Step 1: Tensor Magneto-telluric array
A modern tensor magneto-telluric station needs to record the electric field along 2 perpendicular directions and two horizontal magnetic components. When possible, the vertical magnetic component is also recorded as an additional indicator of resistivity heterogeneity. The electric field is measured through the electric potential difference between two unpolarizable electrodes divided by the distance in-between. The broadband magnetic sensors are coils to be dug and leveled.
Step 2: Equipment checking
Last check before starting campaign.
Step 3: Magneto-telluric data acquisition parameters
The choice of spatial extent and sampling is important to define the imaged zone. But the crucial parameter is the lowest usable frequency in the data. Indeed, it will control the investigation depth. A simple model based on a priori resistivity and geological target depth is used to define the lowest frequency of interest: here, approximately 100 sec.
Because the MT signal is weak especially at the low frequencies, one may face bad results at the depth of interest for geothermal exploration. From previous experiments in island context and reported bibliography (Wawrzyniak, 2012) we fixed the recording duration as presented in the table. At each MT station, the measurement lasted from 36h minimum up to 72h.
Step 4: Remote reference station
A standard filtering nowadays systematically applied is based on a correlation filter using the magnetic signal recorded at a reference station in low level and uncorrelated noise conditions. The location of this remote reference station is also a logistic issue in an island context. This reference station is running continuously 24h/day repeating cyclically the recording table (with a 24h period). The selected remote reference station chosen was located 35 km away at Plateau Boucher (in the mountainous Piton des Carbets area).
