Time lapse electric resistivity tomography to portray infiltration and hydrologic flow paths from surface to cave

This study provides the example of Golgotha Cave, in the south west of Western Australia, where previous long-term hydrological, geochemical and climate monitoring resulted in a theoretical hydrological model of the karst critical zone. We test this model by presenting the results of a 1.5 year monitoring program above the cave using time-lapse electric resistivity tomography (TL-ERT). Between May 2016 and January 2018 several lines up to 200 m long were monitored using a 100 multi-electrode Lipmann 4point light instrument. Various Wenner and Dipole-Dipole arrays provided information of electric resistivity changes at certain points of the year. Laboratory studies measuring volumetric water content versus electric resistivity of various rocks and soils resulted in three different ERT-hydrogeological zones. Besides the annual infiltration patterns, TL-ERT also identified previously unknown, permanent water storages as well as several areas acting as fast hydrological flow pathways most likely caused by pipe structures in the aeolianite. Specific aspects of the TL-ERT study could be correlated with the long term drip water monitoring and thus confirmed and extended the hydrogeological model of the area. A resulting conceptual model of the flow pathways derived from ERT surveys and cave water monitoring results is presented to visualise water flux from the surface to the ceiling of Golgotha Cave. This includes annual recharge to 40 m depth in less than one hydrological year, preferential flow paths to a persistent store of water, and a time lag between soil moisture saturation and seasonal connection of karst stores that indicates a narrower window for seasonal recharge than indicated by soil moisture data.

Automated summary

This study validated a theoretical hydrological model of the karst critical zone by using time-lapse electric resistivity tomography (TL-ERT) to monitor the area above Golgotha Cave in Western Australia. The TL-ERT program identified previously unknown permanent water storages and fast hydrological flow pathways, confirming and extending the existing hydrogeological model. The resulting conceptual model showcased annual recharge patterns, preferential flow paths, and time lags between soil moisture saturation and seasonal connections in the karst system.