Preprints
https://doi.org/10.5194/hess-2021-359
https://doi.org/10.5194/hess-2021-359

  20 Jul 2021

20 Jul 2021

Review status: this preprint is currently under review for the journal HESS.

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

Timothy C. McMillan1,2, Martin S. Andersen1, Wendy A. Timms3, and Gabriel C. Rau1,4 Timothy C. McMillan et al.
  • 1School of Civil and Environmental Engineering, The University of New South Wales, Sydney, Australia
  • 2School of Mineral and Energy Resource Engineering, The University of New South Wales, Sydney, Australia
  • 3School of Engineering, Deakin University, Waurn Ponds, Australia
  • 4Institute of Applied Geosciences (AGW), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

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.

Timothy C. McMillan et al.

Status: open (until 17 Nov 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-359', Anonymous Referee #1, 07 Sep 2021 reply

Timothy C. McMillan et al.

Timothy C. McMillan et al.

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Short summary
This work develops and applies a new method to estimate hydraulic and geomechanical subsurface properties in-situ using standard groundwater and atmospheric pressure records. The estimated properties comply with expected values except for the Poisson ratio which we attribute to the investigated scale and conditions. Our new approach can be used to cost-effectively investigate the subsurface using standard monitoring datasets.