EDO8041 - Hinton St Mary Roman Villa; geophysical survey 2023

‘Earth Resistance Geoscan RM85 PA5 Twin-Electrode Frame: Measurements were recorded over a 30m grid established with a Trimble R8s GNSS (Figure 1) using a Geoscan RM85 earth resistance meter, internal multiplexer, and a PA5 electrode frame in the Twin-Electrode configuration to allow two separate surveys, with electrode separations of 0.5m and 1.0m, to be collected simultaneously. For the 0.5m electrode separation survey readings were taken at a density of 0.5m x 1.0m whilst for the 1.0m separation survey they were taken at a density of 1.0m x 1.0m. Extreme values caused by high contact resistance were suppressed using an adaptive thresholding median filter with a radius of 1.0m (Scollar et al. 1990). Processed datasets are also presented as linear grayscale images after the application of Gaussian high pass filter with a radius of 5.0m. Earth Resistance GST Ltd. SF 2 Trapezoidal Frame: An additional earth resistance survey was conducted using a GST Ltd. SF 2 trapezoidal array and Geoscan RM15 earth resistance meter, resurveying four of the grids over the scheduled area. Readings were taken every 1.0m with a 1.0m traverse separation. Ground Penetrating Radar: 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 DXG1820 vehicle towed, ground coupled antenna array (Linford et al. 2010; Eide et al. 2018). A roving Trimble R8s Global Navigation Satellite System (GNSS) receiver was mounted on the GPR antenna array, that together with a second R8s base station was used to provide continuous positional control for the survey. Data were acquired at a 0.075m x 0.075m sample interval across a continuous wave stepped frequency range from 40MHz 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 75ns), 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. An average sub-surface velocity of 0.118m/ns was assumed following constant velocity tests on the data and was used as the velocity field for the time to estimated depth conversion. Each of the resulting time slices therefore represents the variation of reflection strength through successive ~0.14m intervals from the ground surface. The earth resistance survey has successfully detected a number of anomalies of archaeological, modern and geological origin. The survey identified two buildings, several ditches, drains and a modern pipe. Additional, areas of made or landscaped ground and geological variation were also recorded. Unfortunately, the Ground Penetrating Radar (GPR) survey proved less successful and only identified a limited number of fragmented responses to features mainly identified as drains during the excavation. These results are similar to the previous GPR surveys at the site and suggest a limited dielectric contrast between the structural remains and the subsoil, perhaps also reflected in the limited range of the earth resistance readings too.’