CONSIDERATIONS ON VERY LOW FREQUENCY ELECTROMAGNETIC TECHNIQUES FOR CONTACTLESS SOIL CONDUCTIVITY MEASUREMENTS IN PEDOLOGY

Abstract

To put into practice the conventional determination of resistivity by the galvanic method, requires a relatively large amount of labor and is, therefore, expensive. At the basis of any interpretation are the lateral or vertical variations of resistivity. The high cost of resistivity maps execution generally means that fewer measurements are made than desirable, with the result that, eitheri) the explored area is not large enough to establish a reasonable background, against which the anomaly areas are to be delineated, orthe anomaly areas are obscure and lack definition. The application of electromagnetic techniques for measuring soil resistivity or conductivity has been known for a long time. Conductivity is preferable in inductive techniques, as instrumentation readings are generally directly proportional to conductivity and inversely proportional to resistivity. The operating principle of this method is: a Tx coil transmitter, supplied with alternating current at an audio frequency, is placed on the ground. An Rx coil receiver is located at a short distance, s, away from the Tx coil. The magnetic field varies in time and the Tx coil induces very small currents in the ground. These currents generate a secondary magnetic field, Hs, which 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 directly proportional to terrain conductivity. Measuring this ratio, it is possible to construct a device which measures the terrain conductivity by contactless, directreading electromagnetic technique.This latest technique for measuring conductivity by electromagnetic induction, using Very Low Frequency is a non-invasive, non-destructive sampling method. The measurements can be done quickly and are not expensive. The 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.