4:30 PM - 4:45 PM
[MIS18-17] In-situ stress at the basement under Osaka plain -estimation of stress values by DCDA method in deep seismic obsevation wells-
Keywords:DCDA method, in-situ crustal stress, rock core, Osaka plain, borehole televiewer, breakout
In-situ crustal stress is an important factor to understand earthquake mechanism and tectonic activities. However, the reliable in-situ stress data seems to still be poor, because complicated procedures are necessary for measurement in a borehole. We tried in-situ crustal stress measurements at deep seismic observation wells of NIED in Osaka plain, western Japan. Borehole-televiewer (BHTV) loggings revealed borehole breakout (shear fractures due to stress concentration on the circular borehole wall), and stress orientations are already reported (Omura, 2020, SSJ Fall Meeting). In present study, we applied DCDA (Diametrical Core Deformation Analysis) method to recovered rock core samples to estimate stress values. DCDA method measures the circumferential diameter variation due to stress relief after the core recovery, and use elastic constants of the rock core. Previous studies suggest that the DCDA method can be applied to rock cores from deep boreholes.
We used rock cores from NIED Konohana (34:39:45.92N, 135:23:22.53E, 2038m depth) and Tajiri (34:23:52.14N, 135:17:01.24E, 1535m depth) deep observation wells in Osaka plain. The core depth is 2035.5m from Konohana and 1202.4m and 1494.8m from Tajiri. The circumferential core diameter variation was measured by an apparatus at NIED designed by Funato and Ito, 2017, IJRMMS. We got sine curves of circumferential diameter variation associated with stress relief, suggesting the DCDA method is applicable to these core samples. We used elastic constants of the rock cores, Young's modulus and Poisson's ratio, deduced from rock mass P and S wave velocities by means of P-S logging at the drilling sites assuming the value of densities, 2.5 g/cm3. Then, values of differential stresses are around 60 MPa. We will apply laboratory rock tests to measure the elastic constants of rock cores themselves.
Those results suggest a pair of DCDA method by recovered rock core and borehole breakout observation by BHTV logging is recommended to measure both of value and orientation of in-situ crustal stress in the case of borehole breakout is produced.
We used rock cores from NIED Konohana (34:39:45.92N, 135:23:22.53E, 2038m depth) and Tajiri (34:23:52.14N, 135:17:01.24E, 1535m depth) deep observation wells in Osaka plain. The core depth is 2035.5m from Konohana and 1202.4m and 1494.8m from Tajiri. The circumferential core diameter variation was measured by an apparatus at NIED designed by Funato and Ito, 2017, IJRMMS. We got sine curves of circumferential diameter variation associated with stress relief, suggesting the DCDA method is applicable to these core samples. We used elastic constants of the rock cores, Young's modulus and Poisson's ratio, deduced from rock mass P and S wave velocities by means of P-S logging at the drilling sites assuming the value of densities, 2.5 g/cm3. Then, values of differential stresses are around 60 MPa. We will apply laboratory rock tests to measure the elastic constants of rock cores themselves.
Those results suggest a pair of DCDA method by recovered rock core and borehole breakout observation by BHTV logging is recommended to measure both of value and orientation of in-situ crustal stress in the case of borehole breakout is produced.