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Description
In this study, a comparative analysis of probe-based gas permeability measurements was conducted on a drill core from the Lusatian granodiorite. The investigation forms part of a broader core characterization project related to the planned Einstein Telescope candidacy in the Lusatian Granodiorite (Hasinger et al., 2022). As the facility requires minimal seismic disturbance, understanding the permeability of the surrounding rock mass is essential to identify and mitigate potential influences caused by subsurface fluid flow.
Therefore, the objective was to determine intrinsic gas permeabilities of the granodiorite. For this, a handheld transient vacuum-pulse probe permeameter (TinyPerm 3 by New England Research Inc. (2023)) was used. The quantitative reliability of the TinyPerm 3 was evaluated by comparison with a laboratory-based transient pressure-pulse probe permeameter (here: “PneuBox”) first constructed by Scibek and Annesley (2021). Due to operative constraints and site conditions, a complete hydraulic screening of the drill cores was not feasible. Instead, Lugeon tests were conducted at three locations within the borehole, yielding comparable permeability values.
The PneuBox covers a permeability range from approximately 7E-20 m² to 1E-12 m², whereas the TinyPerm is limited by a detection threshold of about 1E-17 m². The comparison shows that agreement between both methods is strongly permeability-dependent. Good quantitative agreement is observed between approximately 1E-15 m² and 5E-13 m², where deviations remain within Δlog₁₀(K) ≤ 0.5. Outside this range, systematic deviations occur. At low permeabilities (< 4E-17 m²), the TinyPerm consistently overestimates permeability, with increasing deviation towards lower values. A similar, though less pronounced, overestimation is observed at higher permeabilities (> 5E-13 m²).
These findings demonstrate that measurable values do not necessarily imply quantitative reliability. The effective accuracy range of the TinyPerm is therefore restricted to an intermediate permeability window. Within this range, the device is suitable for rapid characterization of higher-permeability features, while laboratory methods remain necessary for reliable assessment of low-permeability matrix domains. In general the results demonstrate the overall very low permeability of all varieties of the Lusatian Granodiorite independent on the degree of alteration.