Tudor BURLAN-ROTAR, Constantin STANESCU, Melania TAMAS
Abstract
Conventional determination of resistivity by galvanic method requires a relatively large amount of labor to apply them in practice and is therefore expensive. It is the lateral or vertical variations of resistivity which form the basis of any interpretation. However the hight cost of resistivity surveying generally means that fewer measurements are made than would be desirable, with the result that either (i) the surveyi area is not made large enough to establish a reasonable background against which the anomalous areas are to be delineated or (ii) the anomalous itself is obscure and lacks definition. The application of electromagnetic techniques for measuring soil resistivity or conductivity is known for a long time. Conductivity is preferable in inductive techniques as instrumentation readings are generally directly proportional to the conductivity and inversely proportional to the resistivity. The operating principle of this method is: a transmitter Tx coil supplied with alternating current at a frequency audio is placed on the ground. A receiver Rx coil is located a short distance s away from Tx coil. The magnetic field varies in time and Tx coil induces very small currents in the ground. These currents generate a secondary magnetic field Hs is sensed by the Rx receiver coil together with primary magnetic field Hp. The ratio of the secondary field Hs to the primary magnetic field Hp (Hs/Hp) is linearly proportional to terrain conductivity. Measuring this ratio, it is possible to construct a contactless, direct-reading linear terrain conductivity meter. This more recent technique for measuring conductivity by electromagnetic induction using VLF (very low frequency), is a non-invasive, non-destructive sampling method. The measurements can be done quickly and inexpensively. Electromagnetic induction technology was originally developed for the mining industry, and has been used in mineral, oil, and gas exploration, groundwater studies, and archaeology. In these applications, differences in conductivity of subsurface layers of rock or soil may indicate stratified layers or voids that could be of interest.
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