Geogas transport in fractured hard rock – Correlations with mining seismicity at 3.54km depth, TauTona gold mine, South Africa

Abstract

An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (the international NELSAM-DAFGAS projects) at the TauTona Gold mine, South Africa. Five boreholes up to 60 m long were drilled at 3.54 km depth inside a 25 m² cubby into the highly fractured Pretorius Fault zone and instruments for chemical and seismic monitoring installed therein. Over the span of four years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and airlike with about 78% N 2 , 21% O 2 , 1% Ar. The geogas components CO 2 , CH 4 , He and H 2 show most interesting trends and variations on the minute-by-minute basis and significantly correlate with seismic data, while the 222 Rn activity remains constant.Time series and cross correlation analysis allow the identification of different gas components (geogas and tunnel air) and the identification of two processes influencing the borehole gas composition: 1) pumping-induced tunnel air breakthrough through networks of initially water-saturated fault fractures; and 2) seismicity induced permeability enhancement of fault fractures to above ~5⋅10 -10 m². The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ≤ 1⋅10 9 Nm, located within 1000 m of the cubby) -and, we suggest, also during induced earthquakesa final goal of the project.

Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?

“…This rate is also more than the 0.2–0.45 n m year −1 sulfate production rate predicted for radiolytic oxidation of pyrite in the Witwatersrand and Ventersdorp Supergroups, respectively (Lin et al ., ). The likely explanation for the higher‐than‐expected geochemical flux of H 2 and sulfate may lie in a more recently discovered release of H 2 during fracturing of the rock formations with mining activity (Lippmann‐Pipke et al ., ). Phylogenetic analyses and sulfur isotopic data suggest that the other deeper fracture water samples are dominated by SRB activity.…”

Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration

“…Cataclastic diminution of silicate minerals in the presence of water can also generate H 2 (Kita et al, 1982) and in the presence of CO 2 , generate CO and O 3 (Baragiola et al, 2011). H 2 release during seismic events has been recorded at 3 km depths in South Africa (Lippmann-Pipke et al, 2011) and H 2 release during rock-crushing at the base on the 3 km thick Greenland ice sheet has been inferred (Telling et al, 2015). The relationship between rock fracturing and/or crushing and subsurface microbial community abundance and activity is not yet resolved and is an avenue of current research in subsurface microbiology.…”

Discriminating Characteristics of Tectonic and Human‐Induced Seismicity

“…Since the beginning of routine earthquake recording in May 1975, the seismicity closely follows the power production and concentrates in the vicinity of the producing steam wells (Marks et al, 1978;Eberhart-Phillips and Oppenheimer, 1984;Oppenheimer, 1986;Majer and Peterson, 2007). (2) The TauTona gold mine in South Africa, which is one of the deepest and best-investigated operating mines worldwide (Boettcher et al, 2009(Boettcher et al, , 2015Lippmann-Pipke et al, 2011), is located in a tectonically dormant region (the Kaapvaal craton) about 90 km southwest of Johannesburg and the seismicity in the mine is primarily associated with blasting activity. We refer to Boettcher et al (2015) for further detail on this region and the examined catalog.…”