Application of geophysical techniques for 3D visualization of regolith hydrogeological architecture and use of this information for management of dryland salinity in Western Australia

Abstract

This thesis demonstrates the use of geophysics to identify the hydrogeological structures and mechanisms responsible for the salinisation of land and water in three different case studies. In addition, it demonstrates the critical importance of the interpreted information products being relevant to the land managers and the management tools and strategies available to them. Three case studies are examined. The common requirement for each study area was the acquisition of detailed sub-surface information on the location of hydrogeological features that could not be interpreted from surface observations or obtained from isolated drill holes. The spatial coverage of the geophysical data is shown to be critical to the quality of location information produced. The interpretation of these data and presentation of information products in terms of the current management tools available to land managers are shown to be essential for successful and cost effective adoption.At Broomehill, the geophysical and other data were interpreted to produce information products that indicated the location of salt stores, sources of water, mechanisms that brought water and salt together in the landscape and mechanisms that brought saline groundwater to the surface. Using these information products, the spatial plan took the form of a farm plan that clearly mapped the location and design of surface water management earthworks, areas of revegetation, water storages, roads and fences such that the salinising processes in each paddock were directly impacted through the considered location of the proposed works. One such plan at Broomehill was promptly implemented in 1996 and the results have been monitored since that time. The farm plan is shown to be more cost effective than comparable farm plans on nearby properties that did not use information from geophysics.At Tammin, ground geophysical surveys (gravity and time domain electromagnetics) were used to identify the location of sedimentary fill in a buried inset valley. This was to provide information on which to base the siting of a production bore such that it would intersect hydrologically transmissive sediments. This bore was used to test the effectiveness of groundwater pumping to lower the watertable and recover agricultural land from salinity.The bore failed to pump an adequate volume of water and the groundwater pumping aspect of the trial was considered a failure. However, the value of this case study lies in observation of the characteristics of the geophysical information collected.The geophysical data were collected along four transects 0.8 km to 2.0 km apart. The processed geophysical data revealed cross-section profiles of the buried valley and the production bore was located on the transect that showed the steepest “V” profile. Given only four transects of information and the difficulty of interpolating over up to 2 km between them, this choice is understandable. The failure of the interpretation of the geophysical transect data to locate a suitable bore site calls into question the usefulness of geophysical survey transects up to 2 km apart. The geophysical information (although it was cheaper) was not much better than having four transects of closely spaced bores. It failed to adequately reveal the hydrogeological architecture of the study area. This case study revealed the importance of survey design and scope to ensure that the data are suitable to produce interpreted information products that are suitable for the purpose.A helicopter-borne, time domain electromagnetic survey was flown over the Lake Warden Wetlands near Esperance. The data were processed and interpreted to reveal potential groundwater flow paths in the main aquifers underlying the wetlands. This information, along with salt storage distribution maps derived from the conductivity product, was provided to the Department of Environment and Conservation (DEC) in the form of the maps shown in this thesis. This information enabled the DEC to formulate a catchment management strategy that focussed on the eastern sub-catchments of Bandy Creek and Neridup Creek as the main sources of saline groundwater. Surface water management plans prioritised diversion of waters contaminated by discharging saline groundwater and could be designed without active discharge occurring at the time. Prior to this survey, the DEC and South Coast Natural Resource Management were promoting generic management strategies, such as revegetation to reduce groundwater recharge, over the whole catchment area.The information products developed in this study enabled the DEC and community to understand mechanisms causing high water tables and salinisation in the wetlands and consequently, to be much more targeted and effective in their investment of limited land conservation funding.In all these case studies, the use of geophysics was essential to identify the hydrogeological architecture and mechanisms responsible for the salinisation of land and water. Furthermore, the interpretation of the data and design of information products is critical as to whether the information is successfully acted upon. The interpretation of the data and design of information products based on location of aquifers and other geological structures is critical to adoption by land managers and the impact and effectiveness of the actions taken.