Bathymetric Sidescan Sonar Research Articles

Direction-of-Arrival Estimation Methods in Interferometric Echo Sounding.

Nowadays, there are two leading sea sounding technologies: the multibeam echo sounder and the multiphase echo sounder (also known as phase-difference side scan sonar or bathymetric side scan sonar). Both solutions have their advantages and disadvantages, and they can be perceived as complementary to each other. The article reviews the development of interferometric echo sounding array configurations and the various methods applied to determine the direction-of-arrival. “Interferometric echo sounder” is a broad term, applied to various devices that primarily utilize phase difference measurements to estimate the direction-of-arrival. The article focuses on modifications to the interferometric sonar array that have led to the state-of-the-art multiphase echo sounder. The main algorithms for classical and modern interferometric echo sounder direction-of-arrival estimation are also outlined. The accuracy of direction-of-arrival estimation methods is dependent on the configuration of the array and external and internal noise sources. The main sources of errors, which influence the accuracy of the phase difference measurements, are also briefly characterized. The article ends with a review of the current research into improvements in the accuracy of interferometric echo sounding and the application of the principle of interferometric in other devices.

Use of the Jiaolong manned submersible for accurate mapping of deep-sea topography and geomorphology

High-resolution bathymetric side-scan sonar (BSSS) performs the functions of traditional side-scan sonar, while also providing a depth-sounding function that allows simultaneous measurement of seafloor topography and geomorphology. Submarine microtopography and microgeomorphology detection ability and advanced underwater acoustic digital communication are important technical capabilities of the Jiaolong manned submersible. High resolution BSSS achieved accurate detection of seafloor topography and geomorphology at a depth of 7000 m, and successful mapping of local microtopography and microgeomorphology in the Mariana Trench.

The improved methods and results of HRBSSS for Mapping the Deep Sea Bottom

High resolution bathymetric sidescan sonar (HRBSSS) is designed by the Institute of Acoustic, Chinese Academic of Sciences (IACAS). HRBSSS have used the technique of direction of arrival (DOA) estimation. It can work near the sea bottom and mapping the deep sea bottom, and get the high resolution bathymetry and sidescan map, concurrently. HRBSSSs have been mounted on the DTA-6000 deep towed body (maximum work depth 6000m) and the Jiaolong HOV (maximum work depth 7000m). In order to upgrading the performance of the system, a new method for improving the depth accuracy on the nadir of the vehicle and an error correct method for minimizing the influence of the inconsistency among the channels have been developed by IACAS. With these method, we have got the good survey results on the area of the seamount (>2000m depth) and on the area of the sea plain (>5000m depth) in the Pacific Ocean in 2011. These methods and results will be introduced in this article.

Improved methods and results of hrbsss for mapping the deep sea bottom

High resolution bathymetric sidescan sonar (HRBSSS) is designed by the Institute of Acoustic, Chinese Academic of Sciences (IACAS). HRBSSS have used the technique of direction of arrival (DOA) estimation. It can work near the sea bottom and mapping the deep sea bottom, and get the high resolution bathymetry and sidescan map, concurrently. HRBSSSs have been mounted on the DTA-6000 deep towed body (maximum work depth 6000m) and the Jiaolong HOV (maximum work depth 7000m). In order to upgrading the performance of the system, a new method for improving the depth accuracy on the nadir of the vehicle and an error correct method for minimizing the influence of the inconsistency among the channels have been developed by IACAS. With these method, we have got the good survey results on the area of the seamount (>2000m depth) and on the area of the sea plain (>5000m depth) in the Pacific Ocean in 2011. These methods and results will be introduced in this article.

Bathymetric Sidescan Sonar Bottom Estimation Accuracy: Tilt Angles and Waveforms

This paper presents a detailed analysis of bottom estimation performance of bathymetric sidescan sonar. Both general and closed-form expressions for the cross correlations of the received backscatter across a receive array are determined for a square pulse, match-filtered square pulse, and a chirp Gaussian pulse. The closed-form expressions clearly show the contributions that the various error mechanisms make to the correlation. Through geometry, simulation, and the Cramer-Rao lower bound (CRLB), it is demonstrated that when the array is tilted, a double-angle region results, which requires two angles to be estimated rather than just one, if the location of the bottom is to be determined correctly. Estimating two angles requires an array with at least three elements as opposed to two elements typically employed in simple relative-phase bathymetric sidescan sonar. It is shown through simulation that multiple angle estimates made with a simple linear prediction algorithm attain estimates near the CRLB and that the CRLB can be used to establish confidence limits. The bottom estimation performance related to a match-filtered Gaussian pulse and Gaussian chirp pulse are compared with that obtained with a match-filtered square pulse. Little difference in performance is found except when the angle estimation accuracy is thermal noise limited and then the chirp pulse performs better because of the higher resultant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">snr</i> . A simple approximation procedure for predicting estimation performance is developed along with conditions for its application. Examples of bottom estimation performance are given for different array sizes, tilt angles, frequencies, and water types.

Wavelet analysis of bathymetric sidescan sonar data for the classification of seafloor sediments in Hopvågen Bay - Norway

In this paper we examine the use of bathymetric sidescan sonar for automatic classification of seabed sediments. Bathymetric sidescan sonar, here implemented through a small receiver array, retains the advantage of sidescan in speed through illuminating large swaths, but also enables the data gathered to be located spatially. The spatial location allows the image intensity to be corrected for depth and insonification angle, thus improving the use of the sonar for identifying changes in seafloor sediment. In this paper we investigate automatic tools for seabed recognition, using wavelets to analyse the image of Hopvagen Bay in Norway. We use the back-propagation elimination algorithm to determine the most significant wavelet features for discrimination. We show that the features selected present good agreement with the grab sample results in the survey under study and can be used in a classifier to discriminate between different seabed sediments.

Side scan sonar using phased arrays for high resolution imaging and wide swath bathymetry

Abstract: The design of a new bathymetric sidescan sonar dedicated to the survey of the continental shelf is described. It uses linear phased arrays in order to perform dynamically the control of the resolution through the focusing of the beams, and beam steering to compensate for yaw and pitch. The 4m long array, working at ll2kHz, yields high resolution imaging. In addition, an interferometric doublet, with a i!h spacing, provides a wide swath bai hymetric capability. Both functions take advantage of a fast digital pulse compression process. Results of sea trials with the hull mounted system (high resolution images and topographic maps) are discussed. -- I Introduction Remote acoustic sensing of the sea floor aims at two complementary goals: imaging for target localisation, bottom classification or small relief Characterisation and sounding in order to draw charts. Two techniques are now widely used for these purposes. Multibeam echosounders are hull mounted systems that are mainly devoted to bathymetric surveys. Most implementations are based on cross shaped phased arrays with a preformed beams technique. The floor is investigated across-track of the surveying ship. The perturbative motions of the ship (i.e. roll and pitch) are compensated dynamically. The soundings are derived from the backscattered echoes by considering either the amplitude or the phase when an interferometric process is performed. Such systems can build images by taking into account the energy backscattered within each beam. Side scan sonar is a much longer established lech- nique. It was originally devoted to imaging only. Antennas point to each side of the ship track and scan, with a single beam, a long and narrow strip of the sea floor. The backscattered strength provides the informa- tion needed to build images with a much higher resolu-

Accuracy of the spatial representation of the seafloor with bathymetric sidescan sonars

When isobath maps of the seafloor are constructed with a bathymetric sidescan sonar system the position of each sounding is derived from estimates of range and elevation. The location of each pixel forming the acoustic backscatter image is calculated from the same estimates. The accuracy of the resulting maps depends on the acoustic array geometry, on the performances of the acoustic signal processing, and on knowledge of other parameters including: the platform's navigation, the sonar transducer's attitude, and the sound rays' trajectory between the sonar and the seafloor. The relative importance of these factors in the estimation of target location is assesed. The effects of the platform motions (e.g. roll, pitch, yaw, sway, surge and heave) and of the uncertainties in the elevation angle measurements are analyzed in detail. The variances associated with the representation (orientation and depth) of a plane, rectangular patch of the seafloor are evaluated, depending on the geometry of the patch. The inverse problem is addressed. Its solution gives the lateral dimensions of the spatial filter that must be applied to the bathymetric data to obtain specified accuracies of the slopes and depths. The uncertainty in the estimate of elevation angle, mostly due to the acoustic noise, is found to bring the main error contribution in across-track slope estimates. It can also be critical for along-track slope estimates, overshadowing error contributions due to the platform's attitude. Numerical examples are presented.

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.

Geomorphology and structural segmentation of the crest of the southern (Pacific‐Antarctic) East Pacific Rise

Geomorphology of the boundary between Pacific and Antarctic plates was mapped with a Sea Beam multibeam echosounder and a SeaMARC II bathymetric side scan sonar, from the southern end of Juan Fernandez microplate at 35°S to Heezen transform at 56°S. There are six spreading center systems separated by two large, left‐stepping nontransform offsets (at 36.5°S and 41.5°S), two right‐stepping transform‐fault systems, and a left‐stepping hybrid structure with equally long strike‐slip and nontransform offsets. Axial rift zones (spreading axes) are generally within 1° of normal to the relative motion predicted by current plate rotation models. Most axial rift zones crop out along the crests of typical East Pacific Rise (EPR) axial ridges which commonly have flat rather than humped long profiles. About 2% of this fast‐spreading (84–100 mm/yr) rise crest has an axial rift valley instead of an axial ridge. The longest rift‐valley segment is midway between Menard and Vacquier transforms near 51°S, where an anomalously deep rise crest may mark a zone of below‐average mantle upwelling. Nontransform offsets structurally similar to those on the tropical EPR, but mostly formed by differential asymmetric spreading, subdivide the four longest spreading center systems. On the northern, transform‐free two thirds of the rise, net rift propagation at migrating offsets has been into the faster Pacific plate. This causes right steps to migrate north, left steps to migrate south, and crust to be transferred to the east flank. This pattern has prevailed for several million years, judging from the oblique fracture zones (pseudofaults) mapped on the rise flanks with older marine data and Geosat altimetry. On this rise crest, the difference in stress on two plates with very different velocities may control the direction of offset migration.

Swath acoustic imaging of the seafloor

Recent developments in bathymetric sidescan sonar systems and in multibeam echosounders have allowed these systems to produce coregistered bathymetry and sidescanned acoustic images of the seafloor in near real time. As a result, in addition to producing geometrically correct images with much less ambiguous information than conventional sidescanned seafloor images, the effects of the local relief and beam patterns on the magnitude of the backscattered signals can be readily compensated to bring out the angular dependence of acoustic backscatter across the swath. By isolating regions of homogeneous tonal appearance in the resulting image, a first-order assessment of the spatial variability in the composition of the bottom can be made. In such areas of the image, where the seafloor is presumably uniform in composition, the angular dependence of acoustic backscatter can be used to infer statistical terrain characteristics such as surface roughness at smaller spatial scales than obtainable with the bathymetry alone. The deterministic approach of shape-from-shading described in a companion presentation by C. Nishimura (same session) can potentially lead to greater bathymetric resolution. Examples are provided with data from multibeam and bathymetric sidescan sonar systems. [Work supported by ONR.]

Seafloor acoustic remote sensing with multibeam echo-sounders and bathymetric sidescan sonar systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.

Differential phase estimation with the SeaMARCII bathymetric sidescan sonar system

A maximum-likelihood estimator is used to extract differential phase measurements from noisy seafloor echoes received at pairs of transducers mounted on either side of the SeaMARC II bathymetric sidescan sonar system. Carrier frequencies for each side are about 1 kHz apart, and echoes from a transmitted pulse 2 ms long are analyzed. For each side, phase difference sequences are derived from the full complex data consisting of base-banded and digitized quadrature components of the received echoes. With less bias and a lower variance, this method is shown to be more efficient than a uniform mean estimator. It also does not exhibit the angular or time ambiguities commonly found in the histogram method used in the SeaMARC II system. A figure for the estimation uncertainty of the phase difference is presented, and results are obtained for both real and simulated data. Based on this error estimate and an empirical verification derived through coherent ping stacking, a single filter length of 100 ms is chosen for data processing applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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 was found, and mapping of differential phase angles into acoustic angles of arrival required an “effective distance” between the two transducer rows almost 50% larger than the physical distance. Implications of these findings for bathymetric measurements and applicability of the phase processing method presented here will be discussed.

Simultaneous operation of the Sea Beam multibeam echo-sounder and the SeaMARC II bathymetric sidescan sonar system

An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented. >

High‐frequency acoustic imaging of the seafloor

For decades, sidescan sonars have been the primary tool to obtain acoustic images of the seafloor. Such images provide qualitative information on the seafloor surveyed based on amplitude variations of the backscattered acoustic signals received. In the 1980s, bathymetric sidescan sonar systems, capable of simultaneously producing acoustic images and measuring depth at numerous points across the swath, added a quantitative description of the seafloor in the form of a depth contour map. Similar claims can be made with multibeam echo sounders well known for their high‐resolution swath bathymetry capabilities. Taking advantage of this high bathymetric resolution, the beamformed acoustic backscatter data can also be displayed as a geometrically correct acoustic image of the seafloor and provide textural information not available in the contoured bathymetry of the same area. Likewise, knowledge of the bathymetry, particularly bottom slopes, is needed to correct for the angular dependence of seafloor acoustic ba...

Stereoscopic visualization and contour mapping of the sea bed using a bathymetric sidescan sonar (BASS)

Besides the backscattered amplitude measurements of a conventional sidescan sonar, the Bathymetric Sidescan Sonar (BASS) makes simultaneous measurements of the depth of sea bed throughout the sidescanned area. It does this by determining the angle of the returning wavefront at each instant in time and combining the result in an appropriate manner with the range measurement. The depth information be used to generate contour maps. It can also be used to generate stereoscopic images of the sea bed. A potential cause of major errors in the depth measurement is surface multipath interference and special features are introduced into the transducer design to avoid this.