A Sea-to-Ground Magnetovariational Method

Abstract

Abstract The magnetovariational (MV) approach consists of recording the natural time-varying magnetic field at two different levels below the ground surface. The MV method can also be used in marine researches using waterproof sensor devices. However, a weak point of marine measurements is the lack of stability of any sensor hanging in seawater. To overcome this difficulty, in this paper we propose an alternative array design consisting of a magnetic sensor device firmly laid down on the sea floor and a second device located on land at a remote station where the subsoil conforms to a 1-D structure. This circumstance and the uniformity of the primary magnetic field over largely extended regions allow the electromagnetic components in seawater to be readily estimated using the measured magnetic field at the remote station. By these estimates, the two-level MV response across the sea water layer can be definitively simulated. The same procedure can be adopted to simulate the magnetotelluric sounding at any level in seawater. Finally, the installation of a second remote station inland is suggested in order to minimize the effect of noise on the estimates of the apparent impedivity functions. Reliability and usefulness of the new sea-to-ground MV method are tested on two synthetic examples. Introduction A pioneering approach to magnetotelluric (MT) sounding with electric and magnetic sensors immersed in seawater at a depth of a few tens of meters dates back to the late 50s1. The recent technological progress makes now possible to carry out more sensitive offshore MT measurements with the sensors lying on the sea floor2. An alternative approach to offshore electromagnetic probing is the magnetovariational (MV) method formulated by Patella and Siniscalchi3. They introduced an appropriate MV apparent impedivity (dispersive resistivity) function, which can be used to retrieve reliable impedivity structural models below the sea water layer. Basically, the MV method requires the recording of only the magnetic field at two different levels in seawater. Fig. 1 shows schematically a seaborne magnetic sensor layout. It consists of a pair of waterproof horizontal two-component magnetic devices, one of which is laid down on the sea floor in S(b), while the other is located close to the air-water boundary in S(t), vertically above S(b). A weak point of this array is the lack of stability of the top sensor device, which can be subject to a motional noise causing unpredictable corrupting effects on the magnetic channels. Fig. 1-Scheme of the magnetic sensors configuration for the two-level magnetovariational method. (Available in full paper)