Transient electromagnetics (TEM)
Measuring the electric conductivity in the underground
The transient electromagnetics (TEM) method was developed in order to gain information on the electric resistivity of the subsurface. It is applied successfully in mineral and geothermal exploration, in hydrogeology, environmental surveys and the like.
TEM is an electromagnetic method measuring in the time domain - in contrast to frequency domain methods - using artifical signals generated with particular transmitters.
A current flowing in a transmitter loop is switched off abruptly. The collapsing electromagnetic field induces eddy currents in the conductive underground according to Maxwell’s equations. This system of eddy currents produces a secondary magnetic field, whose propagation depends on the conductivity distribution in the subsurface (see picture to the left).
The temporal change of the secondary field can be measured at the earth’s surface: the magnetic components Hx, Hy, Hz as induced voltages in coils and the electrical horizontal components Ex, Ey directly as voltages between two electrodes.
From the decaying induced voltage an apparent specific resistivity and an assigned depth as a function of time after switch off of the primary pulse can be calculated. The later the times are, the deeper the current system has penetrated. A great advantage of the method is the temporal separation between the strong primary field and the weak secondary field, allowing high amplification of the latter.
The propagation of the current system is demonstrated in the animated figure to the right. Here, the isolines of the density of the current system induced into the ground after switching off the transmitter current are shown for different times. The example was calculated for a homogeneous half space with a specific resistivity of 30 Ωm and for a circular transmitter coil with a diameter of 100 m. Typical measuring times are in the range micro- to milliseconds.
Setup in the field
There are two types of emitters:
Horizontal electrical dipole: Galvanic coupling means, that a constant current is injected into the ground by two electrodes, generating an electrical field in the underground. Such an emitter is used for example in the LOTEM (Long-Offset Transient Electro Magnetic) method.
Vertical magnetic dipole: In case of inductive coupling, in a big horizontal coil a direct current is flowing, inducing a temporally constant magnetic field into the underground, which can be described as a vertical magnetic dipole. This method is used in measurements by BGR.
Several equipments are available in BGR:
The upper panel of the figure to the left shows the transmitted current as fuction of time and below the transient voltage induced in the receiver coil. The time windows at which the voltages are recorded are marked in green.
A typical field setup is illustrated below.
Frequently used coil configurations are:
Emitter is a vertical magnetic dipole, generated here with the help of a rectangular horizontal transmitting coil, e.g. with a size of 100 m × 100 m. The receiver coil is placed in the center of the transmitter coil.
The same wire is used for transmitting and receiving, which is possible because of temporal separation of the two processes.
Transmitting and the receiving coil are parallel to each other (e. g. bifilar cable).
The receiving coil is located outside the transmitter loop at a certain distance.
The quantity acquired is the voltage measured in the receiver coil as function of time after switching off the primary field. The calculation of an apparent specific resistivity, a much more descriptive quantity is possible. The values are plotted bi-logarithmically by convention; the quantities are normalized by the coil areas and the amperage of the transmitter current.
The field example to the right demonstrates the huge dynamic range of the voltage measured (blue) of six decades. The slope of the curve changes clearly between 10-4 and 10-3 sec. The apparent resistivity demonstrates firstly an increase from 100 Ωm up to almost 500 and then a decrease again down to ~50 Ωm, apparently continuing down further, if longer times were recorded. So a layer with lower resistivity is followed by one with higher resistivity and below that a layer with again lower resistivity follows. Quantitative conclusions are not directly possible from the values measured, however, in particular one cannot estimate the depths where changes are occuring. Model calculations are necessary.
The quantitative interpretation of the data is conventionally accomplished with the help of one-dimensional models consisting of horizontal, infinitely extending homogeneous layers each with a different resistivity. The adjustment of measured data and model responses is usually perfomed using inversion methods.
The sounding curve shown above can well be explained by the simple three layer model plotted to the right: below a thin, only a few meters thick overburden with ~ 8 Ωm follows a thick high resistive layer (1000 Ωm), and below it low resisitve material with again 8 Ωm. A graph of the model is plotted to the very right: the true layer resistivity logarithmically to the right and the depth linearily down. At discontinuously resistivity changes (horizontal lines) a layer limit is found.
The example was measured in southern Chile and shows a hot water aquifer below 300 m thick volcanic rocks with high resistivity.
Applications of TEM are especially the domains of groundwater and landfill surveys, mineral and geothermal exploration and also the detection of burning coal beds (see picture to the right). Case histories can be accessed below.
- Transientenelektromagnetik zur Grundwassersuche in Namibia
- Transientenelektromagnetik zur Erkundung einer Mülldeponie
- Erkundung der Cuxhavener Rinne mit TEM
- TEM surveys in the area of Manzanar, Chile, for exploring a hot water aquifer
- Geothermal as an alternative source of energy for Tanzania
- Airborne- and Ground Geophysics: Report of the project: Sino-German Coal Fire Research
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.
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).
|TEM associates and |
experts in BGR
|Holger Hauswirth||Measuring techniques, measurements||3.3||2405||Holger.Hauswirth@bgr.de|
|Ulrich Kalberkamp||Measurements, processing, interpretation||3.3||2776||Ulrich.Kalberkamp@bgr.de|
|Dr. Ursula Noell||Measurements, processing, interpretation||2.1||3489||Ursula.Noell@bgr.de|
|Dr. Norbert Ochmann||Measurements, processing, interpretation||3.3||2351||Norbert.Ochmann@bgr.de|
|Gert-Ulrich Sandmann||Measuring techniques, firstname.lastname@example.org|
|Dr. Gerlinde Schaumann||Measurements, processing, interpretation||LIAG S2||2781||Gerlinde.Schaumann@liag-hannover.de|
|Dr. Friedrich Schildknecht||Measurements, processing, email@example.com|
|Dr. Annika Steuer||Measurements, processing, interpretation||2.1||2148||Annika.Steuer@bgr.de|