MARGE is a long-period magnetotelluric (MT) survey that aims at imaging the electrical conductivity structure of the Italian lithosphere in three dimensions (3D), with the purpose of identifying and characterizing major geological structures in the crust and upper mantle. The survey results can be combined with other available datasets, including those on gravity, seismic tomography, geochemistry, and geology. The combination of these multidisciplinary datasets will advance our scientific knowledge of the analyzed region's lithospheric architecture and evolution.
The MARGE project’s ultimate goal is to use the modeling of solid-Earth conductivity structure and surface impedance in order to map the magnetic-storm-induced geoelectric fields. Indeed, for the purpose of geoelectric-hazard analysis, the combination of impedance tensors and geomagnetic time series from ground-based observatories or models can be used to estimate geoelectric field variation. This will allow the development of capabilities for both real-time and historical mapping of the geoelectric field over Italy as a result of geomagnetic field disturbance. MT impedance that relates geoelectric fields to geomagnetic fields at the Earth’s surface provides a critical missing piece to estimate geomagnetically induced currents.
It is nothing but a stroke of luck that the MT impedances obtained from MT survey and modeling are exactly the type of data product necessary for space weather hazard mitigation. This is also why the necessity for nationwide MT impedance coverage is becoming more urgent. Nowadays such data serve the electric power grid industry as well as the solid-Earth geophysical community.
Magnetotelluric method
Magnetotelluric (MT) is a passive geophysical technique that can be used to assess the distribution of electrical resistivity in the Earth’s subsurface by examining the variations in the geomagnetic and electric fields that occur naturally. Different types of MT instruments, such as audio, broadband, and long-period, collect data at different frequencies, resulting in an overall investigation depth ranging from tens of meters to hundreds of kilometers.
MT source fields are produced by global thunderstorm activity as well as currents systems flowing in the magnetosphere and ionosphere due to the interactions between the solar wind and the geomagnetic field. These sources provide a broad spectrum of magnetic-field variations in the frequency range from 10-5 to 104 Hz, making them ideal for crust and upper mantle research. The strength of these signals changes over hours, days, weeks, and during the solar cycle.
The time-series measurements of the magnetic and electric fields are analyzed in the frequency domain, with lower-frequency signals corresponding to greater depth of penetration into the Earth’s subsurface. As a result, in order to image the deep crust and upper mantle, very low frequencies must be measured to achieve sensitivity to the lithosphere's base. High performance computers equipped with mathematical inversion algorithms can create a 3D Earth electrical conductivity model using these data.
MARGE data acquisition
A MARGE survey collects MT data using portable instruments on a grid of approximately 55 km. Because it is desirable to have data from both very short and extremely long periods, two different magnetic sensors are combined at each site: induction coils and fluxgate magnetometers.
Thus, in a typical MARGE MT survey station the following equipment is deployed:
- A three-component fluxgate magnetometer and dipoles with electrodes to record continuous magnetic and electric signals in the field. To ensure the MT measurement capture sufficient natural signals at longer periods, data are collected for 4-6 weeks at each site;
- Specific induction coil magnetic field sensors for broadband MT station and dipoles with electrodes to record continuous magnetic and electric signals in the field. Data are collected over the course of one week utilizing the broadband MT station.;
- Acquisition boxes containing data recording units, GPS and 12 volt battery;
- A small solar panel mounted close to the acquisition box to charge the battery.
Figure 1 : scheme of a survey site. The orange cylinders represent the tubes containing the magnetometers sensors, each oriented in
a different direction, and buried 50 cm in the ground. The green lines represent the dipole wires for measuring the ambient electric
field.
Figure 2: the magnetometers sensors, the tubes in which they are inserted and the acquisition box.