Elektrische Verfahren
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Geoelectrics

Geoelectric techniques determine the specific electric resistances in the ground. During the so-called direct current (DC) method, resistivity measurements are taken in the form of current-voltage measurements using various electrode configurations by sending a current through the ground between two electrodes. The potential difference at the surface due to the electric field in the ground is subsequently recorded at two other electrodes. The spacing between the electrodes determines the exploration depth. Seeing as the ground is usually layered, the survey results usually change with the electrode spacing, hence the results are referred to as an “apparent resistivity” for a certain electrode configuration. Different apparent resistivity curves are determined as a function of electrode spacing.

The DC technique can be employed for two purposes: Depth sounding at a fixed location uses the assumption of horizontal layering to determine the resistivity and thickness of the layers (1D image). Mapping along a survey profile determines the lateral resistivity distribution for a fixed exploration depth.

The sounding-mapping technique has proven particularly useful as it combines the two approaches using a multi-electrode configuration. It allows two-dimensional resistivity structures to be modelled (2D image). The simplest form of displaying the results from multi-electrode measurements is the “pseudo-section”, where the measured apparent resistivity values are plotted along a depth section. Because these values are neither true resistivities nor true depths, such diagrams only provide a qualitative image of the actual resistivity distributions. The true values can be determined through two-dimensional computations, where models are iteratively updated and fitted to the measured values.

Electromagnetics

Electromagnetic (EM) techniques encompass a wide range of methods which explore the ground with the aid of electromagnetic signals. The propagation of electromagnetic signals in the soil is determined by the conductivity and dielectric constant of the rocks.

EM signals can be generated in a wide frequency range, starting from less than 1 Hz (magnetotellurics) up to several Gigaherz (radar). The frequencies used depend on the field of application and can either be generated actively by a transmitter (e.g. Slingram, Turam, Controlled Source Audio Magnetotellurics (CSAMT)) or passively using already existing electromagnetic signals which propagate in the underground due to a variety of reasons (e.g. magnetotellurics, audio magnetotellurics (AMT)). Another passive source would be technical signals e.g. electromagnetic waves from military transmitters (Very Low Frequency –VLF-technique) which normally serve the communication of submarines (range up to 5000 km) and can also be used in geophysical applications. VLF measurements can also be conducted actively by using special transmitters.

When using passive methods, the equipment is usually less elaborate as only the appropriate receivers would be required in the field, while active methods require a transmitter to generate the signal.

The exploration depth of EM methods is essentially a function of the employed frequency, where lower frequencies produce greater penetration depths (skin effect). This can range from only a few centimetres (radar) to tens of kilometres (magnetotellurics).

There is a general classification of the technique into near-field methods (low frequency range), where the acquisition geometry is small relative to the wavelength of the EM signal, and the far-field methods (high frequency range), where the employed wavelength is smaller than the object of investigation. An example of the far-field method is the radar technique, which is described under a separate tab. The text below focuses only on near-field methods.

During active near-field methods, an eddy current is generated in the ground by sending an alternating current through a coil. The strength of the eddy current is directly proportional to the conductivity of the ground. The eddy currents themselves generate secondary electromagnetic fields whose strength and phase lag is recorded by a receiver coil. This generates a measurement signal proportional to the conductivity of the ground. In contrast to the DC methods, this technique requires no direct contact with the ground, which allows it to be employed from the air (aeromagnetic methods) and thus provides an efficient means to survey large and possibly inaccessible regions. Even when using ground surveys, the progress of AC techniques is faster than that of DC methods.

Another distinction can be made between EM broad band techniques (transient EM methods), which generate frequencies over a broad spectrum, and methods that only deal with distinct frequencies. The transient techniques allow geometry and conductivity information to be derived, but they are more susceptible to noise. The narrow band methods allow interfering frequencies to be eliminated very effectively by using filters.

One-frequency methods only deliver results on a very limited depth range which is determined by the frequency and the spacing between transmitter and receiver. These methods can thus be used for mapping along a profile or 2D area. In case additional information on different depths is needed, multi-frequency techniques or transient methods have to be employed.