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Effective use of geophysical sensors for marine environmental assessment and habitat mapping

G. E. Jones & G. E. Glegg School of Earth, Ocean and Environmental Sciences, University of Plymouth

Abstract

Given increased concern for sustainable management and its application to marine areas, the aim of this paper is to assess how best sublittoral marine habitats may be surveyed in order that the marine environment may be monitored, in compliance with the requirements of the scientific community, and that of the European Community, in both a cost effective and sustainable manner. Habitat surveys require information on the geomorphology and the biology of an area so that biotope units can be classified. A range of different survey approaches can be used for habitat identification all of which have both advantages and disadvantages. The final product usually results from a combination of at least two different approaches - for example, a remotely sensed sidescan sonar image of the site with ground truthing in the form of physical samples or video or still photography. This project aims to consider the use of the full range of survey and marine habitat mapping techniques and considers their cost effectiveness and their repeatability as part of a sustainable monitoring regime. It includes an assessment of work carried out within Plymouth Sound, a candidate Special Area of Conservation (cSAC), where a range of surveys using differing techniques and sensors has been undertaken in recent years. Keywords: environmental assessment, habitat mapping, sidescan sonar, AGDS systems, multibeam sonar, hydrographic survey, seabed classification.

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8

1 Introduction

Recent years have seen increased concerns for the state of the environment but the marine environment has lagged behind the land, as a result of its being unseen. However politicians are suddenly awakening to this problem, which scientists have been attempting to highlight for a number of years, and are now implementing legislation in order to protect marine sites. For a number of years scientists have been proclaiming a decline in the marine environments, but have been faced with many commercial organisations exploiting marine resources to the detriment of the natural ecosystem. A case in point is that of Strangford Lough (Northern Ireland), which has been in decline for in excess of forty years [1]. Within the UK, in response to environmental concerns, the government initially passed the Wildlife and Countryside Act in 1981, requiring the identification of Sites of Special Scientific Interest (SSSI’s) By 2002, there were in excess of 6,500 such sites within the UK, but only 5% of those are estuarine, and virtually none are below the low water mark. Subsequent legislation, such as the EC Habitats and Strategic Environmental Assessment directives, and their national derivatives, are today making greater provisions for marine areas such that habitat mapping and environmental assessment exercises are becoming a necessity throughout a nation’s continental shelf. Classification of Special Areas of Conservation (cSAC’s), the development of voluntary fishing No Take Zones and voluntary Marine Areas of Conservation are all examples of new approaches to seabed management. However, for their establishment and maintenance, there is a need for repeatable geomorphological surveys. This paper seeks to provide an overview of the methods available and to consider which is most appropriate for long-term monitoring.

2 Survey techniques

Identification of tools for habitat surveys is not unique. A number of institutions have commissioned reports into how assessment may be achieved, with noteworthy reports being those of the California Department of Fish and Game (U.S.A.) [2] and by the SeaMap Research Group, in the UK [3], the latter concentrated on the trailing of a RoxAnn acoustic ground discrimination system (AGDS), occasionally using sidescan sonar, for quality control, along with associated ground truthing. In the UK, the Joint Nature Conservation Committee (JNCC) Natura 2000 – Marine Monitoring Handbook, also, recommends procedural guidelines for mapping [4]. The problem with all of these documents however is that the techniques are all listed as viable methods for a single study of seabed character, but little consideration is given to their repeatability or cost. Each of these methods offers advantages and disadvantages, which are considered below.


6 Coastal Environment V, incorporating Oil Spill Studies

2.1 Divers

Diver surveys are probably the original form of seabed visualisation. Today most dive surveys within the UK follow either the JNCC Marine Nature Conservation Review (MNCR) approach, or the Marine Conservation Society’s Seasearch recording procedure and are completed by a mix of voluntary divers and qualified marine biologist/divers. The major limitation of this survey technique is that it only records conditions at a single point, the location of which may be difficult to recover precisely. The data recorded is also subject to the diver’s ecological knowledge, judgement of importance and survey experience. This last may be overcome, to a degree, by incorporating both still and/or video photography, but will still be restricted to a single location, of say 5-10 metre radius, per dive.

2.2 Aerial and satellite photography

This normally takes the form of LIDAR (Light Detection And Ranging) laser and/or photogrammetry photography from aircraft, but may also make use of multispectral imagery from satellites. It is very similar to techniques used to photographically assess environments on land. Whilst this is a technique recommended for benthic habitat mapping by the U.S. National Oceanic and Atmospheric Administration (NOAA) for a number of habitat types [5], it is severely restricted in turbid waters. Indeed within a recent survey conducted in Plymouth Sound, the maximum attained depth was only 25 metres. SPOT (Systems Pour l’Observation de la Terre) satellite photography has also been used - within the Mediterranean – for the charting of shallow water (up to 30-50 metres) benthic communities [6]. Unfortunately, the technique is again restricted by a lack of water penetration.

2.3 Echo sounders

These instruments are generally intended to measure the depth of water. As such they would appear to be of limited use, however a development to interface to these has been the Acoustic Ground Discrimination Systems (AGDS), as discussed below.

2.4 Acoustic Ground Discrimination Systems (AGDS)

Generically this description is used to identify a group of sensors that interface with echo sounders to measure the strength of the acoustic backscatter signal, the intensity of which is dependent upon the nature of the reflector. The systems also analyse the reflected acoustic signals and, based on an assessment of the signal characteristics, assigns them to a seabed classification. The most widely recognised survey systems within this group are the RoxAnn and Quester Tangent Corporation (QTC) View systems, which have been used by a number of researchers and governments.


Coastal Environment V, incorporating Oil Spill Studies 7

Some commercial operators, however, are sceptical about these systems, which require extensive training and calibration in order to identify the “classes” within which the “sediment-types” will be grouped. In the case of RoxAnn, this is based on hardness and roughness signatures derived from the first and second return waveforms, whilst the QTC technique is based on an analysis of the first waveform return, only. QTC claim that their View system measures many more parameters than the RoxAnn system and thus offers the potential for a more detailed analysis and discrimination of different sediment types.

Another drawback with these systems is that they are specific to echo sounder, frequency, vessel and location, and need retraining within new survey areas and/or within changes of water depth. Logistically this makes the use of these systems more complicated and time consuming, if good results are to be attained. These complications all add to the concerns about the ability to ‘repeat’ comparative surveys or to integrate separate surveys, within a single map or database, especially when even a small change in line position may give a different return (see Section 5, below). In addition, unless they are configured for use with a multibeam sounder, these systems only gives a partial view of the seabed, along the survey lines traversed, and it is then necessary to interpolate between survey lines to obtain an areal picture.

2.5 Sidescan sonar

This instrument normally comprises a pair of acoustic transceivers that obliquely scan the seabed to produce an acoustic image that can be compared to a strongly side-illuminated black and white photograph. High frequency systems, typically 500 kHz, can produce a highly resolved spatial image of the seabed (to 20 centimetric resolution) and identify features such as cobbles and individual sandwaves. Contemporary recording systems, such as the GeoAcoustic GeoPro, and the CODA DA (Data Acquisition) systems also offer facilities whereby a series of parallel and overlapping swathes can be integrated into a mosaic to provide the equivalent of a geo-referenced aerial photograph of the study area and, unlike aerial photography, it is not depth restrained.

One disadvantage of sidescan sonars is that they are aspect and line of sight dependent and accordingly are not always consistent in their returns. However, by following good survey procedures, the observance of 100% overlap, the ability to merge, select and weight images within the final mosaic, as well as its low relative cost, this technique becomes increasingly valuable.

In addition, the development of fractal signal analysis, signal filtering and the use of probability and separation matrices to give a quantitative measure of texture or contrast has led to the development of sediment characterisation capabilities, within the likes of CODA’s GeoKit and GeoAcoustics’ GeoTexture software. These may enable either object-based or pixel-based analysis of the detailed imagery to provide detail and groupings of comparable ecological units. This process, which is similar to that of the AGDS, still requires human intervention since systems are sometimes unable to discriminate between two areas. This may occur simply as a result of poor normalisation of port, starboard



and/or adjacent scans, thus leading to a level of erroneous analysis. Hence ‘classification’ must again be approached with care.

2.6 Multibeam and interferometric swathe bathymetry systems

These systems represent emerging technologies for seabed mapping. Based on development of the echo sounder (multibeam systems) and the sidescan sonar (interferometric systems), these systems can provide isonification of a swathe of seabed, providing both depth and acoustic backscatter strength information, which can be displayed within mosaics and/or composite three-dimensional models of the seabed, with the sidescan imagery overlaying the seabed shape and bathymetry. Their development has been driven by the primary need for bathymetric detail and it is only lately that manufacturers have come to realise the full potential of the imagery available from the acoustic backscatter signal and a potential for seabed classification. Equipment manufacturers have approached developments individually, based on the requirements of their client-base and whilst GeoAcoustics, who produce sidescan sonar systems and the GeoSwathe interferometric sounder, developed GeoTexture as an extension to their GeoSwathe suite of software, QTC developed their Multiview AGDS software in order to interface to multibeam sounders, which they see as the replacement for the single-beam echo sounder.

Whilst swathe bathymetric systems might appear to be the ultimate tool for seabed and habitat mapping, resolution and the quality of the imagery is a function of the spatial separation between the sensor and the seabed. For optimal resolution, lateral ranges should be limited to about 100 to 150 metres since, allowing for a depth to range ratio of 1:10, imagery will start to deteriorate should the separation become greater than 10 to 15 metres. For much of the coastal work this will be adequate, whilst the problem can be overcome by mounting the sounder onto remotely operated vehicles (ROVs), or autonomous underwater vehicles (AUVs) for deeper water. Unfortunately this only increases the costs. There has been a large number of surveys conducted using multibeam, particularly in North America, but at a purchase price and installation cost of between £100,000 and £250,000 for the sounder and operating system alone, this instrumentation may well be beyond the budget of many habitat mappers, who may have jurisdiction over waters up to and in excess of 200 metres water depth. As a result, repeated use of this technique may prove to be unsustainable. In addition the resolution of the backscatter imagery, from a multibeam, is often considered inferior to that of a towed sidescan sonar, which may be acquired for a fraction of the cost and which is not depth restricted.

2.7 Laser line scanners

These instruments are similar to sidescan sonars, but utilise light transmission and imagery, rather than that of acoustic signals. Accordingly they effectively convey true imagery, with resolution that is sub-centimetric. Given this potential, they have been trialed as part of NOAA’s Undersea Research Program [7] and by Science Applications International Corporation [8]. Unfortunately, limitations



upon their use is again likely to be a function of cost and range. Reliant on light penetration, ranging capabilities is again limited to 50 metres in very clear shallow waters, however, this range may be restricted to the order of 3 metres within a harbour, or typical UK waters, where visibility may be poor.

2.8 Video/still photography

As previously stated, this technique of ‘mapping’ can be conducted as part of the process of diver surveying and accordingly be a survey technique in its own right. Still photography can either be based on simple oblique photography or the result of vertical photography, of say a 1 metre square grid, as may be defined by a pre-deployed frame set upon the seabed and within which a species count can be conducted. Unfortunately such a survey technique would only give spot details and interpolation of data between images is usually required. Video photography, as may be achieved by either a ROV mounted camera, or from a towed sled/camera, would give a more dispersed view of changes across the seabed, but would still require interpolation between survey lines.

3 Ground truthing

The bulk of the preceding survey techniques require ground truthing, either via the acquisition of visual imagery and/or physical sampling, for biota and geomorphological identification. Such is the case of AGDS systems, which require pre-survey training in order to enable ecological discrimination, or for sidescan sonar and swathe bathymetric systems, which require post-survey ground truthing. Video imagery is generally becoming the primary form of ground truthing, however, photography on its own is not always adequate. Equally, the acquisition of physical samples by mechanical means may fail to give a full picture of the condition of the seabed and the relationships of any biota. Once again there is also the issue of single-point sampling and its density within the survey area, or whether video transects are more meaningful, given the ability to display the areal changes along the lines of transects.

Ground truthing will add to the survey costs, but is a necessity. Reductions in either the amount of ground truthing or even the density of ‘soundings’, will result in a less efficient survey.

4 Case study – Plymouth Sound

On the South Coast of England, Plymouth Sound lies at the seaward confluence of the rivers Tamar and Plym and was formed at the end of the last ice age as sea level rise flooded the lower river valleys to create a ria-type coastline. Upon the surface it appears to be an open bay, protected by a detached breakwater. Beneath the waves, however, is a complex array of channels of biological and historic importance, ranging in depth from zero to 40 metres and covering an area of some 40 km2, within the offshore limits of the 30-metre contour.



With respect to this area, the first surveys of consequence were those conducted by Hiscock & Moore, in 1986, and the Devon Wildlife Trust (DWT), in 1993, to initially appraise and to compile ecological data, for the area, which ultimately formed the basis of a case for Plymouth Sound to be championed as a cSAC [9, 10]. These surveys consisted of diver surveys at 32 and 24 pre-selected dive sites, respectively. In both cases the survey reports stated the surveys to be incomplete in their assessment. Areas surveyed were described as selective and both surveys reported the sighting of previously unseen species, whilst also acknowledging that previously sighted species were not found. This highlights the limitations of dive surveys.

4.1 AGDS survey

As part of the process of substantiating Plymouth Sound as a cSAC, English Nature commissioned a further survey, in 1996, using a RoxAnn AGDS system with a nominal line spacing of 200 metres [11]. Ground truthing was conducted at 34 stations using a combined diver/video programme and at a further 57 stations, using a combination of a 0.1 m2 Day grab sampler and video (within the whole of the candidate area). Unfortunately the final report again admitted omissions, particularly in a failure to record any eel grass, a flora known to be present. Sublittoral survey findings were assigned to a limited number of ecological classes (6), which were increased (to 9) as a result of the ground truthing.

As would appear to be the trend in cases of producing an environmental audit, the findings of all three surveys, between 1986 and 1997, were compiled by JNCC into a composite assessment (see figure 1), for inclusion within their Marine Nature Conservation Review (MNCR) [12].

4.2 Sidescan sonar

Survey work within Plymouth Sound is frequently conducted by the University of Plymouth for education and research. One such sidescan sonar survey was completed, in 1989, as a part of a sediment study [13], whilst the author has conducted two further surveys (2003), one using a stand alone GeoAcoustic sidescan system and the other a GeoSwathe interferometric system.

The 1989 survey was conducted using a 100kHz Waverley 3000 system, at 100-metres line separation, and presented as a line-drawn interpretation, as was typical at that time (see figure 2). By comparison, the 2003 surveys were conducted at 500kHz and 250kHz, respectively, with 50 metres line spacing. Presentation in both cases was via sidescan sonar mosaics, derived using CODAmosaic and GeoTexture (see figure 3), respectively. In all three cases the sidescan sonar products were believed to show more detail than that portrayed by the 1996 English Nature survey and the JNCC compilation. This is particularly noticeable in the area to the south east of Drake’s Island, which is characterised by sand ribbons, which are ~90 metres in width, ~700 metres in length, running in a 020/2000 direction and showing transitional sediment zones of ~50 metres to



either side. Also prominent, within the 2003 mosaic, is a 50 by 200 metre rock outcrop, known as Dunstone Rock, which is not included on the JNCC chart.

Figure 1: Composite assessment of ecological units within Plymouth Sound. Compiled by Brian Miller, Nic Miller and Ian Reach. Reproduced from the MNCR, with permission from JNCC [12].

Whilst both the artistic line drawing and the mosaic are geomorphologically biased, the variation in acoustic backscatter intensity does represent changes in ecological units. The 1989 surveys were ground truthed by divers, whilst the 2003 surveys were ground truthed by video photography.

2.4.1 Sediment classification As has been stated, a further capability of modern sidescan sonar processing software is for sediment characterisation, either on the basis of object-based (manual), or pixel-based (automatic) selection. Whilst this may be advantageous in portraying classes, making it comparable to the AGDS systems, it is held that



a lot of data may be lost in the interpretation and the assignment of data to a limited number of classes. A figure of between five and seven such classes normally arise from within broad-scale surveys. This may allow for a general assessment and character viewing, however, it does not bear-up when looking for the precise measurements needed in order to monitor change.

Figure 2: Sidescan sonar line-drawn interpretation of sediment classes, 1989. Reproduced with the permission of Fiona Fitzpatrick [13].

5 Habitat surveys

In the UK, we are perhaps ten years behind environmentalists in other parts of the world, notably those of the U.S. and Australia, and it is therefore useful to look at how they conduct habitat surveys. A review of projects, documented upon the internet, shows that most surveys are generally ‘for assessment’ and that there is little comment or indication of any thoughts about repeatability. Further, documented surveys have largely been conducted with multibeam or



single beam echo sounder coupled with an AGDS. Interestingly, some have recorded the use of both single beam echo sounder/AGDS and sidescan sonar, or multibeam/AGDS and sidescan [14, 15]. Within such surveys, the results from the multibeam and/or the AGDS and the sidescan have been described as ‘comparable’. One must therefore ask about the need for ‘all systems’, or if survey costs may be reduced. In addition, one must consider the future source of funding and the potential for repeatability. This last should be considered prior to the initial survey.

Figure 3: GeoSwathe/GeoTexture mosaic of the northern half of Plymouth Sound, showing different ecological units, as completed in 2003.

Whilst support funds would initially appear to be readily available for an opening audit of the environment, provided the commissioning institution can secure matched funding, in the long-term the full costs of monitoring are likely to come from public taxation and, resultantly, monitoring survey costs will need to be kept low, if repeat surveys are to be sustainable.

Given that total isonification is preferable to interpolation, one can understand the utilisation of multibeam systems. These systems do however carry a high cost, being primarily intended as high resolution ‘bathymetry’ systems. Whilst it must be admitted that bathymetry, or depth, is an environmental factor affecting a biotope, it must also be considered that bathymetry, whilst subject to change, is going to be of less importance within the environmental survey. To draw parallels, within environmental surveys upon



land, we look more at what is on the land, than changes in the elevation of countryside, although it has to be acknowledged that the marine topography is more dynamic and subject to change. In addition a lack of confidence in AGDS systems, which was quantified within the SeaMap report as to a figure of between 37% and 57%, raises the question of the acceptability of AGDS surveys for comparative purposes [16].

Accordingly it is this researcher’s consideration that a more qualitative approach should be taken and that use of sidescan sonar as the primary tool should be given increased consideration. If bathymetry is required, then the lower cost interferometric-type swathe sounder, such as the GeoAcoustic GeoSwathe system, should be considered. Interestingly it may be noted that the QTC Multiview (AGDS) software utilises the acoustic backscatter signal that makes up the sidescan sonar-type image within a multibeam system, for its seabed classification, when interfaced to the multibeam system.

6 Conclusions

A large number of current habitat surveys are conducted using multibeam echo sounding and acoustic ground discrimination systems (AGDSs). Not only does the process of classifying the seabed into classes produce a limited number of biotopes, but the results of studies have shown that there the potential for a low correlation between comparable AGDS surveys.

If monitoring of the environment, in this way, is to become a reality, then it is held that a more suitable technique for the geomorphological component, at an acceptable cost and with a higher potential of provide comparable results is that of sidescan sonar and mosaic imagery. Modern processing of the sidescan sonar data offers near photographic imagery in the form of interpretable mosaics. This view is supported by the Devon Wildlife Trust [17], who recently used such techniques to convince the local fishing associations to ‘set aside’ the Beer Ground Reefs, in Lyme Bay, as a protected area.

References

[1] Erwin, D.G., Strangford Lough – 40 plus years of loss of biodiversity under conservation management. Coastal Futures 2003: Proc. of Coastal Management for Sustainability – Review and Future Trends [CD-ROM], ed. R.C. Earll, Coastal Management for Sustainability: Gloucester, 2003.

[2] Kvitek, R., Iampietro, P., Sandoval, E., Castleton, M., Bretz, C., Manouki, T. & Green, A., Final Report – Early implementation of Nearshore Ecosystem Database Project, California Department of Fish and Game: Monterey Bay, pp. 27-56, 1999.

[3] Foster-Smith, R.L., Davies J., & Sotheran, I., Broad scale remote survey and mapping of sublittoral habitats and biota, final technical report on sublittoral mapping methodology, Scottish Natural Heritage: Perth, pp. 18-27, 1999.



[4] JNCC, Natura 2000 – Marine monitoring handbook, Joint Nature Conservation Committee: Peterborough, 2001.

[5] Finkbeiner, M., Stevenson, B., & Seaman, R., Guidance for benthic habitat mapping; an aerial photographic approach, NOAA/CSC/20117-PUB, U.S. NOAA Coastal Services Center: Charleston, 2001.

[6] Belsher, T., Meinesz, A., Lefevre, J. R. & Boudouresque, C-F., Simulation of SPOT satellite imagery for charting shallow-water benthic communities in the Mediterranean. Marine Ecology, 9(2), pp. 157-165,1988.

[7] NOAA, Laser line scan mapping for seafloor habitats, NOAA Undersea Research Program Web Site, Silver Springs, Maryland, U.S.A. http://www.nurp.noaa.gov/Spotlight Articles/laser.html

[8] Storey, P., 2002. Scientists to try laser scanner to detect algae, California Coastal Coalition Web Site, Encinitas, California, U.S.A. http://www.calcoast.org/news/wetlands011302.htm

[9] Hiscock, K., & Moore, J., Surveys of harbours, rias and estuaries in southern Britain: Plymouth area including the Yealm, CSD Report, No. 752, Nature Conservancy Council: Peterborough, 1986.

[10] DWT, Survey Report, Plymouth Sound and approaches – 1993, Devon Wildlife Trust: Exeter, 1993.

[11] Posford Duvivier Environmental, Broad scale biological mapping of Plymouth Sound and estuaries, ENRR 208, English Nature: Peterborough 1997.

[12] Northen, K., Smith, J. & Moore, J., Plymouth Sound (Chapter 12). Marine nature conservation review, section 8, inlets in the western English Channel: area summaries, Joint Nature Conservation Committee, Peterborough, pp. 87-103, 1999.

[13] Fitzpatrick, F., Studies of sediments in a tidal environment, PhD. thesis, University of Plymouth, 1991

[14] Bornhold, B.D., Collins, B. & Yamanaka, L., Comparison of seabed characterization using sidescan sonar and acoustic classification techniques. Proc. of the Canadian Coastal Conference: Victoria, Canada, pp. 893-908,1999.

[15] Saade, E.J., Seafloor habitat mapping nearshore San Diego County, Thales GeoSolutions (Pacific) Inc. Web Site, San Diego, California, U.S.A. http://www.thales-geopacific.com/papers/seafloorhabitat_SD.pdf

[16] Foster-Smith, R.L., Davies J., & Sotheran, I., Broad scale remote survey and mapping of sublittoral habitats and biota, final technical report on sublittoral mapping methodology, Scottish Natural Heritage: Perth, pp. 102-110,1999.

[17] Stanford, R., Personal communication, July 2003, Lyme Bay Reefs Project Officer, Devon Wildlife Trust, Exeter. UK.



effective use of geophysical sensors for marine …...effective use of geophysical sensors for...

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