CONSIDERATIONS REGARDING INDUCTIVE TECHNIQUES FORKARSTIC SOIL CONDUCTIVITY MEASUREMENTS IN THE CONSTRUCTION OF ROADS, BRIDGES AND HIGHWAYS.

Abstract

Abstract: Karstic conditions are a prevalent problem which can impact roads, bridges and highways. the investigations are made to determine the presence of karst (sinkholes and cavities) and their possible impact on existing or new constructions.geophysical measurements are the first investigations before making a new construction. the investigations can be done in areas with historic karst but no recent activity, with sinkholes activity, or unknown karst activity. 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 the contactless electromagnetic 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 technique for measuring conductivity by electromagnetic induction using vlf (very low frequency), is a non-intrusive, non-destructive sampling method. the measurements can be done quickly and inexpensively. The differences in conductivity of subsurface layers of rock or soil may indicate stratified layers or voids that could be of interest.