10:00 AM - 10:15 AM
[MGI32-09] In-situ stress at the basement under Osaka plain (4) - stress values by means of DCDA method -
Keywords:DCDA method, in-situ crustal stress, rock core, Osaka plain
In-situ crustal stress is an important factor to understand earthquake mechanism and tectonic activities. However, the reliable in-situ stress data down to more than 100m depth 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. The circumferential core diameter variation was measured by an apparatus at NIED designed by Funato and Ito, 2017, IJRMMS. 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. We got sine curves of circumferential diameter variation associated with stress relief after more than 10 years passage since core recovery, suggesting the DCDA method is applicable to these core samples. We used elastic constants, Young's modulus and Poisson's ratio, of rock core samples neighboring the DCDA measurement rock cores deduced from laboratory rock strength tests. Then, values of differential stresses are calculated to be around 60 - >100 MPa. These values seem to be rather larger than other results at surrounding area of in-situ stress measurements by means of stress relief and hydraulic fracturing method. The factors of the larger stress values should be considered with DCDA stress measurements of rock cores recovered from other boreholes. Directions of maximum horizontal compressive stress are E-W, which is consistent to tectonic stress direction in broad area in Kinki distinct including Osaka plane.
Those results suggest a pair of DCDA method by recovered rock core and borehole breakout observation by borehole wall image 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. We got sine curves of circumferential diameter variation associated with stress relief after more than 10 years passage since core recovery, suggesting the DCDA method is applicable to these core samples. We used elastic constants, Young's modulus and Poisson's ratio, of rock core samples neighboring the DCDA measurement rock cores deduced from laboratory rock strength tests. Then, values of differential stresses are calculated to be around 60 - >100 MPa. These values seem to be rather larger than other results at surrounding area of in-situ stress measurements by means of stress relief and hydraulic fracturing method. The factors of the larger stress values should be considered with DCDA stress measurements of rock cores recovered from other boreholes. Directions of maximum horizontal compressive stress are E-W, which is consistent to tectonic stress direction in broad area in Kinki distinct including Osaka plane.
Those results suggest a pair of DCDA method by recovered rock core and borehole breakout observation by borehole wall image logging is recommended to measure both of value and orientation of in-situ crustal stress in the case of borehole breakout is produced.