Ground-penetrating radar or GPR is a geophysical method used to explore beneath the ground surface. How far down you can look depends on the characteristics of the soil: in wet or clay ground it's limited to a few meters, but in Antarctic ice it can see hundreds of meters down. Another factor is the wavelength of the radar pulse used—shorter wavelengths show more detail, but cannot penetrate as far. Whereas acoustic methods detect changes in density, GPR reflects from places where the electrical or magnetic properties change.
I spent a summer afternoon with a volunteer crew gathering GPR data near the active trace of the Green Valley fault, and had time to snap a few photos as we surveyed the ground to about 4 meters depth. We used 1-meter antennas (for a 100 MHz signal) mounted in a cart that we pulled by hand as an odometer controlled the radar pulses. Data cables led back to a laptop that recorded the signals. We took turns doing everything, running six lines in about two hours.
