SACLANT Undersea Research Centre Research Articles

NURC/SACLANTCEN milestone experiments toward solving inverse problems in ocean acoustics

This paper reviews milestone experiments conducted by Saclant Undersea Research Centre to support the development and validation of techniques for obtaining and taking into account environmental information in sonar. The experiments brought together underwater acousticians, geophysicists and oceanographers with the aim to collect comprehensive acoustic and environmental ground-truth data. Environmental-adaptive signal processing was first demonstrated in a deep water area (WESTSARDINIA'89&90). From ducted propagation measurements, a model-based matched filter (MBMF) receiver that fully incorporates the physics of wave propagation determined a source range, depth and Doppler. For environmental inversion the broadband extension of matched-field processing was investigated in a shallow water area south of Elba island (YELLOWSHARK'94&95). Geoacoustic properties of the sea bottom were determined by finding the best fit between predicted and observed sound fields at multiple frequencies using genetic search algorithms. The same inversion results were obtained by MBMF processing of broadband linearly-frequency-modulated signals enabling the use of an array of a few hydrophones instead of a dense and large vertical array. This motivated the development of drifting acoustic buoys which were tested successfully over the southern continental shelf off Marettimo island, Sicily (ENVERSE'97&98). A recent experiment demonstrated an integrated concept of Rapid Environmental Assessment (MREA/BP'07) using sparse arrays of hydrophones and pressure/temperature sensors, hand-deployed from small vessels.

Bistatic reverberation benchmarking exercise: BiStaR versus analytic formulas

The newly developed BiStaR range-dependent bistatic reverberation model is benchmarked against analytic formulas for bistatic reverberation intensity in range-dependent environments. The BiStaR model is based on the C-SNAP range-dependent normal mode propagation model with a narrow-band approximation time domain extension and a coherent patch interaction model. The code is also capable of producing incoherent reverberation predictions that here are directly compared to the analytic formulas. The analytic formulas have been derived using ray-based intensity formulas for propagation in range-dependent waveguides and a generalized class of Lambert law type scattering kernels. Comparison between the BiStaR and the analytic predictions show good agreement for a variety of range-dependent bistatic scenarios. Such agreement generates confidence in the closed form formulas, which are particularly valuable for generating insight into the simple mechanisms which control the dominant characteristics of range-dependent bistatic reverberation, and in the BiStaR model, which has been developed to model bistatic reverberation in 3-D environments with high resolution environmental characterizations. [Work supported by ONR and NATO SACLANT Undersea Research Centre.]

Qualitative correlation of marine mammals with physical and biological parameters in the ligurian sea

In support of its acoustic risk mitigation policy, NATO SACLANT Undersea Research Centre (SACLANTCEN) is sponsoring a series of sea trials, entitled to collect a multiyear integrated oceanographic, biological, and hydrographic data set, the goal being to explain, based on these parameters, the distribution of marine mammals found in specific locations. By understanding how ocean dynamics affects the distribution and behavior of whales and the organisms forming the food web upon which the whales feed, it may be possible to conduct acoustic exercises in areas of low cetacean density. The first two Sirena multidisciplinary cruises were conducted in the Ligurian Sea in late summer time frame during 1999 and 2000. The focus of this analysis is to determine whether remotely sensed satellite data can indicate nutrient-rich regions in areas where the oceanography is known and to determine if these regions of higher productivity, coupled with knowledge of cetacean presence from all available sources, could be used as an indicator of marine mammal presence for acoustic risk mitigation purposes. For the two years of data examined, cooler sea-surface temperature data correlated with high levels of chlorophyll production as seen by remotely sensed images. This remotely sensed data correlated well with measured subsurface values of the same parameters. Coincident sightings of three species of marine mammals indicated that fin and sperm whales generally preferred the deep, nutrient-rich portion of the basin while Cuvier's beaked whales preferred a submarine canyon where there is a frontal influence, as indicated from satellite data and historical oceanography. This paper is intended as a contribution to the longer term objective of developing the means to accurately predict cetacean presence from physical oceanographic characteristics.

Bistatic reverberation modeling for range-dependent waveguides

The BiStaR bistatic reverberation model has been developed at SACLANTCEN to model general reverberation scenarios in range-dependent waveguides. Features of the model include coherent propagation to and from the scattering patch, a coherent scattering patch model which includes anisotropy in the scatterer correlation length scales, and explicit inclusion of system parameters such as bandwidth and source and receiver directivity. The model is built upon the C-SNAP coupled mode model developed at SACLANTCEN by Ferla and Porter. In this talk the theoretical features of the model are presented and the characteristics of coherent and range-dependent bistatic reverberation predicted with the model are discussed. The model is also exercised to help interpret reverberation phenomena observed during the Geoclutter/Boundary Characterization Cruise conducted off the coast of New Jersey during the spring of 2001, including reverberation from beyond the shelf break and scattering from features buried beneath the sediment. a)Previously at the SACLANT Undersea Research Centre, La Spezia, Italy.

Results from a series of acoustic variability experiments

A series of experiments were conducted in recent years by the SACLANT Undersea Research Centre to measure the effect of the environment on acoustic propagation. These broadband, shallow water experiments used a moored vertical array with both towed and fixed sound sources. During the fixed experiments the sound source was mounted on a steel frame tower that was resting on the seabed. This design was used to isolate the acoustic fluctuations caused by the environment from those due to source motion. During transmissions extensive measurements of the environment were made including range-depth profiling of the ocean sound speed along the propagation track using a vertically towed CTD (conductivity, temperature, and depth) chain. The seabed types at the sites varied from soft clay (sound speed less than in the water) to a highly reflective sand-gravel (sound speed about 1750 m/s). The temporal fluctuations and spatial structure of the received acoustic signals varied greatly between the sites. This had a big impact on propagation modeling for transmission loss; matched-field processing; and model-based, geo-acoustic inversion. In this paper an overview of these acoustic variability experiments will be described along with results showing the effect of the environment on model-based processing.

The Boundary Characterization 2001 Experiment

The weakest link in performance prediction for naval systems operating in coastal regions is the environmental data that drive the models. In shallow water downward refracting environments, the seabed properties and morphology often are the controlling environmental factors. In the Boundary 2001 Experiment, seabed, surface, and biologic scattering, seabed reflection, propagation, reverberation, and ambient noise data were collected in order to develop and refine measurement techniques for key environmental model inputs. Both Rapid Environmental Assessment (REA) methods and high-resolution measurement techniques were employed from 0.1–10 kHz. Supporting oceanographic, geologic, and geophysical data were also collected. The experiment was conducted in May 2001 in two littoral regions: the New Jersey shelf and the Scotian Shelf. This paper provides an overview of the experiment objectives, hypotheses and conduct. [Research supported by NATO SACLANT Undersea Research Centre, ONR, and DRDC-A.]

Coastal seabed tomography by inversion of drifting acoustic buoys data

A buoy field, specifically designed for shallow-water acoustic tomography, was deployed in a complex coastal environment on the western Sicilian shelf (enverse 97, Saclant Undersea Research Centre). Eight buoys drifted away from a fixed sound source receiving its repeated, broadband transmissions on their single hydrophone at fixed depths and known positions (DGPS). The 1-day run sampled, over kilometers, the acoustic impulse response (the Green’s function) of the environment as a function of range and azimuth from the source. In this paper, the mapping of sediment properties and bottom types from the ‘‘synthetic horizontal aperture’’ measurements is investigated. Statistical analyses of the frequency-dependent, mode interference patterns associated with the buoy tracks allow to isolate range-azimuth sectors of limited acoustic variability and to define regions of similar bottom conditions. Average geoacoustic parameters of these regions are determined by maximizing the processing gain of a model-based matched filter receiver [J.-P. Hermand, IEEE J. Oceanic Eng. 24, 41–66 (1999)]. The preliminary inversion results, including P-wave speed and thickness of the sediment cover, are congruent with the ground truth of a dense grid of seismic reflection profiles and sediment cores (enverse 98). [Work supported by the Royal Netherlands Navy.]

Range-dependent seabed characterization by inversion of acoustic data from a towed receiver array

The MAPEX2000 experiments were conducted by the SACLANT Undersea Research Centre in the Mediterranean Sea in March 2000 to determine the performance of acoustic inversion for seabed properties using a towed, horizontal receiver array. Towed systems are advantageous because they are easily deployed from a ship and the moving platform offers the possibility of estimating spatially variable (range-dependent) seabed properties. Previous research has successfully applied matched field processing (MFP) geoacoustic inversion techniques to measured acoustic data, however, in nearly all cases the inverted data were collected on moored, vertical receiver arrays. In the MAPEX2000 experiments acoustic data were collected on both towed and moored vertical receiver arrays. Results will be presented showing that seabed properties (e.g., seabed sound speed, sediment layer thickness and attenuation constant) can be extracted using MFP inversion of acoustic measurements from a towed array of receivers. These seabed properties agree with those inverted using data received simultaneously on a vertical array. These findings imply that a practical technique can be developed to map range-dependent seabed properties over large areas using a towed acoustic system. An example of such a range-dependent inversion is given for two sites from the MAPEX2000 experiments.

Comparisons of high-frequency acoustic and shallow-water ocean temperature fluctuations

Dynamic ocean processes produce spatial and temporal fluctuations in the ocean’s index of refraction along the propagation path that cause acoustic propagation fluctuations. This paper reports a second set of measurements designed to determine environmental limits on ping-to-ping amplitude and phase variability of direct path acoustic fluctuations in very shallow water. In September 1997, the Naval Research Laboratory in conjunction with the NATO SACLANT Undersea Research Centre conducted this series of experiments, in shallow water off American Beach, located between Pisa and Livorno, Italy. Established equations for the variance of the acoustic amplitude and phase as a function of the index of refraction variance and the ocean characteristic length scale under conditions of isotropic turbulence are used to predict acoustic fluctuations for comparison with measurements. In these results, measured phase fluctuations are more consistent with sea swell predictions than isotropic turbulence predictions. Phase fluctuations are relatively insensitive to water column stability and anomalous phase fluctuations correlate with wind direction. Amplitude fluctuations are consistent with isotropic turbulence predictions and anomalous amplitude fluctuations correlate with changing stability. [Work supported by ONR.]

Acoustical study of the prey field and supporting food web for whales in the Ligurian Sea north of Corsica

The Sound, Oceanography, and Living Marine Resources (SOLMAR) program is a study coordinated by the NATO SACLANT Undersea Research Centre to develop ways of mitigating risks to whales from anthropogenic sound sources. One objective of the SOLMAR program is to examine the correlation between the distribution of whales in parts of the Mediterranean Sea and that of their prey, lower trophic levels, and basic oceanographic parameters. A multiship field study was conducted in the Ligurian Sea north of Corsica in August 1999, as will be a follow-up study in August 2000. The primary role of the R/V Ammiraglio Magnaghi is to map basic physical oceanographic parameters with a CTD; phytoplankton with a fluorometer; small zoo-plankton (copepods and smaller) with a six-frequency TAPS (Tracor Acoustic Profiling System) and small plankton net; and large zooplankton (euphausiids and larger) with a 120-kHz split-beam echosounder. Measurements in 1999 were consistent with a counterclockwise circulation in the basin causing Ekman pumping of nutrients in the center. Phytoplankton were concentrated in the center, small zooplankton were around the periphery, and larger zooplankton were associated with the shelf break and basin center. Results from the two field seasons will be discussed and compared.

The Atlantic Ionian Stream

This paper describes some preliminary results of the cooperative effort between SACLANT Undersea Research Centre and Harvard University in the development of a regional descriptive and predictive capability for the Strait of Sicily. The aims of the work have been to: (1) determine and describe the underlying dynamics of the region; and, (2) rapidly assess synoptic oceanographic conditions through measurements and modeling. Based on the 1994–1996 surveys, a picture of some semi-permanent features which occur in the Strait of Sicily is beginning to emerge. Dynamical circulation studies, with assimilated data from the surveys, indicate the presence of an Adventure Bank Vortex (ABV), Maltese Channel Crest (MCC), and Ionian Shelf Break Vortex (IBV). A schematic water mass model has been developed for the region. Results from the Rapid Response 96 real-time numerical modeling experiments are presented and evaluated. A newly developed data assimilation methodology, Error Subspace Statistical Estimation (ESSE) is introduced. The ideal Error Subspace spans and tracks the scales and processes where the dominant, most energetic, errors occur, making this methodology especially useful in real-time adaptive sampling.

Extracting sea-bottom information from reverberation data

Reverberation measurements contain information about both the scattering and propagation effects. This paper will discuss our efforts to extract bottom loss and bottom scattering from reverberation data, including at-sea results obtained during three recent Rapid Environmental Assessment (REA) trials with the SACLANT Undersea Research Centre. There is often a strong interrelationship between the bottom loss and scattering, and attempts to deal with the uniqueness of the extracted parameters are discussed. Earlier attempts using another simplified, but automated, inverse scheme indicated very good agreement with independently measured bottom loss. In the recent REA trials the initial bottom loss was obtained using some a priori geologic information for the experiment area. Then the bottom parameters were manually adjusted to obtain a good fit to the data from several nearby locations and array headings. The best fits were obtained by first adjusting the bottom loss to fit the rate of decay, and then adjusting the strength of the Lambert coefficient to fit the level.

Rapid response, a demonstration of ocean assessment capabilities

The suitability of sound waves as a carrier of information in the ocean is known to be highly variable. In the past, the assessment of the controlling environmental parameters has typically been subject to post-cruise analysis and often took years. SACLANT Undersea Research Centre and NATO’s organization for military oceanography agreed upon a three-year series of surveys in the Mediterranean Sea aimed at the development of a methodology for rapid environmental assessment (REA). A large number of research ships and patrol aircraft were provided by various nations. Researchers of numerous institutes were engaged in the REA trials. Time-invariant parameters such as bottom properties were measured directly or determined by inversion. The results were transferred for optional replacement in a standard database. Time-dependent parameters such as the sound velocity structure were predicted by means of real-time ocean models initialized and updated with survey data. The infrastructure for timely delivery of REA products and forecasts was developed, the main requirements being appropriate data channels and data fusion capacity. Rapid response was the first demonstration by the research community that the critical characteristics of the acoustic environment can be provided by the end of a mission.

Rapid assessment of undersea ambient noise

Undersea ambient noise is a critical parameter in determining the performance of passive sonar systems, and sometimes active systems. There are many properties of the ambient noise that must be considered in a rapid ambient noise field assessment, including mean and percentile levels as a function of frequency, temporal and spatial variation, vertical and horizontal directionality, and depth dependence. In addition, rapid interpretation and extrapolation of the measured and analyzed ambient noise results require a credible onboard ambient noise prediction model and collateral noise model input measurements of oceanographic, acoustic, meteorological, and noise source parameters, e.g., sea and swell height, wind speed, and the positions, speeds, courses, and lengths of near and distant ships. Finally, turnkey data processing programs and time proven analysis procedures must be utilized with onboard graphics and word processing capabilities to generate the assessment document onboard in final form. The methods and techniques used by the SACLANT Undersea Research Centre (SACLANTCEN) and the Naval Research Laboratory (NRL) for rapid onboard assessment of the ambient noise are presented and discussed. [Work supported by Project 01 of SACLANTCEN and the Office of Naval Research through the management of the WISPR Filter Development and Evaluation Project by NRL.]

Rapid assessment of bottom backscattering

There is a strong need to obtain information about the expected acoustic reverberation environment that will be encountered in various shallow-water sites to aid in planning and evaluating the effectiveness of operational systems. A new technique is being developed at the SACLANT Undersea Research Centre to quickly sense the mean reverberation levels of a shallow-water area. A prototype device and analysis algorithms have been developed and used as part of rapid response operations. The system transmits and receives acoustic signals from an 80-kHz transducer and simultaneously calculates the quantitative acoustic response of the seafloor. The reverberation measurements made so far have concentrated on mapping the mean seafloor scattering level (backscattering strength) and amplitude statistics over an area for various grazing angle regimes. Differences in scattering strength between high and low reverberation zones within an operational area were significant, with differences in mean level reaching 10–12 dB. Additionally, the reverberation measurements were compared with other techniques to verify relevant bottom-type classification. The experimental technique and data obtained from different areas are presented.

Measurements of acoustic scattering from partially and completely buried thin-shelled targets

Conventional high-frequency imaging sonars become less effective in detecting and classifying bottom-laid targets when the targets become buried into seabottom sediments. Some recent work at the NATO SACLANT Undersea Research Centre has concentrated on investigating the use of lower-frequency sonars (2–16 kHz) in order to better exploit scattering features of buried targets that can aid in their detection and classification. Part of the recent GOATS’98 experiment performed near Elba Island, Italy, involved carefully controlled monostatic measurements of scattering by thin-shelled objects (spheres and cylinders) that were partially and completely buried in sand, as well as in free-field. Preliminary results are presented which show promising comparisons of the data to existing models. In addition to examining peak levels of the returns for detection purposes, special attention is paid to resonance effects (examined in the time and frequency domain), and the effect of burial on this part of the target response. Scholte–Stoneley and Lamb-type waves can be clearly identified in the backscattered signals. The issue of inverse scattering for classification purposes is also addressed. [Work supported in part by EC MAST Programme and ONR.]

Data-model comparisons of reverberation at three shallow-water sites

Reverberation measurements made by the SACLANT Undersea Research Centre at three shallow-water sites (130-190-m depth) are compared with each other and with estimates from the DREA normal-mode reverberation model OGOPOGO. The experiments over silt-clay and sand seabeds were conducted at slightly bistatic geometries (0.7-6.0-km source-receiver separation), using explosive sources detonated at mid-water depths. The signals were received on hydrophones of either a vertical or horizontal array and analyzed in one-tenth-decade frequency bands from 25 to 1000 Hz. The data are compared with each other to investigate the site differences and frequency dependencies, and with the estimates from the reverberation model OGOPOGO to interpret the data and to obtain a qualitative measure of the scattering. For modeling purposes, geoacoustic models of the seabed were assumed, and the reverberation data were fitted by adjusting the Lambert bottom scattering coefficients. Good model agreement was obtained with both individual hydrophone and data. Though somewhat sensitive to the geoacoustic the Lambert coefficients give a measure of the frequency dependence of the scattering. For the silt-clay bottom, the scattering is weak but is independent of frequency; for the sand bottoms, the scattering is stronger and increases with frequency. These results are compared with estimates from other experiments.

Development of an arctic ray model.

Recent additions to a previously developed ray model [E. K. Westwood and P. J. Vidmar, J. Acoust. Soc. Am. 81, 912–924 (1987); E. K. Westwood and C. T. Tindle, ibid. 81, 1752–1761 (1987)] have been completed. The inclusion of top layering above the user-specified ocean and bottom layer environment makes the updated model (ICERAY) applicable to ice-covered regions. Sources are specified in the ocean, while receivers may be located in either the ocean or top layers. The implementation consisted of the calculation of several ray types, and energy contributions from the various ray paths were analyzed independently. Compressional and shear rays were included in the ice. The analysis of evanescent waves showed them to be the most important for propagation to receivers in the ice from sources at longer ranges. The evanescent contribution was implemented by calculating an eigenray to a receiver just below the ice–water interface at the location of the actual ice-embedded receiver and including an appropriate transmission coefficient and exponential decay factor. Comparisons to the full-wave model SAFARI [H. Schmidt, SafariUser’sGuide, SACLANTCEN SR-113, NATO SACLANT Undersea Research Centre, La Spezia, Italy (1989)] demonstrated the accuracy of the ray model for various source and receiver geometries starting with simple environments and evolving to a realistic arctic environment. [Work supported by the Office of Naval Research and the Internal Research and Development Program of the Applied Research Laboratories, The University of Texas.]