object detection capabilities of the bathymetry systems utilised for the 2015 cds

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soundings Issue 66 Winter 2016 11

The Object Detection Capabilities of the Bathymetry Systems Utilised for the 2015 Common Dataset

by Luke Elliott Bathymetric Appraisal Officer, United Kingdom Hydrographic Office

Introduction

hallow Survey was established in 1999, in Sydney, Australia and is the international conference series for high-resolution hydrographic surveys in shallow water. The fundamental aim of the series is to promote

progress in survey techniques within the coastal zone. Of the many topics covered at the conference, considerable emphasis is given to the Common Dataset (CDS). The CDS allows manufacturers, relevant to shallow water surveying, to test equipment against market competitors over the same survey area. This is not limited to, but includes, conventional echo sounding and remote sensing techniques for acquiring bathymetry, backscatter and water column data. Sub-bottom profilers, sidescan sonars and laser scanners, amongst other equipment, have featured in previous datasets. The key objective of the CDS is to allow those within the hydrographic community to make comparisons between the latest survey technologies. This enables judgement to be made upon the various approaches employed by manufacturers to the many elements of shallow water surveying.

System Release

Date Cost1 Technology2 BDM3

Max. Frequency

No. of Beams

Max. Swath Angle

Beamwidth4

Al-tT x Ac-tR

Kongsberg5 EM2040DRX

2009 High MBES A, P 400kHz 5126 200° 0.5 x 1.0

Teledyne RESON SeaBat 7125

2007 High MBES A, P 400kHz 512 165° 1.0 x 0.5

Teledyne RESON SeaBat T20P

2013 Medium MBES A, P 400kHz 512 165° 1.0 x 1.0

EdgeTech 6205

2014 Medium MPES P 550kHz N/A 200° 0.5 x N/A

Kongsberg GeoSwath Plus 500kHz

2013 Low PMBS P 500kHz N/A 240° 0.5 x N/A

Kongsberg Mesotech M3

2014 Low MBES A, P 500kHz 256 120° 3.0 x 1.6

Teledyne Odom Hydrographic MB1

2012 Low MBES A, P 220kHz 512 120° 3.0 x 4.0

WASSP WMB3250

2013 Low MBES A, P 160kHz 224 120° 3.5 x 0.54

Notes:

1. Cost: Low £0-50,000; Medium £50-100,000; High £100,000+

2. MPES (Multiphase Echo Sounder), MBES (Multibeam Echo Sounder), PMBS (Phase Measuring Bathymetric Sonar)

3. Bottom Detection Method: A = Amplitude; P = Phase. Primary is given first

4. Al-tT (Along-track Transmit), Ac-tR (Across-track Receive). Phase measuring sonars (EdgeTech 6205, Kongsberg GeoSwath Plus 500kHz) do not have a calculated across-track footprint (resolution) as for these systems; the across-track resolution is a function of bandwidth (1/Pulse Width) rather than beamwidth. For this reason they cannot be compared like for like against the MBES systems in terms of beamwidth.

5. There were two datasets analysed for the Kongsberg EM2040DRX; one completed in single swath mode and one in dual swath mode.

6. 512 actual beams but the system can acquire 800/1600 depths per ping in single swath and dual swath modes respectively. This is because the system creates extra bottom detections (named “soft beams”) by analysing the phase signal of the return, thus increasing the data density.

Table 1: Manufacturers and systems that completed the TDT for the dataset

Following on from a paper by Andrew Talbot comparing the systems utilised in the 2005 CDS, this study provides an analysis of all the systems used to compile the latest dataset (collected in Plymouth, UK during the summer of 2014 and spring 2015). The key theme of the paper is the Target Detection Task (TDT), a new element previously not utilised. The task was instigated to test the object detection capabilities of the systems. Table 1 lists the manufacturers and systems that took part. A 250kHz GeoSwath system was also used but unfortunately was, at the time, unreadable in Caris HIPS and SIPS and thus removed from the study.

Unlike the 2005 CDS, where only one vessel was used in a five day window, this time four vessels were used in a two month period (July to August 2014). Each had various positioning and motion reference systems fitted, reducing the commonality between the datasets. Further to this, Kongsberg returned in March 2015 to add an additional two systems to the dataset. Commonality was thus in the specification provided to those undertaking the TDT. Strict line plans were set, consisting of three lines for each target (Figure 1) with the following criteria: 140° swath coverage sector (±70° from nadir); 6kn speed over ground (SOG); North to South orientation with an offline tolerance of 5m.

Figure 1: TDT within Task Area 1, Plymouth Sound. The location chart (top-right) shows Areas 1 and 2 within the Sound. Later, lines are referred to in relation to this orientation

Targets for Comparison

Target 1 is thought to be the remains of a WWII loading jetty used during the D-Day landings. Due to the stringent algorithms used by many manufacturers, mid-water objects can be mistaken for noise or not

S

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12 soundings Issue 66 Winter 2016

detected at all. One of the steel girders is horizontal in the water column and is the object analysed in this section. It stands 6m proud of the seabed in approximately 14m of water.

Targets 2 and 3 are believed to be the wrecks of two ammunition barges in depths of approximately 9 and 30m respectively. Accurate dimensions of the barges were not available, making it impossible to complete a like-for-like comparison of number of hits on the target. To avoid any bias caused by subjectivity, the targets were therefore analysed qualitatively.

Target 4 is a 2m cube laid for the 2005 CDS collection in order to test the capability of systems to detect 2m objects in <40m of water as per IHO Order 1a Standards, LINZ MB-1 and UK CHP specifications.

Data Analysis

Each manufacturer supplied datasets in a variety of formats. For the purpose of this study, full density unprocessed (raw) datasets were used for analysis. This was in order to offer each system an unbiased potential of getting a maximum number of detections on each target and avoid the subjectivity of data cleaning. This was suited to the MBES and MPES systems with little outliers. The interferometric GeoSwath system required basic processing to remove apparent noise and errors within the data in order to make the targets suitable for analysis (Figure 2). Bathymetric data only was used for this comparison. Although some systems were able to collect backscatter and/or water column data, analysis of this data was beyond the scope of this study.

Top: Raw, unprocessed dataset

Bottom: Dataset with obvious noise removed

Figure 2: Noise levels in the GeoSwath dataset. Both diagrams are orientated in the same direction

All datasets were analysed using Caris HIPS and SIPS software. Supplied tide files and sound velocity profiles (SVPs) were applied to the datasets as necessary. Some manufacturers included lines additional to those set in the line plan. In the majority of cases these were removed from the study and only those adhering to the line plan were analysed, unless that option was not available.

Model Lines

completed

Line speed

(5+ knots)

N-S Orientation

Maximum Offline Distance (m)

EM2040DRX Dual Swath 12/12 12/12 12/12 7.76

EM2040DRX Single Swath 12/12 6/12 12/12 11.54

SeaBat 7125 12/12 5/12 12/12 5.72

SeaBat T20P 12/12 3/12 12/12 4.93

6205 12/12 11/12 10/12 4.78

GeoSwath Plus 500kHz 12/12 3/12 6/12 6.49

Mesotech M3 12/12 3/12 7/12 7.03

MB1 12/12 3/12 12/12 5.73

WMB3250 9/12 9/9 12/12 2.17

Table 2: Abiding by the specification for the TDT. Red values indicate where companies did not adhere to the specification

All manufacturers, except WASSP, completed the set line plan over each target, but the majority did so with a maximum offline distance greater than the required 5m and at speeds other than the 6kn specified (Table 2). It was noted that the observed maximum tolerances were in the extremities of the lines and not over the targets themselves, so no lines were rejected from the analysis on the basis of offline tolerance. Plymouth Sound is a complex body of water with a series of strong tidal streams. This makes achieving a constant speed difficult so, for the purpose of this study, any speed over 5kn was acceptable. Slower speeds tend to be advantageous to the target detection capability of bathymetric swath systems, whereas higher speeds tend to result in a lower target detection capability, hence no upper limit was set. Manufacturers were asked to keep a swath angle of 140° (±70° either side of nadir). The swath angle value could not be displayed in Caris, in which case it is assumed that manufacturers followed the specification for the swath width (provided the system was capable of a 140° swath). Three of the systems: M3, MB1 and WMB3250 could achieve only 120° (±60° either side of nadir) and thus could not fully abide with the specification. An unbiased like-for-like comparison of the systems was not possible for two reasons. Firstly, datasets were collected at different times of the year, under different meteorological and tidal conditions. Secondly, only one company (WASSP) fully observed the specification for each completed line (Table 2). Unfortunately, due to time constraints, WASSP only completed nine of the twelve lines.

Target 1: Objects in the water column

The images in Figure 3 show the centre line over the target as run by each system [other lines are available from the author on request] and Figure 4 indicates the number of hits on the horizontal girder per line. The GeoSwath system had the most hits on the horizontal girder in both the port and centre lines. However, it was the only system to require cleaning in order to define the structure (Figure 2). In the square area used to take each sample almost 18% of points were removed from the GeoSwath dataset. The M3 system did particularly well in both the centre and starboard lines and the MB1 had similar values to the SeaBat 7125 in the port and centre lines, with considerably more hits in the starboard line. The EM2040DRX failed to pick up the girder in either single or dual-swath mode in the centre line, as did the 6205 in the starboard line. The WMB3250 failed to pick up the girder in any of the lines (Figure 4).

Target 2: Ammunition barge at 9m

All systems detected the ammunition barge in all lines, with varying detail, noise levels and definition. The images in Figure 5 show the centre line over the barge as run by the systems.

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Figure 3: The centre line over Target 1 as run by each system

Figure 4: Number of hits on the horizontal steel girder in the water column per line for each system


Target 3: Ammunition barge at 30m

The images in Figure 6 show the centre line over the barge as run by the systems. All systems got hits on Target 3 in at least one line. Owing to the greater depth than Target 2, many of the systems failed to produce clear visualisation. In reality, if this was an unknown target it wouldn't necessarily be

detected. Due to time constraints WASSP did not submit a centre line for this target.

Target 4: The 2m cube

The images in Figure 7 show a consolidation of all three lines over the cube as a shoal depth true position surface. Seven of the nine systems clearly detected an object at the known location of the 2m cube. The 6205 shows a small patch of noise in the correct location but would not be substantive enough to be identified as a target by a data processor. The GeoSwath surface is the noisiest dataset but shows the presence of an object in the correct area.

Six of the nine systems clearly detected the target on every completed line (Figure 8). WASSP only had time to collect data for the centre line. In all but two cases systems which detected the object met the LINZ MB-1 and UK CHP specification. The M3 and GeoSwath systems each had a line, port and centre respectively, that detected the cube without meeting the requirements of the LINZ or UK CHP specification (Table 3).

Although, statistically, the 6205 met the specification (Table 3), detections were not significant (Figure 9). The points are only 35cm from the seabed and do not clearly define the cube surfaces, making the detection questionable.

As previously noted, many manufacturers supplied additional lines. Figure 10 shows that the 6205 can detect the cube despite not appearing substantially in the set line plan.

Discussion

Target 1

Overall the 7125 and T20P provide the sharpest images of the horizontal girder, although not as high a point density as the others, suggesting that greater quality comes from the more expensive systems and data density isn't always the key to detection. Contrary to this, the EM2040DRX didn't fully detect the girder in any of the lines in either dual or single swath. Considering the system works at the same frequency as the 7125 and T20P, it would be reasonable to assume that it is the bottom detection algorithms employed by Kongsberg rather than the system itself that are causing the lack of detection. This said, the bar may have been detected within the collected water column data but this wasn't examined as part of this study. The M3, one of the low cost systems also manufactured by Kongsberg, did particularly well at producing a sharp image of the girder in two of the lines and with a very high point density.

continued over page

Kongsberg 2040DRX – Dual Swath Kongsberg 2040DRX – Single Swath Teledyne Reson Seabat 7125

Teledyne Reson Seabat T20P Edgetech 6205 Kongsberg GeoSwath Plus 500kHz

Kongsberg Mesotech M3 Teledyne Odom MB1 WASSP WMB-3250




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Targets 2 and 3

The trend throughout is that the detail of Target 2 is far higher than Target 3. As depth increases, beams become more spread out, increasing the beam footprint size, which lowers the resolution and quality of the image. By zooming in on the images it is possible to see the differences between the along-track hit spacing (known also as profiles). The spacing is a lot wider in Target 3 owing to the greater depth, hence less detail is expressed by all datasets.

All systems detected the barges with varying levels of detail. Throughout the images of the two targets the trend is that the more expensive systems (EM2040DRX, 7125, T20P) gave the finest detail. However, in three of the six lines run over the two barges, the M3 (one of the cheaper systems) had comparable levels of detail to these systems. The GeoSwath system gave good levels of detail in the outer lines, but not in the centre lines. This highlights the common issue with PMBS systems: a lack of coverage in the nadir region due to a blind spot directly beneath the system.

For both targets the cheaper systems (M3, MB1 and WMB3250) show extensive divergence in quality between the centre and outer lines. These systems are limited to 120° swath angle which, it is assumed, was used over the targets. It is well known that MBES systems lose data quality in their outer beams due to an increased beam footprint size, which is highlighted with these targets in particular. The more expensive systems have much smaller beam footprints throughout the swath than the cheaper systems (Table 1).

Target 4

All the systems detected the cube in at least one pass, proving that all the systems are capable of achieving IHO Order 1a. Only four datasets got meaningful detection on all three passes: EM2040DRX (Single/Dual Swath mode), 7125 and T20P. The GeoSwath and M3 systems each had a line that detected the object but failed to meet the LINZ MB-1 and UK CHP specifications. The GeoSwath got eight hits on target (Figure 11: top); nine hits are required to satisfy the specifications.

The M3 system got only one hit on the cube in the port line, giving no definition of its shape or dimensions (Figure 11: bottom). If this hadn't been an object of known location it would have been removed as noise. It may have triggered the processor to analyse the water column, backscatter (MBES) or co-registered sidescan (PMBS/MPES). This highlights the subjectivity of data cleaning and the fact that simply detecting a target is not a stringent enough standard; it needs to be detected consistently with a reasonable number of depths. This demonstrates the importance of survey specifications, such as the ones set by LINZ and the UK CHP,


Figure 7: A 1m resolution shoal depth true position surface of all three lines for each system. Each image represents an area of 33m x 30m

Figure 8: Number of hits on the cube per line for each system



Data Not Supplied


Kongsberg 2040DRX

Dual Swath Kongsberg 2040DRX

Single Swath Teledyne Reson Seabat

7125

Teledyne Reson Seabat

T20P Edgetech 6205

Kongsberg GeoSwath Plus 500kHz


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Port Line Centre Line Starboard Line

Detected Spec. Detected Spec. Detected Spec.

EM2040DRX Dual Swath Yes Yes Yes Yes Yes Yes

EM2040DRX Single Swath Yes Yes Yes Yes Yes Yes

SeaBat 7125 Yes Yes Yes Yes Yes Yes

SeaBat T20P Yes Yes Yes Yes Yes Yes

6205 No No No No *Yes * *Yes*

GeoSwath Plus 500kHz Yes Yes Yes No Yes Yes

M3 Yes No Yes Yes No No

MB1 No No Yes Yes No No

WMB3250

Yes Yes

Table 3: Did the systems detect the object and pass the LINZ MB-1 and UK CHP specifications? The results are purely statistical i.e. minimum of nine hits on target, three across-track by three along-track

Figure 9: Starboard line over the cube as run by the 6205. Although there are hits on the cube, no definition of its surfaces is apparent and the minimum depth is not detected

Top: Detection of the cube by the 6205

Bottom: Location of the additional line (P: Port, C: Centre, S: Starboard). The square is the 33m x 30m area used to take all samples of Target 4

Figure 10: Additional line completed by EdgeTech

Top: Centre pass completed with the GeoSwath 500kHz system

Bottom: Port line pass for the M3 system

Figure 11: Detecting a target but not at the LINZ MB-1 and UK CHP specifications

specifying detection in greater detail; for example three along-track by three across-track hits.

Analysis of the 2m cube also reiterates the outcome of Target 1: hits on the target alone are not a good measure of a system's quality. In the port and starboard lines for instance, the GeoSwath system has the third and second highest value for hits on target respectively but fails to define the surfaces of the cube. The 7125 has fewer hits than the T20P but with superior definition of the cube detected in both the outer lines.

The M3 and MB1 systems detected the cube in the centre line but not in the outer lines. This is explained by examining the calculated beam footprints of the systems (Table 4). In order to get a reliable detection, the beam footprint of the system must be smaller than the target being detected. It was found that the cube is at approximately 38m depth, directly below the system (at nadir, 0°) in the centre line and approximately 45° swath angle when passed in the outer lines.

Footprint dimensions (m) at 0°

Footprint dimensions (m) at 45°

EM2040DRX 0.33 x 0.66 0.47 x 1.33

M3 1.99 x 1.06 2.81 x 2.12

MB1 1.99 x 2.65 2.81 x 7.53

SeaBat 7125 0.66 x 0.33 0.94 x 0.94

SeaBat T20P 0.66 x 0.66 0.94 x 1.88

WMB3250 2.32 x 0.36 3.28 x 1.01

Table 4: MBES beam footprint at 38m depth (approx. depth of the cube) at 0° (centre line) and 45° (average swath angle for the appearance of the cube in the port and starboard lines)

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Table 4 shows that the beam footprint dimensions at both angles for the EM2040DRX, 7125 and T20P remain below 2m hence detection of the cube in all lines. For the M3 and MB1 the beam footprint is larger than the cube in both dimensions at 45°; hence detections were not made.

This said; the M3 had the single most hits in a line, with 200 in the centre line. A possible explanation for this is due to the large 3° along-track beamwidth. In the nadir and inner beams, MBES tend to use only amplitude detection and thus cannot differentiate where exactly within the 3° the target is, as it measures the range for the angle series within the swath. For a strongly reflecting target, like the cube, off-beam hits could also be reported as in-beam. If this hypothesis is correct, it also explains why both the M3 and MB1 show the width of the cube as 3.2m and 3.8m respectively. The T20P has a 0.5° wider across-track beamwidth than the 7125 and thus could be the reason for it having more hits on the cube despite being the lower specification system of the two.

Although statistically the 6205 detected the cube in the starboard line, points were only 35cm from the seabed and there is no definition on the surfaces. Regardless, statistically speaking, the requirements for passing the LINZ MB-1 and UK CHP specification were achieved (Figure 9). Had the cube been an unknown object, it would not have been detected making it the only one to fail the detection in all lines. However in an additional line (Figure 10), the cube was detected when approached from a different angle to the line plan. This line provided comparable definition of the top of the cube to any of the other systems. The average speed for the additional line was 5.17kn, 0.89 - 1.43kn slower than the lines in the set line plan. This could suggest that the 6205 should operate at lower speeds in order to fully comply with IHO Order 1a.

An anomaly was apparent in the port lines between the single and dual swath datasets for the EM2040DRX (as shown in Figure 8). The single had over three times the hits than the dual swath dataset. The centre and starboard line show the dual swath mode giving approximately double the points acquired in single mode as expected given the technology. The anomaly could have occurred due to a change in vessel speed between the port lines, the line offset between the two, a difference in tide or a drastic change in environmental conditions between lines, i.e. changing currents or similar.

With regards to the vessel speed, the single swath line was completed at a faster average speed seemingly putting it at a disadvantage. Over the cube, the maximum line offset between the two was 1.5m and the tide difference between completing the two lines was 0.6m both of which are negligible. Further analysis of the speed was completed for the EM2040DRX (Table 5). By measuring the average distance between pings surrounding the cube and multiplying by the ping rate for the line gives the speed the vessel was travelling at a specific point, as opposed to assuming the average for the line.

Port Ping

Spacing (m) Ping Rate (pulse/sec)

Speed of line (m/s)

Calculated speed at cube (kn)

Average speed of line (kn)

Dual Swath 0.6 6.39 3.83 7.45 5.66

Single Swath 0.2 7.01 1.40 2.73 6.00

Centre

Dual Swath 0.5 6.76 3.38 6.57 6.23

Single Swath 0.5 6.97 3.49 6.77 5.95

Starboard

Dual Swath 0.6 6.40 3.84 7.46 6.06

Single Swath 0.4 6.68 2.67 5.19 5.64

Table 5: Further analysis for the speed of the line over the cube for the EM2040DRX. In reality, the ping rate for the dual swath is double the value presented in the table

It is possible to see that for the centre and starboard lines, the actual speed over the cube is within ±1.5kn of the average. The port line differs considerably more; in dual swath mode the cube was being passed 1.79kn faster than the average for the line and in single swath mode it was passed at a speed 3.27kn slower than the average line speed. Therefore the single swath port line was completed at approximately a third of the speed hence having almost tripled the points on the cube in comparison to the system in dual swath mode.

Conclusion

The aim of this paper was to compare bathymetry systems used for the 2015 Common Dataset (CDS). A comparison of this sort is difficult to complete without subtle bias from external influences. The most unbiased method would be to use only one vessel, on one day and have a system either side of it operating concurrently. However this would limit the dataset to only two systems and they might interfere with each other. The CDS has made it possible for unlimited companies to take part in a practical manner. The ideal situation would have been to have one vessel at different times through the year; this would still have natural bias with regards to tide and meteorology but greatly reduces human induced bias toward the systems. Four vessels with various motion compensation and navigation systems were used to complete the CDS meaning subtle bias will be inherent to particular systems. In reality every vessel setup is different and each bathymetry system should work to its full potential regardless, making the comparisons viable.

Many companies completed the Target Detection (TDT) without adhering to the criteria set out in the original specification. The required swath angle could not be adhered to by all systems and could not easily be proven within the software. As a result no system judgement should be made without taking into account the line statistics: heading; ping rate; average speed of line; and swath width. Suggestions for future CDSs are to:

keep line plans with the same stringency

allow lines to be run in either direction, i.e. into tidal streams, for more accurate speed keeping

reduce both swath angle and survey speed to allow all systems to operate under more optimum conditions.

For the reasons above, a like for like comparison to determine which system is best at the various elements of the TDT is not viable. It does however provide analysis of each dataset and the conditions for which the task was carried out, allowing judgement on each system to be made. This study proves that all systems that took part have the ability to achieve IHO Order 1a standard set out in the LINZ MB-1 and UK CHP specifications through analysis of Target 4; the 2m cube. This analysis emphasises the subjectivity of data cleaning and that simply 'detecting' a target is not a stringent enough standard. This highlights the importance of survey specifications like the LINZ MB-1 and UK CHP.

In general, the more expensive systems (EM2040DRX, 7125 and T20P) produce the best results for target detection. However the results for

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Target 1 suggests that Kongsberg need to adjust the bottom detection algorithms for the EM2040DRX to better detect mid-water contacts. Of the lower cost systems the M3 appears to consistently produce good results although not as sharp as the more expensive systems. This said the system completed 8 out of 12 lines slower than any other system suggesting it to be working at more optimum conditions.

Acknowledgements

Firstly, thanks to Gwyn Jones, Tim Scott and Andrew Talbot for their extensive advice, guidance and teaching during the writing of this paper.

Thanks also for your co-operation to all manufacturers that took part in the CDS, along with everyone involved in the data collection.

Special thanks to Lisa Brisson (EdgeTech), Peter Hogarth, Craig Wallace (Kongsberg), Pim Kuus (Teledyne RESON) and Justin Kiel (WASSP), my principal contacts with regards to the CDS, who provided answers to the never-ending series of questions.

Finally, many thanks to Nolwenn Collouard at Caris for sharing her extensive knowledge of the company's software, which enabled me to bring the various wide-ranging datasets together and provide advice when errors occurred.

The Author

Luke Elliott is a Bathymetric Appraisal Officer at the United Kingdom Hydrographic Office. He is responsible for validating datasets primarily collected for the Civil Hydrography Programme prior to use in Chart compilation.

He has recently completed the MSc Hydrography programme at Plymouth University. He also holds a BSc in Geography with Ocean Science.

Luke previously worked for MMT UK as an offshore surveyor and data processor during an internship in 2014 and looks forward to further surveys as his career progresses.

He is Hon. Secretary of The Hydrographic Society's South West Region.

He is a keen ultramarathon runner and spends most of his spare-time training for the next event.

[email protected]

https://uk.linkedin.com/in/lse89

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object detection capabilities of the bathymetry systems utilised for the 2015 cds

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