Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Characterising the access constrained valley by using non-destructive investigation methods – Thirlmere lakes case study (401)

Katarina David 1 , Wendy Timms 2 , Peter Hatherly 3 , Tim Cohen 4
  1. UNSW, Sydney, NSW, Australia
  2. Deakin University, Melbourne, VIC, Australia
  3. Retired geophysicist, Sydney, NSW, Australia
  4. University of Wollongong, Wollongong, NSW, Australia

The Thirlmere lake system comprising four lakes is part of the Greater Blue Mountains World Heritage area located about 100 km south of Sydney, NSW. Large changes in water levels in the lakes in the past 50 years have prompted a multidisciplinary investigation to understand how this lake system functions including underlying geology. Given the logistics of drilling access near the lakes, there has been limited investigation of the underlying rock and geological structures that could influence the hydrology of the lakes.

This study was conducted using three different geophysical techniques to improve understanding of the depth, extent and variability of unconsolidated sediments and underlying sandstone. This included undertaking two transects by electromagnetics (EM34) across the sills, 12 km of ground penetrating radar (GPR) surveys along and across the lakes and sills, eight resistivity transects between the lakes. EM34 was found to provide relatively rapid means of measuring shallow electrical conductivity of the ground but lacked detail and resolution.

A combination of GPR and resistivity have been the most successful methods to date. These surveys have identified dry peat horizons to a depth of around 2 m, a clear boundary with the underlying saturated clay layer at a depth between 2 to 5 m below ground and a layer of sandier sediment beneath the clay and along the valley sides. Typical resistivity of less than 30 Ωm and presence of fresh water indicates a saturated clay layer, with variable thickness between lakes. Across the lakes and partially across the sills, the clay layer has attenuated the GPR signal, but enabled clear differentiation from the overlying peat layer.  Strategic drilling has enabled some verification of the resistivity transects with additional targets identified for further work. In addition, GPR has revealed areas of steeply dipping beds in the northern part of the lake system and possible structures, in areas where outcrop is limited.