C. de Moustier

Angular dependence of 12-kHz seafloor acoustic backscatter

The angular dependence of seafloor acoustic backscatter, measured with a 12‐kHz multi narrow‐beam echo‐sounder at two sites in the central North Pacific with water depths of 1500 and 3100 m, respectively, has been determined for incidence angles between 0° and 20°. The acoustic data consist of quadrature samples of the beamformed echoes received on each of the 16 2.66° beams of a Sea Beam echo‐sounder. These data are subjected to adaptive noise cancelling for sidelobe interference rejection, and the centroid of each echo is determined. After corrections for the ship’s roll and raybending effects through the water column, the angles of arrival are converted to angles of incidence by taking athwartships apparent bottom slopes into account. For each beam, the mean echo power received is normalized by the corresponding insonified area that depends on the transmit and receive beam patterns, the ship’s roll angle and the local bottom slope. For lack of system calibration, the data are presented as relative mean...

Evaluation and verification of bottom acoustic reverberation statistics predicted by the point scattering model

The point scattering model offers a parameterization of the reverberation probability density function (pdf) in terms of the coefficient of excess (kurtosis) and a coherent component represented by a harmonic process with random phase. In this paper the potential utility of this parametrization is investigated in the context of seafloor characterization. The problem of separating out the effect of each parameter is discussed. Computer simulations are used to verify model predictions on the reverberation quadrature, envelope, and phase pdf. As part of the verification study, the scatterer density was determined from the kurtosis of the reverberation quadrature pdf. A statistical analysis of this procedure points to reduced estimate accuracy with decreasing kurtosis. Additional computer simulations show that the chosen pdf family, developed under the assumption of a Poisson scatterer distribution, is flexible enough to fit reverberation data generated by non‐Poisson scatterer distributions exhibiting a degr...

Detailed Bathymetric Surveys Offshore Santa Rosa Island, FL: Before and After Hurricane Ivan (September 16, 2004)

Two weeks before Hurricane Ivan reworked the shores and nearshore seafloor of the northeastern Gulf of Mexico, swath bathymetry surveys were conducted with high-frequency (300 and 455 kHz) multibeam echosounders in three areas offshore Santa Rosa Island, FL, an 80-km barrier island extending west from Destin to Pensacola Bay, FL. These surveys were repeated in late October 2004, six weeks after the passage of the hurricane, allowing for quantitative pre- and posthurricane seabed comparisons. Bathymetric difference maps (0.2-0.3-m grid cells) show that sediment accretion exceeded 1 m in areas near the 6-7-m isobaths, where a submerged longshore bar was formed below the breaker zone of large storm waves. Accretion of sediment continued seaward tapering off near the 11-12-m isobaths, with evidence of slight seabed erosion (0.1-0.2 m) seaward of this boundary. Between the 6- and 12-m contour lines, the increase in sediment volume is about 279 000 m 3/km 2. Grab samples obtained in the area by Vaughan [IEEE J. Ocean. Eng., vol. 34, no. 4, pp. 495-515, 2009] support the hypothesis that the added material is fine sand eroded from the beach and dunes on Santa Rosa Island by the overwash and inundation associated with Ivans storm surge and eventually deposited offshore by storm-surge ebb currents. Two-dimensional bottom roughness power density spectra computed from colocated east-west (EW) bathymetry swaths near the 12-m isobath show a post-Ivan threefold increase in root mean square (rms) roughness over the [0.104, 0.495] m-1 spatial wave number band. Bottom roughness spectrograms computed along individual north-south (NS) survey track lines perpendicular to the shoreline and extending 10 km offshore indicate that Ivan-induced waves and currents reworked the seabed to water depths of at least 22 m, with a twofold to fourfold increase in rms roughness over the [0.023, 0.156] m-1 spatial wave number band.

Variable bandwidth filter for multibeam echo-sounding bottom detection

The accuracy of a seafloor map derived from multibeam swath bathymetry depends first and foremost on the quality of the bottom detection process that yields estimates of the arrival time and angle of bottom echoes received in each beam. Filtering of each beam with a fixed bandwidth filter, with the bandwidth based on the length of the transmitted pulse, reduces the error associated with the time-angle estimates. However, filters of this type can not be optimal over the wide range of operational environments encountered. Better results are obtained with a processing scheme that varies the filter bandwidth across the swath width using detected time and angle information from the previous ping. This method is evaluated using sonar data obtained with a Reson SeaBat 8111ER and the results compared with those obtained using a fixed bandwidth filter.

Detection of Direct‐path Arrivals for Multi‐Narrowband Sequences (3–30 kHz) In Shallow Water

In an effort to measure underwater acoustic transmission loss over direct‐path lengths ranging from a few hundred meters to ten kilometers in shallow water, a sequence of 16 gated pure tones (3–30 kHz) was transmitted every 10 s from a lowed source and received at moored sonobuoys. The magnitude of multipath arrivals often exceeded that of direct‐path arrivals, resulting in variable detection performance of simple matched filtering techniques. More reliable signal recognition was obtained via iterative least square time constraints on the arrival times across all frequencies in a sequence, based on the known time intervals between transmitted tones. Signal detection improvement was obtained also by searching for the direct‐path arrival near the global maximum of the sum of the rectified correlograms of the received sequences. These methods allowed detection in environments characterized by multipath interferences, as well as low signal‐to‐noise ratio and fading, and in the presence of other unrelated sonar signals that cause large detection errors. It also improved the direct‐path signal strength estimation, and associated transmission loss computation, by bounding the time interval over which to compute the signals’ autocorrelations and estimate their power. These algorithms were tested on a limited data set recorded in the Southern California Offshore Range, confirming that frequencies below 6 kHz suffered less direct‐path transmission losses than higher frequencies (7–30 kHz).

Near bottom sediment characterization offshore SW San Clemente Island

Normal incidence, 23.5 kHz seafloor acoustic backscatter data and bottom video were measured with the Deep Tow instrument package of the Scripps Institution of Oceanography in 100 meter water depth south of San Clemente Island, CA. The collected data were processed using an echo envelope sediment characterization method, to derive geoacoustic parameters such as particle mean grain size and the strength of the power law characterizing the roughness energy density spectrum of the sediment-water interface. Two regions, sand and silt, were selected based on available ground truth, perceived along-track sediment homogeneity, data quality and tow fish stability. Distinction between sand and fine grain sediments can be accomplished by creation of feature vectors comprised of mean grain size (M/sub /spl Phi//) and interface roughness spectral strength (w/sub 2/). Estimates for mean grain size and roughness spectral strength (M/sub /spl Phi//, w/sub 2/) were (1.5, 0.0095) for sand, and (6.7, 0.0033) for silt, where M/sub /spl Phi// is expressed in PHI units, and w/sub 2/ has units cm/sup 4/. These results are consistent with local ground truth measurements and illustrate the potential of this sediment characterization method in survey mode.

Acoustic determination of seafloor roughness with multibeam echo sounders.

Multibeam echo sounders have been used extensively to gather swath bathymetry in the world’s oceans with co‐registered depth samples a few percent of the water depth apart in the athwartships direction. The resulting maps provide seafloor roughness information down to scales of a few hundred meters. A much finer roughness resolution (cm scale) can be achieved when analyzing the beamformed seafloor echoes received by such sonar systems. One method consists of processing the corresponding acoustic data to remove system dependencies and to correct for bottom slopes and for geometric effects due to the area insonified by the transmitted pulse within the confines of the cross‐fan beam geometry, so as to derive an angular dependence function of the average power in the returns for a given patch of seafloor. By fitting this function with a Helmholtz–Kirchhoff model for acoustic backscatter from a rough surface [D. R. Jackson et al., J. Acoust. Soc. Am. 79, 1410–1422 (1986)], a bottom roughness estimation is obta...

Effects of ship trim and dynamic attitude on the relative noise level measured by a hull‐mounted multibeam bathymetric sonar

Acoustic data recorded in various sea conditions with the hull‐mounted multibeam swath bathymetry system SeaBeam 2000 aboard R. V. MELVILLE have been analyzed to quantify the relationship between the ship’s attitude and the relative noise levels observable in the data. The data consist of concurrent time series of echo magnitude and direction of arrival for each transmission cycle, and of the three attitude components roll, pitch, and heave. Although higher noise levels are usually associated with pitch, in these data heave has the greatest impact on noise level, presumably because of bubble syphoning from the areated surface layer. In addition, a slight change in the trim of the ship from horizontal to bow up results in an average relative noise increase of about 12 dB. [Research supported by ONR.]

Volume scattering measurements with a 12‐kHz multibeam echo sounder

Acoustic volume reverberation measurements were made with a 12‐kHz sea beam multibeam echo sounder by recording quadrature samples of the echoes received on each of the systems sixteen 2.66° beams. For initial inspection, the signal amplitudes from three channels (a center channel and outer channels on either side) were displayed as a function of depth and along‐track distance in a grey level image quantized to 4 bits. Deep scattering layers identifiable in these images were analyzed by integrating the echoes received on each beam over a 50‐m depth slice containing a layer or set of layers and by correcting for beam pattern and time spread effects as a function of angular direction. Results from data recorded during night time periods in the Northern Pacific show a fairly consistent volume scattering picture with variations averaging about 10 dB over 4‐km segments along the ships track, and 2 to 3 dB over a few hundred meters across track. The centroid of the returns in the 50‐m window was also calculat...

On differential phase measurements with the SeaMARC II bathymetric sidescan sonar system

The SeaMARC II sonar system, operated by the University of Hawaii, determines bathymetry by measuring the phase difference produced by seafloor echoes received at two rows of transducers, roughly half a wavelength apart, and by converting these measurements to angles of arrival as a function of time. Such phase measurements are affected by noise and interferences from multiple reflections on the ocean surface and bottom. Furthermore, the noise tends to mask the distortions due to these interferences. Working in the complex domain with acoustic data recorded at sea in the Fall of 1989, a simple time‐averaging filter was found to reduce the noise while preserving distortions due to interferences and hence allowing their investigation. Complex ping stacking, over an area with little change in relief, also gave a clear view of the interference phenomena. As a result, in addition to the expected multiple reflection interferences, evidence of cross‐talk between the port and starboard pairs of transducer arrays ...