CP2 Regional Sea Boundaries: Eastern Channel, Offshore 1:250k: Wight, SeaVoX Sub-Ocean: ENGLISH CHANNEL
Wessex Archaeology was commissioned by English Heritage to undertake a project entitled ‘Wrecks on the Seabed: Assessment, Evaluation and Recording’. The project was supported by that part of the Aggregate Levy Sustainability Fund (ALSF) distributed by English Heritage. The specific aim of this project was to provide industry, regulators and contractors with guidance on the archaeological assessment, evaluation and recording of wreck sites. ‘Assessment, evaluation and recording’ are taken to include various methods of archaeological investigation that are intended to improve the understanding, preservation and appreciation of the historic environment. The term ‘wreck site’ is taken to include the remains on the seabed of both watercraft and aircraft. As part of a variation of this project Wessex Archaeology was commissioned by English Heritage to undertake a geophysical survey of four designated historic wrecks (A1 submarine, Hazardous, Invincible and Mary Rose) which focussed on the development and testing of methodology for rapid in situ recording using geophysical techniques. The geophysical survey included the use of multibeam echosounder, magnetometer and sub bottom profiler. The survey was undertaken from the 12.5 m Aquastar survey vessel Emu Surveyor belonging to Emu Ltd between 11th and 26th June 2003. The geology and geophysics component of the data are archived at British Geological Survey (BGS) MEDIN Data Archive Centre (DAC) for Geology and Geophysics. Data were also provided to other archive centres as appropriate.
The multibeam service provider Netsurvey Ltd. was commissioned to setup and carry out the multibeam sonar survey. The echosounder system used was a Reson SeaBat 8125 multibeam echosounder system with 240 dynamically focussed 0.5º beams, a swath coverage of 120º and a reported depth resolution of 6mm. Components of the system comprised the sonar head itself, a sonar processor unit, sound velocity probes and a dual processor PC with increased hard disk capacity running Reson 6042 acquisition and survey software. The vessel movement was compensated for with an Applanix POS MV (Position Orientation System for Marine Vessels), utilising two Novatel GPS antennas mounted on the A frame of the vessel at the stern to deliver heading and motion information, a gyrocompass for heading information, and a motion reference unit to correct heave, pitch and roll. Positioning was provided by a LEICA SR530 24-channel dual-frequency survey receiver with on-board RTK. A GSM (Global System for Mobile communications) compatible mobile phone was used to receive differential corrections from a base station placed at surveyed locations ashore. The archived data is post-processed. Processing included cleaning of the data to remove bad points and application of tidal parameters. A Geometrics G-881 caesium vapour magnetometer was used for the magnetometer survey and the data were recorded in Maglog software. This was then exported as *.txt format files. Layback positions have been applied to the data and do not need to be added during processing. For the sub-bottom profiler survey an Applied Acoustics AA200 boomer plate was used mounted on a CAT200 catamaran. The AA Capacitor Charging Unit CSP1500 provided the energy for the boomer plate. An Octopus 360 recording unit received all data from the hydrophone and annotated positioning information in the form of fixes every 10 m along the line rather than a unique position for each sounding. Data for archive includes tiff images of the boomer data lines and an associated navigation text file with position for each fix with all offset and layback data applied.
unrestricted use, copyright acknowledgement
Geophysical Survey WESSEX1: (11/Jun/2003 to 26/Jun/2003)
Magnetometer, Multibeam, Surface Tow Boomer
Available under the Open Government Licence subject to the following acknowledgement accompanying the reproduced Wessex Archaeology materials "Contains Wessex Archaeology materials ©Wessex Archaeology [year]"