-dispersion data collected during the 2017 Seabed Characterization Experiment on the New England Mud Patch, an area where the seabed is characterized by an upper layer of mud. The experiment utilized a combustive sound source and a vertical line array of receivers at 5.4-km range. Using a careful combination of source deconvolution and warping time–frequency analysis, modal dispersion data (arrival time as a function of frequency) are estimated for 18 modes between modes 1 and 21. The modal dispersion data are then used to estimate seabed geoacoustic profiles and uncertainties via transdimensional Bayesian inversion. This article demonstrates the capacity to estimate high-order modes using warping. Comparing inversion results obtained with subsets of (lower order) modes to those obtained with the full set of available modes highlights the rich data information content carried by high-order modes. The results suggest a small sound-speed increase over the first 8 m of the seabed, the upper portion of the mud layer, which some earlier studies found to be isospeed. Overall, the inversion results are consistent with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurements, as well as with previous geoacoustic inversion results.

Modal Mapping Experiment and Geoacoustic Inversion Using Sonobuoys

This paper summarizes the results of an experiment whose primary goal was to demonstrate that reliable geoacoustic inversion results can be obtained in shallow water by postprocessing acoustic data acquired by Global Positioning System (GPS)-capable sonobuoys. The experiment was conducted aboard the R/V Sharp on March 5-18, 2011 off the coast of New Jersey using AN/SSQ-53F sonobuoys with a GPS capability as well as GPS-equipped research buoys originally developed under the Modal Mapping Experiment (MOMAX) project, which provided reliable geoacoustic information to which the sonobuoy results could be compared. It is shown that when low-frequency ( 500 Hz) continuous-wave (CW) signals are acquired on the two types of buoys in a colocated configuration, the geoacoustic models inferred from the sonobuoy data are very similar to those obtained from the MOMAX buoy data. The inversion results also compare favorably with bottom models for the region obtained from other experiments. This work is an important milestone toward achieving the ultimate goal of transitioning a basic research method to an operational scenario in which sonobuoy data are routinely used to infer geoacoustic parameters of the seabed.

Model-based seafloor characterization employing multi-beam angular backscatter data—A comparative study with dual-frequency single beam

Sediment geoacoustic inversion results are estimated employing a multi-beam (MB) echo-sounding system operable at 95 kHz. To characterize the western continental shelf of India (off Goa) seafloor, MB backscatter signals were acquired along with grab sediment samples. The substrate type and roughness of the site were estimated using the composite roughness scattering model with the measured backscatter values. The seafloor parameters, namely mean grain size (M(φ)); roughness spectrum strength (w(2)) and exponent (γ(2)); and sediment volume parameter (σ(2)), for coarse and fine grain sediments are estimated by employing the MB system. These parameters have also been estimated at two other frequencies (33 and 210 kHz) and are compared to the ground truth data to provide sufficient support in validating the model results and increasing the understanding of the shelf seafloor processes. Distinct interclass separations between the sediment provinces are evident from the estimated mean grain size M(φ) and water-sediment interface roughness w(2). The seafloor parameters for coarse and fine grain sediments derived from the 95 kHz MB data are consistent with the sediment sample data as well as with the inversion results obtained using backscatter data at 33 and 210 kHz from the same locations.

The impact of water column variability on horizontal wave number estimation and mode based geoacoustic inversion results

The influence of water column variability on low-frequency, shallow water geoacoustic inversion results is considered. The data are estimates of modal eigenvalues obtained from measurements of a point source acoustic field using a horizontal aperture array in the water column. The inversion algorithm is based on perturbations to a background waveguide model with seabed properties consistent with the measured eigenvalues. Water column properties in the background model are assumed to be known, as would be obtained from conductivity, temperature, and depth measurements. The scope of this work in addressing the impact of water column variability on inversion is twofold. Range-dependent propagation effects as they pertain to eigenvalue estimation are first considered. It is shown that mode coupling is important even for weak internal waves and can enhance modal eigenvalue estimates. Second, the effect of the choice of background sound speed profile in the water column is considered for its impact on the estimated bottom acoustic properties. It is shown that a range-averaged sound velocity profile yields the best geoacoustic parameter estimates.

Is higher resolution bathymetry always better for undersea sound propagation modeling?

Bathymetry is one of the most important environmental parameters affecting undersea sound transmission in shallow waters. There has always been a preference towards bathymetry with the highest resolution and accuracy when conducting sound propagation modeling. However, extreme care must be exercised in the selection of the correct bathymetric resolution. It will be demonstrated that the correct resolution depends on the source of the geoacoustic model used. Higher resolution bathymetrics, such as multibeam surveys, introduce steeper slopes that yield larger bottom losses. If a lower resolution bathymetry was implemented to conduct geoacoustic inversion, then the same bathymetric resolution must be used to conduct subsequent sound transmission modeling. In addition, when geoacoustic inversion results are reported, the bathymetric resolution used should also be reported.

Statistical estimation of transmission loss from geoacoustic inversion using a towed array

Geoacoustic inversion estimates environmental parameters from measured acoustic fields (e.g., received on a towed array). The inversion results have some uncertainty due to noise in the data and modeling errors. Based on the posterior probability density of environmental parameters obtained from inversion, a statistical estimation of transmission loss (TL) can be performed and a credibility level envelope or uncertainty band for the TL generated. This uncertainty band accounts for the inherent variability of the environment not usually contained in sonar performance prediction model inputs. The approach follows [Gerstoft et al. IEEE J. Ocean. Eng. 31, 299-307 (2006)] and is demonstrated with data obtained from the MAPEX2000 experiment conducted by the NATO Undersea Research Center using a towed array and a moored source in the Mediterranean Sea in November 2000. Based on the geoacoustic inversion results, the TL and its variability are estimated and compared with the measured TL.

Validation of statistical estimation of transmission loss in the presence of geoacoustic inversion uncertainty

Often the ocean acoustic environment is not well known and sonar performance prediction will be affected by this uncertainty. Here, a method for estimating transmission loss (TL) is proposed which incorporates these environmental uncertainties. Specifically, we derive an approach for the statistical estimation of TL based on the posterior probability density of environmental parameters obtained from the geoacoustic inversion process. First, a Markov chain Monte Carlo procedure is employed in the inversion process to sample the posterior probability density of the geoacoustic parameters. Then, these sampled parameters are mapped to the transmission loss domain where a full multidimensional probability distribution of TL as a function of range and depth is obtained. In addition, TL is also characterized by its summary statistics including the median, percentiles, and correlation coefficients. The approach is illustrated using a data set obtained from the ASIAEX 2001 East China Sea experiment. Based on the geoacoustic inversion results, the predicted TL and its variability are estimated and then compared with the measured TL. In general, there is a good agreement with the percentage of observed number of data points inside the credibility interval.

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.

Localization and geoacoustic inversion by using 20-Hz signals of finback whales on the New Jersey Shelf

One-second duration 20-Hz signals of finback whales were recorded on three vertical line arrays (VLAs) and a bottomed horizontal line array (HLA) during the RAGS03 experiment conducted on the New Jersey Shelf. Modal analysis indicated that most signals, recorded over a 3-week period in December of 2003, contained a weak, early-arriving mode-2 and strong, late-arriving mode-1 component. These modal characteristics are exploited in a broadband matched-field algorithm to localize individual finback whales and to perform geoacoustic inversions. Bearing estimation of individual whales is obtained by performing horizontal beamforming on the HLA data. Range estimation results are verified by time-of-flight triangulation using single hydrophone data from each VLA location. Geoacoustic inversion results were comparable to those previously obtained from chirp sonar data. Localization results indicated that most finback whales traveled near the shelfbreak front where food might be abundant. [This work was supported by the ONR.]

Validation of statistical estimation of transmission loss in the presence of geoacoustic inversion uncertainty

Often the ocean acoustic environment is not well known and sonar performance prediction will be affected by this uncertainty. Here, a method for characterizing transmission loss (TL) is proposed which incorporates these environmental uncertainties. The statistical estimation of TL based on the posterior probability density of environmental parameters obtained from the geoacoustic inversion process is the main theme of this study. First, a Markov chain Monte Carlo procedure is employed to sample the posterior probability density of the geoacoustic parameters. Then, these parameter uncertainties are mapped to the transmission loss domain, where a full multidimensional probability distribution of TL as a function of range and depth is obtained. In addition, TL is also characterized by its summary statistics including the median, percentiles, and correlation coefficients. The approach is illustrated using a data set obtained from the ASIAEX 2001 East China Sea experiment. Based on the geoacoustic inversion results, the predicted TL and its variability are estimated and then compared with the measured TL. In general, the results show that approximately 80% of the observed TL data fall within 90% of the TL probability distribution.

Multistep Matched-Field Inversion for Broad-Band Data From ASIAEX2001

This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data.

Near-real-time geoacoustic characterization during the BOUNDARY 2003 experiment

During the SACLANT Center BOUNDARY 2003 experiment on the Malta plateau, the rapid geoacoustic characterization (RGC) algorithm was applied. This algorithm uses the slope and spacing of shallow-water striations as well as the slope of the received level with range from a passing surface ship to estimate an effective geoacoustic sediment. By searching over a small set of representative sediments, using empirical relationships between grain size and compressional speed, density, and attenuation, the inversion can be carried out in near-real time. Ambient noise measurements of passing surface ships in the Malta plateau lead to geoacoustic inversion results obtained at sea that are comparable to previous work in the area where the inversions were obtained using vertical line arrays and controlled towed sources.

Geoacoustic inversion results of low- to high-frequency source tow data from the ASIAEX East China experiment

During the 2001 ASIAEX East China Sea experiment, source tow data was collected by a 16-element, 75-m aperture, autonomously recording vertical line array in approximately 105-m-deep water. Transmissions from two similar 6-km-long tracks were transmitted. In the first, CW tonals at 95, 195, 295, 395, 805, 850, and 905 Hz were transmitted from a J-15 transducer at a nominal depth of 46 m. In the second, CW tonals at 1.6, 2.4, 3.5, and 4.4 kHz were transmitted from an ITC-2015 transducer at a nominal depth of 49 m. An environmental model in the study area initially was derived by matched-field geoacoustic inversion using the low-frequency track. In this study, the high-frequency data from the second track are included in the parameter assessment. A multiple-stage geoacoustic inversion scheme is applied. Inversion results for seafloor geoacoustic parameters from these transmissions will be presented. [Work supported by ONR.]

Range-dependent geoacoustic inversion: results from the inversion techniques workshop

Model-based geoacoustic inversion in range-dependent underwater environments is a challenging task constrained by data quality (synthetic or measured) and propagation-model efficiency and accuracy. The Inversion Techniques Workshop (ITW), held in Gulfport, MS, May 15-18, 2001, was organized for the acoustics community to present state-of-the-art numerical geoacoustic inversion capabilities in range-dependent shallow-water environments. The organizers defined five range-dependent test cases (three synthetic and two experimental cases). Two of the synthetic cases were adopted for geoacoustic inversion in this paper. The first test case (TC1) is a monotonic down-slope bathymetry problem and the adiabatic normal-mode model PROSIM was applied for the inversion in this case. The second test case (TC3) is a flat-bottom case with an intrusion. The forward model used in this case was RAMGEO. The global optimization package SAGA was used for geoacoustic inversion of the synthetically generated reference solutions for TC1 and TC3. In general, the geoacoustic inversion results are in good agreement with the true solutions provided by the organizers. The results obtained demonstrate the feasibility of performing geoacoustic inversion in synthetic range-dependent shallow-water environments. However, results show that the propagation model choice in the inversion is strongly dependent on the specific range-dependent environment.

Geoacoustic inversion and source localization from multisource broadband HLA data

In underwater acoustics research, source localization efforts are often hampered by incorrect environmental information, and geoacoustic inversion results can be limited by errors in the source description. In addition, any data set can be contaminated by sound from other acoustic sources in the ocean. To overcome these problems, an iterative rotated coordinates inversion method [T. B. Neilsen, ‘‘An iterative implementation of rotated coordinates for inverse problems,’’ J. Acoust. Soc. Am. (submitted)] is employed in conjunction with a broadband bearing estimator [J. Krolik and D. Swingler, IEEE Trans. Acoust., Speech and Signal Process. 37, (1989)] and matrix filters [C. S. MacInnes, IEEE J. Ocean. Eng. (submitted)] to obtain estimates of the source locations and the sensitive geoacoustic parameters from multisource broadband data received on a horizontal line array (HLA). In the iterative inversion method, subsets of rotated coordinates are used to vary both the source and the sensitive environmental parameters in a series of simulated annealing inversions. The information contained in the rotated coordinates and corresponding eigenvalues determine which parameters are varied significantly in each simulated annealing inversion. [Work supported by ONR.]

MFP Benchmark Inversions via the RIGS Method

This paper discusses the geoacoustic inversion results obtained on the Workshop '97 benchmark data via the Refined Iterated Grid Search (RIGS) method. We find that the method can be highly accurate in estimating geoacoustic and source localization unknowns for the ten test scenarios attempted. The original method formulation using KRAKEN as the embedded propagation code produced the results presented at the workshop where we found that some parameters are extremely well determined, i.e. sediment attenuationα sed (within 1% in all 3 cases: ATa, ATb, ATc), sediment densityρ sed (within 5% in all ten subcases: SD, AT, SO, WAa), source ranger sou to within 40 m and source depthz sou to within 2 m (in all four subcases SO and WAa). Moreover, sediment sound-speed gradients γ were also well determined (within 8%) except for very thin sediments/subcase ATb where sediment thickness h sed ≈ 13 m). However, more recent, post-workshop results using ORCA embedded in the RIGS inversion software has resulted in even better results. Five of the original ten subcases have been re-done, and those results are also shown in this paper. In particular, we find that for the new results: r sou is now accurate to within 11 m, z sou to within 0.2 m, and all geoacoustic properties (except for half-space density ρ hsp ) are accurate to within 5%.