Delineation of Damage Zones From 3 km Downhole Geophysical Logs in the Koyna Seismogenic Zone, Western India

• Published in: Journal of geophysical research. Solid earth 2019-06, Vol.124 (6), p.6101-6120
• Main Authors: Goswami, Deepjyoti, Roy, Sukanta, Akkiraju, Vyasulu V.
• Format: Article
• Language: English
• Subjects: acoustic anisotropy; Acoustic properties; Anisotropy; Azimuth; Basement rock; Boreholes; Compressive properties; Damage; Delineation; Dipoles; Drilling; Earthquake damage; Electrical resistivity; fault zone; Fault zones; Faults; Fractures; geophysical logging; Geophysics; Horizontal orientation; Koyna seismogenic zone; Physical properties; Porosity; Rock properties; Rocks; S waves; scientific drilling; Seismic activity; Seismic velocities; Seismicity; Shear wave velocities; stress orientation; Wave velocity
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB017257

Delineation of subsurface faults and damage zones is a major goal of scientific drilling projects in seismically active areas. Geophysical logs acquired in a 3‐km deep scientific borehole KFD1 in the Koyna seismogenic zone, a site of recurrent reservoir triggered seismicity over the past 55 years, provide an unprecedented opportunity to investigate the rock properties and delineate the fault zones. KFD1 passed through 1,247‐m thick Deccan traps and continued for 1,767 m into the underlying granitic basement rocks that host the seismic activity in the depth range 2–10 km. We have studied the physical properties and acoustic behavior of basement granitoids from the analysis of geophysical logs from 1,500 to 3,000 m. Salient results are as follows. (1) Seven anomalous zones are identified below 2,100‐m depth based on electrical resistivity, caliper, density, neutron porosity, self‐potential, and sonic data. (2) The anomalous zones are characterized by significant shear wave velocity anisotropy (up to 25%), as revealed by cross‐dipole sonic data. (3) Dispersion analysis of dipole flexural modes confirms that the anisotropy is primarily stress induced; fast polarized shear wave azimuth (FSA) therefore indicates the orientation of maximum horizontal compressive stress SHmax. (4) Comparison of FSA with independent estimates of SHmax orientations obtained from drilling induced tensile fractures and strike of inclined fractures in the anisotropic zones shows that FSA is controlled mainly by the stress regime. Therefore, the stress rotations inferred from anisotropy analyses in the anomalous zones indicate their association with subsurface fault damage zones. Key Points Seven major zones of anomalous physical and mechanical properties below 2,100‐m depth are identified from geophysical logs The anomalous zones are characterized by stress‐induced anisotropy as inferred from analyses of cross‐dipole sonic logs Stress rotations in four anomalous zones indicate strong association with subsurface fault damage zones