Passive Subsurface Characterisation (PSC): Using the groundwater response to Earth tides and atmospheric pressure

Characterising subsurface hydraulic and geomechanical properties is a prerequisite for Earth resource management. Traditional approaches such as hydraulic testing are costly and require specific infrastructure as well as expertise which limits general testing capabilities. Passive Subsurface Characterisation (PSC) uses the groundwater response to natural forces, such as Earth tides and atmospheric pressure changes, to determine state of confinement and estimate hydro-geomechanical properties of the subsurface in-situ. This approach only requires standard measurements of groundwater hydraulic head and barometric pressure as well as theoretical Earth tides which can be calculated. This presentation gives an overview of the state of the science. Calculation of hydro-geomechanical subsurface properties from standard groundwater monitoring datasets can be done conveniently using the new python package HydroGeoSines (HGS). HGS contains key methods from the peer-reviewed literature and therefore allows anyone with python skills to apply PSC to their datasets. PSC offers cost-effective estimations and can be applied to existing datasets provided they meet minimum quality criteria. Further analysis applied to monitoring bores at different field sites around the world exemplifies that, apart from basic hydraulic properties (hydraulic conductivity, specific storage, barometric efficiency), the full poroelastic parameter space (porosity, shear, Young’s and bulk moduli, Skempton’s and Biot-Willis coefficients and undrained/drained Poisson’s ratios) can be determined. Since PSC is an underutilised tool, this presentation aims to raise awareness as well as for the need to update groundwater monitoring practice to maximise the benefits of PSC.