Year 2 (Geophysics)

The geophysics section of the project was split into two parts. Firstly, an area survey tested different survey specifications over an area of seafloor thought to contain the wreck of the Thomas Lawrence, an 18th Century schooner. Secondly, surveys were made of three charted wrecks sites in deep water.
 
Do you want to know more about the Project Objectives?
 

Project Background

The aim of Round 2 of Wrecks on the Seabed is to provide industry, regulators and contractors with a framework for a staged approach to the investigation of wreck sites. Such a framework is important when considering the time and cost of marine investigations. It is also helps effective communication between industry, regulators and contractors. The project design for Round 2 of Wrecks on the Seabed builds on the experience of Round 1 of the project.
 
The project addresses the following aspects of wreck site investigation:
  • A geophysical survey of a 2x2km area of seabed presents an opportunity to develop methodologies for the survey of ephemeral wreck sites.
  • With the dredging industry moving into deeper water, one of the key points emerging from industry is the applicability of wreck investigation methods to greater water depths. The geophysical and Remotely Operated Vehicle survey of three unknown wreck sites in water depths between 46 and 60m will serve as a basis for the development of effective methods for assessing, evaluating and recording wreck sites in deeper water.
  • As most of the problems encountered during the diving fieldwork of Round 1 were related to the size of the diving vessel, a larger Remotely Operated Vehicle / diving support vessel will be deployed in Round 2 of the project. An analysis of the costs and benefits using larger vessels will be carried out.
  • Four of the wreck sites investigated by divers in Round 1 of the project will be revisited with an Remotely Operted Vehicle in Round 2. This will allow a direct comparison of Remotely Operated Vehicle and diver based results.
  • The project results will be made available to the public through web pages, brochures, lectures, conferences and diver information packs for recreational divers.
As the project is specifically tailored to the requirements of marine aggregate dredging, the study areas and sites chosen are all situated close to existing or intended marine aggregate extraction areas.

Objectives

The objectives of the geophysical surveys were divided into two groups;Area Methods and Ephemeral Sites, and Sites in Deeper Water.
 
Area methods and ephemeral sites
  • Refine and develop methodologies related to area survey methods and the survey of ephemeral sites;
  • Compare and contrast substantial, purposely-collected datasets from relevant and independent survey scenarios;
  • Standardise the approach to reviewing typical geophysical datasets and to discerning whether an acoustic signal is anomalous or not;
  • Inform briefs and specifications for the Levels of Archaeological Recording (i.e. Level 1, especially Level 1b) established in Round 1.
 
Sites in deeper water
  • Develop and refine methodologies for assessing, evaluating and recording wreck sites in deeper water;
  • Refine the use of geophysical techniques for sites in deeper water and assess the effects on data quality (including positional issues) that may arise from their use in deeper water;
  • Assess the effectiveness of undertaking archaeological recording (to Levels 2 and 3) using Remotely Operated Vehicles (ROV) and geophysical techniques only;
  • Assess environmental issues (e.g. depth, distance from port, fetch) in respect of working on sites in deeper water;
  • Assess infrastructure issues (e.g. anchoring, tow cable handling, and umbilical handling) in respect of working on sites in deeper water.

Methodology

Dummy

Survey Vessels

Titan ExplorerTitan ExplorerThe geophysical surveys for this project were conducted from dedicated survey vessels.
 
The sidescan sonar and magnetometer surveys over the 2km x 2km area were conducted aboard the survey vessel Titan Explorer operated by Titan Surveys Ltd. The Titan Explorer is a catamaran 9.7m in length and 2.9m in breadth with a draft of 0.6m.
 
Wessex ExplorerWessex ExplorerThe sidescan sonar and magnetometer surveys of the three wreck sites in deep water were conducted aboard the survey vessel Wessex Explorer operated by Hayes Marine Ltd. This vessel is 15m in length and has a draft of 2m.
 
The multibeam echosounder surveys for this project were conducted from the Wessex Explorer.

Sidescan Sonar

 
Sidescan sonars emit narrow beams of acoustic energy from either side of the towfish. This energy is reflected back from the seafloor which produces an image of the seafloor. Hard objects such as metal reflect more energy than soft objects such as sand and mud.
 
Objects standing up from the seafloor produce an acoustic shadow. On some shipwrecks this can show individual features and parts of the vessels’ superstructure.

Multibeam Echosounders

Multibeam sonar surveys ensonify the seabed beneath and to either side of the survey vessel. They record continuous and very accurate spot heights for thousands of points on the seabed as the vessel moves forward.
 
From multibeam data, 3D terrain models are created. Depressions and features that project from the seabed can be displayed and measured accurately.
 
For this project a Reson SeaBat 8125 multibeam echosounder, provided by NetSurvey Limited, was used and the data was processed by Wessex Archaeology using the IVS Fledermaus visualization system.
 
The results were 3D terrain models, 3D fly through movies and 2D georeferenced images. The 2D images were used as a base for site plans.

Magnetometers

Geometrics G-881 caesium vapour magnetometerGeometrics G-881 caesium vapour magnetometerMarine magnetometers detect variations in the Earth's magnetic field. These variations may be caused by the presence of ferrous material such as iron on or under the seabed, geological features or the daily variations in the Earth's magnetic field that are caused by the sun. Marine magnetic surveying is now a standard technique for mapping ferrous material on the seabed.
 
As the survey vessels create a magnetic disturbance themselves, the magnetometer is usually towed behind it.
 
Marine Magnetics ExplorerMarine Magnetics ExplorerDuring the 2005 fieldwork, a Geometrics G-881 caesium vapour magnetometer was used for the 2km x 2km area survey while a Marine Magnetics Explorer was used on the three deep wreck sites.
 
In the post processing the magnetometer data was imported into a Geographical Information Systems (GIS) and overlaid on the results of the sidescan sonar interpretation to compare the results of the two surveys.

Survey Areas and Aims

Dummy

Areas and Aims

The geophysical surveys were divided into two groups; an area survey of a 2km square, and surveys of three wrecks in deeper water. Click on areas in the map to see the survey results.

Overview Map

2km x 2km Area Survey

Side Scan Survey MosaicSide Scan Survey MosaicThe 2km x 2km survey area was south of the Hastings Shingle Bank off the Sussex coast in the English Channel. It was chosen because it was thought to contain the wreck of the Thomas Lawrence and it is adjacent to a dredging area. The Thomas Lawrence was an 18th Century Danish schooner which is thought to have been lost in 1862 and was discovered by divers in 1983.
 
 
The aim of surveys was to address objectives 1-4 by testing variations in standard survey methodologies. These variations included; line spacing, range setting, and the height of the magnetometer towfish above the seafloor.
 
The conclusions show how area surveys undertaken by the aggregate industry can best produce data that are suitable the location and identification of ephemeral wreck sites within potential aggregate licence areas using archaeological techniques.
 

Deep Wrecks Surveys

The three wrecks in deeper water were selected from United Kingdom Hydrographic Office (UKHO) wreck data for the south coast for the same area studied in Round 1 of the Wrecks on the Seabed Project. The choice was narrowed to wrecks that were unidentified and lying in approximately 50m of water.
 
The chosen wrecks were:
 
Wessex Archaeology No.
UKHO Comments
Depth
1001
Small wreck – intact - wooden
56m
1002
Lying on starboard side – masts lie alongside
49m
1003
Intact – probably two masts
46m
 
Standard surveys were conducted over each wreck using a sidescan sonar, magnetometer and multibeam echosounder similar to the surveys conducted in Round 1 of the project.

Results

View the 2km x 2km Area Survey results or the Deep wrecks survey results

2km x 2km Area Survey

Dummy

Sidescan Sonar Data

1394

The sidescan sonar data sets for the 2km x 2km area were divided into eight data sets. This was to show the effect that frequency and range setting have on the ability to detect small or ephemeral sites.
 
The sidescan sonar used for this project was capable of collecting both high and low frequency data simultaneously. The high frequency setting is able to detect small object and provided detailed images of objects on the seafloor but the energy is not normally capable of getting data at ranges greater than 100m. The low frequency setting is able to provide data out at much larger ranges than the high frequency but at a lower resolution.
 
The high frequency data set produced the best quality images for the interpretation but only at short range settings (50m or 75m). This is because the high frequency acoustic energy is attenuated before it reaches the end of the longer range settings.
 
The low frequency data could be transmitted to the end of the 150m range. Although this was suitable for geological interpretation, it was not suitable for detecting the small objects required for archaeological interpretation.
 
While it was still possible to detect the presence of small objects on both frequencies and at most ranges, only the data sets with high frequency and short range settings allowed the interpretation of whether an anomaly was natural or artificial in origin.
 
The position of ephemeral sites is also affects whether the sites will be detected, as is shown by the Thomas Lawrence.
 
Sites like this wreck, with low relief and lying in areas of complex bathymetry, can be difficult to identify as they will not always reflect enough acoustic energy to produce an anomaly. Such sites highlight the need for every anomaly in the data to be recorded and not just those which are identified from more than one survey line.
 
This means that it is not possible to determine whether an anomaly is a real acoustic signal or noise by simply determining if the anomaly is observed from two adjacent survey lines.
 
While it has not been possible to develop a system for the automatic discrimination of anomalies in sidescan data as part of this project, it was possible to get an impression of the scale of work that would be involved.
 
Any such system would have to be able to learn the difference between geological and archaeological anomalies and also discriminate against acoustic noise.
 
On this basis, it is likely that archaeological assessments of sidescan sonar data are going to continue to be best conducted by experienced geophysicists rather than by software or by people inexperienced with the problems and practicalities of sidescan sonar data.
 
A final factor highlighted by this survey was that in areas where the sea floor shoals rapidly along a survey line, it is not always possible to keep the towfish at the optimum height above the sea floor.
 
Where the towfish gets too high, the range is effectively shortened and so the expected data coverage is not achieved. It is advisable that the line spacing is less than the expected minimum range so as to ensure that all areas of the sea floor are surveyed at least twice.
 
The greatest number of anomalies found in any of the eight sidescan sonar data sets was in the high frequency, 50m range data. All the other data sets had less resolution and so fewer anomalies were identified. This implies that surveys specifically designed for archaeological purposes should use high frequency sidescan systems at low range settings. This project has shown that by using high frequency sidescan sonar systems at either 75m or 100m ranges up to two-thirds of the anomalies within an area may be missed.
 
At short range setting the low frequency setting did detect slightly less anomalies than the high frequency data set. The main difference between the resolutions of the two data sets was that certain anomalies classified as wrecks in the high frequency data set were described as seafloor disturbances in the low frequency data sets.
 
The interpretations of different sidescan sonar data sets underline the need for sidescan sonar surveys to be carefully designed so that the correct range and frequency are selected to detect the smallest anomalies expected to be of interest in the survey area. Also the combination of range setting and line spacing needs to be such that at least 220% coverage of the area is achieved so that all areas of the seafloor are ensonifed at least twice, from different orientations.

Magnetic Data

 

 

1395

The magnetic data was examined in detail to understand the effect of having the magnetometer at different depths in the water column and varying line spacings. This showed that when the magnetometer was kept closer to the seafloor, it detected smaller magnetic anomalies that were not observed when the magnetometer was higher.
 
This ability to detect smaller magnetic anomalies when the magnetometer is closer to the seafloor was expected. However, there was a difference of less than 10m between the depth of the magnetometer for the shallow and deep data sets but it made a noticeable difference to the number of anomalies that could be identified. This large effect of a small difference in height is noteworthy.
 
This relationship between the height of the magnetometer above the seafloor and the number of anomalies which it is possible to detect also explains why more anomalies were found in the north of the area than in the south. The magnetometer was kept at a constant depth below the sea surface on any given survey line but the seafloor within this area shoals by up to 23m from south to north, so the magnetometer was closer to the seafloor in the north.
 
As it may not be possible to alter the depth of the magnetometer during a survey, areas with rapidly changing bathymetry are likely to pose a problem to marine geophysical surveys.
 
The effect of increasing the line spacings also reduced the number of magnetic anomalies identified, which again was to be anticipated. Small magnetic anomalies will only be detected when the magnetometer passes almost directly over the causative object. Large magnetic anomalies such as the metal wreck outside the SW corner of the survey area produce large magnetic anomalies which can be detected a few hundred metres away from the causative object. Therefore, in order to detect small magnetic fields the closest possible line spacing must be used.
 
However, a balance needs to be found between practicality and efficiency in magnetometer surveys as it is not always possible to collect large amounts of magnetic data with a narrow line spacing.

Multibeam Data

 

The multibeam data acquired over the 2km x 2km area was not able to detect the Thomas Lawrence or any of the other small or ephemeral sites. This is perhaps surprising given that this data set was collected using the highest possible resolution system available and with archaeological considerations as a primary concern.
 
Therefore, marine surveys for detecting small or ephemeral sites of archaeological interest are best conducted using a sidescan and magnetometer with the closest line spacing considered to be affordable. The sidescan sonar used should be a high frequency system and the range setting should be as short as possible to still allow at least 200% coverage when run at the line spacing considered to be affordable.
 
Multibeam surveys do add information to such investigations but it is not advisable to use multibeam systems on their own or as a means for prospecting for archaeological sites. Where they are used, though, then the system should be a high resolution system with the narrowest beamwidth affordable. Also, any wreck sites should be surveyed with the vessel at low speed and the ping rate of the multibeam system as high as possible. The archaeological geophysicist should then review all the data and not just the filtered data produced for hydrographic interpretation.

Thomas Lawrence

Introduction

The Thomas Lawrence was a Danish, 18th Century schooner, which sank off the south coats of England in 1862. It was discovered by divers in 1983 and subsequently excavated in 1985. Since then the position of the wreck has been unclear and it has proved difficult to re-locate despite a number of attempts. The wreck was reported to be located in the south-western part of the survey area for this stage of the project and initial documentary research provided five possible locations for the site. As part of the area survey Wessex Archaeology found a wreck believed to be the Thomas Lawrence and have been able to provide its precise location.
 

Detailed Results

The wreck was identified from the sidescan sonar datasets during initial processing, though the site was not consistently interpreted as being a wreck. The site was only located on two of the datasets after correlation of all the data in the GIS. Images of the site from each of the sidescan sonar data sets demonstrate the varying quality of the data.
 
 
In all, the Thomas Lawrence was identified 21 times out of a possible 36 in the sidescan sonar data with the positions from these sites covering an area approximately 60m (N/S) by 55m (E/W).
 
The wreck was only positively identified as a wreck or possible wreck site on two of the datasets (500kHz and 100kHz both at 50m range settings) during the initial processing.
 

1396

Although the site was constantly identified as an anomaly at both high and low frequencies and on all range settings, it was not consistently identified as a wreck. Only the data acquired with the 50m range setting provide sufficient resolution for structures to be identified within the wreck, although these structures are just linear reflectors and not identifiable parts of the wreck.
 
In addition, the nature of the survey pattern should have ensured that the wreck site was surveyed multiple times within each sidescan data set. This was not the case. The reason for this becomes apparent when the position of the Thomas Lawrence is viewed within the context of the surrounding bathymetry. The wreck is located on the side of a large sandwave that may be covering part of the site.
 
From sidescan sonar survey lines to the west of the site, the acoustic energy striking the Thomas Lawrence would have a low angle of incidence and very little energy would be reflected back to the sidescan towfish. Therefore only sidescan sonar survey lines to the east of the site, which would have a high angle of incidence to the site and consequently a large amount of energy, would be reflected back to the towfish producing good images of the wreck.
 
The position of the Thomas Lawrence is such that it is also near the base of the Hastings Shingle Bank and so survey lines oriented E-W, and passing to the north of the wreck site, would not have produced a clear sidescan image. This problem, when combined with the fact that the wreck is partially buried and also has no discernable magnetic signature during this survey, have given the Thomas Lawrence it’s reputation for being very difficult to locate.

Deep Wrecks Survey

Dummy

Deep Wreck Site WA1001

Introduction

Side Scan of WreckSide Scan of WreckWreck WA1001 was described by the UKHO as being a small wreck, intact and possibly wooden, at a charted depth of 56m with the surrounding seafloor at only 57m – 58m deep shoaling to 50m to the northwest of the wreck.
 
The wreck was located during this survey using the sidescan sonar and found to be within 10m of its recorded position.
 

Detailed Results

Sidescan Sonar and Magnetometer Data

Side Scan of Whole SiteSide Scan of Whole SiteThe sidescan sonar data consisted of two prospection lines and nine survey lines each approximately 600m long, orientated E-W with a 75m range setting and covering an area approximately 600m x 330m.
 
A further three lines orientated N-S were surveyed over the wreck itself. Forty-eight anomalies were identified.

 

 

Archaeological PotentialNumber within Study Area
High1
Medium6
Low8
Very Low0
Archaeological Potential Rating of geophysical anomalies within the WA1001 Study Area
 
After the sidescan sonar survey, magnetometer survey lines were acquired with nine survey lines orientated N-S and six survey lines orientated E-W. Two magnetic anomalies were identified.
 
The 48 sidescan sonar and two magnetic anomalies were then viewed in the GIS and grouped into 15 sites. Each was given a level of Archaeological Potential:
 
The largest piece of wreckage in this area (WA6100) measured approximately 25m x 10m. It is characterised by an area of reflectors which are brighter than the surrounding seafloor, with evidence of possible structure.
 
Magnetometer ResultsMagnetometer ResultsThis shows that the material is reflecting less energy than the surrounding sediment. These results are typical of saturated wood lying on sand or gravel. This supports the UKHO record that the wreck is wooden and it is independently confirmed by an associated magnetic anomaly of only 4.82nT.
 
From 25m to the SW of the wreck site to 50m to the NE there were four sites of medium archaeological potential (WA6101, WA6102, WA6113 and WA6114) which were all likely to be debris associated from the wreck. The largest of the four, WA6102, was not described as debris but as a seafloor disturbance. This may be because it was a large section of partially buried wreck rather than a piece of debris on the surface.
 
Approximately 180m to the north-east of the wreck is another large piece of wreckage (WA6106) classified as being of medium archaeological potential. It measures approximately 6m x 1m and has a magnetic anomaly of 6.47nT.
 
The final site of medium archaeological potential (WA6112) is 125m to the south of the wreck and is a large area of bright reflection which may also have been part of the wreck.
 
We cannot be sure that all six sites of medium archaeological potential and the number of other smaller anomalies scattered over the area all belong to the same wreck.
 
But if they do, it suggests that, contrary to the UKHO record, the wreck is broken up and covers an area of approximately 300m x 200m.
 
Of the 15 sites identified within the WA1001 survey area, only 3 have relief of around 1m high in the sidescan sonar data. The majority of the sites lie flat on the seafloor and may be slowly being covered by seafloor sediments.
 

Multibeam Data

The multibeam data covers an area of approximately 740m x 540m. The depth of the seafloor ranges from 49m to 58m. As with all multibeam systems, the resolution of the data is a function of water depth and the beamwidth of the system. The Reson 8125 used for this project has a beamwidth of 0.5° and therefore the footprint of the beams over this site was approximately 0.7m.
 
Multibeam ResultsMultibeam ResultsThe main wreck site (WA6100) was located within the multibeam data only a few metres from the position determined by the sidescan sonar data. The multibeam data shows that this section is approximately 23m x 7m; slightly smaller than the measurements from the sidescan data.
 
The multibeam data also shows that the wreck is standing 0.6m proud of the seafloor. This was not observed from the sidescan sonar data.
 
Viewing the raw soundings rather than the gridded surface provided no further information about the wreck.
 
The area is covered by sandwaves that are orientated NW-SE. This implies that the main direction of the currents is perpendicular to this. Immediately to the SE of the wreck is a small scour pit approximately 1m deep, which was not detected in the sidescan sonar data.
 
information on the ROV survey of this site.

Deep Wreck Site WA1002

Side Scan of Steamer Wreck (40m x 6m x 4m)Side Scan of Steamer Wreck (40m x 6m x 4m)Wreck WA1002 was described by the UKHO as lying on its starboard side, with its bow to the southwest and mast lying alongside the wreck. The charted minimum depth over the site is 49m with the surrounding seafloor 60m deep.
 
The centre of the wreck was located using the sidescan sonar and found to be approximately 15m NE from its recorded position.
 

Detailed Results

Sidescan Sonar and Magnetometer Data

Side Scan of Whole SiteSide Scan of Whole SiteThe sidescan sonar data consisted of 11 lines, each approximately 800m long and orientated E-W with a 75m range setting. The area covered was approximately 800m x 350m. A further seven lines orientated NE-SW were surveyed along the length of the wreck. Forty-five sidescan sonar anomalies were identified.
 
After the sidescan sonar survey, magnetometer survey lines were acquired over the wreck site. The survey lines for this were orientated N-S. One magnetic anomaly was identified.
 
The 45 sidescan sonar and the magnetic anomalies were viewed in the GIS and grouped into seven sites. Again, each was given a level of Archaeological Potential:
 
Archaeological PotentialNumber within Study Area
High1
Medium2
Low4
Very Low0
Archaeological Potential Rating of geophysical anomalies within the WA1002 Study Area

The wreck is lying one its side in one piece (WA6200) and measures approximately 73m x 7m. It stands up to 7m above the seafloor. While it is difficult to determine which end is the bow, it is thought likely that it is to the south-west which implies that the wreck is lying on its starboard side. This agrees with the UKHO record.

 
Magnetometer ResultsMagnetometer ResultsThe sidescan sonar data show considerable evidence of structure remaining on the wreck although only one image shows an object that could be the mast recorded as lying alongside the wreck by the UKHO. To the stern of the wreck is a large structure that may be an A-frame and which could be an indication of the type of vessel.
 
This site (WA6200) has a magnetic anomaly of 1312.49nT, which confirms that the vessel is metal. The size of the magnetic anomaly produced by the wreck obscures any other smaller magnetic anomalies that may lie nearby.
 
Although wreck WA6200 appears intact, there are six sites around it, one of which is 230m away, which are interpreted as associated debris.
 

Multibeam Data

The multibeam data covers an area of approximately 1000m x 460m with the depth ranging from 52m to 60m. The Reson 8125 used for this project has a beamwidth of 0.5° and therefore the footprint of the beams over this site was approximately 0.8m.
 
Multibeam ResultsMultibeam ResultsThe main wreck site (WA6200) was located within the multibeam data only a few metres from the position as determined using the sidescan sonar data. The multibeam data shows that the wreck is approximately 63m x 10m x 8m.
 
The width and height are very similar to those recorded in the sidescan sonar survey but there is a large difference between the length measurements. The length of the wreck as measured from the sidescan sonar data ranged from 58m to 73m but the multibeam measurement of 63m is likely to be the most accurate.
 
From both the sidescan sonar and multibeam data sets, it can clearly be seen that a section of the hull is either damaged or missing from the midships area of the wreck. It is not possible to determine from the geophysical surveys whether the damage was caused at or shortly after the time that the ship sunk or by the deterioration of the hull on the seafloor.
 
Viewing the raw soundings rather than the gridded surface did not provide any further detail that would help in the identification of the vessel, but it did show that a large amount of sediment had built up against it, covering a large portion of the deck.
 
For information on the ROV survey of this site click here.

Deep Wreck Site WA1003

Introduction

Side Scan of Sub Wreck (81m x 5m x 2m)Side Scan of Sub Wreck (81m x 5m x 2m) Wreck site WA1003 is described by the UKHO as being intact, possibly with two masts, at a charted depth of 46m with the surrounding seafloor being 55m deep. The wreck was located using the sidescan sonar and found to be approximately 10m due east of its recorded position.
 

Detailed Results

Sidescan Sonar and Magnetometer Data

Side Scan of Whole SiteSide Scan of Whole Site The sidescan sonar data consisted of 11 lines, each approximately 500m long and orientated E-W with a 75m range setting. The survey covered an area of approximately 500m x 350m. A further 19 lines orientated N-S were surveyed along the length of the wreck. Seven sidescan sonar anomalies were identified.
 
Magnetometer survey lines were also surveyed over the wreck, with lines orientated N-S. A single magnetic anomaly was identified.
 
The two sidescan sonar and the magnetic anomalies were compared in the GIS and grouped into two sites, both of which ascribed a level of Archaeological Potential:
 
Archaeological PotentialNumber within Study Area
High1
Medium0
Low1
Very Low0
Archaeological Potential Rating of geophysical anomalies within the WA1003 Study Area
 
The wreck is sitting upright on the seafloor in one piece (WA6300). The sidescan surveys show it to be approximately 81m x 5m, standing up to 2m above the seafloor. While it is difficult to determine which end is the bow, it is thought likely that it lies to the north.
 
Magnetometer ResultsMagnetometer Results The acoustic shadow produced by the sidescan sonar shows that a significant amount of structure is still present on the deck of the vessel. However, it is not clear exactly what those structures are.
 
This site (WA6300) has a magnetic anomaly of 2869nT, confirming that this vessel is metallic. The size of the magnetic anomaly produced by this wreck obscures any other smaller magnetic anomalies that may lie nearby.
 
Although the wreck appears to be intact, one site (WA6301) that lies just 5m away is interpreted as a small piece of associated debris.
 

Multibeam Data

The main wreck site (WA6300) was located within the multibeam data only a few metres from the position as determined using the sidescan sonar data. The multibeam data shows that the wreck is approximately 58m x 5m x 3m.
 
Multibeam ResultsMultibeam Results The width and height measurements are very similar to the sidescan sonar measurements but there is a difference between the length measurements. The length of the wreck as measured from the sidescan sonar data ranged from 70m to 87m, all greater than the multibeam measurement of 58m, which is likely to be more accurate.
 
The differences between the length measurements are due to the different angles at which the wreck has been imaged and the resulting image distortions. The sidescan surveyed it at an oblique angle from different directions while the multibeam surveyed it from above.
 
Multibeam results presented as pointsMultibeam results presented as points The multibeam survey covered an area of approximately 650m x 450m. The depth of the seafloor ranged from 52m to 58m. The Reson 8125 used for this project has a beamwidth of 0.5° and therefore the footprint of the beams over this site was approximately 0.7m.
 
Viewing the multibeam data as points rather than as a gridded surface does not provide any more detail on the structures present on the deck of the vessel. It does, though, show that the highest point on the vessel, presumably the bridge, is 45m from the bow.
 
For information on the ROV survey of this site click here.