Franjo Šumanovac

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.

ALP 2002 seismic experiment

The ALP 2002 was organized as an international seismic experiment whose scientific objective is to further scientific understanding of the structure and evolution of the lithosphere in the Eastern Alps and surrounding areas. The ALP 2002 experiment included passive seismic monitoring and an active source seismic refraction experiment. Furthermore, local high-density deployments were carried out in Austria and Hungary to investigate local geologic problems. All data will be integrated with the goal of better understanding the geodynamic processes currently at work and the complex tectonic history of this region.

Investigation of underground cavities in a two-layer model using the refraction seismic method

This paper presents a new approach to the investigation of underground cavities. Our technique is based on the refraction seismic method. We have studied a two-dimensional, two-layer geological model. In our model, the lower seismic velocity layer is situated above the higher seismic velocity layer, with a circular cavity positioned within the upper layer. We have investigated the influence exerted by the cavity on the first arrivals of seismic waves. The obtained traveltimes are solutions of the eikonal equation and are presented using the time-distance graph. All refracted waves encountering the cavity have to circumvent it, as it represents an impediment to the propagation of seismic energy. This circumvention causes delays in the first arrivals of the seismic waves at the surface as compared to traveltimes with no cavity present. These delays create a characteristic shape of the time-distance graph, characterized by the peak point in which the plot line has a discontinuous change. Using this graph and analysing the delays of the first arrivals, we have derived expressions for determining both the position and size of the circular cavity. The practical application of the derived relations has been tested on a model test site built in a natural rock setting. This simple method indicates the presence of the cavity. The accuracy of the calculated cavity parameters: the horizontal position x , the depth z and cavity radius r depends on the geophone spacing. For geophone spacing equal to or less than the cavity radius, the accuracy of the method is shown to be acceptable. The maximal estimated error is equal to a half geophone spacing. Finally, we have demonstrated that this method is also applicable in the detection of non-circular cavities.

Defining a general hydrogeological model for Susak Island, Adriatic Sea, Croatia

The island of Susak differs from other Croatian islands in its geological and hydrogeological properties, because it is, for the most part, covered with fine-grained loess-type sands. At the outset of the research it was unclear whether the main aquifer lies within the superficial deposits or within the underlying carbonate rocks. This gave rise to two alternative conceptual models, depending on whether the interface between the superficial cover and the carbonates occurs above or below sea level. Hydrogeological and geophysical investigations were conducted. The primary objective of the geophysical exploration was to determine optimum positions for exploratory boreholes. The results showed that the main aquifer is contained within the carbonate rocks, and that this carbonate aquifer is overlain by a second, confined, aquifer within the superficial sandy deposits. Groundwater within the carbonate aquifer is hydraulically connected to the sea by fault zones. Measurement of electrical conductivity in boreholes established that the mixing zone of fresh and salt water is at depths from 22 to 30 m below ground level. It is necessary to ensure constant control of pumping quantities and control of the brackish water lens by monitoring of the groundwater salinity. For inclusion in public water supply, the groundwater should be treated by desalination.

Exploration of Deep Carbonate Aquifers by Magnetotellurics

Electrical tomography is often considered as the main geophysical method in groundwater investigations, especially in complex geological models, but in case of deep exploration it cannot achieve deeper aquifers and the electrical sounding cannot ensure complete and dense space coverage. In such a case the use of the magnetotelluric method can ensure determination of deep targets, as well as a good space sampling. The capabilities of the MT method are shown in a common hydrogeological model in Croatia which can be found in both regions, in the Pannonian basin and Dinarides. The carbonate aquifer is underlying at depths greater than 100 m below impermeable package of clastic deposits. Two filed examples of groundwater investigations are presented, Gotalovec (north-western Croatia) and Baska (karstic Adriatic island Krk) areas. A main conclusion can be drawn that the MT method provides the most complete data on lithological and structural relationships in the considered model, but it is very sensitive to surface and urban noises. The results of the other geophysical methods can improve the resolution (seismic reflection) and reduce the interpretation ambiguity (electrical tomography).