Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Tidal subsurface analysis using Earth and atmospheric tides: a step forward in the characterisation of the subsurface (248)

Martin S. Andersen 1 2 , Tim C. McMillan 1 3 , Gabriel C. Rau 1 4 , Wendy A. Timms 5
  1. Connected Waters Initiative Research Centre, UNSW, Sydney, NSW, Australia
  2. School of Civil and Environmental Engineering, UNSW, Sydney, NSW, Australia
  3. School of Minerals and Energy Resource Engineering, UNSW, Sydney, NSW, Australia
  4. Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany
  5. School of Engineering, Deakin University, Waurn Ponds, VIC, Australia

The analysis of the groundwater response to naturally ubiquitous Earth and atmospheric tides (EAT) holds the potential to revolutionize groundwater characterisation. Management of this environment is often hampered by sparse hydrogeological data both temporally and spatially to enable reliable groundwater modelling. The sparsity or lack of measured key hydrogeological variables such as permeability or specific storage is often the result of cost, staff and time limitations associated with current investigative techniques (e.g. long-term aquifer tests).  Traditional hydrogeological investigations assume that the matrix of an aquifer is rigid. However, in order to understand EAT influences on groundwater systems, a theory allowing the elastic deformation of both rocks and water must be invoked. Tidal subsurface analysis (TSA) methods are passive techniques which use standard piezometers and pressure transducers to capture the groundwater response to the naturally occurring astronomical and atmospheric forcing (produced by EAT’s) to characterize the subsurface under the premise of Poroelastic Theory. They neither require active hydraulic testing nor bores designed for large groundwater pumps. As such TSA methods can use data from normal monitoring bores which will expand the number of field-determined hydrogeological variables in the subsurface by orders of magnitude. The method also has potential to be applied to long-term groundwater level timeseries so that the variability of hydrogeological parameters can be assessed against long-term groundwater trends (e.g. declining groundwater levels and subsidence). Finally, TSA is far less expensive and resource-intensive than hydraulic aquifer testing. TSA can be used to either complement or, with further development, replace current techniques altogether.  With further development and industry adoption TSA methods could become a routine approach that can add enormous value to existing monitoring programs and represent a potential for a paradigm shift in how we investigate and manage groundwater and subsurface resources globally, significantly exceeding our current capabilities.