Laboratory of Exploration and Applied Geophysics

INSTRUMENTS 

The Laboratory of Exploration and Applied Geophysics was recently renewed and expanded with newly purchased instruments. The Lab is equipped with state-of-the-art instruments for the acquisition of magnetotelluric and geoelectric data:

Magnetotelluric (MT) broad-band equipment (0.0001 Hz – 1000 Hz):

Zonge system. High-resolution, multi-channel 32-bit systems for recording two electric field components (Ex, Ey) and three magnetic components (Hx, Hy, Hz). MT and AMT data can be acquired for imaging the subsurface resistivity.

Components:

• 3 ZEN HighRes receiver broad-band (0.0001 Hz a 1000 Hz). The receiver may acquire 6 synchronized channels at different sampling frequencies from 256 Hz to 4096 Hz. Each channel has mass storage of 8 GB, allowing continuous recording for various days.
• 8 magnetic antennas ANT/4 of ZONGE.
• Sets of non-polarizable electrodes of various types: Pb-PbCl (Wolf) and Ag/AgCl (Stelth)
• Miscellaneous accessories

ABEM system. A standalone solution, multi-channel system for high power current transmission and accurate electrical potential measurements for resistivity/IP surveying:

Components:

• 1 Terrameter LS 2, 10 channels (up to 12), 48 electrodes (up to 81). Maximum output voltage 600 V, maximum output current 2500 mA, maximum output power 250 W.
• 2 multipole cables 24 takeouts each and 10 m spacing (480 m total length)
• Set of steel electrodes

The Laboratory is also equipped with very old instruments, some of them stored in the Lab’s historical corner (such as the old V5 Phoenix equipment), whereas other instruments will be next assessed for a possible recovery:

Hybrid Natural and Controlled-Source Audio Magnetotelluric (CSAMT and AMT) equipment (0.1 Hz - 100 kHz):

Stratagem system by Geometrics. Stratagem’s hybrid-source technique exploits a hybrid source of signal from a combination of natural MT signals and man-made transmitter signals for the imaging of the subsurface resistivity

• 1 transmitter STRATAGEM with vertical loop antennas.
• 1 STRATAGEM receiver
• magnetic sensors

Magnetotelluric (MT) low-frequency equipment (0.0001 Hz - 0.03 Hz, 24 bit digital sampling):

2 NIMS (Narod Intelligent Magnetotelluric Systems) Receivers.

3 magnetometers (full 3-components fluxgate)

Gravity measurements:

1 Gravimeter

Equipment for data analysis and modelling:

Laptops for in-situ MT data processing

Desktop computer for high-performance data processing, editing, modelling

Access to HPC by CNR-IGG

Software

3 licences for Winglink (Schlumberger) for data processing, editing, 1D/2D modelling and 3D forward modelling of geophysical data

1 licence for Petrel (Schlumberger) for 3D geological modelling and data integration (managed by CNR-IGAG)

1 licence of Zonge software for data processing, editing, 1D and 2D modelling

1 licence of MODEM for 3D inversion of magnetotelluric data

Various open-source software for processing and inversion of geophysical data

In-house software developed by our Lab’s researchers for processing, editing, analyzing and inverting (also jointly) geophysical data. Computational Intelligent-based algorithms are the focus of such software

METHODS

MAGNETOTELLURICS

Magnetotellurics (MT) is a geophysical electromagnetic passive method for determining the subsurface electrical resistivity by measuring the natural magnetic and electrical fields on the Earth’s surface with frequencies ranging between 10^4 and 10^-5 Hz. The energy source corresponds to the natural time-varying electromagnetic waves generated in the atmosphere and magnetosphere by different physical processes such as: i) Schumann resonance of lightning discharges, for higher frequencies fields, and the interaction between the Earth’s magnetosphere and the solar wind plasma, for lower frequencies fields.

Radio-magnetotellurics (RMT), Audio-Magnetotellurics (AMT), Broad-Band Magnetotellurics (BBMT) and Long-Period Magnetotellurics (LMT) differ only for the measured frequencies. Being the method based on electromagnetic induction and diffusion processes, the investigated depth depends upon the measured frequencies (and the subsurface resistivity) according to the skin effect. The higher the frequencies, the shallower the investigation depth. This method is suitable for imaging the resistivity from tens of meters to hundreds of kilometres. AMT has a shallow investigation depth, usually limited to 1-3 km.

Controlled Source Magnetotelluric (CSMT) uses the same theoretical base of MT and an artificial signal transmitter, and is used to prevent low signal-to-noise ratios where cultural noise and a weak natural signal can be present.

Simultaneous measurements of orthogonal components of the electric and magnetic fields at the surface provide data for computing the transfer function (impedance) as a function of frequency. Impedance, in turn, is used to construct earth resistivity models.

Five synchronized measurements are recorded at each MT site: the electric field in two horizontal directions, the magnetic field in the two horizontal and the vertical directions, the horizontal directions being orthogonal. A typical MT station for data acquisition consists of two pairs of electrodes set up as orthogonal dipoles with a length between 50 and 100 m, and three magnetometers also set up in orthogonal (two horizontal and vertical) directions.

The magnetotelluric method has not environmental impacts either visually since sensors are located in holes and little trenches. The survey area is explored using several sites, whose location is chosen to try to cover the area uniformly. Care is also taken to avoid noisy, difficult access and rough topography sites.

Among the main application fields of the MT prospection method, CNR-IGG has worked in particular in:

• Geothermal exploration to investigate fluid pathways and heat sources of geothermal resources
• Volcanological exploration, to investigate the depth and volumes of magmatic chambers and volcanic edifices in active Italian volcanoes (Vesuvius and Etna)
• Hydrogeological exploration
• Deep crustal exploration

GEOELECTRIC

The geoelectric is one of the oldest and most commonly used geophysical exploration methods. The first pioneering applications were made almost contemporary by Conrad Schlumberger and Frank Wenner at the beginning of the 1900’s. The method is widely adopted for earth resources exploration, environmental and civil engineering, and archaeological investigations. Since its first use till the late 1980's, it has been used essentially as a one-dimensional (1-D) mapping method, progressively replaced by multi-channel and multi-electrode acquisition systems and multidimensional algorithms for modelling and inversion. State-of-the-art systems allow data acquisition with automatic protocols to retrieve 2D or 3D tomographic reconstruction. This technique is called electric resistivity tomography (ERT).

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

A typical acquisition implies the injection of a current into the ground through two current electrodes AB (usually metal stakes) and measuring the difference in the resulting voltage at two potential electrodes MN.

Through internal (programmable) switching circuitry, multi-electrode resistivity meters, automatically select the appropriate 4 electrodes for each measurement. Hence, any array configuration can be used. By making measurements with different spacing at different locations, a subsurface 2-D profile (or 3D model) is obtained.

As a rule of thumb, the depth of investigation is about 1/5 to 1/6 of the layout length.

Among the main application fields of the geoelectric prospection method, CNR-IGG has worked in particular in:

• Geothermal exploration to investigate fluid pathways
• Hydrogeological studies (coming soon also a survey in Artic environment)

INVERSE MODELLING

In geophysics, a complete understanding of the physics behind the methods (forward operator) allows the measured data to be related to the model parameters of the earth. The inverse problem is to find the model parameters “m”, given certain observed data “d”. By solving the inverse problem, the indirect esteem of the electrical resistivity of the subsoil is retrieved from the analysis of the data measured at the surface (magnetotelluric, geoelectric, etc.). The problem in magnetotelluric is

ill-posed due to its instability and can demand very high computational resources. The methods for solving the inverse problem can be classified into deterministic and probabilistic methods. Deterministic methods are the conventional means of estimating the resistivity models.

The MT response is particularly affected by the dimensionality of the measured impedance tensor. The literature is rich in deterministic schemes for 1D, 2D, and 3D inversions, with 3D inversion being the main focus of current scientific research. The model parameters are generally solved iteratively by minimizing an objective function operator and exploiting differential operators.

The procedure may reach a local minimum, also depending on how close the starting model is to the global minimum.

In the Exploration and Applied Geophysics Lab, we exploit state-of-the-art codes for the inversion of MT and geoelectric data in 1D-2D-3D.

Conversely, the probabilistic approach is less conventional and is still a matter of research. The probabilistic methods do not imply any differential operator, but the model space is sampled randomly or according to some strategy.

Many earth models are proposed in the probabilistic methods, and the theoretical data are compared with the observed data. Then, the minimization function is directly estimated to retrieve the best model. The philosophy of the probabilistic approach, which can be considered an optimization procedure, is to explore a broader space solution to seek a global solution to the problem.

In the Exploration and Applied Geophysics Lab, in collaboration with other partners, we develop Computational-Intelligence based algorithms to solve the geophysical inverse problem also jointly.