TEM
Transient electromagnetics (TEM)
The transient electromagnetic (TEM) method was developed in ore exploration to determine the electrical conductivity of the subsurface at depths of up to several hundred meters. Recent developments are aimed at enabling this method to be used for mapping near surface electrical conductivites at shallower depths up to 50 - 80 meters. TEM is successfully used in hydrogeological or geothermal investigations and in ore exploration. It is an active electromagnetic time domain method - in contrast to frequency domain methods that operate with periodic excitation.
Through the sequence of various induction processes, which are described below, a voltage that decays over time (the decay voltage or transient) is ultimately measured, the course of which reflects the characteristics of the resistivity distribution in the subsurface.
Transmitters
There are two types of transmitters:
Horizontal electric dipole (HEM):
With galvanic coupling, a constant transmitting current is fed into the soil via two electrodes. This generates an electric field, with the electrical horizontal components Ex and Ey being measured directly as voltage between two electrodes. Such a transmitter is used, for example, for the LOTEM (Long-Offset Transient Electromagnetics) method.
Vertical magnetic dipole (VMD):
In the case of inductive coupling, a constant direct current flows through a transmitting coil of various sizes, which is laid out horizontally on the surface. This current generates a magnetic field, which approximately has the geometry of a vertical magnetic dipole.
The vertical magnetic dipole is the standard configuration for most TEM applications in practice. The constant current within the transmitter loop is switched off as abruptly as possible, causing the constant primary magnetic field to collapse. This induces so-called eddy currents in the conductive subsurface according to Maxwell's equations. The diffusion, their propagation into depth is determined by the conductivity distribution within the subsurface. Due to ohmic losses, this current system decays, which in turn produces a secondary magnetic field that also decays. This change of this secondary field induces a current in the receiving coil on the surface. It is proportional to the temporal change of the vertical component Hz of the secondary magnetic field and can be measured over a period of several microsecond up to a few milliseconds. The length of this so-called transient is device-dependent and varies from application to application. The resulting exploration depth theoretically depends only on the duration of the recorded transient and the ambient noise. In this way, TEM measurements provide a depth sounding for every site. Due to the temporal separation of the weak secondary field and the strong primary field, the measuring system is less distance-sensitive and enables high measurement accuracy.
In addition to the temporal change in the Hz component as the most important measured variable, the changes in the horizontal components Hx and Hy can also be measured. From the decay curve, an apparent resistivity and also an apparent depth can be calculated for each time point. The later the measurement is taken after the current is switched off, the deeper the current system has penetrated. The resistivity is the reciprocal of the electrical conductivity and is used equally. The process of switching off the transmit current and measuring the decay voltage in the time window in which no primary field is present is repeated many times, even with reversed polarity, in one measurement (t0 to t1). The receiver coil can be set up in different orientations depending on the measurement setupUsually, it lies horizontally on the Earth's surface (VMD) and is located in a defined position and at a specific distance from the transmitting coil.
Frequently used configurations of the measurement setup:
Central Loop
(also called in-loop), where the receivercoil is placed in the center of the transmitter coil. The transmitter coil (VMD) usually has a rectangular or quadratic shape for practical reasons, e.g. 100 x 100 m. Due to the high dynamic range of the decay voltage recorded in the receiving coil, measurements are taken in different time domains with different amplifications (the smaller the signal, the higher the amplification).
Single Loop
where the same coil of wire is used for both transmitting and receiving (this is possible because the two processes are separated in time).
Separate Loop
where the receiver coil is installed outside the transmitter coil at a certain distance.
Coincident Loop
where the transmitter and receiver coils run parallel and next to each other but use different wire loops.
Available measuring equipment:
- ABEM WalkTEM 2
- MONEX GeoScope terraTEM24
- AEMR TEM-Fast
Measurement results
The measured value is the voltage induced in the receiver coil by the time-varying secondary magnetic field as a function of time after the primary pulse is switched off. Conversion to the more descriptive value of apparent resistivity is possible. These values are represented in the conventional double-logarithmic manner, normalized by the coil torques and the strength of the transmit current. The measurement example shows the large dynamic range of the measured voltage (blue) of approximately six decades. The slope of the curve changes significantly between 10-4 and 10-3 sec. The apparent resistance (red) initially shows an increase from approximately 100 to almost 500 Ωm, followed by a decrease to approximately 50 Ωm, which would obviously have led even further downwards if later times had been recorded; thus, one sees a good conductor at the surface, a poorer conductor below, and then a good conductor again. Based on the measurement data itself, only qualitative statements can be made. To estime the depth at which changes occur, further model calculations are required for quantification.
Interpretation
The quantitative evaluation of the data is usually carried out using one-dimensional models; these consist of horizontal, infinitely extended, homogeneous layers, each with a different resistivity. The fitting of measured values and model response is now usually performed using inversion methods. The sounding curve shown can be well explained using the simple three-layer model shown on the right: a thin, a few-meter-thick cover layer with a resistance of approximately 8 Ωm above a thick, poorly conductive layer with a resistance of 1000 Ωm, and below that, from 300 m down, a good conductor with a resistance of approximately 8 Ωm. The model is shown in the graph on the far right: the true layer resistance increases logarithmically to the right and the depth decreases linearly. At the points where the resistance changes abruptly (horizontal lines), a layer boundary is present. The example was measured in southern Chile and confirms a hot water aquifer beneath a 300 m thick layer of volcanic rock.
Applications
TEM applications include groundwater, geothermal, and landfill exploration, mineral exploration, and the detection of burning coal seams (image).
Literature
Greinwald, S. & Schaumann, G., 1997: Transientelektromagnetik, in: Knödel, K., Krummel, H. & Lange, G.: Handbuch zur Erkundung des Untergrundes von Deponien und Altlasten, Band III: Geophysik. Berlin, Heidelberg (Springer).
Nabighian, M.N. & Macnae, J.C., 1991: Time Domain Electromagnetic Prospecting Methods, in: Nabighian, M.N., Electromagnetic Methods in Applied Geophysics - Application, Part A, Vol. 2, Society of Exploration Geophysicists, Tulsa, Oklahoma.