Target And Reverberation Experiment 2013 Research Articles

Validation and verification of three-dimensional finite element target scattering models and Green's functions

A configurable three-dimensional (3D) finite element framework is used for modeling acoustic scattering from various elastic targets on seabed and for the numerical determination of Green's functions used in far-field target scattering prediction. The 3D selection is chosen for possible inclusion of complex seafloor geometries, such as rough, rippled, cluttered, or variably layered seabeds, in which two-dimensional modeling approaches cannot capture the full acoustic interaction with the target and the seabed. Model results are verified and validated against analytic scattering theories and measured results from the 2013 Target and Reverberation Experiment (TREX) and the 2017 CLUTTEREX experiment.

Underwater Unexploded Ordnance (UXO) Classification Using a Matched Subspace Classifier With Adaptive Dictionaries

This paper is concerned with the development of a system for the discrimination of military munitions and unexploded ordnance (UXO) in shallow underwater environments. Acoustic color features corresponding to calibrated target strength as a function of frequency and look angle are generated from the raw sonar returns for munition characterization. A matched subspace classifier (MSC) is designed to discriminate between different classes of detected contacts based upon the spectral content of the sonar backscatter. The system is exclusively trained using model-generated sonar data and then tested using the measured Target and Reverberation Experiment 2013 (TREX13) data sets collected from a synthetic aperture sonar system in a relatively low-clutter environment. A new in situ supervised learning method is also developed to incrementally train the MSC using a limited number of labeled samples drawn from the TREX13 data sets. The classification results of the MSC are presented using standard performance metrics, such as receiver operating characteristic curve and confusion matrices.

In situ observation of sediment sound speed and attenuation from coarse to fine-grained sediments

The long-term objective is to address the impact of spatial variability of sediment properties on sound propagation and reverberation from a few hundred hertz to 10 kHz. In this band, the frequency dependences of sound speed and attenuation are important for applications, but cannot be inferred from high-frequency data, hence direct measurement is necessary. In this talk, a summary of in situ measurements of sediment sound speed and attenuation collected from three major field experiments, i.e., Shallow Water 2006 (SW06), Target and Reverberation Experiment 2013 (TREX13), and Seabed Characterization Experiment 2017 (SCE17), are presented. Sediment sound speed and attenuation within the surficial 3 m of sediments were obtained through penetration, using the Sediment Acoustic-speed Measurement System (SAMS). In TREX13, i n situ measurements were carried out at five sites along a 5-km track, with sediment types ranging from coarse sand to a mixture of soft mud over sand. SAMS was deployed at 18 sites within the 10 × 30 km SCE17 study area. Significant variation of sediment geoacoustic properties was observed in range (TREX13) and depth (SCE17), respectively. Preliminary sediment acoustic modeling work is presented, using SAMS data taken from the wide range of sediment types. [Work supported by ONR.]The long-term objective is to address the impact of spatial variability of sediment properties on sound propagation and reverberation from a few hundred hertz to 10 kHz. In this band, the frequency dependences of sound speed and attenuation are important for applications, but cannot be inferred from high-frequency data, hence direct measurement is necessary. In this talk, a summary of in situ measurements of sediment sound speed and attenuation collected from three major field experiments, i.e., Shallow Water 2006 (SW06), Target and Reverberation Experiment 2013 (TREX13), and Seabed Characterization Experiment 2017 (SCE17), are presented. Sediment sound speed and attenuation within the surficial 3 m of sediments were obtained through penetration, using the Sediment Acoustic-speed Measurement System (SAMS). In TREX13, i n situ measurements were carried out at five sites along a 5-km track, with sediment types ranging from coarse sand to a mixture of soft mud over sand. SAMS was deployed at 18 sites within ...

Overview of Midfrequency Reverberation Data Acquired During the Target and Reverberation Experiment 2013

The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2–10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, wind-generated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface conditions.

Characterizing Transmission Loss Variability During the Target and Reverberation Experiment 2013

A significant driver of uncertainty in sonar performance is the variability in underwater acoustical propagation caused by environmental fluctuations and uncertainty in the position of sources, targets, and receivers. A set of echo-repeat experiments was conducted during the Target and Reverberation Experiment 2013 (TREX13), a sea trial that took place in April to May 2013 in the Gulf of Mexico near Panama City, FL, USA. The variability in measured transmission loss (TL) was characterized using two different methods: Variability with respect to a mean observed TL, and variability with respect to modeled TL. Both one-way and quasi-reciprocal two-way TL measurements at 2250 and 7500 Hz were analyzed to characterize the variability at timescales ranging from less than one second to several days, with the results indicating that the acoustic propagation fluctuates stochastically on all these time scales. The results of statistical tests suggest that the TL variability can be treated as Gaussian fluctuations about a central TL obtained from an acoustic propagation model, with standard deviations of 5 dB over timescales up to one day, or 10 dB over timescales from one to six days.

Direct Measurements of Sediment Sound Speed and Attenuation in the Frequency Band of 2–8 kHz at the Target and Reverberation Experiment Site

The sediment acoustic-speed measurement system is designed to measure in situ sediment sound speed and attenuation within the surficial 3 m of sediments in the frequency band of 2–8 kHz. Measurements were carried out during the Target and Reverberation EXperiment 2013 (TREX13) off Panama City, FL, USA. During TREX13, nine deployments at five selected sites were made along the 20-m isobath, termed the main reverberation track. The sediment types at the five selected sites ranged from coarse sand to a mixture of soft mud over sand, and the measured results show a spread of 80 m/s in sediment sound speed among the different types of sediments for all frequencies. Between 2–8 kHz, about 3% dispersion was observed at the sandy sites, whereas little dispersion was observed at the sites with mud. Preliminary attenuation results show 0.5–3.3 dB/m at the sandy sites, and 0.5–1.0 dB/m at the sites with mud in the same frequency band.

Guest Editorial Target and Reverberation Experiment 2013 (TREX13)—Part II

Guest Editorial Target and Reverberation Experiment 2013 (TREX13)—Part II

Guest Editorial Target and Reverberation Experiment 2013 (TREX13)—Part I

The articles in this special section focus on research and development related to the Target and Reverberation Experiment (TREX13) conducted in the Gulf of Mexico off Panama City, FL, USA, in spring 2013. The primary goal of the experiment was to measure midfrequency (2–10 kHz), shallow-water reverberation with full companion environmental measurements such that models and data could be compared without ambiguity. While a number of other major field efforts, such as the Acoustic Reverberation Special Research Program and the Acoustic Clutter Reconnaissance Experiment, have been conducted to study low- to midfrequency reverberation, it has been difficult to measure and account for all of the relevant reverberation mechanisms in a single experiment.

Extracting elastic target information from acoustic scattering data of underwater munitions

Synthetic Aperture Sonar (SAS) acoustic scattering data from a variety of underwater munitions was collected in the Target and Reverberation Experiment 2013 (TREX13), Bay Experiment 2014 (BayEX14) and from the Acoustic Test Facility at Naval Surface Warfare Center Panama City Division. One data product from these tests is two dimensional templates of back scattered acoustic intensity as a function of aspect angle and frequency. A common method of target classification utilizes normalized template cross-correlations as the input into classification algorithms. This is typically done in a manner agnostic to observable physics-based scattering phenomena derived from the physical size, material composition, and geometry of the targets that might help improve classification performance. This presentation discusses methods explored to process the available data to make target elastic phenomena easier to recognize so that associated information can be extracted and used as target classifier input. The methods ex...

Mid-frequency clutter and reverberation characteristics of fish in a shallow ocean waveguide

Horizontal-looking, mid-frequency sonar systems allow synoptic underwater observation over kilometer scales and are useful for a wide range of applications. However, quantitative assessment of fish aggregations using such systems in a shallow water environment is challenging due to the complexity of sound interactions with the surface and seafloor. Based on data collected during TREX13 (Target and Reverberation Experiment 2013), this study investigates methods to reliably distinguish fish echoes from background contributions to reverberation. The data were collected on a fixed, horizontal receiving line array from a source transmitting linear frequency-modulated signals in the band between 1.8 and 3.6 kHz. The experimental site was nearly isodepth (approximately 20 m) and characterized by a well-mixed, isothermal water column. Fish echoes were ubiquitously found in the data, with noticeable differences between day and night. In a particularly interesting case, a large aggregation of fish was observed emerging from a shipwreck, evolving in space, obscuring the wreck echoes temporarily, and eventually dispersing and disappearing into the overall reverberation background. Using a physics-based approach, the scattering level, statistical features, and spatial characteristics of both the fish echoes and background reverberation in this data set were analyzed and modeled. [Work supported by the APL postdoctoral fellowship and ONR.]

Testing the feasibility of a concurrent comparison of continuous and pulsed active sonar

The performance of continuous active sonar (CAS) was compared to conventional pulsed active sonar (PAS) during the TREX13 (Target and Reverberation Experiment 2013) sea trial. The approach was to execute a one-hour CAS run followed closely by a one-hour PAS run to limit the environmental variability between runs and allow a fair comparison. This approach was possible because the sonar source and receiver were fixed to the seabed. A different approach was required for a more recent sea trial, LCAS15 (Littoral Continuous Active Sonar 2015), where the source and receiver were towed from a ship. Ship motion increases variability in sonar performance, therefore a simultaneous comparison of CAS and PAS was desired so that any motion effects were the same for both waveforms. The approach of transmitting CAS and PAS concurrently in two separate frequency bands was taken. The risk with this approach is that potential differences in propagation in the two bands could bias the comparison. A run in which equivalent CAS waveforms were transmitted simultaneously in the two bands was therefore performed to establish a baseline for the performance difference between the bands. Doppler-corrected replicas were required in all of the correlation processing due to the high Doppler sensitivity of the long-duration, wideband CAS waveforms. Preliminary results from the LCAS15 sea trial will be presented.

SONAR Equation perspective on TREX13 measurements

Modeling shallow water reverberation is a problem that can be approximated as two-way propagation (including multiple forward scatter) and a single backward scatter. This can be effectively expressed in terms of the SONAR equation: RL = SL-2 x TL + SS, where RL is reverberation level, SL is the source level, TL is the one way transmission loss, and SS is the integrated scattering strength. In order to understand the reverberation problem at the basic research level, both propagation and scattering physics need to be properly addressed. A major goal of TREX13 (Target and Reverberation EXperiment 2013) is to quantitatively investigate reverberation with sufficient environmental measurement to support full modeling of reverberation data. Along a particular reverberation track at the TREX13 site, TL and direct-path backscatter were separately measured. Environmental data were extensively collected along this track. This talk will bring together all the components of the SONAR equation measured separately at the TREX13 site to provide an assessment of the reverberation process along with environmental factors impacting each of the components.

Propagation measurement using source tow and moored vertical line arrays during TREX13

The objective of TREX13 (Target and Reverberation EXperiment 2013) is to investigate shallow water reverberation by concurrently measuring propagation, local backscatter, and reverberation, as well as sufficient environmental parameters needed to achieve unambiguous model/data comparison. During TREX13 the Marine Physical Laboratory (MPL) conducted propagation and forward scatter measurements. The MPL effort during TREX13 included deploying three, 32-element (0.2 m element spacing), vertical line arrays along the Main Reverberation Track at a bearing of ~128° and ranges ~2.4 km, ~4.2 km, and ~0.5 km from the R/V Sharp, where reverberation measurements were being made. In addition, MPL carried out repeated source tows in the band of 2–9 kHz along the Main Reverberation Track, using tonal and LFM waveforms. The experimental procedure is described and the resulting source-tow data is examined in the context of Transmission Loss and its implications for reverberation.

Overview of reverberation measurements in Target and Reverberation Experiment 2013

The Target and REverberation EXperiment 2013 (TREX13) was carried out off the coast of Panama City, Florida, from 22 April to 16 May, 2013. Two fixed-source/fixed-receiver acoustic systems were used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Beamformed RL data are categorized to facilitate studies emphasizing (1) bottom reverberation; (2) sea surface impact; (3) biological impact; and (4) target echo. This presentation is an overview of RL over the entire experiment, summarizing major observations and providing a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis will be placed on the dependence of RL on local geoacoustic properties and sea surface conditions. [Work supported by ONR.]

Transmission loss and reverberation variability during TREX13

The ONR-funded Target and Reverberation Experiment 2013 (TREX13) took place in the Northeastern Gulf of Mexico near Panama City, Florida, during April and May of 2013. During this trial, which took place in a shallow water (20 m deep) environment, several sets of one-way and two-way acoustic transmission loss and reverberation data were acquired. Closed form expressions are derived to trace the uncertainty in the inputs to a Gaussian beam propagation model through the model to obtain an estimate of the uncertainty in the output, both for transmission loss and for reverberation. The measured variability of the TREX environment is used to compute an estimate of the expected transmission loss and reverberation variability. These estimates are then compared to the measured acoustic data from the trial.

Persistence of sharp acoustic backscatter transitions observed in repeat 400 kHz multibeam echosounder surveys offshore Panama City, Florida, over 1 and 24 months

The Target and Reverberation Experiment 2013 (TREX13), conducted offshore Panama City, FL, from April to June 2013, sought to determine which environmental parameters contribute the most to acoustic reverberation and control sonar performance prediction modeling for acoustic frequencies between 1 kHz and 10 kHz. In that context, a multibeam echosounder operated at 400 kHz was used to map the seafloor relief and its high-frequency acoustic backscatter characteristics along the acoustic propagation path of the reverberation experiment. Repeat surveys were conducted a month apart, before and after the main reverberation experiment. In addition, repeat surveys were conducted at 200 kHz in April 2014. Similar mapping work was also conducted in April 2011 during a pilot experiment (GulfEx11) near the site chosen for TREX13. Both experiments revealed a persistent occurrence of sharp transitions from high to low acoustic backscatter at the bottom of swales. Hypotheses are presented for observable differences in bathymetry and acoustic backscatter in the overlap region between the GulfEx11 survey and the TREX13 surveys conducted 2 y apart. [Work supported by ONR 322 OA.]

Environmental measurements collected during TREX13 to support acoustic modeling

The major goal of TREX13 (Target and Reverberation EXperiment 2013) was to quantitatively investigate reverberation with sufficient environmental measurements to support full modeling of reverberation data. The collection of environmental data to support reverberation modeling is usually limited by the large ranges (10s of km) involved, the temporal and spatial variability of the environment and the time variation of towed source/receiver locations within this environment. In order to overcome these difficulties, TREX13 was carried out in a 20 m deep shelf environment using horizontal line arrays mounted on the seafloor. The water depth and well controlled array geometry allowed environmental characterization to be focused on the main beam of the array, i.e., along a track roughly 5 km long and 500 m wide. This talk presents an overview of the efforts made to characterize the sea surface, water column, seafloor, and subbottom along this track to support the modeling of acoustic data collected over the course of the experiment. [Work supported by ONR Ocean Acoustics.]

Effect of channel impulse response on matched filter performance in the 2013 Target and Reverberation Experiment

Active sonar performance is determined by the characteristics of the target, the sonar system and the effect of the environment on the received waveform. The two main influences of the environment are propagation effects and the contamination of the target echo with a background. The ambient noise and reverberation are mitigated by means of signal processing, mostly through beamforming and matched-filtering. The improvement can be quantified by the signal to noise ratios before and after processing. Propagation effects can have a large influence on the gains obtained by the processing. To study the effect of the channel on the matched filter performance, broadband channel impulse responses were modeled and compared to measurements acquired during the Office of Naval Research-funded 2013 Target and Reverberation Experiment (TREX). In shallow water, a large time spread is often observed, reducing the effectiveness of the matched filter. TREX data show, however, a limited time spread. Model predictions indicate that this could be caused by a rough sea-surface, which while increasing propagation loss, at the same time increases matched filter gain.

Using physical oceanography to improve transmission loss calculations in undersampled environments

The vertical sound speed profile (SSP) is a critical input to any acoustic propagation model. However, even when measured SSPs are available they are frequently noisy “snapshots” of the SSP at a single moment in time and space and do not fully capture changes such as solar heating and wind-driven mixing that can significantly affect shallow water propagation on time scales of less than a day. Furthermore, SSPs measured in the field may not extend to the ocean bottom and are often based on measured profiles of temperature with an implicit assumption of constant salinity. In April–May 2013, the Target and Reverberation Experiment (TREX) was conducted in the Northeastern Gulf of Mexico near Panama City, Florida, a region strongly affected by local wind forcing, freshwater inputs, and the presence of a warm-core Gulf of Mexico Loop Current eddy ("Eddy Kraken") offshore of the experimental site. “Synthetic” SSPs were constructed for the trial area by combining knowledge of the physical oceanography and water masses in the area with the measured SSPs that were available. Transmission loss was modelled using both synthetic and measured SSPs and the results will be compared with measured transmission loss.

Modeling reverberation in a complex environment with the finite element method

Acoustic finite element models solve the Helmholtz equation exactly and are customizable to the scale of the discretization of the environment. This makes them an ideal candidate for reverberation studies in complex environments. In this study, reverberation is calculated for a realistic shallow water waveguide. The environmental parameters are taken from the extensive characterization completed for the Target and Reverberation Experiment (TREX) conducted off the coast of the Florida panhandle in 2013. Measured sound speed profiles, sea surface roughness, bathymetry, and measured ocean bottom roughness are included in the model. Measurements of the normal incidence bottom loss are used as a proxy for range dependent sediment density. Results are compared with a closed form solution for reverberation. [Work sponsored by ONR, Ocean Acoustics.]