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.On leave at the Naval Research Laboratory, Code 7420, Washington D.C. 20375-5350, U.S.A.     

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.     

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 Hopvågen 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.     

Multi-angle backscatter classification and sub-bottom profiling for improved seafloor characterization

This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa >0.7) with the Bayesian approach which also correlates well with ground truth data (r²?>?0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings.     

Processing of high-frequency multibeam echo sounder data for seafloor characterization

Processing simultaneous bathymetry and backscatter data, multibeam echosounders (MBESs) show promising abilities for remote seafloor characterization. High-frequency MBESs provide a good horizontal resolution, making it possible to distinguish fine details at the water-seafloor interface. However, in order to accurately measure the seafloor influence on the backscattered energy, the recorded sonar data must first be processed and cleared of various artifacts generated by the sonar system itself. Such a preprocessing correction procedure along with the assessment of its validity limits is presented and applied to a 95-kHz MBES (Simrad EM 1000) data set. Beam pattern effects, uneven array sensitivities, and inaccurate normalization of the ensonified area are removed to make possible further quantitative analysis of the corrected backscatter images. Unlike low-frequency data where the average backscattered energy proves to be the only relevant feature for discriminating the nature of the seafloor, high-frequency MBES backscatter images exhibit visible texture patterns. This additional information involves different statistical distributions of the backscattered amplitudes obtained from various seafloor types. Non-Rayleigh statistics such as K-distributions are shown to fit correctly the skewed distributions of experimental high-frequency data. Apart from the effect of the seafloor micro-roughness, a statistical model makes clear a correlation between the amplitude statistical distributions and the signal incidence angle made available by MBES bathymetric abilities. Moreover, the model enhances the effect of the first derivative of the seafloor backscattering strength upon statistical distributions near the nadir and at high incidence angles. The whole correction and analysis process is finally applied to a Simrad EM 1000 data set.     

Trend analysis and its tectonic implications using digital bathymetric data

Abstract

Quantitative and objective trend analysis of bottom topography in order to detect the tectonic structures has become available by use of the processed Seabeam data. The following two procedures of trend analysis are introduced.

(1) Edge detection procedures in digital image processing are applicable to the analysis of topography for extraction of the lineament of tectonic structures and prediction of the existence of faults based on the digital bathymetric data.

(2) Automatic calculation of water flow using the topographic grid data is used for estimation of not only water flow pattern and volume but also the construction of the ridge or trough axis by calculating the accumulated water volume. This method was also applied to the Seabeam bathymetric data. This is quite useful for detection of offset structures and hidden faults.

These two methods are applied to the topographic data obtained in the North Fiji Basin, which is characterized by active spreading ridges. The regional tectonic structure of the North Fiji Basin was found to be expressed by the topographic trend of the central axis.     

A porosity mapping survey in Hecate Strait using a seafloor electro-magnetic profiling system

A newly developed marine electromagnetic (E-M) system was used to create apparent porosity maps of the uppermost 20 m of sediments on the seafloor. Measurements of electrical conductivity were interpreted to give the porosity of the bottom sediments and underlying units. The data were collected continuously in a surveying mode using two receivers at different spacings to improve the resolution of porosity as a function of depth. The variations in apparent porosity over the area correlate well with information obtained from cores and acoustic profiles.

Acoustic profiles indicated the presence of biogenic gas in the central region of the survey area. The absence of distinct changes in apparent porosity over regions of gas allows us to conclude that the concentration of gas trapped within the Queen Charlotte muds does not exceed about 4% of the sediment by volume if we assume that the gas displaces fluid in the sediment.

A buried, lower porosity layer was detected, its position reflecting changes in the depth to the glaciomarine layer. Mapped variations in apparent porosity are well correlated with features on the acoustic record. This illustrates the system's ability to obtain continuous profiles of apparent porosity over seafloor features. The marine E-M survey provided rapid areal coverage, and, combined with the acoustic profiles, information on the porosity of deeper units.     

Quantitative estimation of seafloor features from photographs and their application to nodule mining

Methods developed for quantitative estimation of seafloor features from seabed photographs and their application for estimation of nodule sizes, coverage, abundance, burial, sediment thickness, extent of rock exposure, density of benthic organisms, and their lebensspuren have been presented. Digitization of the photographs shows variable nodule size (< 1 to 10 cm), coverage (< 1 to 75%) and abundance (< 1 to 20 kg/m²). Nodule population is inversely proportional to the coverage of the sediment (10–100%) and its thickness (0 to > 10 cm), which causes differential burial (0–100%) of nodules. Correlation between nodule parameters (diameter and coverage) in the photographs and grab recovery is used to evolve empirical relationships for estimating nodule abundance in different seabed settings. The rock outcrops (basalts) with a coverage of 6–100% are the sources of nuclei for the nodules, the distribution of which is controlled by the local topography. Higher concentrations of nodules are observed along the slopes, followed by the crests of seamounts, and are lowest in the valleys and plains. A population density of 6–7 benthic organisms per 100 m² belonging to 7 different phyla is observed, with a high frequency of lebensspuren (4–12 traces/m²) in association with nodules. Estimation of these parameters can be used as important inputs in the design of the nodule collector, as it will have to encounter a variety of seafloor conditions, such as patchy nodule distribution, rock outcrops, steep slopes, and frequent microtopographic changes, as well as benthic life. The distribution and relation of various features with one another can also be used to understand the possible impact of nodule mining on the seabed. Estimates show that for a yield of 3 million tonnes of nodules per year, the volume of sediment disturbed will be between 200 × 10⁷ and 500 × 10⁷ m³over an area of 300–600 km², depending upon the average abundance of nodules. Hence, the nodule collector will have to be a self‐propelled system, with photographic and acoustic sensors, to enable selective mining and avoid unfavorable areas.     

Reverberation fluctuations from a smooth seafloor

High-frequency shallow-water reverberation statistics were measured from a smooth, sandy, featureless seafloor. The reverberation statistics are presented as a function of source frequency (20-180 kHz), grazing angle (30°, 20°, 9.5°), and source beamwidths (1.2°-2.75°). Generally, the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics exhibited a complex behavior that ranged from near Gaussian to beyond log-normal. The results show that small changes in the source frequency, grazing angles, and beamwidths caused large variations in the model dependence of the reverberation statistics     

Quantitative bathymetric analyses of selected deepwater siliciclastic margins: receiving basin configurations for deepwater fan systems

Comparative bathymetric analyses of four siliciclastic continental margins reveal important distinguishing characteristics in deepwater receiving basin configurations. These characteristics have potential impact on deepwater sedimentation patterns and associated fan development. Present-day bathymetry was analysed quantitatively, using advanced software and digital bathymetry data. Various physical attributes of dip, basin geometry, and accommodation were quantified and mapped in 3D. Results show significant differences in receiving basin configurations between salt-based and shale-based continental margins in the Gulf of Mexico, Angola, Nigeria, and NW Borneo, especially in the type, amount, and distribution of accommodation. Salt-based systems have more ponded accommodation than shale-based systems; however, all the margins have a relatively small percentage of ponded to total accommodation. Two exceptions to this are the central portion of the Northwest Gulf of Mexico where ponded accommodation constitutes 55% of the total accommodation and the Kwanza Basin of Offshore Angola with 39% ponded accommodation. Shale-based systems may be more prone to bypass on the upper to mid slope than salt-based systems. Evidence for this is found in the linear grade trend analyses where large below-grade areas (sinks) are pervasive on the upper and mid slope of salt-based systems for deepwater sediment to accumulate, while shale-based systems show extensive above-grade highs across the entire slope. Ponded accommodation trends and drainage analyses also demonstrate that shale-based systems are more susceptible to extensive bypass than salt-based systems. The tectonically active margin of NW Borneo has steeper slope profiles than the passive margins, with areally small ponded accommodation largely restricted between active toe-thrusts on the lower slope and locally distributed on the shelf. These and other quantitative analyses suggest that the type of substrate and overall tectonic setting are important factors to consider for receiving basin configuration and its potential impact on deepwater sedimentation patterns.     

Characterizing Indian Ocean manganese nodule-bearing seafloor using multi-beam angular backscatter

Application of quantitative angular backscatter modelling to manganese nodule-bearing areas of the Central Indian Ocean Basin (CIOB) has been initiated at NIO during the year 1998. Studies were aimed to establish the suitability of seafloor backscattering in delineating seafloor parameters characteristic of nodule-rich sediments. In this paper, processed Hydrosweep multi-beam backscatter data from 45 spot locations in the CIOB (where nodule samples are available) were analysed to estimate seafloor and sediment volume roughness parameters. The application of a composite roughness model to a nodule-bearing region (6,600 km²) of the CIOB, to determine seafloor interface roughness parameters from a multi-beam backscatter dataset, shows only four power law sets. The results attest 80% of the nodule-bearing seafloor to be smooth in terms of interface roughness parameters at micro-topographic level. The sediment volume roughness parameters are dominant only in 29% of the smooth interface roughness sites. This indicates that 51% of the seafloor area possesses negligible (interface and volume) roughness. A critical analysis using pseudo-side-scan records from 12 selected locations in the study area affirms the combined importance of the seafloor interface and sediment volume roughness parameters for precise determination of manganese nodule abundance.     

Signal processing strategies for a bathymetric sidescan sonar

A dominant source of errors in swath bathymetry is acoustic interference. In 1989 the author published an analysis of these errors and predicted depth accuracies for a system which reduced their effect by averaging. This present paper shows how a considerable improvement in performance may be obtained by a variety of signal processing strategies that include the use of several widely spaced receivers and the elimination of the most unsatisfactory measurements before averaging. Simulations show how impressive sea bed profiles can be produced with a single ping, even at low signal-to-interference ratios     

Results of a known seafloor instability event

A submarine sediment instability event (landslide) occurred at Kitimat, British Columbia, in 1975. Recent high-resolution surveys provide details of the resulting seafloor morphology. The effects of the slide include modification of the fjord head delta-front slopes, transport of delta sediments into deep water and mixing with deep water, fjord bottom clays. Distinctive features include the results of shallow rotational sliding, tearing and shearing, compressional folding and long distance block gliding at the downslope slide terminus.     

Backscatter calibration of high-frequency multibeam echosounder using a reference single-beam system,on natural seafloor

The calibration of multibeam echosounders for backscatter measurements can be conducted efficiently and accurately using data from surveys over a reference natural area, implying appropriate measurements of the local absolute values of backscatter. Such a shallow area (20-m mean depth) has been defined and qualified in the Bay of Brest (France), and chosen as a reference area for multibeam systems operating at 200 and 300 kHz. The absolute reflectivity over the area was measured using a calibrated single-beam fishery echosounder (Simrad EK60) tilted at incidence angles varying between 0° and 60° with a step of 3°. This reference backscatter level is then compared to the average backscatter values obtained by a multibeam echosounder (here a Kongsberg EM 2040-D) at a close frequency and measured as a function of angle; the difference gives the angular bias applicable to the multibeam system for recorded level calibration. The method is validated by checking the single- and multibeam data obtained on other areas with sediment types different from the reference area.     

Optimal localization of a seafloor transponder in shallow water using acoustic ranging and GPS observations

This study utilized circular and straight-line survey patterns for acoustic ranging to determine the position of a seafloor transponder and mean sound speed of the water column. To reduce the considerable computational burden and eliminate the risk of arriving at a local minimum on least-squares inversion, the position of a seafloor transponder was estimated by utilizing optimization approaches. Based on the implicit function theorem, the Jacobian for this inverse problem was derived to investigate the constraints of employing circular and straight-line survey patterns to estimate the position of a transponder. Both cases, with and without knowledge of the vertical sound speed profile, were considered. A transponder positioning experiment was conducted at sea to collect acoustic and GPS observations. With significant uncertainties inherent in GPS measurements and the use of a commercial acoustic transponder not designed for precise ranging, experimental results indicate that the transponder position can be estimated accurately on the order of decimeters. Moreover, the mean sound speed of the water column estimated by the proposed optimization scheme is in agreement with that derived from conductivity, temperature, and density (CTD) measurements.     

一种分层海底反向散射模型

In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface,and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency(0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency(below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency(10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect.     

Study of the sedimentation and self-weight consolidation behavior of seafloor sediments using a radioisotope densitometer

ABSTRACT

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p’) at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (s_u) values are almost the same when the density values are less than 1.14?g/cm³, and the s_u values increase linearly with an increase in density when the density values are in the range of 1.14–1.2?g/cm^3.     

Estimation of mean grain size of seafloor sediments using neural network

The feasibility of an artificial neural network based approach is investigated to estimate the values of mean grain size of seafloor sediments using four dominant echo features, extracted from acoustic backscatter data. The acoustic backscatter data were collected using a dual-frequency (33 and 210 kHz) single-beam, normal-incidence echo sounder at twenty locations in the central part of the western continental shelf of India. Statistically significant correlations are observed between the estimated average values of mean grain size of sediments and the ground-truth data at both the frequencies. The results indicate that once a multi-layer perceptron model is trained with back-propagation algorithm, the values of mean grain size can reasonably be estimated in an experimental area. The study also revealed that the consistency among the estimated values of mean grain size at different acoustic frequencies is considerably improved with the neural network based method as compared to that with a model-based approach.