REFERENCES
1 ASTM D6639–01 (2008). Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Investigations.
2 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.
3 McNeill, J.D. (1980). Electromagnetic terrain conductivity measurements at low induction number. Technical note TN‐6. Geonics Ltd. (www.geonics.com).
4 McNeill, J.D. (1980). Applications of transient electromagnetic techniques. Technical note TN‐7. Geonics Ltd. (www.geonics.com).
5 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.
6 Parasnis, D.S. (1979). Principles of Applied Geophysics. Third edition, Chapman and Hall.
7 Sharma, P.V. (1997). Environmental and engineering geophysics. Cambridge University Press.
2.3. TIME DOMAIN ELECTROMAGNETIC (TDEM) METHOD: OPERATIVE PRINCIPLE AND THEORY
2.3.1. TDEM (Time Domain Electromagnetic)
It dates back in the 70s when the electromagnetic technique known as Time Domain Electro Magnetic Method (acronym TDEM) has been developed, almost at the same time, by the Russian, Canadian, and Australian technical and scientific community, for mineral search purposes.
Later in the 80s, TDEM found a wider application for hydrogeological purposes, and recently it has been applied for geotechnical (stratigraphic interpretation of the subsoil) and environmental studies (pollutants search, UXO search) (A. Menghini et al., 2010).
The aim of a TDEM sounding is the reconstruction of a 1D model of the subsoil to detect layers having the same characteristics in terms of electrical resistivity. The depth of investigation of a TDEM survey depends upon the characteristics of the instrumentation employed, the stratigraphic and geological conditions of the medium to be investigated and the background and or other sources of noise.
The main advantage of this technique with respect to other EM techniques (for example respect to the FDEM method), is that the TDEM method employs transient EM field: this means that the measurements of the secondary EM field is done at the receiving coil, when the transmitting coil is switched off for a very short time, called time off. During the time off, the secondary is sampled in wider “time windows,” corresponding to deeper portion of the subsoil.
The above‐described setting put the conditions to avoid (as it happens instead in the FDEM methods) to measure a very small quantity (the secondary magnetic field) in presence of another, much larger, one: the primary magnetic field, produced by the transmitting coil (P.V. Sharma, 1997; Kearey et Alii, 2002; J.D. Mc Neill, 1980). Hence, the signal measured at the receiver coil, is likely to be only due to the “contribution” given by the secondary field associated to the soil material, and not to the signal directly coming from the transmitting coil.
In general, to perform a TDEM sounding one must lay an electric cable describing the shape of either a square or a rectangle (the transmitting loop). Inside the transmitting loop, known electric current flows as it is connected to a transmitting unit fed by a battery, depending on the system and its configuration (Figure 2.3.1) it is possible to inject current into the ground from 2 to more than 150 A, depending on systems design. Also, a grounded transmitter may be employed allowing great exploration depth to be reached.
The dimension of the transmitting loop varies depending on the required investigation depth, and it generally ranges between a few tenths of meters to a square whose side is 500 meters long, for investigation depth down to more than 200 meters (P.V. Sharma, 1997) (Figures 2.3.2–2.3.3). Actually, by using bigger transmitter loop with grounded transmitter, larger exploration depth can be achieved, as deep as 1000 meters below ground level. On this purpose, it is also possible using the so‐called multi‐turn loop to increase the energizing moment (M). In Fact, M = nIA where n is number of turns, I is the current, and A is the area of the loop. As for this aspect, the most important and characterizing parameter is given by the product nA.
Figure 2.3.1 Typical TDEM acquisition scheme.
Figure 2.3.2 A Prototype acquisition TDEM system. All the essential parts of the system are shown.
Figure 2.3.3 Prototype acquisition ProTEM manufactured by Geonics Ltd. (www.geonics.com). All the essential parts of the system are shown.
Receiving loop dimension is in the order of one meter (side or diameter, depending on its shape see Figures 2.3.4–2.3.5).
In the acquisition scheme depicted in Figure 2.3.1 an acquisition system defined as Central Loop is illustrated, where the receiving loop is concentrically positioned with respect to the transmitting loop. In contrast, in the Loop‐Loop or Slingram mode, the transmitter and receiver are coaxial but not concentric: the receiver loop is external with respect to the transmitter loop.
The energizing current is normally injected as a square wave (Figure 2.3.6). To each positive pulse (time on) there follows an equal period during which the current is switched off (time off); then, the direction of the current in the transmitting coil is changed (negative portion of graph in Figure 2.3.6 a), and following this the electric current is again switched off for an equal amount of time. The entire cycle just described, is repeated many times with repetition frequency varying from 0.25 to 250 Hz.
In Figure 2.3.6 b, it is highlighted how in the moment in which the electric current is switched off (turn off), as illustrated in the Faraday’s law, a primary magnetic field is produced, and this tends to become null within a very little time (a few milliseconds). This magnetic field interacting with the subsoil generates induced currents in it, which propagates deeper in the subsoil as time passes (
