EDO8243 - Poole Water Recycling and Transfer, Sturminster Marshall; geophysical survey 2025

OASIS: A fluxgate gradiometer survey (Magnetometry) was considered the most appropriate method for this survey. The non-magnetic hand-pushed Sensys MX PDA cart-based gradiometer system uses five Sensys FGM 650/3 fluxgate gradiometers, spaced 0.5 m apart. Data is collected with an effective sensitivity of ±8 µT over ±1000 nT range at a rate of 100 Hz, producing intervals of 0.01 m along parallel transects spaced 2.5 m apart. The system uses a Carlson BRx7 GNSS instrument, which receives corrections from a network of reference stations operated by the Ordnance Survey. Such instruments allow for positions to be determined with a precision of 0.02 m in real-time. Data is collected by the MXPDA data acquisition unit and combined with the incoming GNSS NMEA stream on a Carlson RT4 rugged tablet PC mounted at the rear of the cart. The system uses the GNSS positioning and the programmable cart geometry to provide a real-time geolocation of collected traverses for the user to ensure correct navigation, without the need for physical markers. The survey was performed in a zig-zag method of straight, parallel lines producing maximum coverage and minimal gaps.