Sidescan Sonar System Research Articles

Postprocessing and corrections of bathymetry derived from sidescan sonar systems: application with SeaMARC II

A procedure for postprocessing bathymetry data provided by a phase-measuring sidescan sonar system is presented. The data were collected with the SeaMARC II system, and are generally characterized by a high level of noise and uneven spatial sampling. Before any spatial filtering is applied, data are selected to remove most of the obvious artifacts and to retain instantaneous depth profiles whose slant ranges increase monotonically from a central location to the edges of the swath. An extrapolation scheme, patterned after a potential field, is proposed to fill gaps in the coverage or to extend the bathymetric swath to that of the corresponding sidescan image when regridding the data to a rectangular frame. To fill the near nadir gap typically found in these data, a specific interpolation methodology is developed that takes into account the slant range of the first bottom return as received by the sidescan sonar itself or by a shipboard echo-sounder. Spatial low-pass filtering is applied through convolutions with parabolic windows whose width is proportional to the footprint of the acoustic beam along track and roughly 1/8 of the swath width across track. Mismatches of contour lines between adjacent tracks are reduced through a statistical method design to correct systematic profile errors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Towfish orientation and position estimation

The towfish location and orientation problems that arise in using side-scan sonar to detect objects on the sea bottom are treated separately. Data which locate the towfish relative to the ship are usually deteriorated by multipath receptions and other effects. In order to overcome this serious degradation in the location measurements, a modified Kalman filter is proposed. An estimate of the state transition matrix for this filter is derived, and a means of switching between two Kalman gains is suggested. The feasibility of the proposed filter is justified by a case study. Improved estimates of towfish pitch and heading measurements are obtained by a separate system employing model identification and subsequent Kalman filtering. Application of these methods to data from similar towed side-scan sonar systems should yield significant gains in object location accuracy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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>

PROCESSING REQUIREMENTS FOR SYNTHETIC APERTURE SONAR SYSTEMS

This paper develops expressions which relate the parameters of a sonar to the processing power and memory required for the system. These expressions can be used when designing a sonar t o ensure that realistic demands are placed on the processing engine. Benchmark test results are also presented for simulation algorithms running on a variety of machines. RESUME Ce document ddveloppe des expressions mettant en relation les paramgtres d'un sonar avec la puissance et la memoire traitements requises pour le systhme. Ces expressions peuvent dtre utilisBes lois de I'Btude du sonar de f a ~ o n B s'assurer que les exigences r6elles sont placees sur I'appareil traitant. Les resultats des tests de reperage sont Bgalement donn6s pour les simulations algorithmiques d'une vari6tB de machines. INTRODUCTION High resolution mapping of the sea-bed has traditionally been met by side-scan sonars. In radar, synthetic aperture processing is applied t o side-scan systems t o improve the resolution and this technique is now being investigated for side-scan sonar systems. The side-scan technique involves towing a transducer array along a straight path at a constant velocity. Sound is transmitted in a beam perpendicular to the motion of the transducer (fig 1). Range (acrosstrack) resolution is obtained by either transmitting a short pulse, or by transmitting a longer modulated pulse which can later be compressed into a short pulse. Azimuth (along-track) resolution is obtained by using a narrow beam, which means that a traditional side-scan sonar needs to operate at a high frequency which in turn reduces the operating range or with a long antenna which makes the system more costly and more cumbersome. Scanning Fleorns SIDE VlEW PI

Sidescan survey results from a multibeam sonar system—sea beam 2000

Abstract Over the last decade, the oceanographic community has witnessed rapid development in deep sea survey technology. Multibeam sonar systems have virtually replaced the traditional single beam echo sounders by providing higher‐density and higher‐quality topographic data. Until now, surveys generally were complemented by first‐order (flat bottom assumption) ground range corrected sidescan sonar systems providing seafloor imagery and textural information. Traditionally this meant surveying the same area twice for the complete description of the seafloor. This provided the impetus for trying to obtain both types of information from the same survey system. Research by different investigators yielded varying degrees of success in extracting bathymetric information from sidescan surveys, or sidescanlike images from the multibeam sonars in postprocessing of the field data and in blending the two for full topographic correction. This article presents the sidescan images generated during the sea trial of Sea Beam 2000, a new generation multibeam system that covers an angular range of — 60 to 60 degrees from vertical with 121 beams. The images, comparable in quality to traditional sidescan systems, are fully corrected in real time for topographic and ship speed variations. They demonstrate the breakthrough in technology for obtaining both topographic and textural information simultaneously from a precision multibeam sonar system.

Recent tectonics around the island of Timor, eastern Indonesia

Eastern Indonesia is the site of active collision between the Banda Arc and the Australian continent. The zone of collision is a poorly understood area and, in particular, controversy exists regarding the sites and styles of Holocene deformation in the vicinity of the island of Timor. This paper describes evidence for late Holocene to historically active sea-floor and shallow subsurface tectonic activity in the area, mapped using the GLORIA sidescan sonar system and single-channel seismic reflection data. Our main conclusion is that the zone of plate contact and major compressional deformation, which lies along the Java Trench west of 119° 45′E, continues directly eastward into the Timor Trough. There is little evidence for recent upper plate shortening north of this major plate boundary in the vicinity of the islands of Savu and Timor. Changes in deformational style, from small-scale uniform deformation at the Java Trench to mud diapirism and broad thrust-bounded folds further east, partly disguise the continuity of the deformation zone. Lateral changes in the style of deformation appear to be controlled only by the thickness and facies of the subducting sedimentary section. Where thin, such as at the Java Trench, uniform small-scale folding results. Further east, in the Timor Trough, a thicker sedimentary sequence deforms into buckled thrust sheets. The location of a large mud diapir field in the Timor Trough south of Savu appears to correlate with a thick sequence of Early Cretaceous marine shale that underlies the Australian shelf. The same shale may be the source of mud diapirs on Timor, demonstrating that, within the convergence zone, sediment facies rather than precise tectonic setting is the principal control on the distribution of the diapirs. A zone of strike-slip, thrust, and possibly normal faults observed to the north of Timor, between the islands of Alor and Wetar, appears to have a complex origin. Some evidence suggests that it is part of a left-lateral offset which cuts across the entire arc, through Timor. However, the apparent south-easterly thrusting of the Banda Sea beneath Wetar, and the deep pull-apart basin developed south-east of Alor, seems also to indicate eastward translation of Wetar relative to the arc further west. This may be driven by deformation of the Banda Sea under north-south compression. We see no evidence for major thrusting north of Wetar, indicating that, at this longitude, little cross-arc compression is accommodated by shortening on its northern side.

Quantitative estimation of seafloor backscattering strength using long-range sidescan sonar data

In ocean water depths of a few kilometers, long-range sidescan sonar systems generate “acoustic brightness” images of seafloor swaths 10 km or more in width using energy acoustic energy returned from the sea floor by monostatic scattering. When simultaneously acquired seafloor bathymetry is available, as it is for some modern systems, the acoustic received level versus time (imagery) data from the towed sonar “fish” can be inverted to extract estimates of seafloor backscattering strength and associated seafloor grazing angle. These estimates can be made for the entire “swath” of seafloor for which imagery is usually generated. Such inversion has been performed for sidescan imagery data from a portion of the Monterey Fan off the coast of California. Published sidescan imagery for the same region [Atlas of the Exclusive Economic Zone, Western Conterminous United States (U. S. Geological Survey Miscellaneous Investigations Series 1–1792, 1986)] indicates a strongly scattering seafloor for much of the region investigated. Consistent with this, the backscattering strength values estimated from data inversion are higher than typical values from the literature in the bright image regions, though consistent with typical values over much of the local seafloor. [Work supported in part by the Geodynamics Research Inst., Texas A&amp;M Univ.]

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.

GLORIA image processing: The state of the art

This chapter presents a summary of the image-processing techniques being used at present in the Institute of Oceanographic Sciences Deacon Laboratory's GLORIA long-range sidescan sonar system. It begins with a brief review of the development of GLORIA, and then describes in outline the present shipboard data acquisition, recording and replay system, including simple image-processing techniques that can be used on-board ship. Next, a detailed form of the sonar equation is developed, and this is evaluated factor-by-factor, to demonstrate the effects of beam directivity, refraction and water depth on the form of intensity variation to be expected in the final image. Finally, we discuss recent developments in shore-based image-processing. These include the development of improved radiometric corrections to normalize range-dependent intensity variations, recovery of true backscattering levels and estimation of backscattering coefficients, and combination of GLORIA with other data sets into single, colour digital images. As an example of the last process we show a digital mosaic of sonar data from the Southwest Indian Ridge, coloured as a function of depth derived from Sea Beam data in the same area.

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. >

Constrained iterative deconvolution applied to SeaMARC I sidescan sonar imagery

Images collected by any sidescan sonar system represent the convolution of the acoustic beam pattern of the instrument with the true echo amplitude distribution over the seafloor. At typical low speeds, the 1.7 degrees beam width of SeaMARC I (seafloor mapping and remote characterization) results in multiple insonification of individual targets, particularly at the outside of the swath. A nonlinearly constrained iterative deconvolution technique developed for radar applications can be applied to SeaMARC I imagery to reduce the effect of the beam pattern and equalize the spectral content of the image across the swath. Since the deconvolution is implemented in the along-track direction, the registration of individual scan lines must be precisely corrected before the operator is applied. The deconvolution operator must be modeled to account for beam shape, vehicle speed, swath width, slant range, and ping rate. The method is numerically stable and increases the effective resolution of the image, but results in some loss of dynamic range. The technique is applied to target recognition and imagery from volcanic terrains of the central Juan de Fuca Ridge.

A Giant Submarine Slope Failure on the Insular Slope North of Puerto Rico: A Response of Arecibo Basin Strata to Tectonic Stress: ABSTRACT

An amphitheater-shaped scarp, approximately 55 km across in water depths from about 3,000 m to 6,700 m was imaged on the northern insular slope of Puerto Rico (southern slope of the Puerto Rico Trench) using the GLORIA side-scan sonar system. This scarp represents the removal of more than 1,500 m{sup 3} of Tertiary Arecibo basin strata. The head of the scarp coincides with the location of a fault zone observed on nearby seismic-reflection profiles. Interpretation of the GLORIA imagery, and a review of available bathymetric, geophysical, and stratigraphic data and tectonic-framework models suggest that the scarp formed as a consequence of slope failure induced by tectonic oversteepening of the insular slope. The oversteepening may be a result of the most recent episode of convergence of the Caribbean and North American plates, which began approximately 4 million years ago. The Arecibo basin strata have been tilted approximately 4{degree} to the north and are apparently gravitationally unstable under the present seismic regime. The volume of material involved in this slope failure is comparable to the material displaced in tsunamogenic submarine landslides along the Peru Trench and Hawaiian Ridge. Therefore, if the slope failure north of Puerto Rico was catastrophic, it was largemore » enough to have generated a tsunami that would have flooded the low ground of northern Puerto Rico.« less

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...

Remote Sensing in Marine Environment--Acquiring, Processing, and Interpreting GLORIA Sidescan Sonar Images of Deep Sea Floor: ABSTRACT

The US Geological Survey's remote sensing instrument for regional imaging of the deep sea floor (> 400 m water depth) is the GLORIA (Geologic Long-Range Inclined Asdic) sidescan sonar system, designed and operated by the British Institute of Oceanographic Sciences. A 30-sec sweep rate provides for a swath width of approximately 45 km, depending on water depth. The return signal is digitally recorded as 8 bit data to provide a cross-range pixel dimension of 50 m. Postcruise image processing is carried out by using USGS software. Processing includes precision water-column removal, geometric and radiometric corrections, and contrast enhancement. Mosaicking includes map grid fitting, concatenation, and tone matching. Seismic reflection profiles, acquired along track during the survey, are image correlative and provide a subsurface dimension unique to marine remote sensing. Generally GLORIA image interpretation is based on brightness variations which are largely a function of (1) surface roughness at a scale of approximately 1 m and (2) slope changes of more than about 4/degrees/ over distances of at least 50 m. Broader, low-frequency changes in slope that cannot be detected from the Gloria data can be determined from seismic profiles. Digital files of bathymetry derived from echo-sounder data can be mergedmore » with GLORIA image data to create relief models of the sea floor for geomorphic interpretation of regional slope effects.« less

Spatial variability of bottom types in the lower Chesapeake Bay and adjoining estuaries and inner shelf

The spatial distributions of the bed textural and morphologic properties that influence boundary-layer roughness characteristics in the lower Chesapeake Bay, the lower portions of the York, James and Elizabeth Rivers, and the adjacent inner continental shelf were systematically mapped. A high resolution, fully-corrected side-scan sonar mapping system (100 kHz) was used for remote acoustic detection of bottom roughness, supported by ‘ground-truthing’ by direct in situ observations by divers. These complementary methods proved to be especially effective in detecting a wide range of roughness-controlling bed surface properties at various scales. Fine-scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in Nikuradse (skin friction) roughness. A wide variety of small- and intermediate-scale morphologic elements provide meso-scale and small-scale distributed (form drag) roughness. Depending upon location, the distributed roughness may be either biogenic or hydrodynamically induced (by currents and waves), although anthropogenic roughness prevails in certain instances (e.g. port areas). In terms of particular combinations of roughness scales and types, combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types, each of which is associated with a particular type of subenvironment.

Hydrothermal chimneys and associated fauna in the Manus Back‐Arc Basin, Papua New Guinea

Between December 6, 1985, and January 9, 1986, the Hawaii Institute of Geophysics Research Vessel Moana Wave surveyed the Manus Basin, north of New Britain, Papua New Guinea. This basin occupies a back‐arc position with respect to the New Britain arc‐trench system and contains an active plate boundary (Figure 1; also Taylor [1979]). Organized under the aegis of the Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/SOPAC), the cruise was part of a program to assess the mineral potential of southwest Pacific marginal basins and included a National Science Foundation‐sponsored study of crustal accretion processes in a fastspreading (greater than 100 mm/yr) back‐arc basin. The tectonic fabric of the Manus Basin was mapped and defined with the Sea MARC II sidescan sonar and bathymetric mapping system. Then dredge samples and cores were collected, and a towed bottom camera system was used to search for signs of hydrothermal activity.

GLORIA II Sonograph Mosaic of the Western U.S. Exclusive Economic Zone

In 1983 the United States declared sovereign rights and jurisdiction over living and nonliving resources in an area extending 200 nautical miles (370 km) seaward from its shores. In response to the establishment of this Exclusive Economic Zone (EEZ), the U.S. Geological Survey (USGS) has implemented a program, called EEZ‐Scan, to systematically map the EEZ, using the Geological Long‐ Range Inclined ASDIC (GLORIA) II longrange side scan sonar system developed by the Institute of Oceanographic Sciences (IOS) of Great Britain [Somers et al, 1978]. The first part of the EEZ‐Scan field program was completed in the summer of 1984, when USGS and IOS scientists surveyed the EEZ off the western conterminous United States aboard the British research vessel Farnella (Figure 1). The west coast survey, requiring 96 days of ship time and four separate legs, has resulted in virtually total sonograph coverage of the sea floor from the continental shelf break to the 200‐nautical mile limit between the Mexican and Canadian borders, an area of about 850,000 km2 . Other data collected on the cruises included two‐channel digital seismic reflection and 3.5‐kHz highresolution and 10‐kHz bathymetric profiles, as well as towed magnetometer data along approximately 20,000 km of trackline spaced nominally at 30‐km intervals.

Geomorphic Features off Southern California as Seen by GLORIA Side-Scan Sonar System: ABSTRACT

Approximately 165,000 km2 of the sea floor off southern California was mapped during May 1984, as part of a USGS/IOS cooperative program to study the newly proclaimed Exclusive Economic Zone (EEZ) of the United States Pacific margin. The area was insonified using the Geological Long-Range Inclined Asdic (GLORIA), a long-range side-scan sonar system. Images were corrected for water-column velocity anomalies, for along-track distortions caused by variations in ship speed, and for slant-range distortions caused by acoustic ray travel paths. A photo-mosaic of the overlapping sonographs has been compiled at a scale of 1:375,000. The basins of the inner California continental borderland are characterized by both sinuous channel and fan complexes and by feathery acoustic patterns indicating active sediment transport. In contrast, outer borderland basins appear to be more sediment starved, exhibit large areas of sediment failure, and show significant structural influence. West of Patton Escarpment, the sonographs are dominated by acoustic patterns showing volcanic ridges and seamounts and by deposits of the Monterey and Arguello Fans. Arguello Fan, for example, exhibits multiple sinuous channels that have transported sediment 60 km south from the canyon mouth. These channels coalesce into a single 100-km long, westward-meandering channel that terminates in a 600-m deep box canyon. A zone of sediment failure is identifiable on the north levee of an upper fan channel. Tectonic trends associated with oceanic basement are highlighted by the terminus of the west-trending Murray Fracture Zone and by the prevailing northeast trend of volcanic ridge and seamount chains. End_of_Article - Last_Page 251------------