CP2 Regional Sea Boundaries: Irish Sea, Western Channel & Celtic Sea, Offshore 1:250k: Cardigan Bay, Lundy, SeaVoX Sub-Ocean: CELTIC SEA, IRISH SEA
Wessex Archaeology (WA) was commissioned by Cadw to acquire, process and interpret marine geophysical data over a number of known and suspected wreck site locations off the coast of Wales. This survey was conducted under the Protection of Wrecks Act (1973) contract and the data is Crown Copyright©. The data were acquired and processed by Wessex Archaeology. The marine geophysical surveys targeted a total of nine suspected wreck locations in Milford Haven area, off the coast of South Wales, seven of which represent the locations for the remains of 19th century Welsh vessels associated with the coal mining industry. Additional survey data were acquired over two 20th century sites within the Milford Haven, the Thor and the Sunderland. The acquisition of these data aimed to assist RCAHMW maritime team and the Sunderland Trust with on-going archaeological research and fund-raising. The marine geophysical surveys also targeted a total of ten suspected and known wreck site locations off the coast of Lleyn peninsula in North Wales. These included eight unconfirmed positions of 19th century Welsh vessels associated with the slate mining industry, two designated sites the Diamond and the Tal-Y-Bont, and the marine hazard of St Patrick’s Causeway with the aim of discovering new wreck sites. The project aimed to acquire marine geophysical data consisting of high resolution sidescan sonar and magnetic gradiometer data over a total of total of 21 known and suspected wreck positions and along the known marine hazard of St. Patrick’s Causeway. Since the sites were located in two different areas, they were surveyed during two separate surveys from the Pembroke based vessel Blue Shark. The first session surveyed sites mostly off Milford Haven in south Wales between 17th and 18th April 2010. The second session surveyed sites off the coast of Lleyn peninsula and along St. Patrick’s Causeway in north Wales between 23rd April 2010 and 2nd May 2010 with a day of mobilisation and a day of demobilisation at the start and end of the survey session. 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.
Positions were provided using Leica 500 Trimble RTK beacon receiving corrections via satellite or a Trimble DGPS, which received differential corrections via land stations to provide a position to within 0.6m. Positions were recorded using World Geodetic System 1984 (WGS84) Datum and projected as Universal Transverse Mercator (UTM) Zone 30 North co-ordinates. The navigation data were recorded using Hypack (2008 v.2). The sidescan sonar data were acquired with a Klein 3900 towfish system. This system uses the latest technology in digital sidescan sonar imaging and can operate at either low (445kHz) or high (900kHz) frequency to provide high resolution images. The data were digitally recorded in xtf format using SonarPro Software. The sidescan sonar towfish layback values were recorded on survey logs and also applied to the raw data in SonarPro as it was recorded. The magnetic gradiometer data were acquired using Geomatrix’s G882 gradiometer system consisting of two marine caesium vapour magnetometers arranged horizontally 1.5m apart on a transverse gradiometer tow frame. The gradiometer frame was deployed independently of the sidescan sonar system, permitting the separate adjustment of fish heights. A fin and lead weight positioned under the centre of the frame were used in addition to those attached to each of the magnetometers in order to aid the submersion and stability of the gradiometer. This system was chosen for its high performance in shallow water with high sensitivity and fast sampling rate with a resolution of 0.02nT at 0.1 second sample interval and an absolute accuracy of <3nT throughout the range. The system only suffers a heading error of +/- 1nT error through a 360° arc. The gradiometer data were recorded in Geomatrix software Maglog and exported into ASCII files in real-time, where the raw and corrected position, field value, altitude and depth for each of the magnetometers were recorded in a single string. All magnetometer layback values were applied to the raw data in Maglog during recording and detailed on survey logs. In cases where the layback calculation failed the gradiometer data were corrected during the processing stages.
unrestricted use, copyright acknowledgement
Geophysical Survey WESSEX5: (17/Apr/2010 to 02/May/2010)
Available under the Open Government Licence subject to the following acknowledgement accompanying the reproduced Cadw materials "Contains Cadw materials ©Crown Copyright [year]"