Exploratory study of nuclear magnetic resonance well logging technology on the hydrogeological characteristics of deep geological formations

In-situ measurements of the hydrological properties (e.g. porosity, pore size distribution, and permeability) of geological strata is crucial for the safe assessment of potential disposal sites for high-level radioactive nuclear waste. We applied proton nuclear magnetic resonance (NMR) well logging to a boring site down to 280 m in depth in a coastal region of Japan, consisting of unconsolidated alluvial fan sediments and basaltic lava with high susceptibility. The NMR results combined with in-situ aquifer tests, borehole wall resistivity imaging, and core analysis revealed that (i) the porosity was successfully measured for the intact basalt formation, (ii) the hydrological aquitards consisting of unconsolidated clay and silt were almost successfully detected by analyzing the distribution of the proton transverse relaxation times, (iii) the permeability was measured with a reasonably small error for the depth interval of the unconsolidated sand-gravel sediments, and (iv) the fracture aperture was calculated for the depth interval of the fractured basalt, suggesting that this interval with a high permeability could serve as a rapid groundwater flow path within the regional hydrological system. This case study provides invaluable data for the geological disposal project to elucidate the hydrogeology of the deep-seated strata in the coastal region of Japan.