Detecting fracture zones in granite with seismic and audio-magnetotelluricmethods

Abstract

Geological disposal is a feasible and safe method for dealing with the high-level radioactive waste problem at present. The Beishan area is the key area preselected for high-level radioactive waste geological disposal in Gansu Province, China. The Jijicao rock block is currently the most extensively studied area, where there are several fracture zones in borehole BS15. In 2011, a remarkable conductive anomaly near the BS15 was detected using a 4-km-long audio-magnetotelluric (AMT) profile with 50 m station space crossing the borehole. To study the anomaly, a short seismic survey was shot along the part of the AMT profile. This paper presents an example for detection of the fracture zones in granite using AMT and seismic methods at Beishan. The AMT data inversion model agrees with the borehole well-logging data and weak reflections are related to the fractured zones. The seismic detection may help in interpreting the cause of the conductive anomaly, and the conductive anomaly could determine whether there are fracture zones within the weak reflection areas. The simultaneous surveys can complement each other for detecting fracture zones. Besides this, such joint detection may be used to estimate whether there exist aquiferous fracture zones at depth.