Geophysical survey techniques, including electromagnetic and resistivity, were undertaken to assist with the identification of the size and extent of the freshwater lens beyond the known locations of the existing monitoring boreholes or wells.
Electromagnetics
Electromagnetics is a survey technique that relies on measuring the ground conductivity of the subsurface. Ground conductivity values obtained were used to interpret possible geological and hydrogeological models. A Geonics EM34 electromagnetic survey system was used during the Bonriki, Tarawa Kiribati and Vaitupu, Tuvalu field investigations. The EM34 uses two coils — a transmitter and a receiver — to measure ground conductivity. The transmitter coil radiates an alternating electromagnetic field that induces electrical currents (termed eddy currents) in the earth below the coil. These eddy currents in turn generate a secondary magnetic field. The receiver coil detects and measures both the secondary and primary magnetic fields and calculates the apparent conductivity based on the ratio between the two fields.The two coils are held by operators connected by a cable of one of three defined reference lengths, 10 m, 20 m or 40 m. The coils are placed in the vertical or horizontal dipole position depending on the investigation design of the survey, ground conditions, and targets. The depth of exploration depends on the separation between the transmitter coil and the receiver
coil, as well as on the coil orientation (coil axis/dipole horizontal or vertical). Geological factors such as moisture content, salinity of the pore water, temperature, and composition of colloids in the ground will influence the conductivity and the success of the survey.
This technique does not produce a unique result or solution. Measured values can be interpreted in a number of different ways, and the interpretation relies on an understanding of local ground conditions. A calibration technique used in atoll environments for groundwater exploration is the comparison of the measured apparent conductivity values against actual salinities measured in monitoring boreholes to develop a logarithmic profile of the freshwater lens’ thickness.
Resistivity
Resistivity profiling using a SuperSting R1/IP with 56 electrodes from Advanced Geophysics Inc. was used for resistivity investigations at Bonriki and Vaitupu. Electrical resistivity profiling involves measuring the apparent resistivity of soil and rock as a function of depth over a section from which an interpretation of the geology and hydrogeology of the subsurface can be made. The resistivity of soils is a function of porosity, permeability, ionic content or salinity of the pore fluids, and clay mineralisation. Resistivity is useful in determining freshwater lens thickness and shape.
Electrical current is injected into the ground through the use of stainless steel current electrodes, and then measured as a voltage in the potential electrode. Different spacing and sequencing of injection and measurement electrodes result in different arrays that are used for different investigations, depending on the depth and resolution of interest. At Bonriki, dipole–dipole and Wenner arrays were used for the surveys because they provided sufficient depth and resolution for the available time of the survey. Wenner arrays yielded the best data recovery at Bonriki and were relied on for the majority of the survey.
