Howard, B. (2023). Whitecross Offshore Windfarm, Braunton, Devon - Geophysical Survey Report. Wessex Archaeology. https://doi.org/10.5284/1113880. Cite this using datacite

Title
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Title:		 Whitecross Offshore Windfarm, Braunton, Devon - Geophysical Survey Report
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Series:		 Wessex Archaeology unpublished report series
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wessexar1-511561_211975.pdf (426 kB) : Download
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DOI
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DOI
https://doi.org/10.5284/1113880
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Publication Type:		 Report (in Series)
Abstract
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Abstract:		 Fieldwork methodology (Gradiometer) The cart-based gradiometer system used a Leica RTK GNSS instrument, which receives corrections from a network of reference stations operated by the Ordnance Survey (OS). Such instruments allow positions to be determined with a precision of 0.02 m in real-time and therefore exceeds European Archaeologiae Consilium recommendations (Schmidt et al. 2015). The detailed gradiometer survey was undertaken using either: four Bartington Grad-01-1000L gradiometers spaced at 1 m intervals and mounted on a non-magnetic cart; or, four Sensys FGM650/3 gradiometers spaced at 1 m intervals and mounted on a non-magnetic cart. Data were collected with an effective sensitivity of 0.01 nT – 0.03 nT at a rate of 10 Hz – 100 Hz, producing intervals of 0.15 m along transects spaced 4 m apart. Fieldwork methodology (EM) The EM survey was conducted using a GF Instruments CMD Explorer. This is a multi-receiver EM conductivity instruments with pairs of coils (one as transmitter and the other as a receiver) at three inter-coil separations (1.48, 2.82 and 4.49 m). This provides measurements from several depths consecutively, up to approximately 6.7 m when collected in the horizontal coplanar (HCP) formation. It has measuring ranges of 1000 mS/m for the apparent conductivity and ± 80 ppt for the magnetic susceptibility. The EM survey works by measuring the conductivity of different subsurface materials by transmitting electrical currents into the ground with a transmitter coil and measuring the secondary induced magnetic field with a separate tuned receiver coil. Readings are simultaneously recorded for the quadrature component (apparent conductivity) and the in-phase component (magnetic susceptibility). High conductivity values can be associated with clays and silts, whilst low conductivity values are likely to relate to deposits of sands and gravels, which have higher electrical resistance. If anomalies of both high magnetic susceptibility and high conductivity are coincident, then it is likely that the feature is metallic and therefore likely to be modern or artificial in origin. The EM data was collected in transects at 5 m intervals with 5 readings taken per second throughout all survey areas using the zig-zag method. A SBAS GPS system was used in order to facilitate continuous measurement which is precise to ±0.3 m. The location of these areas was compared against survey data collected using a Leica RTK GNSS GPS instrument, which is precise to approximately 0.02 m, in order to ensure accurate correspondence between the datasets. Gradiometer survey results The presence of possible archaeological activity towards the north of the site, in the form of positive linear anomalies and areas of increased magnetic response, are likely attributable to military activity during the Second World War. That portion of the survey area is believed to have been utilised by the military for possible barrack blocks, with associated infrastructure, as shown on aerial photography from 1946. This activity would have been in support of the documented training efforts by the US military throughout the dunes system, in preparation for the D-Day landing offensive. Further possible archaeological activity is noted to the south, both immediately north and south of the Taw Estuary, which bisects the southern portion of the site. The possible archaeological features north of the estuary, may be attributable to extraction activity. However, further information is not available, and these anomalies may be the by-product of military activity, modern agricultural practices, or variation in the geo-morphology of the site. The possible archaeological activity south of the estuary, may be associated with archaeological ditch features, such as land or animal management boundaries.
Author
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Author:		 Brett Howard
Publisher
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Publisher:		 Wessex Archaeology
Other Person/Org
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Other Person/Org:		 Devon Historic Environment Record (OASIS Reviewer)
Year of Publication
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Year of Publication:		 2023
Locations
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Locations:
County: Devon
Parish: Instow
District: North Devon
Country: England
Parish: Braunton
Grid Reference: 247717, 131494 (Easting, Northing)
Grid Reference: 245654, 137580 (Easting, Northing)
Subjects / Periods
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Subjects / Periods:
MILITARY TRAINING SITE (Monument Type England)
20TH CENTURY MILITARY TRAINING SITE (Tag)
MAGNETOMETRY SURVEY (Event)
GEOPHYSICAL SURVEY (Event)
ELECTROMAGNETIC SURVEY (Event)
20TH CENTURY (Historic England Periods)
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OASIS Id: wessexar1-511561
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Created Date
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Created Date:		 20 Oct 2023