Bathymetric Sidescan Techniques for Near Shore Surveying

Dr C. R. Bates
School of Geography and Geosciences, University of St Andrews.
P. W. Byham
Submetrix Ltd.

Abstract

Ten years ago, bathymetric sidescan surveying was only affordable for ocean-scale research projects and some large budget oil and gas operations.  Recently however bathymetric sidescan equipment has seen significant developments where a combination of decreasing electronic costs and increasing power of personal computing has made the technique become a reality for lower budget, high resolution near shore surveying.  For example, the high degree of bathymetric resolution and complete 3D coverage offered by the technique is providing new insights into complex sea floor geology and also allowing evaluations of dynamic sediment movements.  In environmental habitat studies, both the bathymetry and co-located amplitude information is proving invaluable for habitat appraisal when the technique is used in combination with other acoustic methods and appropriate ground control from video, diver and sampling programmes.  The method is outlined and limitations for surveying are discussed before three case histories are used to illustrate some of the potential applications of the technique.

Introduction

A bathymetric sidescan system is one that is used to measure the depth to sea floor and amplitude of sonar return from the sea floor along a line extending outwards from the sonar transducer at right angles to the direction of motion of the sonar (Geen et al., 1993). As the sonar platform moves forwards, a profile of sweeps is defined as a ribbon-shaped surface of depth measurements known as a swath in a similar manner to a sidescan image of the seafloor. The final deliverables from a bathymetric sidescan system are similar to those from a multi-beam survey however the mechanism for recording the data are different. The use of multibeam systems for both bathymetric mapping and analysis of seafloor type have been extensively reviewed in the literature (for example, Clarke et al., 1996; de Moustier, 1989). A basic outline of the bathymetric sidescan technique is given here however only a brief comparison is made between bathymetric sidescan and multi-beam as this has been discussed elsewhere (Davis et al., 1986; de Moustier and Matsumoto, 1993; Blacquiere and Van Woerden, 1998).

The acoustic signal is produced by the sonar in a similar manner to a sidescan acoustic pulse and is narrow in azimuth (that is, viewed from above), and wide in elevation (viewed from the side) (Geen et al., 1993). The difference between a sidescan and bathymetric sidescan system is in the recording of the acoustic energy (De Moustier and Matsumoto, 1993). In a bathymetric sidescan system a number of transducers or transducer staves are used to record the returned energy that is back-scattered from the seafloor. When this back-scattered sound is detected at the transducers, the angle it makes with the transducer is measured by recording the phase difference between transducers and a reference signal. Multiple staves ensure that both the angular measurement and the overall phase resolution are measured with high precision. The range for a reflector is calculated from the travel time to the reflector and back and the range and phase angle pair enable the location of the ensonified seabed patch to be known relative to the sonar transducer thus creating a 3D bathymetry map of the seabed. The range is measured using travel times to typically better than 0.05m and transducer angles to better than 0.05degree. As the transmit beam spreads in the water away from the sonar in a similar manner to the sidescan, the size of the footprint will also increase away from the sonar. Thus a footprint can be calculated with 234kHz transducers to about 0.87m at near range and 5.2m at 300m range along track and 5cm across track (Table 1). The 117kHz transducer has an along track footprint of 1.5m at near range and 8.9m at 300m range with a 7.5cm across track dimension.

                                            Across track rangeAcross track range

Frequency (kHz)/

beam width (degree)  50              300            50              300

117/1.7                              1.5             8.9             0.075        0.075

234/1.1                              0.9             5.8             0.050        0.050

Table 1: Across track and along track resolution

Because phase difference is recorded a major advantage of the bathymetric sidescan sonar is realised in that there is no footprint spreading along the beam i.e. in the across track dimension. However, it should be noted that it may not be possible to achieve these across track dimensions in practice at far offsets due to energy loss. The maximum range limit is dictated by the nature of the seafloor and the grazing-angle limit where most of the energy is reflected away from the seafloor. Bottom types such as soft mud or peat can reduce the expected range by as much as 30% however, sand, rock and shingle all give good sonar backscattering. A typical far range limit is about 7.5 times the water depth giving a total swath width of approximately 15 times the water depth. For seafloor classification this is an important issue as it is vital that similar size areas of the bottom are surveyed across a sonar record in order to be able to make meaningful comparisons (Monterey Bay, 2000). It should also be remembered that if there are slopes on the seafloor that fall away from the direction of the sonar beam, these areas will fall into shadow zones and it is unlikely that they will be ensonified. Thus, obtaining true 100% coverage of the seafloor is rarely a practical achievement.

Similar to the sidescan sonar, the number of pings or hits on a target is defined by the ping rate and speed advance of the sonar over the seafloor of the survey. The ping rate is determined by the furthest range limit and speed of beam in the water (Table 2). High survey speeds will result in poorer target definition or poorer quality images of the target as discussed for the sidescan sonar. Some bathymetric sidescan systems alternate transmitting between each side of the sonar to avoid interference, thus the ping rate is effectively halved. However, the distance between pings indicated in the table can be halved if only one side or transducer is used.

                                      Range

Survey Speed (kts)50              100            200

4                                   0.26           0.53           1.07

8                                   0.53           1.07           2.10

Table 2: Distance between pings (alternate pinging for port and starboard transducers)

In addition to a determination of the location of a reflecting target on the seafloor, the amplitude of the returned signal can be measured with the bathymetric sidescan system (Geen, 1998). This amplitude data can be used in one of two ways. Either it can be treated as a sidescan record, that is as a time series to produce a qualitative image of the seafloor, or it can be processed using the bathymetric information for the point on the seafloor from which each individual reflection is measured. In this latter case the recorded amplitude is compared with the source signal after compensation for energy losses during the travel path such as loss of signal to the water column, spherical beam spreading and the incidence angle for scatter or reflectance from the sea floor (Goodfellow, 1996). It is only since the development of this type of sonar with high fidelity co-location of bathymetry and amplitude that these compensations for amplitude loss have been possible. The processing of this type of amplitude data is currently the focus of research activities at a number of institutions and may represent and important new use of the bathymetric sidescan and multibeam sonar (Clarke et al., 1996; Canepa and Bergem, 1997).

Because of the high fidelity of depth measurements with bathymetric sidescan systems they have been used for a number of survey objectives in a variety of locations over the last 5 years. The types of survey include offshore hydrocarbon pipeline and telecommunication cable route planning, dredge analysis of harbours, estuaries and ports, resource evaluation and environmental surveying (Geen, 1998 and 1999). The range of users includes engineering companies, hydrographic surveyors and environmental assessors. Three examples are given to illustrate the use of bathymetric sidescan systems. Different survey parameters were used for each survey but each data set was processed using similar protocols which included final bathymetric surfaces interpolated using a using triangular irregular network (TIN).

Plymouth sound

A bathymetric sidescan survey was conducted at Plymouth Sound for English Nature using the Submetrix System 2000 sonar with 234kHz transducers. The primary objective of the survey was to produce a data set of bathymetry and sea-bottom amplitude for comparison with other remotely sensed geophysical data and ground truth data of biological habitats derived from video and grab sampling. The final deliverables included a bathymetric chart for a 5.5km north-south strip and a 3km east-west strip of the Sound together with amplitude data over key areas of bottom habitat. The survey was conducted over a calm two day period in January, 2000.

 Fig. 1: Bathymetry for Plymouth Sound

For this survey a Leica RTK differential GPS system was used for navigational information. The bathymetric sidescan system was deployed in a bow-mount configuration together with DMS 205 motion reference unit. Tidal corrections were provided by a continuous-logging tide gauge and models from the Hydrographic Office. Sound velocity measurements were made before and during the survey for correction of the sonar data. Survey lines were located to give 100% overlap between swaths running both north-south and east-west. The acquisition parameters resulted in final position errors of less than 1m over the entire survey and in places less than 0.4m. Height information gave errors of less than 1m for all the area with less than 0.2m for at least 40% of the area.

Results - Bathymetric Data

The bathymetric chart for the total Plymouth Sound data set is shown in Figure 1. A number of interesting features are shown on the chart. To the north of the outer breakwater, two large circular depressions were mapped that have been formed by the differential scour produced by mooring buoys from the Navy warship moorings. The depressions have a 0.5m topographic expression with a further 1m scour indicated at the point that the chains are attached to the seafloor. Heterogeneous rock skerries were mapped extending from each of the major promontories on the east side of the Sound with gently sloping bathymetry from the north to the south over the remainder of the survey. One aspect of the survey program was to determine the repeatability of the bathymetric sidescan system. In order to test this, a key area at the centre of the site was chosen for repeat surveying over the two day survey period using a number of different line orientations and directions. Figure 2a shows the bathymetry for the calibration area surveyed in a north-south direction and Figure 2b shows the same area surveyed in an east-west direction.

Fig. 2a: Bathymetry from north-south lines through calibration area
Fig. 2b: Bathymetry from east-west lines through calibration area

Results - Amplitude Data

The amplitude data was processed in a manner similar to that for a sidescan sonar with time varying gain applied to each transducer stave individually. The final sidescan output was then compared at a number of different grid resolutions (0.2, 0.5 and 1.0m bins) in order to test the system resolution and also the errors associated with the position fixing. While the 0.2m bin showed some fine detail, significant areas were missing between the lines (shown as empty pixel spaces) and the data was further compromised at the end of the lines by large data gaps. No discernible differences were seen between the 0.5m and 1m bins. Because of the navigational errors associated with the Trimble RTK GPS, grid resolutions less than 1m give a false impression of the data having a higher positional precision than it actually does. Thus for habitat appraisal based on the bathymetry and amplitude combined response, a final data grid size of 1m² was used appropriate.

The results of amplitude data for the area are given in Figure 3 as a mosaic summary equivalent to the bathymetric charts. From this chart it can be seen that even with a 100% overlap in swaths, amplitude artefacts were obvious as along-track signatures. These survey artefacts will have a profound influence on the data for classification if full amplitude correction is not conducted as the artefacts represent the strongest events in the records. None the less, even with this data, many bottom features can be identified such as the complexity in the rock skerries striking out from the two headlands in a southwest direction.

 
Fig. 3: Amplitude returns for Plymouth Sound

Results - Seabed Classification

Supervised classification was conducted following image processing techniques on the combined data sets of bathymetry and amplitude with 67% weight for bathymetry and 33% for amplitude. First histograms of combined spectral peaks were produced to identify significant classes within the data. These significant classes, representing different acoustic responses from the bottom, were then used to pick training sites for the classification routines. Representative sites for training were manually located on typical or type example locations. Three training sites were chosen per class with the sites chosen where the bottom type was know from grab or video sampling, however, the acoustic classification map (Figure 4) represents only acoustic signatures of bathymetry and amplitude. A polygon was digitised around each of the training sites with a minimum of 400, 1m² pixels in each polygon. The parallelepiped method was used to classify individual pixels with relation to the know signatures at the training sites. The classification scheme produced six distinct groups or clusters of bottom type. Broadly these types were located based on their bed rock outcrop (Group 1), the bedrock outcrop/sand or mud boundary conditions (Group 2), sand or coarser grained regions (Group 3), regions of mud (Group 4), an unknown region (Group 5) and regions where the sonar data was beyond the survey limits of the equipment usually in very shallow water (Group 6).

Fig. 4: Bottom type classification for Plymouth Sound based on bathymetry and amplitude.

Discussion

While the theoretical limits of ensonified areas for the Submetrix 2000 system give individual bottom pixel or image patches on the sea floor of 12cm sides, the practical resolution of the system is limited by a number of other survey parameters. These parameters such as the errors in navigation information and the angles of the beams compounding errors at the end of the beams are present with any sonar system. When the errors are compounded, it can be demonstrated that positional accuracy for features on the bottom is often only known at best to 50cm and on average to 1m. For this reason, any comparison of remote sensing techniques should be made at 1m. A further issue with sea floor classification into habitat types is introduced with the ground truth data. The ground truth sampling locations are only as good as the navigation system plus the error in not knowing exactly where on the bottom the sample has been taken from. Despite these limitations, the bathymetric sidescan mapping provided high resolution, repeatable bathymetry and images of the seafloor that were acquired in a short survey period.

Loch Sunart

The western sea-lochs hold important information for evaluating past climatic changes in northern latitudes. This information includes high fidelity undisturbed records of sedimentary sequences together with palaeo-landscape relics that indicate the actions and extent of previous glacial limits. The bathymetric sidescan survey was conducted using the 234kHz transducers with the primary objective of resolving topographical features on the bed of the loch. A secondary benefit resulted from the survey subsequent to the acquisition, the data provided valuable information for biologists studying critical habitats in candidate Special Areas of Conservation (an EC protected habitat area). The survey was conducted during a relatively calm period in June, 2000.

 

Fig. 5: Inner and mid-loch bathymetry for Loch Sunart together with bedrock geology and the limits of ice re-advance during the last re-advance.

For this survey navigation was provided by a Racal Skyfix differential GPS and tide control from the Hydrographic Office. The sonar was bow-mounted together with a DMS 205 motion reference unit. Survey lines were oriented parallel to the long dimensions of the loch with line separations of 100m. This line spacing was necessary for the deep sections where depths in excess of 100m result in areas of greatly reduced ensonification. Unfortunately in the deepest part of the loch the return sonar signal from the fine-grained muds was very weak and it is recommended that for future work in these conditions lower frequency transducers are used.

Bathymetric results

The inner and middle sections of the loch are shown in Figure 5 together with summaries of the bedrock geology. The inner loch shows clear over-deepening at its western end as a result of glacial pinching controlled by the more resistant pelitic bands at the narrows before the loch turns to the northwest. It is postulated from observations made on land (Greene, 1995) that this represents the seaward (western) limit of the Loch Lomand re-advance in this part of Scotland and this is collaborated by the shallow bathymetry and sea-bottom features that are evident to the west of the narrows. Similar overdeepening with a shallow lip or sill has been observed in many fjords in Norway. At the eastern end of the inner loch, De Geer type glacial moraines are inferred from their shape which is similar to debris deposited by grounded ice in front of glaciers, Baffin Island (Boulton, 1986). These moraine ridges with steep proximal sides and shallow distal sides are also clearly identified from an analysis of the amplitude data (Figures 6a, and 6b) and bathymetric slopes (Figures 6c, 6d and background image). The high resolution offered by the bathymetric sidescan survey has allowed not only a new bathymetric chart for the inner and middle lochs but new insights for environmental reconstruction of the Quaternary history of the area.

Megget Reservoir

In 1999 a bathymetric survey was conducted of the Megget Reservoir in the Southern Uplands of Scotland. The reservoir has a volume of 63,722 Megalitres and was constructed in 1983 as the main drinking water supply serving Lothian Regions and the city of Edinburgh. In 1997-98 the reservoir was drained to 25% of its capacity in order to conduct repair work on the face of the dam. A number of slump features were noted during the drainage process and it was postulated that the process of refilling could trigger further slumps. The primary objectives of the bathymetric survey were to assess if further slumping had occurred during re-fill and to map the amount of sedimentation since the reservoir was constructed, in the deep part that was not visible on draining. The final bathymetric charts would therefore be compared to initial engineering maps of the reservoir bottom, that is the old land surface prior to flooding of the river valley.

Fig. 6a: and 6b: Amplitude response over de Geer moraine features in Loch Sunart. 
6c: Slope angle measured from bathymetry.
6d: Slope aspect measured from bathymetry over moraine features. Backdrop is bathymetry of inner loch.

The bathymetric chart (Figure 7a) shows the reservoir floor to be gently sloping to from the west to the dam in the east with a step approximately half way down the reservoir where depths drop to greater than 40m. Many features were observed that remain from conditions before the glen was flooded such as the old road in the western half of the reservoir and a number of burrow pits. The meander channel of the western river courses can also be seen crossing the floor of the reservoir in the shelf area at the western end. A number of features were also evident particularly on the south shore of the reservoir and on steeper slopes of the north shore that were not present before the reservoir was constructed. The geometry of these features is characteristic of slumping events and this is reconfirmed by the difference or cut-fill map (Figure 7b) from the old topography and the bathymetry from the bathymetric sidescan survey.

Discussion

The bathymetric sidescan sonar system provides very high resolution bathymetry when used in conjunction with differential GPS and a motion reference unit. The sonar can be deployed on a wide range of vessels of opportunity for a variety of survey objectives. The 234kHz sonar transducers have an effective survey depth of 100m and the 117kHz transducers a survey depth of 250m. Under typical survey conditions this manifests itself as 0.25m² and 1m² ensonified patches with 25 and 50cm depth resolution where full differential GPS and motion reference are available. The high bathymetric survey fidelity and the repeatability of the survey method are of particular advantage to users who wish to record small bathymetric variations and also need to measure changes in bathymetry over time. Furthermore, with nearly 100% coverage at swath widths of up to 15 times the water depth the method can be highly cost-effective for surveying large areas. The method has some bathymetric resolution limitations at very steep bottom slope angles but is particularly effective where slopes are gentle. The amplitude data, which is recorded simultaneously and co-located with the bathymetric information, contains additional semi-quantitative information on bottom type. Current research programmes at a number of institutions aim to utilise this data for full quantitative remote bottom classification.

Fig. 7a: Bathymetry of the Megget Reservoir in elevation above OD

Fig. 7b: Net loss and gain (cut and fill) within reservoir

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