[S02-P] Poster

[S02-P-07] An integrated estimation of the stress field in seismogenic zones in South African gold mines

^○Akimasa Ishida¹, Hiroshi Ogasawara^{1, 9}, Yasuo Yabe^2,9, Akio Funato³, Takatoshi Ito², Shuhei Abe², Raymond Durrheim^4,9, Siyanda Mngadi^4,9, Gerhard Hofmann^7,9, Dave Roberts^5,9, Harumi Kato⁶, Alexander Milev^6,9, Makoto Naoi^8,9 (1.Ritsumeikan University, Shiga, Japan, 2.Tohoku University, Miyagi, Japan, 3.Fukada Geologcal Institute, Tokyo, Japan, 4.University of the Witwatersrand, Johannesburg, South Africa, 5.CSIR, Johannesburg, South Africa, 6.3D Geoscience., Tokyo, Japan, 7.Anglogold Ashanti ltd, Johannesburg, South Africa, 8.Kyoto University., Kyoto, Japan, 9.JST-JICA Science and Technology Research Partnership for Sustainable Development, Tokyo, Japan)

Although it is very important to know the details of the stress state in seismogenic zones for both seismology and safety in deep South African gold mines, it has not been feasible due to technical difficulties.
The Diametrical Core Deformation Analysis (DCDA) technique, proposed by T. Ito and A. Funato, uses an easy-to-make laboratory measurement of the diameters of drilled core, to estimate the differential stress on a plane normal to the borehole axis, which is calculated assuming that variations in core diameter are caused by axi-symmetric elastic expansion during drilling. We carried out DCDA for a total of 90 cores drilled from eighteen boreholes at three seismogenic zones in three deep-level South African gold mines in order to evaluate the reliability of the DCDA method and to clarify technical problems. Our measured cores included those from a high-stress-concentration area at 1.0 km depth in a shaft pillar where quasi-static growth of ruptures was elucidated by the SATREPS's acoustic emission observation, and from a M3.5 fault area at 3.3 km depth. We also conducted DCDA with cores that were recovered from over-coring stress measurements, and compared these results. It was observed that drilling quality sometimes affects DCDA results, as South African geological drilling crews don't care about the smoothness of the core surface because the main purpose of geological exploration drilling is to identify the lithology. However, if we scrutinize cores that are straight and smooth enough for DCDA, the DCDA results were in coherence with the results of the overcoring stress measurements. If we instruct drilling companies to use a stabilizer to straighten the drilling operation, we will be able to obtain more cores suitable for DCDA to elucidate stress variations along a borehole. We are currently developing a procedure to effectively select reliable DCDA results. At IAG-IASPEI 2017, we are going to make a follow-up report.