T011-0007
In-situ crustal stress in inland Japan with application of DCDA method to rock core samples of seismic observation wells
In-situ crustal stress in inland Japan with application of DCDA method to rock core samples of seismic observation wells
Tuesday, 8 December 2020
Poster
Abstract:
In-situ crustal stress is an important factor to understand the rheological properties of crust associated with seismicity and tectonic activities. However, the in-situ crustal stress measurements seem to be difficult, because complicated procedures are necessary in a borehole for the measurements. Recently, a new method for in-situ crustal stress measurement, DCDA (Diametrical Core Deformation Analysis) method, has been successfully proposed (Funato and Ito, 2017, IJRMMS). We tried to apply the method using boring rock core samples from seismic observation wells. DCDA method measures the circumferential diameter variation and elastic constants of core samples. We expect the method can be applied to old-time samples from NIED (National Research Institute for Earth Science and Disaster Resilience) seismic observation (Hi-net) wells, and we may collect global in-situ crustal stress data. We used recovered 7 hard rock core samples drilled more than 10 years ago from the depth about 100m - 200m (one is from the depth of 2000m). The circumferential core diameter variation was measured by an especially designed apparatus that consists of an optical micrometer, a pair of motor-driven rollers and a data processing system (Funato and Ito, 2017, IJRMMS). We got the sine curves of circumferential diameter variation of some of the core samples associated with stress relief. The data suggest that the DCDA method is applicable to old-time boring core samples from shallow wells.
To decide the value of stress, elastic constants of the core samples, Young's modulus, and Poisson's ratio, were estimated from rock mass P and S wave velocities by means of P-S logging at the time of boring. Assuming the value of density is 2.5 g/cm3 as a representative value, we calculated the elastic constants of the core samples. In the limited cases, we obtained the elastic constants by laboratory rock mechanical tests. Assuming of various kind of rocks composing the crust, the differential stresses are around 10 MPa. Those results suggest DCDA method is suitable to measure in-situ crustal stress in global area with existing rock core samples. In the next step, we will try accumulate DCDA measurements with laboratory rock tests of available rock core samples to explore the in-situ crustal stress distribution in inland area.
To decide the value of stress, elastic constants of the core samples, Young's modulus, and Poisson's ratio, were estimated from rock mass P and S wave velocities by means of P-S logging at the time of boring. Assuming the value of density is 2.5 g/cm3 as a representative value, we calculated the elastic constants of the core samples. In the limited cases, we obtained the elastic constants by laboratory rock mechanical tests. Assuming of various kind of rocks composing the crust, the differential stresses are around 10 MPa. Those results suggest DCDA method is suitable to measure in-situ crustal stress in global area with existing rock core samples. In the next step, we will try accumulate DCDA measurements with laboratory rock tests of available rock core samples to explore the in-situ crustal stress distribution in inland area.