Geological investigations using cosmic ray muons; A Trial to detect fault at the Mizunami Underground Research Laboratory

Sasao, Eiji   ; Suzuki, Keiichi*; Yamada, Nobuto*; Kuboshima, Koji*

We performed investigation of a fault with thick and clay-altered damaged zone in granitic rock using cosmic ray muons at the Mizunami Underground Research Laboratory. Geology of the Laboratory consists of sedimentary rock and underlying granite with unconformable contact at the 170 meters below ground level (G.L.). A vertical fault with a thick, clay-altered damaged zone is present in the granite. The muon telescopes were settled at the G.L.-200 and -300 meters to estimate densities of granite and fault. Densities of granite, fault and sedimentary rock are calculated as 3.38, 2.88 and 1.99 g/cm, respectively. The obtained density is obviously higher than absolute value, though the reason of such difference is not clear at the moment. If the density of granite is assumed to be 2.6 g/cm, then the densities of fault and sedimentary rock are re-calculated as 2.2 and 1.5 g/cm. This result indicates that cosmic ray muons have good potential to detect geological structure.