In-situ physical and elastic properties of Archaean basement granitoids in the Koyna seismogenic zone, western India from 3 km downhole geophysical well logs: Implications for water percolation at depth

Abstract

The Koyna region in western India is characterized by recurrent artificial water reservoir triggered seismicity since the impoundment of the Shivaji Sagar lake in 1962. Physical and mechanical properties of the crystalline basement rock beneath the Koyna seismogenic zone are determined from the downhole geophysical logs in a 3 km research borehole. The borehole, KFD1, is located in the proximity of the Donichawadi fissure zone (DFZ), the surface manifestation of the fault associated with the 1967 M 6.3 Koyna earthquake. KFD1 penetrated 1247 m thick Deccan traps and continued 1767 m into the underlying granite-gneiss basement. Geophysical logs provide a rare opportunity to characterize the basement granitoids, which host the recurrent seismicity. Several major zones of anomalous physical and mechanical properties were delineated below a depth of 2 km from the analysis of downhole geophysical logs. High Poisson's ratio and high neutron porosity across anomalous zones imply the presence of water saturated rock. The analyses of Nuclear Magnetic Resonance log reveals the presences of clay bound and free fluid across these zones. Sharp decrease in dynamic Young's modulus with increasing Poisson's ratio imply the presence of fractures saturated with water. These hydraulically conductive fractures are potential locations for fault reactivation. Additionally, strike of major and minor fractures associated with the anomalous zones are compared with the strike of Donichawadi fault. High angle between strike of the major shear fractures in the anomalous zone and strike of the Donichawadi fault imply that borehole KFD1 may have either intersected subsidiaries of the main fault at multiple depths or a new hidden fault system.