Phase 2a West Midlands to Crewe: EI-711 Geophysical and Archaeological Surveys

Methods Survey Methodology A full methodology for each survey type undertaken as part of EI-711 is detailed in the Written Scheme of Investigation (WSI) for each survey package. Survey locations Survey locations were selected using a methodology detailed in the Written Scheme of Investigation (WSI) for each survey package. Survey Instruments Detailed gradiometer survey (Magnetometry) The gradiometer surveys were conducted using Bartington Grad-01-1000L or SenSYS FGM650 fluxgate gradiometers, which have a vertical separation of 0.65 m between sensors, and four of these sensors were attached to a Bartington cart system, with horizontal separations of 1 m. Data were collected at 0.25 m intervals along transects spaced 1 m apart with an effective sensitivity of 0.03 nT, in accordance with EAC guidelines . In areas where small features, such as pits, are expected or where a high level of detail is required, the horizontal separation can be reduced to 0.5 m. Caesium vapour The caesium vapour magnetometer surveys were conducted using two Geometrics G-864 sensors mounted 1 m apart on a non-ferrous cart. Data was collected at intervals no greater than 0.25 m along transects spaced 1 m apart with an effective sensitivity of 0.02 nT, in accordance with EAC guidelines . A Geometrics G-857 base station was also used to correct for diurnal magnetic drift when processing the data. All survey data was recorded in OS National Grid coordinates and heights above Ordnance Datum Newlyn (aOD), as defined by OSGM15 and OSTN15, with a three-dimensional accuracy of 0.3 m. Ground Penetrating Radar (GPR) The majority of GPR surveys were conducted using an Impulse Radar CrossOver4080 system, or an Impulse Radar PinPointR system, with a dual-frequency antenna. This antenna has the option of being mounted on a rough terrain cart or in a pull sled. These can be fitted with an odometer wheel to measure horizontal distance along the ground surface or set for continual triggering using GPS to position data. Data were collected along traverses spaced 0.5 m apart, with samples taken every 0.02 m, and effective time windows of 100 ns for the 400 MHz antenna and 50 ns for the 800 MHz antenna. Alternative GPR systems were also used on some sites. The GSSI 350 MHz HS GPR system uses hyper-stacking signal technology to effectively extend the depth range capability of the antenna. Data continued to be collected along traverses spaced 0.5 m apart, with samples taken every 0.02m, and effective time windows of 100 ns. A MALA X3M GPR system was used, with associated 500 MHz antenna, in a cart-based system. Data continued to be collected along traverses spaced 0.5 m apart, with samples taken every 0.02m, and effective time windows of 100 ns. Resistivity Resistivity survey data was collected using a Geoscan Research RM85 attached to an MSP25 cart system. The cart instrument has four electrodes arranged in a squared array attached to metal wheels spaced 0.5 x 0.5 m apart. The probes measure the current resistivity across various configurations of the probes and is recorded at 0.25 m intervals along traverses spaced 1 m apart. This spacing produces and effective reading interval of 0.25 m x 1 m. Where ground conditions were not suitable for the use of the MSP25 cart system, data were collected using a Geoscan Research RM15-D or RM85 with a twin-probe array system. The instrument has two electrodes spaced 0.5m fixed horizontally apart to the portable frame, and two further electrodes anchored 15 - 50 m away from the survey grid. The probes work in pairs (one mobile, one anchored) to measure the current and potential resistivity through the subsurface. This arrangement of electrodes suppresses any double peaking effects. Resistivity surveys undertaken by Wessex Archaeology depend on the establishment of an accurate 30 m site grid. These grids are arranged across the survey areas to collect maximum data.