Geoelectric exploration and monitoring in rock salt for the safety assessment of underground waste disposal sites

The safety of an underground waste disposal site depends to a large extent on the presence and migration of the water in the rock. Geoelectrics is the most suitable method particularly in underground mining conditions to explore and monitor the moist zones in many different rocks. Recent developments in hardware, inversion schemes and petrophysical interpretation of the resistivity enable reliable and useful measurements to be conducted with direct current geoelectrics for exploration and monitoring also in salt rock environment with rather high resistivities and resistivity changes. Examples are presented for resistivity exploration and monitoring in rock salt. The measurements are carried out in the research mine Asse in North Germany which is used for investigations into handling, storage, disposal and geological interaction of nuclear waste. A fully automated geoelectric system suitable for salt rock environment and long-term monitoring was used with a large number of electrodes installed permanently. Logistical conditions allow measurements only in profiles so that two-dimensional inversion schemes had to be used and their suitability and limitations are shown. At one site where the moisture is visible in a limited area at the wall the extension of the moist zone in the rock is explored and determined. This helps to estimate the possible changes that might occur in future and which measures have to be taken. At another site in a large area in the rock salt the resistivity has been monitored over several months. The resistivity distribution in the area shows local variations indicating changes in the state of rock salt which is otherwise usually homogeneous. The changes are related to disturbed rock zone at near surface around the voids due to the stress induced by the mining, to the neighbouring cavities and also disturbed zones in the deeper rock due to the stress redistribution in the last 30 years since the excavations took place. Also significant changes of resistivity with time are detected for which an estimate of water content can be given. These are attributed to fluctuations of the water content in the disturbed rock areas. q 2000 Elsevier Science B.V. All rights