© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
An overview of seabed-mapping technologies in the context of marine habitat classification
a CEFAS Burnham Laboratory Remembrance Avenue, Burham-on-Crouch, Essex CM0 8HA, UK
b Geological Survey of Sweden, Division of Marine Geology Box 670, S-75128, MUppasala, Sweden
c GEMEL, Stnd'Etude en Baie de Somme, Quai Jeanne d'Arc 80230 St Valery-s/Somme, France
d Geological Survey of Canada, Bedford Institute of Oceanography PO Box 1006, Dartmouth, Canada NS B2Y 4AZ
e Netherlands Institute of Applied Geosciences, TNO PO Box 80015, NL3508 TA Utrecht, The Netherlands
f ICIT Heriot-Watt University, Old Academy Back Road, Stromness, Orkney, KW16 3AW, UK
*Correspondence to A. J. Kenney. e-mail: a.j.kenny{at}cefas.co.uk.
A wide range of seabed-mapping technologies is reviewed in respect to their effectiveness in discriminating benthic habitats at different spatial scales. Of the seabed attributes considered important in controlling the benthic community of marine sands and gravel, sediment grain size, porosity or shear strength, and sediment dynamics were highlighted as the most important. Whilst no one mapping system can quantify all these attributes at the same time, some may be estimated by skilful interpretation of the remotely sensed data. For example, seabed processes or features, such as bedform migration, scour, slope failure, and gas venting are readily detectable by many of the mapping systems, and these characteristics in turn can be used to assist a habitat classification (and monitoring) of the seabed. We tabulate the relationship between "rapid" continental shelf sedimentological processes, the seabed attributes affecting these processes, and the most suitable mapping system to employ for their detection at different spatial scales.
Keywords: seabed-mapping technologies, swath system, single-beam system
Received 2 May 2002; accepted 12 December 2002.
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Introduction |
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 Top Introduction Principal acoustic seabed...Area of seafloor mapped...Relevant geological attributesConclusionsReferences |
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Maps revealing the geophysical characteristics of the seabed represent an essential tool for the effective management of the marine environment, because they allow the wide-scale geology and present-day (Holocene) sedimentary processes to be determined and understood (Laban, 1998). This helps scientists to predict accurately the impact of mans' activities on the seabed and, in particular, impacts on those habitats of high-nature conservation and/or ecological value. In addition, offshore sediment dynamics play an essential role in the long-term stability and geomorphology of the coastline, important considerations when planning flood and coastal sea defence schemes.
Imaging of the seabed was revolutionized in the 1940s when, for the first time, relatively high-frequency echo-sounders were positioned in such a way so as to insonify (expose to sonar energy) a swath of seabed (Fish and Carr, 1990). These early systems yielded the first sidescan sonar (SSS) sonographs (hard copy output of sonar data). The first sonographs had low resolution and could only reliably be used to detect large physical targets such as shipwrecks. However, the 1970s and 1980s witnessed rapid developments in acoustic electronics, which allowed the phase and amplitude properties of the acoustic signal to be precisely controlled, leading to high-resolution images of the seabed of almost photographic quality to be obtained. Most recent developments in acoustic mapping during the 1990s have been associated with the increase in digital processing power offered by modern computers. This has enabled acoustic engineers to incorporate digital electronics in transducers designed to make them more efficient. In addition, new software applications are continually being developed, offering greater data control functions, with most systems now supporting real-time visualization of sonar data as true (geo-corrected) mosaic seabed maps.
Irrespective of post-processing applications, important differences exist between available acoustic devices in relation to user-needs. The many acoustic devices that are currently on the market generally fall into one of the following categories; (i) broad-acoustic beam (swath) systems, such as SSS (Fish and Carr, 1990; Kenny, 1998; Newton and Stefanon, 1975), (ii) ground-discriminating single-beam echo-sounders (AGDS) such as RoxAnn® and QTC-View® (Foster-Smith and Gilliland, 1997; Greenstreet et al., 1996; Magorrian et al., 1995) and fish finding echo-sounders, (iii) multiple narrow-beam swath bathymetric systems (Hughes Clarke, 1998; Loncarevic et al., 1994), and (iv) multiple-beam (interferometric) SSS systems (Green and Cunningham, 1998).
The most commonly used and versatile systems are SSS and multi-beam swath bathymetric devices (here called multi-beam echo-sounders, MBES). These systems are subsequently described in more detail and a tabulated comparison with other devices, such as ground-discriminating single-beam echo-sounders (acoustic ground discrimination systems, AGDS), is provided highlighting their advantages and disadvantages for various seabed-mapping applications.
Sub-bottom profilers provide high-resolution definition of sediments down to a maximum of about 50 m in soft sediment and much less in coarser sediments or in shallow waters. The sound source is generally a pressure compensated boomer or sparker generating a high-intensity, short-duration pressure pulse with well-defined directional characteristics. These devices offer the potential to map sediment thickness, infaunal communities and to examine interactions between benthic fauna and sediments. However, an appraisal of these systems is not within the remit of this article.
While aerial and satellite imagery are widely used in the discrimination and mapping of terrestrial habitats, these techniques have a much more limited application to seabed-habitat mapping, where they are restricted to very shallow waters (always <15 m, and usually <6 m in temperate waters). Even under these conditions, surface effects, light attenuation (which varies with depth and wavelength), and other water column effects, combined with the general problems encountered in the use of entire aerial and satellite imageries, hinder consistent interpretation of images. Aerial photography provides better resolution, and systems like compact airborne spectrophotographic imager (CASI) have been employed in several studies of seabed habitats, such as coral reefs (Mumby, 1999) and sea grass meadows (e.g. Kendrick et al., 2002). In general, the use of such systems is limited in temperate waters to the littoral and sub-littoral coastal fringes, but in very shallow waters, where it is not practicable to deploy acoustic systems, may provide the only means of acquiring remotely sensed data. The focus of this review is acoustic systems and these remote optical sensing techniques are not considered further. A "brain-storming" sub-group of the ICES "Working Group on the Effects of Extraction of Marine Sediments on the Marine Ecosystem (WGEXT)" was convened in 1999 to discuss as to which of the ("measurable") seabed geological attributes are most important in determining the type of seabed benthic faunal assemblages.
We describe the techniques, which are presently used to map the shape of the seafloor and physical properties of surficial sediments, based on which the habitat (mainly physical attributes) and biotope (habitat and community) mapping classifications may be developed, with the aim of identifying as to which acoustic-mapping techniques are most suitable for mapping seafloor habitats at different spatial scales. There are many complimentary benthic "ground-truthing" sampling methods, such as grabs, corers, and underwater photography, but a detailed appraisal of these techniques is beyond the scope of this review.
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Principal acoustic seabed-mapping technologies |
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TopIntroductionPrincipal acoustic seabed...Area of seafloor mapped...Relevant geological attributesConclusionsReferences |
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Sidescan sonar
SSS has been defined as an acoustic imaging device used to provide wide-area, high-resolution pictures of the seabed. The system typically consists of an underwater transducer connected via a cable to a shipboard recording device. In basic operation, the SSS recorder charges capacitors in the tow fish through the cable. On command from the recorder, the stored power is discharged through the transducers that in turn emit the acoustic signal. The emitting lobe of sonar energy (narrow in azimuth) has a beam geometry that insonifies a wide swath of the seabed, particularly when operated at relatively low frequencies (typically <100 kHz). The returning echoes from the seafloor are received by the transducers over a short period of time (from a few milliseconds to 1 s), amplified on a time-varied gain curve and then transmitted to the recording unit. Most technological advances relate to the control of the phase and amplitude of the emitting sonar signal and in the precise control of the time-varied gain applied to the return signals. The recorder further processes these signals. In the case of a non-digital transducer, the analogue signal is converted into a digital format, the proper position for each signal in the final record (pixel by pixel) is calculated and then these echoes are printed on electro-sensitive or thermal paper, one scan or line at a time.
Modern high (dual) frequency digital SSS devices offer high-resolution images of the seabed on which objects on the order of tens of cm may be detected at a range of up to 100 m either side of the tow fish (total swath width 200 m). However, precision and accuracy depend on a number of factors. For example, the horizontal extent of the image is affected by the frequency of the signal and the grazing-angle of the signal to the bed, which is determined by the altitude of the transducer above the sea floor. Some typical limits associated with SSS are as follows: under optimal seabed conditions and altitude above the bed, a range of 300 m (600 m swath) can be obtained at a frequency of 117 kHz and typically 150 m at a frequency of 234 kHz. Accuracy increases with decreasing range. For example, 0.1 m accuracy is typically obtained at a range of 50 m (100 m swath), while only 0.3 m accuracy is obtained at a range of 150 m.
A major advantage is that under optimal conditions, SSS can generate an almost photo-realistic picture of the seabed. Once several swaths have been mosaiced, geological and sedimentological features are easily recognisable and their interpretation provides a valuable qualitative insight into the dynamics of the seabed. However, the quality (or amplitude) of the data varies. For example, the grey-scale (signal amplitude) between swaths covering the same area of seabed differs often noticeably. The variation in signal amplitude for the same area or type of seabed causes problems when trying to classify the sonograph, if ground-truth samples reveal the seabed to be the same, while the sonograph indicates differences. SSS does not normally produce bathymetric data, but provides information on sediment texture, topography, and bedforms, and the low-grazing angle of the beam makes it ideal for object detection.
There is a general trade-off between the area that can be mapped in a given time and the resolution or detectability of seabed features within the mapped area. For example, a SSS operating at 500 kHz potentially detects features measured in dm, but this can only be achieved along a narrow swath of about 75 m per channel and, therefore, the area mapped per hour is relatively small. By contrast, systems that operate at lower frequencies of around 50 kHz have much greater range and can be towed at faster speeds. Table 1 compares the footprint resolution versus range for two particular system settings.
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The recently developed "chirp" and synthetic aperture sonars (SAS) provide high-resolution sonar images at greater range (McHugh, 2000). These systems emit more energy by generating longer-duration and wide-bandwidth pulses, with the resolution of the sonar depending on the bandwidth and not the pulse length, as is the case with traditional SSS. Table 2 compares the relative performance of varying system configurations.
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Multi-beam echo-sounders
MBES provide a relatively new seabed-mapping technology that can be applied to an understanding of the marine habitats, aggregate resources and seabed processes. Through digital processing techniques, the data provide shaded-relief topographic maps. Echo-strength data (reflectance) can be extracted and presented as seabed backscatter maps that display information on sediment types. Slope maps can also be provided. From a combination of both shaded-relief bathymetry, slope analysis and backscatter maps, the seabed can be interpreted in terms of both relict and recent processes. Multi-beam data processing can also enhance subtle aspects of relief elements through shading techniques for an understanding of erosive and depositional processes.
There are many manufacturers of MBES for operating in water depths of a few meters to full ocean depths. Higher resolution systems for continental shelf depths provide resolution in dm. Interpreted maps of seabed geology and the relief and processes from these systems help to provide the foundation for the assessment and mapping of seabed habitats.
A major advantage of MBES over SSS is that MBES generate quantitative bathymetric data that are much more amenable to classification and image processing, but the narrow beam width (ideal for quantitative analysis) makes them less useful for detection of small objects (<1 m; Brissette and Hughes Clarke, 1999). A typical high-resolution set-up would be a 1.5° degree beam width in 30 m of water providing a 0.8 m diameter nadir footprint.
Two factors control the potential bathymetric target resolution capability of a multi-beam echo-sounder: distance between soundings (both cross and along track), and size of the nadir footprint. Table 3 presents the results of two MBES systems, one with a 3.3° beam width and the other with 1.5° beam width (modified after ICES, 1999). The two systems are compared while operating under varying conditions of water depth and speed. Note that the higher-resolution system (EM3000) is not appropriate for applications in deeper water (>400 m), and indeed, for detecting objects of about 1 m2 the optimum operating conditions would be survey speeds of up to 12 knots in 50 m of water.
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Acoustic ground discrimination systems
Normal incidence single-beam echo-sounders may be used to obtain a variety of information about the reflective characteristics of the seabed. They send a pulse of sound at a particular frequency (usually within 30–200 kHz) that reflects from the seabed and the echo is picked up by the transducer. RoxAnnTM, an AGDS that has been frequently used for environmental studies off the UK (Foster-Smith and Gilliland, 1997) uses echo-integration methodology to derive values for an electronically gated tail part of the first return echo (E1) and the whole of the first multiple return echo (E2). While E2 is primarily a function of gross reflectivity of the sediment (hardness), E1 is influenced by small- to meso-scale backscatter and is used to describe bottom roughness. By plotting E1 against E2, various acoustically different seabed types can be discriminated (Chivers et al., 1990; Heald and Pace, 1996). With appropriate ground-truth calibration, AGDS can be remarkably effective at showing where changes in seabed characteristics occur. However, great caution should be exercised in trying to directly compare readings taken during different surveys, as it is difficult to ensure that the sounder is delivering the same power level into the water column, especially when intervals between the surveys are in the order of months or years. This problem has, in part, been addressed in the design of the Echoplus (Chivers et al., 1990), where a greater consideration has been given to using narrow-beam geometry and stabilising the power output.
Although AGDS is relatively simple to use, the output requires considerable interpolation to generate a broad-scale map of the seabed with 100% coverage. The area insonified directly under the vessel depends on beam angle and depth. For example, a sounder with a beam angle of 15° at a depth of 30 m would insonify an area with a radius of about 7 m. This limits the ability of the system for accurate discrimination. For example, a 7 m track that is composed of sand with one or two cobbles would have a different E1/E2 value compared with an adjacent 7 m track of sand with say six or seven cobbles. However, the habitat in both cases would be the same: that is a sandy bottom with cobbles. Table 4 summarizes the characteristics of three AGDS. At present, AGDS are also being developed or are under consideration, based on other sonar types, like MBES and forward-looking sonar.
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Recent comparative studies between SSS and two AGDS (RoxAnnTM and QTC-ViewTM) showed that accurate, continuous spatial-mapping swath systems out-performed single-beam systems in identifying habitats. There was a general agreement between the two AGDS, although this depended largely on the post-processing methods applied (Brown et al., 2001).
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Area of seafloor mapped versus object resolution |
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TopIntroductionPrincipal acoustic seabed...Area of seafloor mapped...Relevant geological attributesConclusionsReferences |
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Three important factors to consider when selecting the most appropriate and cost-effective acoustic system for habitat mapping are: (i) size of the area to be mapped, (ii) depth range over the survey area, and (iii) object detectability required. Table 5 provides a comparison between an MBES and a high-frequency SSS. Because the MBES has its transducers rigidly mounted to the hull of the vessel, the footprint diameter (or resolution) is significantly reduced as the depth increases beyond about 50 m. This in turn will determine the maximum coverage achievable within a given time. By contrast, the SSS is towed above the seabed at a constant height to ensure that a low-grazing angle is always maintained. Coverage, and hence the resolution achieved, is, therefore, independent of depth.
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There are many other technologies capable of mapping the seafloor (Table 6). At one extreme, these include benthic corers, grabs and probes that sample small areas of seabed, but allow microstructure and composition to be investigated in detail. In most mapping surveys, these devices will be used to ground-truth acoustic data (Rumohr, 1995).
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Clearly, the choice of system will depend on survey objectives and scale of the area to be mapped. For baseline broad-scale mapping of the continental shelf, where relatively large geological features, such as sand waves and reefs are of interest, the quantitative data offered by MBES in conjunction with object detection in the order of tens of meters (at 200 m depth) is often the preferred choice. However, for inshore areas and depths <50 m where identification of small (<10 m) habitat features may be required, a combination of MBES and SSS ensures that both quantitative bathymetric data (1 m resolution) and qualitative, high-resolution habitat relief data (10 cm resolution) are obtained.
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Relevant geological attributes |
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TopIntroductionPrincipal acoustic seabed...Area of seafloor mapped...Relevant geological attributesConclusionsReferences |
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Geological attributes that may be relevant for classifying and mapping marine habitats are: micro-relief (cm–dm), macro-relief (m–hm), grain size (gravel, sand, silt and clay), lithology (rock composition, carbonate content), patchiness (local variability, shape, spatial patterns), sediment distribution, sediment sorting, porosity (pore spaces and packing), shear strength, grain shape, stratigraphy, dynamic processes (relict to recent and combinations thereof), bedforms and bedform migration, sediment transport pathways, sediment thickness, regional setting (e.g. sandbank, moraine, beach ridge, basin), geological history (origin), anthropogenic features (shipwrecks, anchor marks, extraction pits, and dredge material mounds and trawl marks).
This list has been evaluated against available monitoring techniques to find which method of detection (at varying spatial scales) best suites the mapping of "key" habitat attributes thought to control the benthic faunal assemblages (Table 7). Of the geological attributes listed, sediment grain size, porosity or shear strength, and sediment dynamics were considered particularly important for habitat classification in relation to benthic faunal assemblages of marine sands and gravels.
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Bioherms and biogenic accumulations (e.g. shells, maerl, mussel-beds) represent special cases where the biota influence the nature of the seabed to the extent that they may be detected by remote acoustic sensing techniques. As such, bioherms have been treated as a separate environmental condition of the seabed. Table 7 categorizes the relationships between sediment, its stability and relevant processes of physical disturbance and also indicates as to which technique, or combination of techniques, is best suited for identifying each of the conditions described.
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Conclusions |
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TopIntroductionPrincipal acoustic seabed...Area of seafloor mapped...Relevant geological attributesConclusionsReferences |
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Several conclusions may be drawn about technical advantages and disadvantages of various devices for habitat mapping. Swath systems (such as SSS) are most likely to provide the best high-resolution maps, particularly over wide areas. They provide information on sediment texture and bedform structure and allow dynamic processes (e.g. sediment transport) to be deduced. Disadvantages associated with swath systems are their high costs and the need to have skilled interpretation. In addition, the output often requires considerable post-processing time and expense to obtain appropriate classifications. By contrast, single-beam systems (such as AGDS) cost much less and are generally simple to operate. The disadvantage of single-beam sounders is that they require intensive calibration (ground-truthing) to discriminate between habitats. The beam often has a large acoustic footprint resulting in low resolution. Also the lack of swath coverage results in the need to undertake extensive spatial interpolation to provide full-coverage maps of the seabed.
The value of one system versus any other depends on survey objectives, but as a general guide, the high-resolution capability of SSS and their ability to discriminate small-scale habitat features (0.3–1 m), together with providing information on habitat stability makes them most suitable for most detailed biotope-mapping applications. Single-beam AGDS consistently detect gross differences in substrate. Although experience suggests that more subtle differences in the acoustic properties of the seabed may be detected, these are often difficult to define or calibrate.
For broad-scale mapping of aggregate habitat (>1 km2), SSS and MBES were considered to offer the most cost-effective means of discriminating different sediment types and dynamic processes. For small-scale habitat classification (<1 km2), high-resolution SSS, underwater cameras, and grab-sampling methods are considered to be the most appropriate mapping tools. Finally, it is highly recommended that appropriate biological ground-truthing is undertaken where remote-sensing technologies are to be used for habitat-mapping purposes.
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Acknowledgements |
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The authors are grateful to the Geological Survey of Sweden and Canada for providing data on the comparative performances of SSS and MBES systems and to Ron McHugh, of Heriot-Watt University, for additional data on MBES and SAS, and to all the members of the ICES Working Group on the Effects of Extraction of Marine Sediments on the Marine Ecosystem (WGEXT) for their contributions to the text and review during the Annual Meetings of the Group in 1999 and 2002.
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Footnotes |
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This article was compiled and reviewed by the members of the ICES Working Group on the Effects of Extraction of Marine Sediments on the Marine Ecosystem (WGEXT). In addition to the editors (cited as authors), contributions were made by: E. Andrulewicz, H. Bokuniewicz, S. E. Boyd, J. Breslin, C. Brown, J. Costelloe, C. Dijkshoorn, R. Courtney, S. Freeman, S. J. de. Groot, L. Galtier, S. Helmig, H. Hillewaert, J. C. Krause, B. Lauwaert, H. Leuchs, G. Markwell, M. Mastowske, A. J. Murray, P. E. Nielsen, D. Ottesen, R. Pearson, M-J. Rendas, S. Rogers, T. Simpson, A. Stolk, S. Uscinowicz, and M. Zeiler. 
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References |
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