Laboratory of Exploration and Applied Geophysics

Magnetotellurics

Magnetotellurics (MT) is a passive electromagnetic geophysical method used to investigate the electrical resistivity of the subsurface. The method is based on the measurement of natural electric and magnetic fields on the Earth's surface with frequencies of 10 Hz and 10 Hz. The energy source corresponds to natural electromagnetic waves generated in the atmosphere and magnetosphere by various physical processes, such as: i) the Schumann resonance of lightning discharges for the higher frequencies and ii) the interaction between the Earth's magnetosphere and the solar wind for the lower frequencies.

Radio-magnetotelluric (RMT), audio-magnetotelluric (AMT), broadband magnetotelluric (BBMT), and long-period magnetotelluric (LMT) methods differ in the frequencies they measure. Since the MT method is based on the electromagnetic processes of induction and diffusion, the depth of investigation depends on the measured frequencies (and the subsurface resistivity) according to the skin effect. The higher the measured frequency, the shallower the depth of investigation.

The method is capable of investigating resistivity from tens of meters to hundreds of kilometers. The AMT method is generally limited to a depth of about 1-3 km.

The controlled-source magnetotelluric (CSMT) method is based on the same theoretical basis as the MT method and uses an artificial transmitter of the electromagnetic signal. The method improves the signal-to-noise ratio when anthropogenic noise or a weak natural signal may be present.

Simultaneous measurements of the orthogonal components of the electric and magnetic fields at the surface provide the data to calculate the transfer function (impedance) as a function of frequency. The impedance, in turn, is used to reconstruct subsurface resistivity models.

At each MT site, five synchronized measurements are recorded: the electric field in two directions, the magnetic field in the same directions and in the vertical direction; the horizontal directions are orthogonal. A typical MT data acquisition station consists of two pairs of electrodes arranged as orthogonal dipoles with a length between 50 and 100 meters and three magnetometers, also arranged orthogonally (two horizontal and one vertical).

The magnetotelluric method has no environmental impact, even visually, as the sensors are buried underground. A study area is explored using multiple measurement sites, whose locations are chosen to uniformly cover the area, taking care to avoid difficult-to-access or noisy sites.

Among the various fields of application of the magnetotelluric prospecting method, CNR-IGG researchers have mainly worked on:

• Geothermal exploration to study fluid circulation pathways and heat sources in geothermal systems
• Volcanological studies, to study the depth and volume of magma chambers in active Italian volcanoes (Vesuvius and Etna)
• Hydrogeological exploration
• Deep crustal exploration

Geoelettrica

The geoelectric method is one of the oldest and most widely used methods for geophysical exploration. The first pioneering applications were conducted almost simultaneously by Conrad Schlumberger and Frank Wenner in the early 1900s.

The method is widely used for georesource exploration, and for environmental, civil engineering, and archaeological studies. From the early days of the methodology until the 1980s, the 1D investigation method was widely used, but was gradually replaced by multichannel and multielectrode acquisition systems and multidimensional modeling algorithms.

Recent systems use automatic data acquisition protocols to obtain 2D and 3D tomographic reconstructions of the subsurface.

This technique is more properly called: electrical resistivity tomography (ERT).

The theory is based on Ohm's law which relates the (unknown) electrical resistivity to the current and the electric potential.

A typical acquisition involves injecting current into the ground through two current electrodes AB (usually metal stakes), measuring the voltage difference at the two potential electrodes MN.

A multi-electrode georesistivity meter automatically, after specific programming, selects the two pairs of electrodes for each measurement, allowing the use of any acquisition configuration.

Measurements with different spacing and different positions of the electrode pairs can obtain 2D or 3D models of the subsurface.

As a first approximation, the investigation depth is approximately 1/5 to 1/6 of the total length of the array.

Among the various fields of application of geoelectric prospecting, CNR-IGG researchers have worked in particular on:

• Geothermal exploration to investigate fluid circulation pathways
• Hydrogeological studies (a campaign in the Arctic environment will be conducted soon)

Inverse Modeling

In geophysics, a thorough understanding of the physical principles (direct operator) behind the prospecting method allows for the correlation of measured data with Earth model parameters. The inverse problem involves defining the model parameters "m" given a set of observed data "d." Solving the inverse problem allows for an estimate of the subsurface electrical resistivity from the analysis of surface-measured data (magnetotelluric, geoelectric, etc.). The problem is generally ill-posed (e.g., for the MT method) due to its instability and requires significant computational resources.

Methods for solving the inverse problem can be classified as deterministic and probabilistic. Deterministic methods are the conventional approach for estimating resistivity models.

Since the MT response is particularly influenced by the dimensionality of the measured impedance tensor, deterministic schemes for 1D-2D and 3D inversion are available in the literature, with 3D inversion being the subject of the most recent scientific research.

In general, model parameters are calculated by minimizing an objective function using differential operators. The procedure can reach a local minimum, depending also on the distance of the initial model from the global minimum.

In the Exploration and Applied Geophysics Laboratory, advanced codes are used for the inversion of MT and geoelectric data in 1D-2D-3D.

On the contrary, the probabilistic approach is less conventional and still a matter of research.

The probabilistic method does not require differential operators, but the model space is sampled randomly or based on some kind of strategy.

In the probabilistic method, many terrain models are proposed and theoretical data are compared with observed data. The minimization function is directly calculated to obtain the best model. The philosophy of the probabilistic approach, which can be considered an optimization procedure, is to explore larger model search spaces to seek a global solution to the problem.

Researchers at the Laboratory of Exploration and Applied Geophysics, in collaboration with other partners, are developing algorithms based on Computational Intelligence to solve the inverse geophysical problem, including joint problems.