In-situ estimation of subsurface hydro-geomechanical properties using
the groundwater response to 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 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.