The use of inverse geotherms for determining underground water flow at the Ombla karst spring near Dubrovnik, Croatia

Abstract

At the site of the Ombla permanent karst spring, near Dubrovnik, Croatia, it is planned that an underground hydroelectric power plant with an installed capacity of 60 MW be built. This assumed power capacity is based on the results of measurements of average annual discharge of the spring, which is approximately 24 m 3 s −1. The positions and dimensions of all the basic structural elements of this power plant have now, after 20 years of interrupted investigations, been outlined. However, one very important problem remained unsolved, concerning the degree of impenetrability of the planned underground dam and especially of its lower part, which is located inside the rock massif below sea level. One of the main results of the extensive hydrogeological and geophysical investigations, including drilling works, was the finding that all karst water discharging into the Ombla definitely flows by means of siphons passing through a roughly semi-circular vertical plane defined by the vertical boreholes drilled into the massif from the investigation gallery, some of which reached the impermeable barrier of the flysch base-rock lying some hundreds of metres below. The length of this plane is somewhat greater than 1000 m. The results of the first temperature loggings in the boreholes drilled from the investigation gallery disclosed a rare natural phenomenon: inverse geotherms, i.e., geotherms with negative geothermal gradients. This phenomenon has been observed at those underground locations and depths where the temperature of the rock or the fluid has a lower temperature than that of the surrounding rock at the same depth. When such a location is encountered, the temperature begins to fall with increasing depth. In the case of the investigated site, the special tectonic, hydrogeological and hydrogeothermal conditions needed for the occurrence of negative geotherms are fulfilled. On the basis of the large number of boreholes which have been drilled in the direct vicinity of the site, and the observed inverse geotherms, it has been possible to estimate the position of the underground karst channels beneath the investigation gallery of the planned dam.