Determining Crack Aperture Distribution in Rocks Using the ¹⁴C‐PMMA Autoradiographic Method: Experiments and Simulations

Abstract

Because cracks control the global mechanical and transport properties of crystalline rocks, it is of a crucial importance to suitably determine their aperture distribution, which evolves through alteration processes and rock weathering. Due to the high variability of crack networks in rocks, a multiscale approach is needed. The 14 C-PMMA (polymethylmethacrylate) method was developed to determine crack apertures using a set of artificial crack samples with different controlled apertures and tilt angles and also using Monte Carlo simulations. The experiments and simulations show the same result: the estimation of apparent aperture w A was successful regardless of tilt angle, even if the estimates are less accurate for low tilt angles (<30°). The uncertainties on the estimation of the real crack aperture w R arise from the unknown tilt angle. The ability of the 14 C-PMMA autoradiography method to estimate crack aperture distributions in rock samples was successfully confirmed on a sample of Grimsel granodiorite.

Weathering of Viamão granodiorite, south Brazil: Part 2 – Initial porosity of un-weathered rock controls porosity development in the critical zone

“…To quantify the connected porosity of the rock at different stage of weathering with a spatial resolution of ~20 µm, the samples were impregnated with the 14 C-MethylMethAcrylate (MMA) liquid monomer (Siitari-Kauppi, 2002;Oila et al, 2005;Sardini et al, 2006Sardini et al, , 2009Mazurier et al, 2016;Bonnet et al, 2020;Delayre et al, 2020). The unweathered rock (GRV01), saprock (GRV03, GRV04) and saprolite (VIA03) samples were dried in a vacuum in an oven set at 50 • C for 11 days to remove all residual water before impregnation.…”

“…pixel of 21.17 µm) in 8 bit (256 grey levels) giving a connected porosity digital map of each studied rock section (Supplementary Fig. 1a) (Oila et al, 2005;Kelokaski et al, 2006;Leskinen et al, 2007;Voutilainen et al, 2012;Robinet et al, 2012;2015;Bonnet et al, 2020). The detailed procedure is provided by Siitari-Kauppi (2002).…”

“…However, connected microcracks with a small aperture (<1 μm) are also revealed because they appear as a continuous line of a point source in the autoradiograph (Sardini et al, 2014;Bonnet et al, 2020).…”

“…Because fractures/cracks formed during weathering are important to quantify as they are preferential and fast fluid pathways in the rock, controlling its permeability and the accessibility of solutes to interaction sites, an effort was made using these imaging methods for characterising the proportion and aperture distribution of fractures during weathering. A specific imaging method developed by Bonnet et al (2020) was employed for estimating fracture porosity, aperture and density generated at different weathering stages.…”

Quantifying the Porosity of Crystalline Rocks by In Situ and Laboratory Injection Methods

“…Porosity calculation was performed according to the equations described above. The 14 C-PMMA porosity method was developed at the University of Helsinki and is described in [29][30][31][32][33]. The method allows for visualising and quantifying the spatial distribution of pore spaces in the rock at the micrometre to centimetre scale by evaluating autoradiographs of rock samples after impregnation with a 14 C-doped methyl-methacrylate (MMA) resin.…”