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 are ubiquitous and can be used to characterise the subsurface. We illustrate that disentangling the groundwater response to Earth and atmospheric tidal forces in conjunction with 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/drained Poisson's ratios) are comparable to values reported in the literature, except for consistently negative drained Poisson's ratios which are surprising. Our results reveal an anisotropic response to strain, which is expected for a heterogeneous (layered) lithological profile. Closer analysis reveals that negative Poisson's ratios can be explained by differing in-situ conditions 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. Our method can be used to improve our understanding of the relationship between geological heterogeneity and geomechanical behaviour.
Highlights
IntroductionThe main reason for this challenge is the subsurface’s heterogeneous nature and that the sampling density necessary to describe it may be prohibitively expensive (e.g. by drilling and testing of core)
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
We illustrate that disentangling the groundwater response to Earth and atmospheric tidal forces in conjunction 5 with hydraulic and linear poroelastic theories leads to a complete determination of the whole hydro-geomechanical parameter space for unconsolidated systems
Summary
The main reason for this challenge is the subsurface’s heterogeneous nature and that the sampling density necessary to describe it may be prohibitively expensive (e.g. by drilling and testing of core). This issue is further exacerbated by the difficulty in approximating in-situ environments in laboratory testing in regards to both scale and subsurface pressures (Hoek and 25 Diederichs, 2006; Cundall et al, 2008; Bouzalakos et al, 2016). These difficulties may be overcome by in-situ characterisation of hydro-geomechanical properties of the subsurface (Villeneuve et al, 2018). The in-situ pressure, stress conditions, and the scaling and inclusion of heterogeneities can achieve a more representative estimate than possible from selective laboratory testing
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