Electrical resistivity (or its reverse, conductivity) is an important physical parameter for subsurface exploration. In the geothermal exploration, for example, it provides one of the most used and powerful diagnostic method, since electrical resistivity of geothermal reservoirs is usually lower than that of surrounding and hosting rocks (excluding clay caps). Its value depends on various factors such as temperature, porosity and permeability, fluid salinity. Strong resistivity contrasts are useful indicators of underground structures. Electromagnetic (EM) methods are able to detect electrical resistivity over large areas at relatively low cost, if compared to other geophysical prospection, e.g active seismic.

Magnetotelluric (MT) and Audio-frequency MT (AMT) are electro-magnetic survey and imaging techniques that use naturally-occurring ionospheric current sheets and lightning storms — passive energy sources — to map geologic structures from near-surface to depth of hundreds of kilometers.

INSTRUMENTS 

Magnetotelluric laboratory is equipped with tools able to acquire MT data in a large frequency range (0.0001 Hz - 1000 kHz). Equipment was partially designed and realized in collaboration with West Systems S.r.l. (http://westsystem.eu).

1) MT broad-band equipment (0.0001 Hz - 1 kHz):

A High-resolution, multi-channel 32-bit equipment for recording two electric field components (Ex, Ey) and three magnetic components (Hx, Hy, Hz), and to acquire MT and AMT data.

Components:

  • ZEN HighRes Receiver of ZONGE (http://zonge) broad-band (0.0001 Hz a 1000 Hz), sampling frequency 256 up to 1024 Hz. The receiver may acquire 6 synchronized channels at a sampling frequency of 1024 Hz. Each channel has a mass storage of 4 GB, allowing continuous recording for various days.
  • 4 magnetic antennas (coils) ANT/4 of ZONGE.
  • a set of non-polarizable (Pb-PbCl) electrodes WM of Wolf Ltd developed for MT, geoeletrical and Induced Polarization surveys.
  • cables, batteries (12 V, 42 Ah), spare parts.

2) MT low-frequency equipment (0.0001 Hz - 0.0003 Hz, 24 bit digital sampling):

  • 2 NIMS Receivers.
  • 5 West-IGG Receivers.
  • 3 magnetometers (3-components fluxgate)
  • electrodes, cables, batteries (12 V, 42 Ah), spare parts.

3) AMT (high-frequency MT) equipment (0.1 Hz - 100 kHz):

  • 1 transmitter STRATAGEM for CSAMT.
  • 2 West-IGG and 1 STRATAGEM receiver
  • 4 magnetic sensors (coils)
  • 8 metallic and 4 non-polarizable electrodes, cables, batteries (12 V, 42 Ah), spare parts.

4) Equipment for data analysis and modelling:

  • 2 laptop computers for in-situ MT data processing
  • 1 desktop computer for high performance data processing, editing, modelling
  • 3 Winglink (Geosystem) software licence for data processing, editing, 1D/2D modelling and 3D forward modelling
  • 1 licence of Zonge software for data processing, editing, 1D and 2D modelling
  • in-house software for processing, editing, 1D and 2D modelling

The team is developing a MatLab-based software for MT data inversion using innovative optimization methods

STAFF

Dr. Adele Manzella (CNR Researcher - Head Laboratory)
Dr. Serena Botteghi (CNR Researcher)
Dr. Alessandro Santilano (CNR Researcher fellow)

CONTACTS

Phone:

050 6213417 (Laboratory)
050 6212392 (Dr. Adele Manzella)
050 6212387 (Dr. Serena Botteghi e Dr. Alessandro Santilano)

E-mail:

adele.manzella(at)igg.cnr(dot)it
serena.botteghi(at)igg.cnr(dot)it

alessandro.santilano(at)igg.cnr(dot)it

METHODS

Magnetotelluric (MT) is a passive geophysical method for inferring the earth's subsurface electrical resistivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface, with frequencies ranging between 106 and 10-6 Hz. The natural source fields are due to electric currents flowing in the Earth and the magnetic fields that induce these currents. The magnetic fields are produced mainly by the interaction between the solar wind and the magnetosphere. In addition, worldwide thunderstorm activity causes magnetic fields at frequencies above 1 Hz. AudioMagnetoTelluric (AMT) is a high-frequency magnetotelluric technique for shallow investigations. 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 weak natural signal can be present.

The electromagnetic signal recorded at the surface is the sum of a primary field arriving from the atmosphere and a secondary field, induced in the underground. Simultaneous measurements of orthogonal components of the electric and magnetic fields at surface provides data for computing the transfer function (impedance) as a function of frequency. Impedance, in turn, is used to construct earth resistivity models.

Investigation depth ranges from a few tens of meter below ground level (by recording higher frequencies) down to 10,000 m or deeper with long-period soundings. Investigation depth is also proportional to the resistivity of rocks. AMT and CSAMT have a shallow investigation depth, usually limited to 1-3 km depth.
At each MT site, five measurements (channels) are recorded: the electric field in two horizontal directions, the magnetic field in the two horizontal and the vertical directions, the horizontal directions being orthogonal (ad es., nord-sud e east-ovest). A typical MT station for data acquisition consist of two pairs of electrodes set up as orthogonal dipoles with length between 50 and 100 m, and three magnetometers also set up in orthogonal (two horizontal and vertical) directions.

When layout is complete, the MT site is essentially invisible, since sensors are located in holes and little trenches so that they are not disturbed during overnight recordings. The survey area is explored using several sites, whose location is chosen trying to uniformly cover the area, but taking also care to avoid noisy, difficult access and rough topography sites..

Recording time has to be long compared to the period (inverse of frequency) of interest, which is time depended on the depth to be investigated in order to get enough signal and ensure high-quality data. Each recorded channel is a time series (time variation) of electric or magnetic signals.

MT data processing consists of Fourier transformation from time domain to frequency domain and the computation of impedance tensor, from which apparent resistivity, phase, strike, tipper, skew and other parameters are derived. Data are then modelled using 1D, 2D and 3D forward and inverse modelling, integrating available information.

A peculiarity of MT method is that data acquired at each site do not probe only the resistivity along the vertical below the site, but hemispheric (in uniform ground. More complex shapes result in anisotropic media) volumes whose ray increases with decreasing frequency.

the main application fields of the MT prospection method

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 depth and volumes magmatic chambers and volcanic edifices in active Italian volcanoes (Vesuvius and Etna)
  • Hydrogeological exploration
  • Deep crustal exploration

PROJECTS

The MT laboratory was involved in various scientific projects, and collaborates with many universities and reasearch centers at national and international level.

Recent scientific projects:

  • IMAGE (EU-FP7 Project, 2013-2017): "Integrated Methods for Advanced Geothermal Exploration
  • I-GET (EU-FP6 2005-2009): “Integrated Geophysical Exploration Technologies”
  • Si.Ri.Pro (Progetto PON 2004-2010): "SIsmica a RIflessione PROfonda, Metodologie integrate per l'esplorazione geofisica avanzata lungo un transetto crostale, Sicilia (Deep Reflection Seismic, Integrated methodologies for advanced geophysical exploration along a crustal transect in Sicily)
  • INTAS EU Project (2004-2006): "Three-dimension electromagnetic and thermal tomography of the active crustal zones".

Recent scientific collaborations and applications:

  • Geothermal exploration in: Balmatt (Belgium), 2015); Canino (Viterbo), 2011 e 2014; Argentera (Cuneo), 2010;
  • Hydrogeological exploration in: Equi Terme 2008-2009); Monte Amiata, 2003-2006.

Other activities

  • Crustal investigation in CROP Alpi Centrali, CROP03, CROP18 projects. Volcanological exploration in INGV projects (Vesuvio, Etna)
  • Geothermal investigation in Larderello and Amiata (Tuscany Italy, also in collaboration with Enel), in Tibet, Slovakia and Australia

SCIENTIFIC INTEREST

  • Development of optimization algorithms for data processing
  • Integrated numerical modelling
  • Geothermal, crustal and volcanological exploration

SELECTED PUBLICATIONS

  • Santilano A., Donato A. Galgaro A. Montanari D. Menghini A. Viezzoli A. Di Sipio E., Destro E. Manzella A. An integrated 3D approach to assess the geothermal heat-exchange potential: The case study of western Sicily (southern Italy). Renewable Energy, 97, 611-624, 2016
  • G. Molli, M. Doveri, A. Manzella, L. Bonini, F. Botti, M. Menichini, D. Montanari, E. Trumpy, A. Ungari & Luca Vaselli Surface-subsurface structural architecture and groundwater flow of the Equi Terme hydrothermal area, northern Tuscany Italy. Ital. J. Geosci., Vol. 134, No. 3, pp. 442-457, 2015
  • Santilano A., Godio A., Manzella A., Menghini A., RizzoE., Romano G and Viezzoli A.. Electromagnetic and DC methods for geothermal exploration in Italy – case studies and future developments. First Break volume 33, pp. 81-86, August 2015.
  • Nimalsiri T. B., Suriyaarachchi N. B., Hobbs B., Manzella A., Fonseka M., Dharmagunawardena H.A., Subasinghe N. D. Structure of a low-enthalpy geothermal system inferred from magnetotellurics — A case study from Sri Lanka. Journal of Applied Geophysics 117, 104–110, 2015.
  • Oskooi, B. and Manzella, A.: 2D inversion of the Magnetotelluric data from Travale geothermal field in Italy. Journal of the Earth & Space Physics. 36, No. 4, Pages 1-18, 2011.
  • Manzella A., Ungarelli C., Ruggieri G., Giolito C., Fiordelisi A.: Electrical resistivity at the Travale geothermal field (Italy). Proc. World Geothermal Congress, 2010.
  • Bianchi A., Bovini L., Botti F., Doveri M., Lelli M., Manzella A., Molli G., Montanari D., Pierotti L., Ungarelli C., Ungari A., Vaselli L.: Multidisciplinary approach to the study of the relationships between shallow and deep circulation of geofluids. Proc. World Geothermal Congress, 2010.
  • Spichak V., and Manzella A. (2009). Electromagnetic sounding of geothermal zones, Journal of Applied Geophysics, 68, 459-478.
  • Armadillo E., Bozzo E., Caneva G., Manzella A., Ranieri G. Imaging deep and shallow structures by electromagnetic soundings moving from the Transantarctic Mountains to the Wilkes Subglacial Basin. Terra Antartica Reports. Issue 13, Pages 65-74, 2007.
  • A. Manzella and A. Zaja: Volcanic structure of the southern sector of Mt. Etna after the 2001 and 2002 eruptions defined by magnetotelluric measurements. Bulletin of Volcanology 69 (1), 41-50, 2006.
  • Oskooi, B., Pedersen, L.B., Smirnov, M., ¡rnason, K., Eysteinsson, H., Manzella, A., and the DGP Working Group. The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland. Phys. Earth Planet. Int., 150, 183-195, 2005.
  • Manzella, A. Resistivity and heterogeneity of Earth crust in an active tectonic region, southern Tuscany, Italy. Annals of Geophysics, 47, 107-118, 2004.
  • Manzella, A., Volpi, G., Zaja, A., and Meju, M. Combined TEM-MT investigation of shallow-depth resistivity structure of Mt. Somma-Vesuvius. J. Volc. and Geoth. Res., 131, 19-32, 2004.
  • Volpi, G., Manzella, A., and Fiordelisi, A. Investigation of geothermal structures by magnetotellurics (MT): an example from the Mt. Amiata area, Italy. Geothermics, 32, 131-145, 2003.
  • Manzella A.: Electrical resistivity structures of southern Tuscany geothermal areas, Italy, Proceedings 24th NZ Geothermal Workshop, 141-146, 2002.
  • Manzella, A.: Resolution of the magnetotelluric method in the crustal investigation of southern Tuscany. Studi per l'interpretazione del profilo sismico CROP18, Stato di Avanzamento, 15-19, 2002.
  • Manzella A., Volpi G., Corsi F., Fiordelisi A.: Distribution of resistivity in the upper crust of the Larderello geothermal area. Studi per l'interpretazione del profilo sismico CROP18, 2° Stato di Avanzamento, 19-24, 2002.
  • Manzella A., Volpi G., and Zaja A. New magnetotelluric soundings in the Mount Somma-Vesuvius volcanic complex: preliminary results. Annali di Geofisica, 43 n.2, 259-270, 2000.
  • Fiordelisi A., Manzella A., Buonasorte G., Larsen J., Mackie R.: MT methodology in the detection of deep, water-dominated geothermal systems. Proc. World Geothermal Congress, Kyushu-Tohoku, Japan, 1121-1126, 2000.
  • Manzella A., Mackie R., and Fiordelisi A. MT survey in the Amiata volcanic area: A combined methodology for defining shallow and deep structures. Physics and Chemistry of the Earth (A), 24, No. 9, 837-840, 1999.
  • Di Mauro D., G. Volpi , A. Manzella, A. Zaja, N. Praticelli, V. Cerv, J. Pek and A. De Santis
  • Magnetotelluric investigations of the seismically active region of northern bohemia: preliminary results, Annali di Geofisica, 42, n.1, 39-48, 1999.
  • De Angelis, M., Fiordelisi, A., Manzella, A., and Zaja, A. Two-dimensional analysis of a magnetotelluric profile in the CROP 03 area in southern Tuscany. Memorie della Società Geologica Italiana, 52, 295-304, 1998.
  • Fiordelisi, A., Mackie, R., Manzella, A., and Zaja, A. Electrical features of deep structures in southern Tuscany (Italy). Annali di Geofisica, 41, 333-341, 1998.
  • Baldi P., Bellani S., Buonasorte G., Fiordelisi A., Manzella A.: Geothermal exploration in Tuscany (Italy) for high temperature resources. WREC 1998, 2733-2736, 1998.
  • Cerv V., Manzella A., Pek J., Praus O., Zaja A. Magnetotelluric and deep geomagnetic induction data in the Bohemian Massif. Annali di Geofisica, 15, 413-422, 1997.
  • J. C. Larsen, R. L. Mackie, A. Manzella, A. Fiordelisi, and S. Rieven . Robust smooth magnetotelluric transfer functions. Geophys. J. Int., 124, Pag. 801-819, 1996.
  • Manzella A., Fiordelisi A.: Resistivity structure on the western side of the Mt. Amiata region. Proc. World Geothermal Congress 1995, 2, 881-886, 1995.
  • Fiordelisi A., Mackie RL., Madden T., Manzella A., Rieven S.: Application of the magnetotelluric method using a remote-remote reference system for characterizing deep geothermal system. Proc. World Geothermal Congress 1995, 2, 893-898, 1995.
  • Larsen J., Mackie RL., Fiordelisi A., Manzella A., Rieven S.: Robust processing for removing train signals from magnetotelluric data in central Italy. Proc. World Geothermal Congress 1995, 2, 903-908, 1995.
  • Zaja A., Morbin F., Manzella A.: 2-D MT- resistivity interpretation along CROP 8803 seismic profile, Southern Central Alps. Proceedings of the Symposium “CROP-Alpi Centrali” Sondrio, 20-22 October 1993. Quaderni di Geodinamica Alpina e Quaternaria, 2, 213-223, 1994. ISBN 88-86596-00-6
  • Manzella A., Bellani S., Brogi L., Jong Q., Pinna E., Rossi A. Magnetotelluric measurements in the Monte Amiata region. Annali di Geofisica, 37, 1229-1239, 1994.
  • Manzella A., Patella D., Roberti N., Rossi A., Siniscalchi A. Tramacere A. Prime misure magnetotelluriche effettuate in zone di interesse del CROP 03. Studi Geologici Camerti, Special Vol. 1991/1, 71-74, 1991.