Electromagnetic Methods in Geophysics. Fabio Giannino

Electromagnetic Methods in Geophysics - Fabio Giannino


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with time. Eventually, the signal disappears into the system and the background noise, and further measurement is impossible. This is the maximum depth of exploration for “that” particular system.

      In the case of TDEM soundings, on the other hand, it was observed earlier that as time increased, the depth to the current loops increased too, and this phenomenon is used to perform the sounding of resistivity with depth. Thus, Equation (2.3.1) can be inverted to read (since ρ = 1/σ):

      (2.3.2)equation

Image described by caption.

      As for the investigation depth, this depends upon the geoelectric section explored and its geoelectric characteristics.

      On this matter, however, the transient electric field reaches a maximum at the diffusion depth (dd) which is what the skin depth ∂ is to FDEM (Ranieri, 2000):

      (2.3.4)equation

      Finally, it is now important to describe a process relating to the following: let us assume that a confined object of given dimension and resistivity is buried in a homogeneous half space at a given depth below ground surface.

      At the moment when the primary electric field at the transmitter is off, this will generate a current in the ground (Eddy current) because of its associate magnetic component. At this very time, the current flow shall be distributed solely on the surface of the object mentioned above. The magnetic field in the object shall be exactly the same as that due to the primary. This moment is called Early Time.

Schematic illustration of an example of decaying curve of the measured tension with time. Schematic illustration of a sketch of the decaying curve of the measured tension with time.

      At this moment, the current starts to stabilize towards the center of the object, decreasing outwards to the edge of it. At the same time the associated magnetic component starts to decay exponentially with time, with a time constant τ that is given by (McNeill 1980):

      (2.3.5)equation

      This moment is known as Late time.

      The behavior described above, can be recognized in the 1D soundings as a result of the TDEM survey, and the analysis and forward modeling of the recorded data is addressed at defining a model based on the information that is directly dependent upon the shape, dimension, orientation, burial depth, and electrical resistivity of the target(s).

      1 Giannino, F. (2014). Metodi Elettromagnetici in Geofisica applicata. Acquisizione, analisi e interpretazione dei dati FDEM, TDEM e AEM in ambito geologico, ambientale e ingegneristico. Dario Flaccovio Editore.

      2 Menghini, A., Pagano, G., Floris S., et al. (2010). TDEM method for hydrothermal water detection. First Break, Vol. 28. EAGE Publications.

      3 Menghini, A. & Viezzoli, A. (2012). Il metodo Airborne EM: un approccio innovativo allo studio del territorio. Geologia Tecnica e Ambiente. Ed. Ordine Nazionale dei Geologi, Roma, MARZO 2012.

      4 McNeill, J.D. (1994). Technical Notre 27: Principles and applications of Time Domain Electromagnetic technique for resistivity sounding. Geonics Ltd.

      5 McNeill, J.D. (1980). Technical Notre 7: Applications of Transient Electromagnetic Techniques. Geonics Ltd.

      6 Nabighian, M.N. (1980). Electromagnetic Methods in Applied Geophysics. Investigation in Geophysics No 2. Volume 2, Application, Parts A and B, ISBN 978‐0‐931830‐46‐4 (Vol.1) 978‐0‐931830‐51‐8. Society of Exploration Geophysics.

      7 Parasnis, D.S. (1979). Principles of Applied Geophysics. Third edition, Chapman and Hall.

      8 Sharma P.V. (1997). Environmental and Engineering Geophysics. Cambridge University Press.

      9 Kearey, P., Brooks, M., & Hill, I. (2002). An Introduction to Geophysical Exploration. Third edition. Blackwell Science.

      10 Ward, S.H., & Hohmann, G.W. (1988). Electromagnetic theory for geo¬physical applications. In: Electromagnetic Methods in Applied Geophysics. Volume 1: Theory (ed. M.N. Nabighian), pp. 130–310. SEG.

      11 Ranieri, G. (2000). Tem‐fast: a useful tool for hydro‐geological and environmental engineers. Annali di Geofisica, Vol. 43, N. 6, December 2000.

      2.4.1. AEM (Airborne Electromagnetic)

      The AEM methods can be considered, as the airborne equivalent of the TDEM (or the FDEM) method, carried out on land. It was developed first for mineral exploration over vast areas in Canada and Australia. For a general overview of the various AEM systems, it is useful to read Siemon et al. (2009).

      The methodology is currently not


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