We present a new marine MT and CSEM receiver system based on fluxgate magnetic sensors and standard long span electric potential electrodes with a new 24 bit converter. Fluxgate magnetometers provide a high precision stable simultaneous measure of the 3 components of the magnetic field at 50 Hz. The receiver was tested for both marine magnetotelluric (MT) and Controlled Source ElectroMagnetic (CSEM). We report on results obtained in sub-basalt (for marine MT) and gas hydrate (for CSEM) geological contexts.
Marine magneto-telluric and marine control source electromagnetic receivers may be combined into a single system recording 5 or 6 components of the electro-magnetic field on the sea-floor (Constable and Snrka, 2007). With the rapid improvement of fluxgate magnetic sensors, in particular in terms of precision at relatively high frequency (above a few 10 Hz) and reduced power consumption, new marine electromagnetic systems become available. Exploration with Marine magnetotelluric (MT) and marine Control Source ElectroMagnetic (CSEM) uses search coil magnetometers, a direct application from long history of land MT work. Marine exploration, in particular for gas and hydrocarbon, involves depths of investigation of several 100 m to several km. Hence the transient magnetic field needs to be measured only at low frequency (less than 1 Hz to a few 10 Hz at most). In this frequency range, current fluxgate magnetometers may be advantageous because of their small size (a few 10 cm or less) and with a same accuracy than search coils while measuring the 3 components of the magnetic field. The bulk of the instrument is therefore reduced. We were interested to upgrade our low frequency magnetotelluric system based on fluxgate magnetometers to use them for crustal investigation in the marine environment. We considered further to use this system as a marine CSEM receiver. The new equipment was tested for marine MT in sub-basalt imaging studies and then improved for high frequency sampling (50 Hz) with a new test experiment for CSEM use.
We developed a new high frequency electromagnetic receiver (Fig. 1) dedicated to both magnetotelluric and CSEM experiments, from shallow to more than 3000 m water depth . It involves both a 3-component high accuracy fluxgate magnetometer, and two 10-meter-span electric field 24-bits converters. The instrument provides high precision measurements up to 50 samples per second. A 100Hz version is under study and will be soon available. We used fluxgate magnetometers because they may achieve very good performance nowadays and they are less sensitive to ocean motion, tidal currents, and much lighter than search coils. The small size of the vector magnetometer allowed us to design an instrument combining flotation and instrument housing. This approach has several advantages. It allowed us to built a light instrument (about 70kg in air) with non magnetic and non metallic parts for more than 99% of the total weight, which is very convenient for electromagnetic measurements, corrosion resistance, and maintenance. Instrumentation housings and flotation are 17“ glass spheres. They provide enough flotation and are fully non magnetic.
