The key requirements for a geophysical survey to improve understanding of the structure Milingimbi Island are that:
- the system is non-invasive;
- the method is relatively rapid;
- it provides sufficient resolution of the subsurface to resolve the hydrogeological features of interest.
The two main geophysical tools that will be used to investigate the aquifer structure in this area are a fast-sampling Time-domain Electromagnetic method (TEM- specifically Zonge Engineering's NanoTEM system) and electrical resistivity (IRIS Syscal Pro Switch 96). The TEM system has been used extensively in the Riverland of South Australia to characterize the contact between highly saline groundwater, and fresh river water (Berens et al., 2009), as well as to characterize hydrogeological conditions immediately below the river bottom (Hatch et al., 2010). Depending on ground conditions, loop size and background noise, this system is typically able to provide information about the conductivity structure of the ground to depths greater than 60 m.
We anticipate running TEM over the island in a stepped approach. The TEM system will be used to collect data over the centre of the island where the main production bores are located in a rough grid pattern and also along transects across the island to identify the freshwater/saltwater interface (Figure 3). These soundings are designed to characterize the aquifer thickness and distribution where the depths and thicknesses of units are anticipated to be there thickest, and the contact to salt water would also be the deepest. Sounding data in this part of the program would be collected using 40 m transmitting loops and 10 m receiving loops in order to maximize the depths to which data may be collected.
These soundings would be followed up with a program of approximately 500 m long continuous profiles that would skirt the edge of the island in order to directly characterize the fresh groundwater / salt water interface around the island's perimeter. Figure 3 shows also the proposed locations of ten 500 m long profiles at various strategic locations around the island.
Additionally, an Electrical Resistivity Imaging system, SYSCAL SWITCH Pro by IRIS Instruments will be used. It consists of a versatile electrical resistivity meter which combines a transmitter, a receiver and a switching unit. It has 480 meters of cable, with takeouts at every 5 meters (96 electrodes). The ERI system will be used to collect data where the TEM system is either not seeing deeply enough, is impractical to use due to dense undergrowth, and other situations where appropriate. At least two of the shorter profile lines will be repeated using the ERI system, to see if this adds information to the TEM data already collected.