EVALUATION OF RESISTIVITY AND SEISMIC METHODS FOR HYDROGEOLOGICAL MAPPING IN KARST TERRAINS

Abstract

A water exploration in a karst area can be hardly imagined without the application of electrical and seismic methods, but karst areas are very difficult environments for any geophysical exploration due to very high surface nonhomogeneities. Dual gradient mapping (measurements performed at two depth levels), two-dimensional (2-D) resistivity imaging, refraction methods and high-resolution reflection methods have been applied on a characteristic karst case in Croatia. General overview on the subsurface resistivity distribution has been reached by using the gradient mapping measurements. This revealed low-resistivity zone points of the fractured rock filled with water. Map of relative changes of apparent resistivities with depth discovers two different zones: resistivity increases with depth in the eastern part and resistivity decreases with depth in the western part of the area. Weak reflections are seen in the stacked seismic reflection section placed with the 2-D resistivity imaging profile, but they can be followed along the entire profile. By combining refraction and reflection data, faulted and fractured zones can be determined at surface as well as at greater depths, which will enable a precise definition of their extensions. Two-dimensional inverse seismic modelling have been carried out to clarify significance of the main reflection horizon, which is probably caused by a lithologic boundary, meaning a contact of dolomites and compact limestones. The model of acoustic impedancies has been derived on the basis of refraction velocities, reflection velocities and geological data from borehole. An accurate geological model has been established on the basis of all the data.Two-dimensional resistivity surveying can be used very efficiently in karst water explorations of shallow targets, but for deeper targets the seismic methods can be very useful by reason of poor resolution of the electrical resistivity methods.