Appendix 6: Geophysical Survey Methodology

Paul Linford, Neil Linford and Andrew Payne

February 2018

This appendix draws on Wessex Archaeology (2016a) for details on the Wessex Archaeology surveys.

Magnetometer survey

Magnetometer data for the Historic England survey were collected using an array of six high sensitivity Geometrics G862 caesium vapour magnetometer sensors mounted on a non-magnetic sledge towed behind a low impact All-Terrain Vehicle (ATV). Five sensors were mounted in a linear array transverse to the direction of travel 0.5m apart and, vertically, ~0.36m above the ground surface. The sixth was fixed 1.0m directly above the centre of this array to act as a gradient sensor. Sensor output was sampled at a rate of 25Hz which, based on the typical travel speed of the ATV (3.5–4.0m/s), gives a sampling density of ~0.15m by 0.5m along successive swathes. Each swathe was separated from the last by approximately 2.5m and navigation and positional control was achieved using a Trimble R8 Global Navigation Satellite System (GNSS) receiver mounted on the sensor platform 1.63m in front of the central sensor with a second R8 base station receiver established using the Ordnance Survey VRS Now correction service. After data collection the corresponding readings from the gradient sensor were subtracted from the measurements made by the other five magnetometers to remove any transient magnetic field effects caused by the towing ATV. The median value of each instrument traverse was then adjusted to zero by subtracting a running median value calculated over a 60m 1D window. This operation corrects for slight biases added to the measurements owing to the diurnal variation of the Earth's magnetic field and any directional sensitivity of the sensors.

For the Wessex Archaeology survey, data were collected using Bartington Grad-01-1000L gradiometers mounted on either a non-magnetic cart or a hand-held frame with samples being taken at 0.25m intervals along transects spaced 1m apart. Where hand-held frames were used, individual survey nodes were established using a Leica Viva RTK GNSS instrument at 30m x 30m intervals with guidelines placed between the nodes to locate successive traverses. The cart-based system used a Leica Captivate RTK GNSS instrument, which receives corrections from a network of reference stations operated by the Ordnance Survey and Leica Geosystems. Post-survey, data were subject to a minimal correction process comprising a zero mean traverse function (with thresholding to protect linear anomalies parallel to the instrument traverses with a magnitude greater than ±5 nT) to correct for any directional sensitivity of the sensors and a de-step function to account for variations in traverse position due to varying ground cover and topography.

Ground Penetrating Radar survey

For the Historic England survey, a 3d-Radar MkIV GeoScope Continuous Wave Step Frequency (CWSF) Ground Penetrating Radar (GPR) system was used to conduct the survey collecting data with a multi-element GX1820 vehicle towed, ground coupled antenna array (Linford et al. 2010). Two Trimble R8 GNSS receivers were used for navigation and positional control in the same way as for the magnetometer survey. Data were acquired at a 0.075m x 0.075m sample interval across a continuous wave step frequency range from 60MHz to 2.99GHz in 4MHz increments using a dwell time of 2ms. Post-acquisition processing involved conversion of the raw data to time-domain profiles (through a time window of 0 to 70ns), adjustment of time-zero to coincide with the true ground surface, background and noise removal, and the application of a suitable gain function to enhance late arrivals. To aid visualisation amplitude time slices were created from the entire dataset by averaging data within successive 2.4ns (two-way travel time) windows (e.g. Linford 2004). An average sub-surface velocity of 0.0968m/ns was assumed following constant velocity tests on the data, and was used as the velocity field for the time to estimated depth conversion.

For the Wessex Archaeology surveys, individual survey nodes were established using a Leica Viva RTK GNSS instrument at 30m x 30m intervals with guidelines placed between the nodes to locate successive traverses. GPR data were collected with a GSSI SIR 3000 control unit using a ground coupled 400 MHz shielded antenna mounted on a tricycle cart with odometer to record horizontal distance. This was deployed across all of the GPR survey areas with data collected along traverses spaced 0.5m apart. Data with the 400MHz antenna were collected at 50 scans per unit (1 unit = 1m) with an effective time window of 50ns. Post-acquisition, data were subject to common impulse radar signal correction processes. These comprise amplitude and wobble (dewow) correction of the radar profile to correct for variance in temperature and soil moisture content, background and band-pass filtering to remove noise in the data from the surrounding area, and XYZ mean line to correct for mosaic effects from variance in the day-to-day conditions during the survey. To determine sub-surface velocity, radar profiles were analysed for suitable hyperbolic reflections which were used to estimate the velocity field for the time to estimated depth conversion (Wessex Archaeology 2016a, appendix C).


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