#### Electrical resistivity surveying

Electrical resistivity is an old geophysical method in which electrodes are placed in the ground in a linear or spatial array, and a DC current injected into the ground, typically around 500 mA. The flow of electricity between the current electrodes creates an electrostatic field whose strength can be measured on surface electrodes. Salt water is very much more conductive than fresh water, and the voltage measurements can be used to reconstruct the salt-water/freshwater interface for an island aquifer.

#### Seismic methods

Seismic methods involve generating waves in the ground using a seismic source like a buried explosive. These waves bounce off and refract through different geological layers. Using accelerometers to measure the wave motions, the subsurface water-bearing layers can be identified.

Off-the-shelf geophysical instrumentation for both electrical resistivity and seismic surveying costs upwards of $10k - 100k and relies on black box software to interpret the data. We build instrumentation from the ground up for a fraction of the cost and write all computer code using the most advanced mathematical techniques on which we have published extensively.

#### Mathematics

Electrical or seismic surveys are not in themselves very helpful. The task of transforming electrical or ground vibration measurements into images of the subsurface is a substantial mathematical problem. This problem of image reconstruction is identical to that encountered in medical imaging.

We use the newest mathematics and computational advances to build accurate images of the subsurface for locating groundwater.