Withdrawn NCGRT/IAH Australasian Groundwater Conference 2019

Hydrogeophysical approach towards SWI investigation in Keep river, NT (319)

Klara Steklova 1 , Anandoroop Ray 1 , Ken Lawrie 1
  1. Geoscience Australia, Symonston, ACT, Australia

Seawater intrusion (SWI) represents a hazard to many groundwater systems, particularly in areas where there has been significant utilisation of groundwater resources in coastal aquifers. Mapping the seawater-freshwater interface is therefore a high priority for groundwater management, especially in aquifers with dynamic SWI interface.

Due to the high conductivity of seawater, SWI is a good target for many geophysical electromagnetic methods, such as airborne electromagnetic (AEM) or direct current resistivity methods. Airborne collected data are able to rapidly map extensive areas, and thus map the extent of SWI on a large scale along the coastlines.

However, looking at local scales, a discrepancy is often found between geophysical estimates and groundwater borehole data, due to different resolution, data sensitivity and also quality of geophysical and groundwater data. Numerous synthetic studies have shown the benefit of approaching the problem by evaluating both types of data in a joint manner. Research in combining the field geophysical and groundwater data for SWI cases is however very limited.

In this contribution, we look at an AEM survey in the Keep river, NT, which was acquired in 2017. It is a dense line survey with spacing of 200- 250m, acquired using the SkyTEM 312FAST system. Due to the character of AEM methods, the estimation of 3D (or 2D) subsurface conductivity is mathematically an ill-posed problem, giving multiple “equally good” models (bulk conductivity of subsurface) with the same data misfit.

The borehole data from this area (such as sonic core and borehole geophysics (NMR, Induction and spectral gamma) provide valuable information about the textures, porosity, salinity etc. We applied this “a priori” information to invert selected lines of AEM data to obtain estimates that fit well the geophysical data but are also plausible with regard to geology, groundwater chemistry and textural data.