of Elsevier)
Figure 2.3.8 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)
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).
REFERENCES
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. AIRBORNE ELECTROMAGNETIC (AEM) METHOD: OPERATIVE PRINCIPLE AND THEORY
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 yet widespread, however there is a growing interest for those applications where the deployment of geophysical techniques over very large areas is required (for example for large‐scale engineering projects or groundwater mapping). At the same time, this technique should guarantee an economical advantage with respect to the application of land‐based techniques, and a comparable resolution in the result.
Further application of the AEM method in hydrogeology, as well as in other field of application where a higher degree of resolution is required, led to the fine‐tuning of systems capable of more detailed definition of the geophysical model, above all in the shallower layers. This need, was also due to the limited contrast in terms of electrical resistivity (or conductivity) that can be found in field of applications different from the mineral exploration (the application where the AEM was developed first), where the ore bodies show electrical resistivity several orders of magnitude lower than the hosting rock.
A good compromise to achieve the required resolution, has been reached by mounting the EM systems over helicopters. This occurrence, allowing for lower flying altitude and slower velocity than aircrafts, contributes to a better quality of the data and a higher resolution. Furthermore, the possibility to perform more complex processing and inversion techniques implemented over dedicated software, allowed to refine the data analysis and interpretation with respect to the more simplistic data analysis originally carried out for mineral exploration purposes, which is known as the so‐called “bump detection.”
In the EM helicopter borne systems (defined as HTEM), both the transmitting as well as the receiving loop, are carried below the helicopter in the same frame, at a height of about 30 to 40 meters above the ground level. In some systems, the receiver frame is independent from the transmitter frame.
AEM soundings are collected at approximately 1.5 seconds interval, which corresponds (taking into consideration the velocity of the helicopter) to a TDEM sounding (on land) approximately every 25 meters along the acquisition line. Actually, the sampling frequency may be defined during the post‐processing phase where a resampling can be performed with a finer stacking up to less than 5 meters.
The spacing between acquisition lines varies, from 50–100 meters to a few hundreds of meters, depending on the degree or resolution required and on the purpose of the survey.
AEM systems, may also host a GNSS, tilt meters, and altimeters used to control the position of the system, the inclination of the loops and the height from the ground, in continuous mode, allowing for the following correction during the data processing phase.
To summarize the phase of an AEM survey, starting from the data acquisition on the field to the data analyzed and visualized in office, the following can be concluded:
1 Once the most appropriate data acquisition system is selected based on the wave form emitted, and the survey is designed according to the purposes of the project, the data can be acquired on site.
2 EM data is visualized as Voltage variation in respect to time (along increasing time window). A first quality control is performed over each single 1D sounding, in order to verify that in no part of the survey area the data must be re‐collected because of a poor signal‐to‐noise ratio.
3 Then, each single 1D sounding is analyzed and filtered to enhance the quality of the signal, if needed. For each 1D signal a 1D inversion routine, is applied.
4 Finally, each single 1D model, is entered as input of a 2D or pseudo‐3D inversion routine to obtain 3D maps of the distribution of the resistivity.
Hereinafter, the principle and the basic equations related to the field quantities measured by AEM systems and the inversion will be presented.
As for the land FDEM and the TDEM method already presented,
