wave hub hydrographic survey for archaeological assessment · halcrow group limited wave hub...

Emu Limited Head Office 1 Mill Court The Sawmills Durley Southampton SO32 2EJ UK Tel: +44 (0)1489 860050 Fax: +44 (0)1489 860051 www.emulimited.com

Wave Hub Hydrographic Survey for Archaeological Assessment

for

Halcrow Group Limited

REPORT NO. 08/J/1/02/1329/0829

January 2009 Job No J/1/02/1329

Halcrow Group Limited Wave Hub Geophysical Survey for

Archaeological Assessment

Report 08/J/1/02/1329/0829-DRAFT Emu Ltd

i

AUDIT INFORMATION

Title: Wave Hub Hydrographic Survey for Archaeological Assessment

Report Number : 08/J/1/02/1329/0829

Job Number : J/1/02/1329

Client Name : Halcrow Group Limited

Client Contact : Helen Needham

Project Manager : Rob Penrose

Project Surveyor(s) : Rob Penrose, Alex Elliott, Ed Henden, Ben Rainbow, Jess Hanham

Data Processor(s) : Rob Penrose, Alex Elliott, Ben Rainbow

Signature Date

Report written by Rob Penrose

Report checked by Richard Cooke

Report authorised by Huw Powell

Report Status Final

Issue Date January 2009




ii

EXECUTIVE SUMMARY

Emu Ltd was commissioned by Halcrow Group Limited to undertake a hydrographic survey

of the proposed wave hub site, located approximately 18 km from St Ives, Cornwall. The

survey included the collection of multibeam bathymetry, sidescan sonar and magnetometer

data throughout the proposed wave hub site and cable route. The primary objective of the

survey was to provide data for archaeological analysis by a third party contracted by the

client.

The survey was carried out using the vessel FPV Morven between 28th August and 20th

September, 2008. The equipment used included a Geometrics G882 marine magnetometer, a

fully motion-aided Reson 8101 multibeam system and an Edgetech 4200 dual frequency

sidescan sonar system. A Coda Octopus F180 motion reference unit with EGNOS differential

corrections was used for horizontal positioning of the vessel. Tides were acquired using a

Leica GX1230 system and post-processed with RINEX data.

Survey lines were run at 50 m spacing with the sidescan and magnetometer systems deployed

off the port and starboard sides respectively. The Reson 8101 swath transducer head was

attached by a pole to the port side of the vessel throughout. Forty-one approximately north-

south lines were run in the main proposed wave hub area, with a further 11 lines run along the

cable corridor. Cross lines were also completed inshore to ensure sufficient coverage was

acquired.

All datasets have been cross referenced with each other for quality assurance and positional

accuracy.

Water depths along the cable corridor increase from the beach to a maximum of

approximately 53.5 m Chart Datum (CD). In the wave hub site water depths range between

approximately 44 m to 60 m CD. Throughout the wave hub development area, sand ripples

are orientated approximately north south. In the south of the main site, there is an area of

outcropping bedrock which extends from approximately 800m from the south to the centre

and to the east of the site. Outside this area, bedrock outcrops are occasionally present. The

rest of the area is relatively featureless with some slight changes in the recorded intensity of

the sonar pulse indicating coarse sediment intersected with finer sediments in a banding

formation associated with the tidal movements.

The cable corridor comprises a mixture of fine sediment inshore, outcropping bedrock

through the majority of the corridor with coarser sediment offshore. An area of outcropping

bedrock is present from approximately 6 km to 20 km offshore. The northern end of the cable

corridor also has occasional outcropping bedrock which coincides with the type of features

seen in the main site. Sand ripples are also observed towards the northern extents of the cable

corridor.

Numerous contacts which are thought to be boulders and cobbles were found within the main

site and cable corridor. No new wrecks or contacts of any significant size were located within

the main site area, however a wreck was located in the cable corridor and this can be seen in

the sidescan sonar, bathymetry and magnetometer datasets. A number of magnetic anomalies

of between 0.1 nT and 20 nT were highlighted at the inshore section of the cable corridor.

Two of the anomalies were identified in a region of fishing gear near the inshore end of the

cable route. Although some other small magnetometer anomalies were found in the area,

these did not coincide with anything on the sidescan sonar or bathymetric records suggesting

they are buried.

The magnetometer survey identified a total of 52 magnetic anomalies in the data collected at

the proposed wave hub site and cable route.




iii

TABLE OF CONTENTS Page

AUDIT INFORMATION

EXECUTIVE SUMMARY

CONTENTS

1. Introduction & Objectives 1.1 Scope of Work 1.2 Summary of Events 1.3 Key Personnel

2. Methodology

2.1 Horizontal Positioning 2.2 Swath Bathymetry Survey 2.3 Sidescan Sonar Survey 2.4 Magnetometer Survey

3. Results

3.1 Positional Accuracy and Data Quality 3.2 Swath Bathymetry Data 3.3 Sidescan Sonar Data 3.4 Magnetometer Data 3.5 Interpreting Magnetometer Data

4. Discussion

5. Recommendations and Conclusions

6. Health and Safety

7. References

APPENDIX A Service Constraints

APPENDIX B Daily Progress Reports

APPENDIX C Tidal Data

APPENDIX D Sidescan Sonar Contacts

APPENDIX E Table of Significant Magnetic Anomalies

APPENDIX F Drawings Part 1

Drawings Part 2 (supplied in a separate folder)

All Digital Data has been provided on the enclosed data CD

i ii iii 1

1 3 3

4

4 5 6 7

8

8 8 9 10

13

18

19 20




iv

List of Tables Table 1.1 Timeline of Survey Events

Table 1.2 Key Personnel

Table 2.1 Summary of Navigation settings

Table 3.1 Typical detection range for common objects with G-881

Magnetometer

Table 3.2 Possible Magnetometer target sizes

List of Figures

Figure 1.1 Overview Map showing the survey lines

Figure 3.1 The boundary between coarse and fine sediments

Figure 4.1 Banding of coarse and fine material

Figure 4.2 Sand ripples indicative of sediment movements

Figure 4.3 Outcropping bedrock at the main wave hub site

Figure 4.4 Outcropping bedrock within the cable corridor

Figure 4.5 Image 1 of the wreck



List of Drawings – Part 1

J.1.02.1329.01a Sidescan Sonar Mosaic of Main Site

J.1.02.1329.01b Sidescan Sonar Mosaic of Offshore Area

J.1.02.1329.01c Sidescan Sonar Mosaic of Inshore Area




v

List of Drawings – Part 2 (supplied in a separate folder)

J.1.02.1329.02a Seabed Classification at the Main Site

J.1.02.1329.02b Seabed Classification at the Cable Corridor Offshore

J.1.02.1329.02c Seabed Classification at the Cable Corridor Inshore

J.1.02.1329.03a Bathymetric Contours of the Main Site

J.1.02.1329.03b Bathymetric Contours of the Cable Corridor Offshore

J.1.02.1329.03c Bathymetric Contours of the Cable Corridor Inshore

J.1.02.1329.04a Shaded Colour Bathymetry of the Main Site

J.1.02.1329.04b Shaded Colour Bathymetry of the Cable Route Offshore

J.1.02.1329.04c Shaded Colour Bathymetry of the Cable Route Inshore

J.1.02.1329.04d Shaded Colour Bathymetry of the Main Site

J.1.02.1329.05a Magnetometer Trackplot with Anomalies at the Main Site

J.1.02.1329.05b Magnetometer Trackplot with Anomalies at the Cable

Route Offshore

J.1.02.1329.05c Magnetometer Trackplot with Anomalies at the Cable

Route Inshore




1

1.0 INTRODUCTION AND OBJECTIVES

Emu Ltd was commissioned by Halcrow Limited to undertake a hydrographical survey of the

proposed wave hub site and cable corridor. The primary objective of the survey was to

determine water depths, seabed bathymetry and to provide data for an archaeological

assessment of the site.

The survey was carried out using the vessel FPV Morven between 28th August and 20th

September 2008. Data were collected using an Edgetech 4200 Sidescan Sonar, a Reson 8101

multibeam system and a Geometrics G882 magnetometer. Software used during the survey

allowed for online quality assurance of data. Navigation information was derived from a

DGPS system and tides were acquired using a Leica GX1230 system and post-processed with

RINEX data.

1.1 Scope of work

The scope of work was outlined in proposal No. P/1/02/08/0671 and submitted for approval in

July 2008.

The scope of work specified the collection of simultaneous multibeam bathymetry, sidescan

sonar, and magnetometer data throughout the proposed survey area. It required data to be

collected along the proposed cable route and main site, with 50m line separations and infill to

provide bathymetric data of a quality to meet IHO Sp44 Order 1. Paper charts have been

produced for contoured and colour shaded bathymetry, sidescan sonar mosaic, seabed features

with sidescan targets and magnetometer trackplot with anomaly size and location.

All relevant digital data have also been provided with this report.




2

Figure 1.1 Overview map showing the survey lines.




3

1.2 Summary of events

The vessel FPV Morven was mobilised on 28th August, 2008 to begin the survey on the next

available period of good weather. The survey was completed on 20th September, 2008 and all

survey operations with FPV Morven were completed with the vessel demobilised on the 20th

September, 2008.

Timeline of Events Date

Vessel mobilisation: 28/08/2008

Survey: 08/09/2008 – 09/09/2008,

16/09/2008 – 20/09/2008

Standby: 29/08/2008, 30/08/2008 –

07/09/2008, 10/09/2008 –

15/09/2008

Vessel demobilisation: 20/09/2008

Table 1.1 Timeline of Survey Events

1.3 Key Personnel

Personnel Name

Project Manager Rob Penrose

Hydrographic Surveyors Rob Penrose, Jess Hanham, Ben Rainbow

Geophysicists Edward Henden, Alex Elliot

Data Processors Rob Penrose, Alex Elliott, Ben Rainbow

GIS John Lonsdale, Helen Cole

Reporting Rob Penrose, Alex Elliott

Table 1.2 Key Personnel




4

2.0 METHODOLOGY

This section includes a brief factual description of the methods used to achieve the project

objectives.

2.1 Horizontal Positioning

For the purpose of the survey, navigation information was required to an accuracy of 1-5m.

Positioning information was obtained using a Coda Octopus F180 motion reference unit with

built in vector GPS. The DGPS signal was acquired through the Crescent Hemisphere R120

DGPS and sent to the Coda F180 via an RTCM data string. The receiver was configured to

receive differential corrections from the EGNOS differential network via satellite. A

navigation error check was performed during mobilisation and calibration. Details of this can

be found in the Daily Progress Reports in Appendix B. All relevant instrument offsets were

measured before the survey commenced and entered into the QINSy (version 8) hydrographic

navigation software. Navigation settings are given below in table 2.1.

Navigation Settings

Geodetic Parameters System Source Projection Parameters

Spheroid WGS 1984 System UTM

Semi-major axis 6378137.000000 Zone 30 North

Semi-minor axis 6356752.314245 Central Meridian 3° W

Inverse Flattening 298.257223563 False Easting 500000

Scale Factor 0.9996

Table 2.1 Summary of Navigation settings

2.2 Multibeam Bathymetric Survey In order to measure the depth of all parts of the seabed a Reson Seabat 8101 swath

bathymetry system was employed. The Reson Seabat 8101 swath bathymetry system emitted

a series of 101 equally spaced pulses of acoustic energy across the track of the vessel to

achieve 100% insonification of the seabed throughout the swath. The time it took for each

beam to return to the system was accurately measured to derive the seabed depth at each point

of insonification. The resulting swath soundings provided high resolution information on the

bathymetry of the seabed across the whole area. The seabed bathymetry was measured

throughout the development site and the cable corridor to shore.

Attitude and motion were measured by the Coda Octopus F180 system giving heave, roll,

pitch and yaw data. This information was fed directly into the QINSy system for real time

correction of the bathymetric data. The swath system was calibrated prior to the survey with a

standard patch test operation. This revealed the fixed errors in heading, pitch and roll and also

the time delay in the position data. Sound velocity profiles through the water column were

taken at the start and end of the survey and at regular 12 hour intervals. The sound velocity

profiles were applied online during acquisition of the data.

Post-processing was performed at Emu Ltd’s Durley office using QINSy software. At this

stage all corrections and filters were applied including the patch test calibration results,




5

removal of outliers with automatic and manual filtering and tides. The cleaned data were then

gridded using a 0.5m bin size and exported in XYZ format.

All bathymetric data were reduced to Chart Datum at St.Ives (3.4 metres below Ordnance

Datum Newlyn). WGS84 elipsoidal heights were recorded on board the survey vessel by

using a Leica GX 1230 RTK GPS system configured to collect post processed kinematic

(PPK) data. Once the survey was completed the PPK GPS heights were processed in Leica

Geo Office at 5 second intervals using the five nearest Ordnance Survey Active RINEX

stations. All data with a height quality of under ±0.2m were selected and imported into

Microsoft Excel for further processing. Elipsoid heights were converted to Ordnance Datum

(Newlyn) and then reduced to Chart Datum at St.Ives. A smooth curve was then fitted through

the PPK data and compared with observed tide gauge data at St Ives. Good agreement was

obtained and it was decided that the smoothed PPK data were a reasonable representation of

the tide levels recorded on board the survey vessel (tidal data has been presented in Appendix

E).

Bathymetric sounding data were reduced using the PPK tidal data for September 16th to 20th.

On September 9th GPS tidal heights were not available due to equipment failure of the GPS

unit. Therefore, for September 9th tidal levels were obtained from the Environment Agency

(EA) tide gauge at St. Ives. For quality control purposes the St. Ives tide gauge values have

been plotted against GPS tide values recorded on board the survey vessel on the 16th to 20

th

September (shown in Appendix E). These plots showed that the EA tide gauge data agreed

closely with GPS tide levels recorded during this period.

Details of when each line was surveyed have been provided in the swath bathymetry logs in

Appendix B. These logs can be cross referenced with the Magnetometer Track plot to give

precise information on the vessel track (drawings J.1.02.1329.05a – c).

2.3 Sidescan Sonar Survey High resolution sidescan sonar data was collected in order to identify any objects that may be

of archaeological interest. An Edgetech 4200 Dual Frequency (nominally 120kHz and

400kHz) sidescan towfish was deployed from the survey vessel. The configuration of the

system, with wide vertical and narrow horizontal beam angles, enabled a swath of the seabed

to be imaged by each pulse. Qualitative interpretation of the strength of the acoustic response

enabled the identification of both topographic and sediment changes within the surveyed area

and indicated the presence of seabed anomalies such as bedrock or large boulders. Further

quantitative interpretation of the sediment class can differentiate areas with different sediment

content.

Throughout the survey the sonar was operated on a 100 metre range for high frequency and

200m for low frequency data collection. The 50m line spacing allowed for a maximum of

200% ground coverage in high frequency, and 400% coverage in low frequency.

Survey data was acquired on board the survey vessel using Discover acquisition software.

SonarWiz was used for post-processing where optimum gain settings were selected, laybacks

applied and the data mosaiced for presentation in GIS. Full QC procedures were adhered to

throughout data collection and processing.

2.4 Magnetometer Survey

A magnetometer survey was undertaken in order to detect the presence of ferrous objects on

or just below the seabed. A Geometrics G882 caesium vapour marine magnetometer was




6

utilised for this aspect of the survey. The system incorporates a towed fish which houses a

total magnetic field sensor. The unit provides absolute readings of total magnetic field, with a

resolution of 0.01 nT (nanotesla), which is more than sufficient for this type of survey. These

readings are ‘point-specific’ i.e. taken individually they do not give any directional indication

of where any feature causing anomalous magnetic field values may lie.

In operation, the magnetometer is towed with navigation supplied from the Coda Octopus

F180 to a specific tow point on the vessel. The cable is then laid out and recorded with the

magnetometer layback entered manually online into the MagLog acquisition software.




7

3.0 RESULTS

Results from the survey are presented in this section. Reference is made to supporting

information presented in the appendices and on the drawings. All water depths are referenced

to Chart Datum (CD) at St. Ives (3.4 metres below Ordnance Datum Newlyn).

3.1 Positional Accuracy and Data Quality

The horizontal positioning data from the DGPS system were of good quality throughout the

duration of the survey, with a positional accuracy of greater than 2 m. The sea bed features

identified in the sidescan sonar data are estimated to have an accuracy of approximately ±10

m across track due to the manual layback method and the influence of environmental factors

such as currents. The magnetometer will have a similar accuracy of approximately ± 10 m, as

it would be affected by the manual layback method and environmental influences.

All datasets are found to be of very good quality, this being assisted by generally good sea

conditions at the time of survey, despite strong south/westerly winds. The bathymetric data

were reduced using GPS tidal data measured onboard the survey vessel with the exception of

tidal data collected on the 16th September which was acquired from the EA tide gauge at St.

Ives.

Fishing gear onsite made obtaining full coverage difficult. However full coverage was

obtained with the use of infill lines where necessary.

3.2 Swath Bathymetry Data

A Reson 8101 multibeam echo sounder was used to map the bathymetry of the proposed

wave hub site. The data has also been cross referenced against the other data sets to provide

information on items of debris and seabed types. The processed data is presented in the form

of a colour shaded relief chart; Drawings J.1.02.1329.04a, J.1.02.1329.04b and

J.1.02.1329.04c and a 0.5m contoured relief chart; Drawings J.1.02.1329.03a, J.1.02.1329.03b

and J.1.02.1329.03c. Digital data in XYZ format, suitable for use in archaeological analysis,

has been provided on the enclosed data CD.

The swath dataset achieved full coverage using the line plan approved by the client, except in

the inshore shallow water. Infill lines were run to ensure full coverage of this area. Adjacent

lines, surveyed at the southern end of the cable route do show some variations in depth

(approximately 0.3m variation when compared to adjacent lines). These variations were an

artefact of the vessel movement and tidal effects, which were less well resolved in the inshore

section of the survey area. This section of the cable route was found to be relatively flat and

no noticeable sand dunes or bedform features were found in the swath or sidescan sonar

record.

Cable Corridor

Depths throughout the cable corridor range from approximately 3 m at the inshore extent of

the survey to approximately 53.5 m offshore at the northern extent. The swath bathymetry

dataset closely matches the seabed features depicted in the sidescan sonar dataset.

Between the inshore extent of the survey and 6 km offshore the seabed gently shoals from a

depth of approximately 3 m to 26 m below CD. The seafloor is relatively smooth increasing




8

in depth with occasional patches of exposed bedrock showing a comparatively low relief of

up to approximately 1 m.

From approximately 6 km to 20 km offshore outcropping bedrock dominates. This

outcropping bedrock has a comparatively high relief of up to approximately 3.5 m. These rock

outcrops generally have their perimeters at a depth of approximately 23 m to the south

increasing to 33 m in the north. Within the areas of outcropping bedrock, there are occasional

isolated pockets of sediment.

From approximately 20 km to 21.5 km offshore at the northern extent, the seabed returns to

be dominated by mainly course sediments with occasional outcropping bedrock. Depths in

this area range from approximately 49 m to 53.5 m below.

A wreck was observed in the swath record at approximately 50º;17.2086’ N 005º;34.1721’ W.

It is approximately 2.4 m high and 14 m in length. It appears to be in two pieces and lies at a

depth of approximately 30.5 m below CD on an area of uneven ground.

Main Site

Depths through the wave hub site range from approximately 44.4 m to 60 m below CD. An

area of outcropping bedrock is present approximately 800m from the south to the centre and

to the east of the site. These outcrops rise up to approximately 3m above the surrounding

seafloor. Throughout the site, areas of occasional outcropping bedrock are observed within a

mainly sediment dominated seabed.

3.3 Sidescan Sonar Data

High resolution sidescan sonar data was acquired with an Edgetch 4200 sidescan sonar. The

dataset has been used to classify the seabed as well as to identify any items of debris, wrecks

or other seabed features. The processed data have been mosaiced and presented in Drawings

J.1.02.1329.01a, J.1.02.1329.02b and J.1.02.1329.01.c. A seabed features chart has been

produced using the sidescan sonar data and presented in Drawings J.1.02.1329.02a,

J.1.02.1329.02b and J.1.02.1329.02c. A digital copy of the sidescan sonar mosaic in geotif

format has been provided on the enclosed CD.

Numerous contacts were picked with a minimum target size of approximately 1.0 m or

features which looked out of place with the surrounding seabed. In areas of outcropping

bedrock, targets were difficult to identify due to the nature of the sidescan record.

Due to the large amount of fishing gear in the area, it was necessary at times to fly the

sidescan fish close to the sea surface to ensure that fishing and survey equipment was not

damaged. On the sidescan mosaic, these areas appear relatively darker. The sidescan sonar

mosaic will also show some areas which did not follow the line plan, as fishing gear made it

unsafe to do so, though full coverage was still acquired.

Cable Corridor

A wreck was observed in the cable corridor. This coincides with the wreck identified during

the previous geophysical survey of the wave hub cable corridor. This wreck is potentially the

charted wreck Helene. The wreck appears to be in two pieces, with an overall approximate

length of 14 m. The wreck protrudes approximately 4.5 m above the seabed and is lying on a

bearing of approximately 106° as shown in Figure 4.5, 4.6, 4.7. The wreck is a different

height from the bathymetry because the height on the sidescan sonar record is calculated from

the contacts shadow. The wreck is examined further in the discussion, section 4.




9

There is a marked boundary at the south end of the cable corridor between the coarse and fine

sediments and this is highlighted in Figure 3.1. The data show that across the majority of the

site, there are bands of coarse and fine sediment. It is not possible to provide more detailed

descriptions of these sediments without ground truthing.

Figure 3.1 The boundary between coarse and fine sediments

Further offshore the coarse sediment is interrupted by outcropping bedrock which is present

from approximately 6 km to 20 km offshore (Figure 4.4).

Towards the northern extent of the cable corridor, coarse sediments become the dominant

seabed type with occasional outcropping bedrock throughout the area.

Sand ripples are observed mainly towards the northern extents of the cable corridor, however

small pockets through the central and southern central section are present. These tend to be

aligned approximately north-south with wavelengths of between 3 m and 5 m

Main Site

To the centre of the site, there is an area of outcropping bedrock which extends from

approximately the centre to the east of the site. Outside this area, bedrock outcrops are

occasional; this corresponds with the features observed in the bathymetry dataset at the

northern end of the cable corridor. The remainder of the area is relatively featureless with

some slight changes in the recorded intensity of the sonar return.

The wave hub site contains twelve contacts, however these are mainly cobbles, boulders or

outcropping bedrock. No wrecks were observed in the area.

Throughout the main site sand ripples have been identified. These run in band formations

with a bearing of approximately 12° and a wavelength of 3.5m.

fine sediment

coarse sediment




10

3.4 Magnetometer Data

Magnetometer data have been acquired along the proposed cable route and potential wave

hub area. Data were recorded using Geometrics Maglog data acquisition software and

processed using Geometrics magnetometer software MagPick. Layback was applied to each

profile line in real time using Maglog according to the cable length used during the survey

and recorded in the survey logs. Corrected magnetic profile data were then checked for

magnetic anomalies. As part of the quality assurance the magnetometer anomalies were cross-

correlated with the other datasets for the area.

Overall, 52 significant targets have been identified from the magnetometer data. These

anomalies are detailed in Appendix E and shown on magnetometer survey trackplots

presented in Drawings J.1.02.1329.05a, J.1.02.1329.05b and J.1.02.1329.05c. Targets have

been identified by anomalous spikes in the data and separated from background noise through

comparison with adjacent lines.

The geology of the site is dominated by recently reworked soft Quaternary sediments

overlying the Devonian bedrock consisting of siltstones, sandstones, slate and limestones.

These sedimentary rocks are unlikely to cause significant interference and deleterious effects

on the accuracy of interpretations in magnetic surveying. The quality of the interpretations has

been maintained through careful data processing and rigorous quality assurance checks.

Cable Corridor

The magnetometer data identified 47 magnetic anomalies within the cable route. One area

along the central section of the cable route had a large cluster of magnetic anomalies ranging

between 320 nT and 420 nT. This coincides with the approximate position of the wreck.

At the southern end of the cable corridor magnetic anomalies of between 6nT and 20nT were

common, with eleven identified. These did not correspond to any sidescan sonar targets, so it

is likely that the anomalies are buried. One small magnetic anomaly did correspond with the

fishing gear at the inshore end of the cable route, with another small anomaly close by.

Main Site

Within the main wave hub site 5 targets were identified, however these were only between .01

nT and 3.00 nT, suggesting no contacts of any significant size. After comparing with the other

data sets, these did not correspond with any features.

3.5 Interpreting Magnetometer Results

In order to interpret the magnetic anomaly results it is important to understand that the

magnetic anomaly from a given object can vary greatly. Both the size and the shape of a

magnetic anomaly vary with magnetic latitude, the shape of the magnetic body, its attitude

and its orientation with respect to magnetic north (Sheriff, 1978). However the greatest effect

on the anomaly size from a given object is the distance between the object and the sensor.

Magnetic field strength (from a dipole) declines inversely with the cube of the distance from

the objects centre (Breiner, 1999). Table 3.1 below shows typical detection ranges for

common objects with a G-881 magnetometer, and Figure 3.1 below shows the decline in

anomaly size with distance for various ferrous targets.




11

Typical Detection Range For Common Objects

Ship 1000 tons 0.5 to 1 nT at 244 m

Anchor 20 tons 0.8 to 1.25 nT at 120 m

Automobile 1 to 2 nT at 30 m

Light Aircraft 0.5 to 2 nT at 12 m

Pipeline (12 inch) 1 to 2 nT at 60 m

Pipeline (6 inch) 1 to 2 nT at 30 m

100 kg of iron 1 to 2 nT at 15 m

100 lbs of iron 0.5 to 1 nT at 6 m

10 lbs of iron 0.5 to 1 nT at 3 m

Screwdriver 5 inch 0.5 to 2 nT at 4 m

1000 lb bomb 1 to 5 nT at 30 m

500 lb bomb 0.5 to 5 nT at 16 m

Grenade 0.5 to 2 nT at 3 m

20 mm Shell 0.5 to 2 nT at 1.8m

Table 3.1 Typical detection range for common objects with G-881 magnetometer

(from G-882 Cesium marine magnetometer operations manual, 2005)

Figure 3.1 Graph of detection depths versus magnetic signal strength for various

ferrous UXO targets (Stanley, 2003)




12

During the magnetometer survey the magnetometer fish depth was measured with onboard

pressure sensors. Over the site, magnetometer fish height varied between approximately 2 to

10 m above the seabed. Targets can be combined with the height range above seabed to give a

range of possible target sizes as shown in Table 3.2 below. This is assuming targets are on the

seabed; buried targets would have an added distance from the sensor and so would appear as

smaller anomalies.

Anomaly Size (nT) Possible target sizes (based on fish height 2-14.2 m)

0-0.5 <0.5 kg iron – 1 kg iron, Bar Magnet (5000 cgs units)

0-1 Bar Magnet (5000 cgs units) – 45 kg iron

1-2 - 50 kg iron

2-5 - 100 kg iron

5-10 > 100kg iron, 6 inch pipeline

10-20 >>100kg, > 6 inch pipeline (<1000 kg iron)

Table 3.2 Possible Magnetometer target sizes (after Breiner, 1999)

It should also be noted that magnetic anomalies can also arise from variation in the chemistry

of the rocks and soil from one position to another. Even at any one place, the Earth's field is

forever changing by a small amount with time. For the purpose of target detection anomalies

due to changes in the ground geology or due to temporal sources are termed "magnetic noise".

Many of the identified targets were identified on more than one line due to their anomaly size.

These have been filtered down to one anomaly where possible by observing similarities in the

shape of the anomaly on each line and taking the largest anomaly as being closest to the

source.




13

4.0 DISCUSSION

Four main seabed types have been classified from the combined datasets as listed below:

• Outcropping bedrock with patches of coarse sediment

• Coarse Sediment with boulders and cobbles

• Coarse Sediments

• Fine Sediments

These classifications have been presented on Drawings J.1.02.1329.02a, J.1.02.1329.02b and

J.1.02.1329.02c.

The cable corridor comprises a mixture of fine sediment inshore, pro-grading into coarser

sediment offshore. This boundary coincides with the outer extent of St Ives Bay where the

two headlands end and the water becomes deeper.

In the main wave hub site, there is evidence on the sidescan record of finer sediments mixed

with coarser sediments in a banding formation (Figure 4.1). The width of the banding varies

across the site.

Figure 4.1 Banding of coarse and finer material

Sand ripples are indicative of mobile sediments and the occurrence of sand ripples increases

at the northern end of the cable corridor and into the main wave hub site (Figure 4.2). The

sediment banding coupled with the sand ripples in the main site and cable route indicate that

the sediment is influenced by tidal current and is moving in an approximate east-west

direction.

fine sediment

coarse sediment

coarse sediment




14

To the north of the cable route, the sediment and bedrock exposure suggests that the sediment

layer is thin; however this layer is expected to thicken slightly to the far north and into the

main site as the rock outcrops become scarcer and the seabed type changes from dominant

rock outcrop to coarse sediment.

Figure 4.2 Sand ripples indicative of sediment movements

Outcropping bedrock is focused in two areas; the central section of the cable corridor (Figure

4.4) and the centre towards the eastern area of the wave hub site (Figure 4.3). In both areas

there are isolated areas of sediment. Bedrock occasionally outcrops outside the two areas

described above. In these areas the seabed is mainly dominated by coarse sediment. It is likely

that the sediment is a relatively thin layer.

sand ripples




15

Figure 4.3 Outcropping bedrock at the main wave hub site.

Figure 4.4 Outcropping bedrock within the cable corridor.

outcropping bedrock

outcropping bedrock

sediment




16

No new wrecks or contacts of any significant size were located within the main wave hub site,

however a wreck was located in the cable corridor. This has been identified in approximately

the same position for all the datasets and can be seen in the sidescan sonar data as shown in

Figures 4.5, 4.6, 4.7. The wrecks position also coincides with the nearby charted wreck

Helene. The UKHO have its position as approximate and have charted this wreck at 50°

17.034’ 005° 36.060’, approximately 1.5 km away from the actual site. This wreck was also

identified in a previous geophysical survey of the cable corridor.

The Helene is a riveted iron steamer built in 1896 and was sunk in 1918 by a U boat. The

vessel was 73m in length, had twin boilers and a single screw. Further investigation of this

site is needed to clarify if this wreck is the Helene.

At the southern end of the cable corridor, the intensity of magnetic anomalies increased. It is

not uncommon for magnetic anomalies to be concentrated inshore as targets may be related to

human activity from Hayle harbour. The size of the targets in this area ranged from 0.01nT to

20nT. The only target that corresponds with the sidescan record is a small anomaly in an area

of fishing gear, with another small anomaly close by. It is therefore likely that the other

anomalies are buried. The magnetometer fish would have also been flying very low to the

ground meaning the size of the targets compared to the rest of the site would be exaggerated.

Due to the large amount of fishing gear in the area, the anomalies are possibly related to this.

Due to the inaccuracies of both the sidescan sonar fish and magnetometer fish being

approximately ± 10 m, this may have also offset magnetometer anomalies from sidescan

sonar targets.

Figure 4.5 Image 1 of wreck on cable route.

wreck




17



wreck

wreck




18

5.0 RECOMMENDATIONS AND CONCLUSIONS

Water depths range from approximately 3 m to 53.5 m and 44.4 m to 60 m throughout the

cable corridor and wave hub site respectively. The southern end of the cable corridor changes

from fine sediments to coarse sediments at a noticeable boundary moving further offshore. An

area of outcropping bedrock dominates the cable corridor. At the northern extent of the cable

corridor, coarser sediments return. An area of outcropping bedrock dominates the centre of

the wave hub site while the rest of the wave hub site remains relatively featureless with

occasional rock outcrops with banding of coarse and fine sediments.

Sand ripples are evident in the cable corridor and wave hub site indicating a mobile seabed.

Throughout the areas of coarse and fine sediments, sand ripples run in bands with their axis

orientated approximately north to south with the predominant direction of sediment transport

being from west to east.

Numerous contacts and boulders were found within both the cable corridor and wave hub site.

52 magnetic anomalies were identified within the magnetometer data collected at the

proposed wave hub site and cable corridor. 5 magnetic anomalies were identified within the

proposed wave hub site. One possible wreck was also located within the cable corridor lying

in an area of outcropping bedrock which coincides with 5 magnetic anomalies and a contact

on the sidescan sonar record. A number of magnetic anomalies of between 0.1 nT and 20 nT

were highlighted at the inshore section of the cable corridor. Two anomalies were identified

in the region of fishing gear. No other targets corresponding with anomalies were identified in

the area, suggesting the unidentified anomalies are buried. No other magnetic anomalies

related to features in the other datasets were present.

The survey data acquired offshore were of sufficient quality and in all cases deemed to satisfy

the requirements of the contract. Data processing was also performed to Emu’s QA standards

(ISO 9001: 2000).

As part of Emu’s Quality Assurance procedures and in the interests of our ongoing

development, every contract undergoes a comprehensive review on completion. This process

is designed to identify any areas where improvements can be made. No significant

deficiencies or suggested improvements were identified.




19

6.0 HEALTH & SAFETY

On the commencement of the survey a safety briefing was attended by all survey personnel

whereupon the emergency procedures were explained and emergency equipment

demonstrated. The briefing was repeated for any new staff.

A safety plan and risk assessment was completed prior to survey commencement and can be

viewed on request. All survey and crew members were required to read and sign the safety

plan. Toolbox talks were given regularly to familiarise all personnel with the equipment and

vessel.




20

7.0 REFERENCES Breiner, S., 1999, Applications Manual for portable magnetometers

G-882 Cesium marine magnetometer operations manual, 2005, Geometrics

Sheriff, R., 1978. A First Course in Geophysical Exploration and Interpretation.

Stanley, J.M., 2003, Achieving 100% quality assurance in ferrous explosive ordnance

disposal, Geophysical Research Institute, University of New England, Australia.

APPENDIX A Service Constraints

Service Constraints This report and the data were compiled and carried out by Emu Limited (Emu), in accordance with

Proposal No. P/1/02/08/0671. The services were performed by Emu with the skill and care ordinarily

exercised by a reasonable hydrographic survey specialist at the time the services were performed.

Further, and in particular, the services were performed by Emu taking into account the limits of the

scope of works required by the Client, the time scale involved and the resources, including financial

and manpower resources, agreed between Emu and the Client.

Other than that expressly contained in paragraph 1 above Emu provides no other representation or

warranty whether express or implied, in relation to the services.

The services were performed by Emu exclusively for the purposes of the client. Emu is not aware of

any interest of or reliance by any party other than the client in or on the services. Unless expressly

provided in writing, Emu does not authorise, consent or condone any party other than the client relying

upon the services. Should this report or any part of this report, or otherwise, details of the services or

any part of the services be made known to any such party, and such party relies thereon that party does

so wholly at its own and sole risk and Emu disclaims any liability to such party. Any such party would

be well advised to seek independent advice from a competent hydrographic survey specialist and / or

lawyer.

It is Emu’s understanding that this report is to be used for the purpose described in Section 1.1 of this

report. That purpose was a significant factor in determining the scope and level of the services. Should

the purpose for which the report is used, and / or should the clients proposed development or use of the

site change (including in particular any change in any design and / or specification relating to the

proposed use or development of the site), this report may no longer be valid or appropriate and any

further use of or reliance upon the report in those circumstances by the Client without Emu’s review

and advice shall be at the clients sole and own risk. Should Emu be requested, and Emu agree, to

review the report after the date hereof, Emu shall be entitled to additional payment at the then existing

rates or such other terms as may be agreed between Emu and the client.

The passage of time may result in changes (whether man-made or otherwise) in site conditions and

changes in regulatory or other legal provisions, technology, methods of analysis, or economic

conditions which could render the report inaccurate or unreliable. The information, recommendations

and conclusions contained in this report should not be relied upon if any such changes have occurred or

after the period of 2 years from the date of this report or such other period as may be expressly stated in

the report, without the written agreement of Emu. In the absence of such written agreement of Emu,

reliance on the report after any such changes have occurred or after the period of 2 years has expired,

shall be at the clients own sole risk. Should Emu agree to review the report after the period of 2 years

has expired, Emu shall be entitled to additional payment at the existing rates or such other terms as

maybe agreed between Emu and the client.

The observations, recommendations and conclusions in this report are based solely upon the services

which were provided pursuant to the contract between the client and Emu. Emu has not performed any

observations, investigations, studies or testing not specifically set out or required by the contract

between the client and Emu. Emu is not liable for the existence of any condition, the discovery of

which would require performance of services not otherwise contained in the services.

Where the services have involved Emu’s interpretation and / or other use of any information (including

documentation or materials, analysis, recommendations and conclusions) provided by third parties

(including independent testing and / or information services / laboratories) or the client and upon which

Emu was reasonably entitled to rely or involved Emu’s observations of existing physical conditions of

any site involved in the services, then the services clearly are limited by the accuracy of such

information and the observations which were reasonably possible of the said site. Unless otherwise

stated, Emu was not authorised and did not attempt to independently verify the accuracy or

completeness of such information, received from the client or third parties during the performance of

the services. Emu is not liable for any inaccuracies (including any incompleteness) in the said

information, the discovery of which inaccuracies required the doing of any act including the gathering

of any information which it was not reasonably possible for Emu to do including the doing of any

independent investigation of the information provided to Emu save as otherwise provided in the term’s

of the contract between client and Emu.

APPENDIX B Daily Progress Reports

APPENDIX C Tidal Data

APPENDIX D Sidescan Sonar Contacts

APPENDIX E Table of Significant Magnetic Anomalies

Positions are given in metres Easting and Northing to projection WGS84 UTM Zone

30 North.

No: Easting Northing Intensity of Anomaly (nT)

1 324264.7 5567098 1.12

2 326293.8 5564118 8.42

3 326172.7 5564036 2.9

4 326654.1 5565641 1.73

5 326299.6 5564119 13.52

6 326342.9 5565360 5.1

7 325789.9 5566230 9.39

8 325404.9 5566583 1.92

9 326334.2 5564309 2.69

10 323694.2 5567329 0.79

11 321575.7 5567926 0.49

12 326708.2 5564305 0.72

13 326741.3 5564545 6.78

14 326644.3 5565556 0.79

15 326442.2 5565914 2.29

16 326412.2 5564469 4.74

17 326425.7 5564536 6.25

18 326514.2 5564928 6.81

19 326385.9 5564314 2.45

20 326069.1 5565893 0.76

21 326128.3 5565803 0.67

22 326743.8 5564858 15.11

23 326739.3 5564818 9.34

24 326749.1 5564941 7.24

25 326735.3 5565294 2

26 326531.5 5565648 8

27 326238.7 5566108 2.96

28 326620.2 5564445 10.3

29 326640.2 5564537 2.87

30 325902.9 5566485 11.53

31 326656.2 5564617 1.33

32 326483 5565630 1.52

33 326018.1 5566141 3.58

34 326546.4 5564469 6

35 326614.8 5565294 3.95

36 326405.1 5565669 3.08

37 325908.2 5566480 4.94

38 316937.9 5573656 320.48

39 316941.2 5573701 351.52

40 316976 5573750 13.07

41 324869.4 5567254 1.99

42 325291 5566967 0.53

43 321770.7 5568253 1.3

No: Easting Northing Intensity of Anomaly (nT)

44 313750.5 5576240 0.78

45 316461.5 5574340 1.35

46 317738 5573473 1.41

47 314619.8 5582219 0.5

48 313971.4 5580527 0.75

49 313744.6 5582324 0.97

50 316892.2 5573615 452.72

51 316856 5573591 17.01

52 313277.1 5583315 0.93

APPENDIX F Drawings

wave hub hydrographic survey for archaeological assessment · halcrow group limited wave hub...

Documents