Acoustic imagery using a multibeam bathymetric system

Historically, measurement and collection of deep‐ocean acoustic imagery are accomplished by towed sidescan systems. Recently, work has been performed to extract acoustic imagery from current hull‐mounted wide‐swath bathymetric sonars with minimal hardware modification. Past work of deriving acoustic imagery from swath sonars has been performed primarily with SeaBeam's sixteen 2^2/3 ° preformed beams. The Navy is investigating the feasibility of extracting an acoustic image from the Sonar Array Survey Systems (SASS), a high‐resolution (1^o beams) wide‐fan (90°) bathymetric system. Due to the large data volume (approximately 1 MB per ping), SASS normally discards the raw acoustic returns once bathymetry is calculated. In early 1991 the Naval Air Development Center (NADC) installed the hardware on board the USNS Maury to capture and record the raw acoustic signal (inphase and quadrature) from the SASS's 144 hydrophones for later inversion to a backscatter image. Preliminary qualitative mosaics of the sidescan images show promising results and warrant further development.     

A generic swath‐mapping data format

There is a pressing need for standardization of data derived from bathy‐metric swath‐mapping systems. Currently several dozen multibeam and sidescan sonar data formats exist within the oceanographic community, and more can be expected as new systems are developed. Without some standardization of swath‐mapping data formats, the capability for use and integration of data from different systems will be severely compromised.

This paper presents a strategy for organizing swath bathymetry data in a logical modular fashion that will allow data from all current swath bathymetric sonar systems to be stored and accessed in a common fashion. We have chosen the approach of defining compact efficient modules for each logically independent portion of a data record and storing it in a manner that is portable between diverse computer architectures and operating systems. This approach is extensible to accommodate new types of data. Although specifically developed for swath bathymetry, this format is also capable of supporting digital sidescan data and other types of swath data.     

Feasibility of interferometric swath bathymetry using GLORIA, along-range sidescan

The feasibility of adding an interferometric swath bathymetric system to GLORIA, a 6.6 kHz long-range sidescan sonar, is discussed. The size of GLORIA's low-frequency transducer arrays and towfish precludes significant modifications, but even without such changes bathymetric errors could be several tens of metres over a usable swath somewhat smaller than the normal GLORIA swath. A swath bathymetry based on GLORIA will have random errors depending strongly on wind speed, water depth, and swath width. Within the range of these parameters, root-mean-square bathymetry errors in the range of 1-100 m can be expected     

System concept and results of the Canadian Aerial Hydrography Pilot Project

Abstract

The Canadian program for obtaining hydrographic data by aerial methods consists of merging laser bathymeter data with photogrammetric depth data. The main deficiency of the photogrammetric approach for bathymetric measurements is that incomplete stereomodels can occur in areas where little or no land appears. This problem is overcome by using an inertial navigation system (INS) hardmounted to the aerial camera to provide the orientation parameters of position and attitude for each photograph. In order to meet the high accuracy requirement, the INS and other complementary navigation data are processed through a post‐mission track recovery software package. The photogrammetric depths are improved further by merging them with the waterline height information and the laser bathymeter depths using a least‐squares adjustment algorithm. The photogrammetric compilation, depth measurements, shoreline plots, and laser bathymeter integration is done in an analytical stereoplotter. This instrument provides an on‐line refraction correction necessary because of the two‐media mode of operation. Results of a recent pilot project indicate that the integrated system is capable of obtaining depth measurements that agree with echo sounder depth measurements to a precision of .65 m (RMS), and that it can position measured depths to a precision of .74 m (RMS) relative to local control.     

Coastal Bathymetric Studies from Space Imagery

Abstract

Studies of coastal bathymetry are important where littoral drift has implications on the planning of fishing and dredging operations. Also, there is a possibility of finding hitherto unknown bottom features in relatively less explored regions of the shallow seas around the globe. High resolution satellite imagery over oceans provides us with quantitative methods for estimating depth in shallow parts of the seas. One of the methods is the analysis of the refraction of coastal gravity waves observed on satellite imagery. A panchromatic image acquired by SPOT with 10 m resolution on March 22, 1986, over Bay of Bengal near Madras Coast, was used for this analysis. The image was enhanced to clearly bring out the wave structure seen on the sea surface. The image was then superimposed with a 1 km × 1 km grid. For each grid cell, 64 × 64 pixels at the center were considered for getting a Fast Fourier Transform to determine the wave spectrum and the dominant wavelength present there. The classical theory of gravity waves was used to relate the shallow water wavelengths obtained as above with the corresponding wavelengths in the deep water. The deep‐water wavelength was estimated to be 110 m using the known chart depths at a set of control points. The resulting depth estimates, when compared with standard bathymetric charts, were found, in general, to be well in agreement up to a depth of 30 m in the sea, with an r.m.s. error of 2.6 meters. The method seems to be very useful for remotely sensed bathymetric work. However, further research is required to reduce the error margin and operationalize the method.     

The filtering and compressing of outer beams to multibeam bathymetric data

Some errors and noises are often present in multibeam swath bathymetric data. Echo detection error (EDE) is one of the main errors. It causes the depth error to become bigger in outer beams and looks like sound refraction. But depth errors due to EDEs have a trumpet-shaped appearance, instead of a curved appearance that is caused by the sound refraction errors. EDEs, including systematic acoustic signal detection errors and internal noises, cannot be removed during the correction of sound refraction. It causes depth inconsistencies between adjacent swaths and degrades precision of outer beams. Sometimes, the bathymetric errors caused by EDEs do not even meet the requirements of IHO (International Hydrographic Organization). Therefore, a post-processing method is presented to minimize the EDEs by filtering outliers and compressing outer beams of multibeam bathymetric data. The outliers caused by internal noises are removed by an automatic filter algorithm first. Then the outer beams are compressed to reduce systematic acoustic signal detection errors according to their depths, the calculated depth line and standard deviations (SDs). The automatic filter process is important for calculating the depth line. The selection of inner beams to calculate the average SD of beam depths is crucial to achieving compressing goals. The quality of final bathymetric data in outer beams can be improved by these steps. The method is verified by a field test.     

Correction for depth biases to shallow water multibeam bathymetric data

Vertical errors often present in multibeam swath bathymetric data. They are mainly sourced by sound refraction, internal wave disturbance, imperfect tide correction, transducer mounting, long period heave, static draft change, dynamic squat and dynamic motion residuals, etc. Although they can be partly removed or reduced by specific algorithms, the synthesized depth biases are unavoidable and sometimes have an important influence on high precise utilization of the final bathymetric data. In order to confidently identify the decimeter-level changes in seabed morphology by MBES, we must remove or weaken depth biases and improve the precision of multibeam bathymetry further. The fixed-interval profiles that are perpendicular to the vessel track are generated to adjust depth biases between swaths. We present a kind of postprocessing method to minimize the depth biases by the histogram of cumulative depth biases. The datum line in each profile can be obtained by the maximum value of histogram. The corrections of depth biases can be calculated according to the datum line. And then the quality of final bathymetry can be improved by the corrections. The method is verified by a field test.     

Postprocessing and corrections of bathymetry derived from sidescansonar 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     

High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m² in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.     

Toward an automated system for a correctly registered bathymetricchart

A scheme has been developed for automated bathymetric registration of multiple overlapping swaths of data collected by a ship equipped with a multibeam echo-sounder device. Because each swath of data overlaps with several others, registration is performed both at local and global levels. The primitives used for local matching are contours of constant depth which are extracted from the data and are represented as a modified chain code. The main heuristic guiding the search for matching contours of equal depth is their proximity to the middle of the apparent (unregistered) overlapping region. The degree to which two contours match is determined by the correlation of their respective chain codes and the geometrical proximity of their nodes. All best matches are considered tentative until their geometrical implications are evaluated and a consistent majority has emerged. To do global matching. a cost function is constructed and minimized     

Advancements in the U.S. army corps of engineers hydrographic survey capabilities: The SHOALS system

In an effort to modernize its hydrographic survey capabilities, the U.S. Army Corps of Engineers has undertaken a joint development program with Canada to construct and field test an operational prototype airborne lidar bathymeter system. The construction and field verification effort of this program began March 1990 with field tests scheduled for winter 1993. The system will be built by Optech, Inc., based on their design of the LARSEN 500, the only commercial lidar system currently producing bathymetric surveys.

The Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system will operate out of a medium‐sized helicopter such as the Bell 212 at approximately 200 meters altitude where the laser scanning system generates a swath width of just over 140 meters. System requirements dictate a laser operating at 200 Hz in both the blue‐green wave length for maximum water depth penetration and the infrared for surface interface recognition. Each laser shot strikes the water surface at a known location where its energy is partially reflected back to the receiver and partially transmitted through the water column. Transmitted energy undergoes scattering and absorption along its path to the bottom where the remaining energy is then reflected back to the receiver.

The Transceiver, Positioning, Acquisition, Control and Display, and Ground Based Data Processing subsystems make up the SHOALS system. These subsystems have been designed, constructed, and currently are being laboratory tested prior to total system integration and field testing. This article presents the system's design and discusses system use following development.     

San Diego trough geotechnical test area

Abstract

The San Diego Trough Geotechnical Test Area, located about 24 km southwest of San Diego in a water depth of about 1.2 km, lies near the base of the Coronado Escarpment directly north of the Coronado Fan. A new bathymetric map delineates a shallow basin in the soft, highly plastic, clayey silts flooring the Test Area. Measurements of shear strength by vane and static cone pene‐trometer, and bulk density by nuclear densitometer, were made in place from the submersible Deep Quest. Sixteen short (< 1.6 m) gravity cores were collected from ships.

The geotechnical properties show little areal variation and generally change uniformly with depth within the 55 km² Test Area. Silt is the predominant grain size, averaging about 62%. In‐place bulk density shows little change with increasing depth, values range from 1.23 to 1.26 Mg/m³; laboratory density values increase with depth, ranging from 1.30 to 1.52 Mg/m³ between the surface and a depth of about 1.1 m. The difference between the in place and laboratory values may indicate sampling densification of the cored sediment. Water content in the cores decreases uniformly within the range of 249 to 43% dry weight. Shear strength increases linearly with depth. The laboratory shear strength values are lower than the in place values, which range from 4 kPa at the surface to about 29 kPa at a depth of 3.27 m. Predictor equations relate Atterberg limits, bulk density, water content, and laboratory and in place shear strength to depth. Sedimentation‐compression e log p curves have an equivalent compression index of 1.5 to nearly 2. Excluding rurbidite layers and sampling disturbance effects, all cores indicate a uniform depositional environment in the surface to 1.6 m of sediment sampled. The geotechnical properties indicate that the sediments in the west central and southwest parts of the Test Area exhibit vertical heterogeneity due to thin silt‐sand layers, presumably of turbidity current origin, that originated from the Coronado Canyon.     

Morphologic investigation of “uncharted” seamount from central Indian basin revisited with multibeam sonar system

Morphology of a seamount at 12°35'E and 76°18.5’ and two abyssal hills in its vicinity is described using the Hydrosweep multibeam‐swath bathymetric system. The height of the seamount is 1350 m, and it occupies an area of 330 km². Its basal width is 22.5 km, and the mount has a gentle and longer western flank and a steep and shorter eastern flank. There is a characteristic terracelike feature on the western flank, about 300 m from the top. A caldera is also observed on top of the seamount. Slope angles in this area are high (over 35"). Results of morphologic studies of the seamount from the multibeam survey are comparable to those from a narrow‐beam echosounding survey. The origin of the seamount may be related to the presence of a fracture zone at 75°45'E.     

Tide Model CST1 of China and Its Application for the Water Level Reducer of Bathymetric Data

ABSTRACT

The performance of global tide models in China's seas is far worse than in deep water, owing to generally larger and more complex tides. An ocean tide model (named CST1) was developed by Princeton Ocean Model (POM) and blending assimilation technique, which includes 13 major constituents with a 1.2' × 1.2' spatial resolution. The nautical charts over the Chinese continental shelves and ETOPO1 database are used to generate the bathymetric grid model. The bottom friction coefficient is parameterized through a linear dependency on the depth of each grid. The weight parameter in blending assimilation is a function of the amplitude of constituent. Compared with 33 tide gauges along the Chinese coastline, the Root Sum Square (RSS) error of 8 major constituents is 12.09 cm, and the RSS of 4 major constituents (M2, S2, K1, O1) is 10.76 cm. CST1 with spatially interpolated residual water level was used to provide water level reducers for bathymetric survey. An example near amphidromic point of semi-diurnal constituents in the Yellow Sea demonstrated its process. Assessment showed that it can meet the required accuracies of standards of hydrographic surveys in China. The application proved that CST1 can predict adequately precise astronomical tide level, and is potentially economical and the best approach to provide water level reducers, although some checks should be made in survey planning stage.     

Fault scarp identification in side-scan sonar and bathymetry images from the Mid-Atlantic Ridge using wavelet-based digital filters

Digital filters designed using wavelet theory are applied to high resolution deep-towed side-scan sonar data from the median valley walls, crestal mountains, and flanks of the Mid-Atlantic Ridge at 29°10 N. With proper tuning, the digital filters are able to identify the location, orientation, length, and width of highly reflective linear features in sonar images. These features are presumed to represent the acoustic backscatter from axis-facing normal faults. The fault locations obtained from the digital filters are well correlated with visual geologic interpretation of the images. The side-scan sonar images are also compared with swath bathymetry from the same area. The digitally filtered bathymetry images contain nine of the eleven faults identified by eye in the detailed geologic interpretation of the side-scan data. Faults with widths (measured perpendicular to their strike) of less than about 150 m are missed in the bathymetry analysis due to the coarser resolution of these data. This digital image processing technique demonstrates the potential of wavelet-based analysis to reduce subjectivity and labor involved in mapping and analyzing topographic features in side-scan sonar and bathymetric image data.     

A new bathymetric model for the central Fram Strait

Based on data from R/V Polarstern multibeam sonar surveys between 1984 and 1997 high resolution bathymetry has been generated for the central Fram Strait. The area insonified covers approx. 36,500 km² between 78–80°N and 0–7.5°E allowing the creation of a Digital Terrain Model (DTM) with 100 m grid spacing. The DTM was utilized for contouring and generation of a new series of bathymetric charts (AWI Bathymetric Charts of the Fram Strait, AWI BCFS) at a scale of 1:100,000. The paper starts with a brief introduction to the regional setting of the study area comprising information on the local links between bathymetry, sea ice transport and water mass exchange. The bathymetric feature names used in this article and how they were chosen is outlined. Next, the input data and processing applied are described. Thereafter the newly created grid and contour data are put into context with existing data sets. Finally the main bathymetric features of the area are characterized and the generated data products available for public disposal are specified.     

Shallow Water Bathymetry Derived from Green Wavelength Terrestrial Laser Scanner

Abstract

Shallow water bathymetry has proved to be a challenging task for remote sensing applications. In this work, Green-Wavelength Terrestrial Laser Scanning (GWTLS) is employed to survey nearshore bathymetry under clear atmospheric and water conditions. First, the obtained seabed points were corrected for refraction and then geo-registration, and filtering processes were exerted to obtain an accurate bathymetric surface. Terrain analysis was performed with respect to a reference surface derived from classical surveying techniques. The overall analysis has shown that the best results stem from 35° to 50° incident angles, whereas for angles higher than 65° measurements are not acceptable, although for the same angle in front and close to the instrument accuracy is considered acceptable due to the high laser power. Also, high resolution micro-topography, shallower than 1?m water depth, was managed to be captured. Systematic experimental approaches are expected to improve the GWTLS technique to detect bathymetry, which is anticipated to assist in mapping very shallow foreshore, tidal, and deltaic environments, to contribute conceptual into developing hybrid observation systems for coastal monitoring, and also to be applied in various maritime applications.     

基于相干原理的多波束测深新算法

为了改善多波束声纳的分辨率,提出了一种基于相干原理的测深新算法,对每一个波束脚印内的信号进行相干处理,获得了大量的海底深度值。在此基础上,采用新算法对仿真数据和某型号多波束测深声纳湖上实验数据进行处理。结果表明,相对于传统多波束测深算法,该算法可显著提高声纳海底测量的分辨率,获得大量的海底深度测量值。     

基于等效声速剖面法的多波束测深系统声线折射改正技术

多波束测深技术是近几年比较流行的海洋勘测手段之一,其具有全覆盖、无遗漏、高精度和高效率等特点。多波束测深系统的声学原理和海水的不均匀性,使得声波在传播过程中发生声线的折射现象,对波束测点的最终位置的归算带来较大的误差。针对多波束数据后处理方法,介绍了利用等效声速剖面法对多波束测深系统的声线折射进行改正的一种新方法,并通过试验对比,证明了该方法可以提高多波束测深系统的整体测量精度。