Bottom Backscattering Research Articles

Environmental measurements and mid-frequency backscatter in shallow water

To understand bottom backscattering mechanism at the mid-frequency range (nominally 3.5 kHz), direct measurements of bottom roughness and sub-bottom heterogeneity were made along with concurrent backscatter measurements in a shallow water site with 105 m water depth. The backscatter was recorded on a vertical line array with 31 elements with a 3.5 kHz source attached at the bottom of the array. Bottom roughness and sub-bottom heterogeneity were measured using an in situ conductivity probe. The roughness measurements cover a one-dimensional profile of approximately 4 m in length with vertical resolution of 4 mm and horizontal resolution of 1.5 to 2.5 cm. Heterogeneity measurements cover a depth of 5 to 10 cm. Ambient noise received by the vertical line array was used to estimate the sound speed, density, and attenuation coefficient of the surficial sediments. A Monte Carlo modeling capability was developed to extensively simulate the backscatter with different environmental inputs. The results of this effort covering all three areas of acoustics, environments, and modeling will be reported.

The Use of Phased-Array Doppler Sonars near Shore

Phased-array Doppler sonars (PADS) have been used to probe an area several hundred meters on a side with 8-m spatial resolution, sampling every second or less with under 2 cm s 21 rms velocity error per sample. Estimates from two systems were combined to produce horizontal velocity vectors. Here, concerns specific to use of PADS in shallow water are addressed. In particular, the shallower the water is, the larger the fraction of bottom backscatter, so the stronger the bias is toward zero Doppler shift in the estimates. First, direct comparisons are made with other current measurements made during the multi-investigator field experiment ‘‘SandyDuck,’’ sponsored by the Office of Naval Research, which took place in the autumn of 1997 off the coast of Duck, North Carolina. The coherences between PADS and in situ current measurements are high, but the amplitude of the sonar response is generally low. To explore this further, a simplified model of wave shoaling is developed, permitting estimates of wave-frequency velocity variances from point measurements to be extrapolated over the whole field of view of PADS for comparison. The resulting time‐space movies of sonar response are consistent with quasi-steady acoustic backscatter intensity from the bottom competing with a variable backscatter level from the water volume. The latter may arise, for example, from intermittent injection of bubbles by breaking waves, producing patches of high or low acoustic response that advect with the mean flow. Once this competition is calibrated via the surface wave variance comparison, instantaneous measured total backscatter intensities can be compared with an estimated bottom backscatter level (which is updated on a longer timescale, appropriate to evolution of the water depth or bottom roughness) to provide corrected sonar estimates over the region.

Extraction of modal backscattering matrix from reverberation data in shallow-water waveguide. Part I. Theory

Extracting the bottom backscattering information from reverberation data in shallow-water waveguide is an attractive but difficult issue. In previous works, some a priori assumption (for instance, the Lambert’s Law, or the separability of the backscattering matrix) has been made in order to solve this problem. In this paper, new approaches are proposed. The modal backscattering matrix can be extracted directly from reverberation data without any a priori assumption on scattering coefficient. The backscattering information inversion is based on the following three components: (1) mode-filtering at the receiving vertical array, (2) changing the point source depth to obtain different incident mode excitation, and (3) using mode stripping in the waveguide as an additional mode filter. For lower frequency the linear inversion is presented, and for higher frequency the sequential inversion with angle interval smoothing is proposed. [Work supported by ONR.]

Broad-band frequency and incident-angle dependence of bottom backscattering on Browns Bank

A two-scale roughness model for bottom backscattering (Novarini and Caruthers) was applied to multibeam sounder data (95 kHz) from Browns Bank (south of Yarmouth, Nova Scotia, Canada). In order to better understand frequency and incident angle dependence of backscattering, acoustic-calibration data (1-6 kHz) were collected from the same area and treated with the same model. The frequency and incident angle dependence of bottom backscattering in the multibeam and acoustic-calibration data were compared. Backscattering due to large-scale roughness was most relevant at near-normal incidence (<7/spl deg/) and it was more dominant in the low-frequency range, and was strongly dependent on incident angle. Volume scattering was least dependent upon incident angle. It was the dominant factor at the large incident angle. Bragg scattering was the most significant over a very wide frequency range and was more important for high frequency (>5 kHz) and small incidence, but not near-normal incidence.

Modeling of bottom backscattering from three-dimensional volume inhomogeneities and comparisons with experimental data.

In this paper, backscattering from 3D volume inhomogeneities in the seabed is modeled and the results compared with experimental data at 250-650 Hz. The experiment was part of the Acoustic Reverberation Special Research Program (ARSRP) and the data were obtained in a sediment pond on the western flank of the Mid-Atlantic Ridge. A volume scattering model based on first-order perturbation theory is developed incorporating contributions from both sound speed and density fluctuations. With the propagators, i.e., the Green's functions, handled accurately through numerical wave number integration and random fluctuations generated effectively by a new scheme modified from the spectral method, the model is capable of simulating monostatic, backscattered fields in the frequency domain as well as in the time domain owing to 3D volumetric sediment inhomogeneities. The model compares favorably and consistently with the ARSRP backscattering data over the entire frequency band, with the fluctuations of sound speed and density in two irregular sediment layers, identified from the data analysis, described by a power-law type of power spectrum.

Seabottom characterization using multibeam echosounder angular backscatter: an application of the composite roughness theory

Composite roughness theory is used to characterize Southern Ocean bottom backscatter (multibeam) data. Spectral parameters based on Helmholtz-Kirchhoff's theory, D. R. Jackson et al. (1986), are determined from measured near-normal incidence values. A splicing technique using Rayleigh-Rice theory, E.Y. Kuo (1964), is employed beyond 20/spl deg/ incidence angles. Estimated roughness parameters for six deep ocean areas correlate with geology.

Analysis of high-frequency acoustic scattering data measured in the shallow waters of the Florida Strait

Experiments were conducted in the Florida Strait region of the United States in July 1995 to investigate the contribution of sediment volume scattering to measured acoustic backscatter levels at frequencies of 7.5 and 15 kHz. Five sites were analyzed for bottom backscattering level in the radial and azimuthal directions. Sediment geophysical parameters were established by a cross-well tomographic measurement and used as inputs for modeling. Both sediment volume and rough interface scattering models were utilized in analyzing the data. Comparison of model predictions with measured data showed that for grazing angles, at the water–sediment interface, from the critical angle to approximately 60 degrees, volume scattering dominated. For angles greater than approximately 60 degrees, surface roughness scattering consistently dominated. In geographic areas where the interface sound speed ratio exceeded one, roughness scattering controled the backscattered level for grazing angles less than the critical angle. Inversion of the measured acoustic backscatter data was performed using a genetic algorithm optimization developed for use with the volume scattering model. Inversion results were found to agree well with measured data and measured environmental parameters that describe the scattering volume.

Seabottom roughness study using a hydrosweep–multibeam system

Seabottom profiling using a multibeam echosounder is a well-known method to acquire a high-resolution and high-density data set for bathymetric mapping of survey area. The use of multibeam echosounder backscatter signals for bottom roughness characterization is a modern technique. Here, the model results of seabottom backscatter data using a hydrosweep-multibeam system, from some of the geologically well-known areas of Southern Oceans, are presented. Using the capabilities of multibeam systems, angular backscatter strengths are determined employing various corrections. Different bottom backscattering modeling techniques like the composite roughness [Jackson et al., J. Acoust. Soc. Am. 79, 1410–1422 (1986)] and two-layer Helmholtz–Kirchhoff model [Talukdar et al., J. Acoust. Soc. Am. 97, 1545–1558 (1995)] for estimation of bottom roughness is applied. Various seabottom parameters like root-mean-square (rms) relief height, correlation lengths, attenuation coefficients, and layer thickness using the two-layer Helmholtz–Kirchhoff model are calculated. The interface roughness parameters, i.e., slope and intercept values, and volume roughness parameters are computed using the composite roughness theory for the same geological areas.

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.

Environmental factors governing shallow-water active sonar

Recent experiments have measured the broadband sound transmission, coherency, reverberation and bottom backscattering strength in a variety of shallow-water environments with sand-silt bottoms. Ancillary range and time-dependent conductivity and temperature versus depth along with bathymetry were also obtained. The measurements were performed with especially designed impulsive sources, and vertical and horizontal receiving arrays. This paper presents key results on the spatial coherency of the received signals, the frequency dependence of the effective attenuation factors and the frequency dependent backscatter strength coefficient. The broadband coherency measurements, although variable from site to site, were found to be consistent with lengths of 30–35 wavelengths at 400 Hz at 40-km range. The frequency-dependent effective attenuation factor varied as frequency to a fractional power ranging between 0.15–0.25 dB/km at 100 Hz and 0.4–0.55 dB/km at 1 kHz. Bottom backscattering strengths at grazing angles on the order of 10 deg were found 10–15 dB lower at 200 Hz as compared to 1 kHz. The results are shown to be consistent with those of other investigators and are shown to place realistic limits on shallow-water sonar parameters. [Work supported by DARPA.]

Spatial heterogeneity of acoustic bottom backscattering model parameters and predicted results

Sediment geoacoustic and roughness measurements were made during a mine burial experiment in the North Sea in November 1997. The experiment site was a sand ridge (20.5–32-m water depth) influenced by strong tidal currents and wave action. Stereo photographs indicated that the sediment roughness was characterized by cross-rippled fine sand that transitioned to straight- and sinuous-rippled mixtures of fine quartz sand and mollusk shell fragments. Box core samples were subsampled with core tubes from which measurements of sediment compressional wave velocity and attenuation, density, and grain size distribution were made. Values for sediment velocity ratio, density ratio, slope of the roughness power spectrum, roughness spectral strength, and other measured and calculated parameters were used as input parameters for the composite roughness model to predict bottom backscattering strengths from various regions within the experiment site. Horizontal and vertical variability in sediment properties is high relative to other shallow sites under the influence of strong hydrodynamic forcing. There is a marked interdependence among sediment geoacoustic properties, water depth and bottom roughness features. The influence of hydrodynamic conditions and sediment type on the acoustic model predictions is demonstrated by the spatial variability in predicted scattering of high-frequency sound from the site.

Extracting in-plane bistatic scattering information from a monostatic experiment

Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. The bottom sediment at the four sites varied from mud/clay to silt/gravel. Using a simple array consisting of a free-flooding-ring projector which was omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800–2400 Hz [Hines and Barry, J. Acoust. Soc. Am. 92, 315–323 (1992)]. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. By deploying the array at a height of 500 m above the seabed, bottom backscatter data were obtained at grazing angles down to 4 degrees before the onset of the first surface return. For data arriving after the first-bottom–first-surface interaction, the geometry approximates an in-plane bistatic experiment. Estimates of the in-plane bistatic scattering strength were obtained for pairs of incident and scattered grazing angles (φi,φs)&amp;lt;(50 degrees,88 degrees), at frequencies of 900, 1200, and 1600 Hz. At one site the grazing angle dependence matched Lambert’s rule; at all other sites the data exhibited a steeper slope than that of Lambert’s rule. There was insufficient data to extract the frequency dependence. The fathometer returns were used to estimate the normal-incidence bottom loss at the four sites. The measured bottom loss ranged from a low of 2 dB to a high of 16 dB. The in-plane bistatic data and the monostatic data were compared to examine the validity of the separable and half-angle approximations [Ellis and Crowe, J. Acoust. Soc. Am. 89, 2207–2214 (1991)]. The separable approximation provided a reasonable fit to the data at three of the four sites; the half-angle approximation did not match the data well at any of the sites.

High-frequency bottom backscattering from a homogeneous seabed

High-frequency acoustic backscattering was measured from a sandy site located in very shallow water (8 m) off Tirrenia, Italy, at a range of frequencies and grazing angles. Concomitant measurements of the seabed indicated that the very fine sand was well sorted, relatively uniform with respect to vertical and horizontal distribution of compressional wave velocity and attenuation, and depleted with respect to biological activity. Bottom scattering strengths were very low and variability of sediment parameters was exceptionally low. Coefficients of variation determined for sediment compressional wave velocity and attenuation were 0.29% and 7.2%, respectively. These data compare with previously recorded minima of 0.30% and 14.8% for the same respective parameters, in nearshore, mobile fine sands from the Gulf of Mexico and the Pacific Ocean. Lack of variability in sediment sound velocity and density concurrent with low acoustic bottom scattering strength measurements would endorse recent modeling approaches that predict scattering from sediment impedance fluctuations. Acoustic data are compared with predictions from the composite roughness model (modified for inclusion of sediment volume scattering parameters) at various frequencies and grazing angles in order to test the model’s response at minimal perturbation of seabed properties.

Analysis of acoustic backscatter in the vicinity of the Dry Tortugas

Experimental measurements of the bottom backscattering strength from carbonate sediments were made with a 200-kHz multibeam sonar mounted on a remotely operated vehicle. Results were obtained from eight different sites, which may be grouped into three categories, labeled soft, medium and hard, according to measured sediment sound speed. Sediment samples were gathered at or near each site to help interpret the acoustic results. The acoustic results are compared with extant published data and with the BOGGART bottom backscatter model. Backscattering strength values measured in the soft and medium sites fell within the main cluster of previously published values from sediments of similar grain sizes. The values from the hard region fell close to the upper limit. Dependence of the apparent backscattering strength on sonar height above bottom, particularly for the lower values of height above bottom, was observed, which suggests that the scattering process is a multiple-scattering one.

Detection of migrating salmon in the Fraser River using 100-kHz sidescan sonars

During a 10-day period in September 1995, field trials were conducted in the Fraser River near Mission, B.C., to evaluate 100-kHz side-looking sonars for salmon detection. A two-element sidescan installation looking transverse to the river flow detected echo traces due to salmon at ranges up to 250 m in water 6-13 m deep. The salmon target trajectories were identified and counted against a background of acoustic reverberation from the river surface and bottom sediments. Occasional, strong interference from boat wakes and wind-induced surface waves and bubbles was observed. Fish echo trajectory analyses allowed extraction of upstream and cross-stream swimming speeds. For some targets acoustic multipaths from surface and bottom reflections were distinguishable from direct echoes at ranges up to 150 m. Acoustic models of surface and bottom boundary backscattering, combined with bubble layer scattering and attenuation under breaking wave conditions, were able to match the observed background echo levels. Comparisons of model and data predict that under ideal conditions fish detection should be possible at ranges up to 250 m, but under windy conditions detection was limited to less than 70 m.

Finite correlation and coherent propagation effects in the normal-mode description of bottom reverberation

A normal-mode scattering formulation that assumes a finite spatial correlation length in the distribution of scattering features is used to compute single-frequency bottom reverberation for bistatic and monostatic scattering geometries in a shallow water and deep water environment. Spatial correlation of the scattering features allows the superposition of modes scattered within each spatially correlated region and produces diffraction in the scattered field that is not predicted in the limit of a zero spatial correlation length (point scattering). For bistatic scattering geometries, the scattered field computed as a function of scattering location in the horizontal plane exhibits a pattern of diffractive maxima and minima for nonzero spatial correlation lengths. The spatial details of the diffraction pattern and its influence on the scattered energy depend on the frequency and spatial correlation length and can result in a significant reduction in the predicted levels of received reverberation. The greatest sensitivity to finite correlation effects occurs for monostatic scattering geometries because the strongest diffractive effects occur in the backscattering direction. The effects of including modal interference in the incident and scattered field propagation are also examined in this paper. The inclusion of modal interference in the propagating fields imposes an interference pattern on the spatial structure of the scattered field in the horizontal plane, and can cause the temporal dependence of the reverberated return to oscillate about the levels of return predicted when modal interference in the propagating fields is neglected. In agreement with previously published results for bottom backscattering, the effects of including modal interference in the propagating fields were found to be significant for deep water environments that exhibit convergent zone propagation and to be of limited importance for shallow water environments in which the energy incident on the bottom is characterized by a large number of multipaths. The present work includes results which show that the effects of including modal interference in the propagating fields can be important for shallow water environments that exhibit significant bottom penetration.

Estimating in-plane bistatic scattering strength from a monostatic experiment

Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. Using a simple array consisting of a free-flooding ring projector, omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800 to 1700 Hz. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. The array was deployed 500 m above the seabed and bottom backscatter data were obtained at grazing angles down to 4° before the onset of the first surface return. For data arriving after the first-bottom first-surface interaction, the geometry approximates an in-plane bistatic experiment. Therefore, the in-plane bistatic data and the monostatic data can be compared to examine the validity of using the separable and half-angle approximations used in some bistatic scattering models [J. Acoust. Soc. Am. 89 (1991)]. In this paper the bistatic geometry is explained and the data are interpreted in light of the bistatic arrivals. Then, an algorithm is presented for extracting the in-plane bistatic scattering strength from the data. Finally, bistatic and monostatic data are compared using the separable and half-angle approximations.

High-frequency reverberation in shallow water

Monostatic reverberation measurements were collected in shallow water, over a coarse gravel and cobble bottom, 100 m deep, off the coast of Nova Scotia. Data were collected at frequencies of 21, 28, and 36 kHz using linear FM pulses of 2-kHz bandwidth and 0.160-s duration. An anchored, high-frequency active sonar array deployed at a depth of 42 m was used to collect the data. The reverberation measurements were compared with estimates computed with the NUWC generic sonar model (GSM). The data were reasonably well modeled for times greater than 0.2 s after pulse transmission by neglecting surface reverberation and using Lambert's rule for bottom backscattering with a scattering coefficient of -27 dB, independent of frequency. At all three frequencies, the data and model show a peak approximately 0.9 s after pulse transmission. This peak results from a focusing effect that the downward-refracting sound-speed profile has on the interaction of the rays with the bottom.

Ocean floor mapping using a forward-looking, high-resolution array

A system for remote sensing of ocean bathymetry using a forward-looking sonar is being developed. The system combines a wide-angle transmit beam (for full volume coverage) with multiple, steered, narrow receive beams for object localization. Sample bathymetry data were collected in Narragansett Bay, RI using a 10-ms-duration, 3-kHz bandwidth, linear stepped FM transmit sonar pulse, centered at 87 kHz. Bathymetry results compare favorably with a sidescan of the test area [M. R. Medeiros and R. N. Carpenter, Proc. IEEE AUV-96, pp. 185–191]. In addition to bathymetry the system returns an echo level map which can, after correcting for system gains, transmission loss, and grazing angle, provide a measure of bottom backscatter. The system is currently under investigation at the Naval Undersea Warfare Center, Division Newport, for use as a bottom detection and collision avoidance forward-looking sonar for unmanned underwater vehicles.

A generalized acoustic model for marine sediments

Current acoustic backscatter models are limited in applicability to specific types of marine sediments. A fine-grain sediment in which particles are suspended can generally be treated as an acoustic fluid, whereas an elastic model is more appropriate over a rigid bottom. Sandy sediments have been modeled successfully as Biot poroelastic media [F. A. Boyle, and N. P. Chotiros, J. Acoust. Soc. Am. 98, 531–541 (1995)]. Since it is unclear how to treat a sediment that straddles the boundary between sediment types, a unified theory for bottom backscatter is needed, from which a seamless model can be developed. Due to the porous nature of marine sediments, a poroelastic model is considered to be most appropriate. Critical parameters that need to be determined, particularly frame moduli and permeability, are inverted from acoustic measurements, to give empirical values as a function of porosity. [Work funded by NRL Code 321 OA.]