In situ estimation of subsurface hydro-geomechanical properties using the groundwater response to semi-diurnal Earth and atmospheric tides

Abstract. Subsurface hydro-geomechanical properties crucially underpin the management of Earth's resources, yet they are predominantly measured on core samples in the laboratory while little is known about the representativeness of in situ conditions. The impact of Earth and atmospheric tides on borehole water levels is ubiquitous and can be used to characterise the subsurface. We illustrate that disentangling the groundwater response to Earth (M2) and atmospheric tidal (S2) forces in conjunction with established hydraulic and linear poroelastic theories leads to a complete determination of the whole hydro-geomechanical parameter space for unconsolidated systems. Further, the characterisation of consolidated systems is possible when using literature estimates of the grain compressibility. While previous field investigations have assumed a Poisson's ratio from literature values, our new approach allows for its estimation under in situ field conditions. We apply this method to water level and barometric pressure records from four field sites with contrasting hydrogeology. Estimated hydro-geomechanical properties (e.g. specific storage; hydraulic conductivity; porosity; shear, Young's, and bulk moduli; Skempton's and Biot–Willis coefficients; and undrained or drained Poisson's ratios) are comparable to values reported in the literature, except for consistently negative drained Poisson's ratios, which is surprising. Our results reveal an anisotropic response to strain, which is expected for heterogeneous (layered) lithological profiles. Closer analysis reveals that negative Poisson's ratios can be explained by in situ conditions differing to those from typical laboratory core tests and the small strains generated by Earth and atmospheric tides. Our new approach can be used to passively, and therefore cost-effectively, estimate subsurface hydro-geomechanical properties representative of in situ conditions and it improves our understanding of the relationship between geological heterogeneity and geomechanical behaviour.