Prediction of Karst Occurrences by Interpreting Borehole Data within the Inception Horizon Hypothesis

The use of borehole logging data is common in engineering geological investigation programs. These data offer a vertical view to the variation of the physical, chemical or lithological properties of the rock series. Concerning karst, the interpretation of borehole data has been, so far, mainly restricted to the detection and characterisation of dissolution voids, which have been usually considered as randomly distributed in space and therefore any prediction of karst occurrences at a more regional scale was supposed as not possible. Our research focussed on the analysis of the 3D geometry of several of the largest cave systems in the World (almost 1500 km of analysed cave conduits). It showed that the development and position of karst conduits under phreatic conditions is remarkably related to a restricted number of so called "inception horizons". An "inception horizon" — a concept introduced by Lowe (1992) — is a part of a rock succession that is particularly susceptible to the effects of the earliest cave forming processes. This concept provides a theoretical framework in which the interpretation of borehole data can be used to characterize and predict karst occurrences up to a regional scale.