Resistivity of rock-salt in ASSE (Germany) and petrophysical aspects

This paper reports that in the Asses salt-mine (Germany), where extensive research is carried out on various aspects of nuclear waste disposal in rock-salt formations, the resistivity of a future construction site for a test seal at a depth of 945 m has been investigated. Measurements have been conducted, using various types of the four-point electrode configuration, on a network of 180 electrodes permanently installed in boreholes. A fully computer-controlled measurement system has been built consisting of a resistivity meter, a switchbox for electrodes and a computer with special control software. The system has interactive and programmable automatic working modes with an extensive capability for documentation and processing including the immediate evaluation of apparent resistivities and is suitable for long-term continuous observations. The average resistivity at the site which consists of almost homogeneous Stassfurt rock-salt, is approximately 0.6 x 106 Ω m with little variation. There is a small anomaly due to a conductive inclusion, probably a local anhydrite band. There is not indication of anisotropy so far and no significant change with time. The specific amount of water in the effective pores and its change can be estimated from the observed resistivities using the extended form of Archie's equation with pore saturation included. In this case data for the specific internal surface from samples are available and the order of permeability may be estimated using the formation factor derived from resistivities