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In-situ estimation of subsurface hydro-geomechanical properties using the groundwater response to Earth and atmospheric tides
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  • Timothy Colin McMillan,
  • Martin S Andersen,
  • Wendy Timms,
  • Gabriel Christopher Rau
Timothy Colin McMillan
University of New South Wales
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Martin S Andersen
University of New South Wales
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Wendy Timms
Deakin University
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Gabriel Christopher Rau
Karlsruher Institut fur Technologie

Corresponding Author:[email protected]

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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 characterize 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 parameter space for unconsolidated systems. Further, the characterization of consolidated systems is possible when using literature estimates of the grain compressibility. While previous field investigations have assumed a Poisson’s ratio, our new approach allows for its estimation under in-situ conditions. We apply this method to water level and barometric pressure records from four field sites with different hydrogeology. Our results reveal the anisotropic response to strain, which is expected for a heterogeneous lithological profile. Estimated hydro-geomechanical properties (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. Closer analysis reveals that this can be explained by the fact that in-situ conditions differ from typical laboratory core tests. Our new approach can be used to passively, and therefore cost-effectively, estimate subsurface hydro-geomechanical properties representative of in-situ conditions. Our method could be used to improve understanding of the relationship between geological and geomechanical subsurface heterogeneity.