Water can drastically affect the attenuation of the GPR signal in soil and in large concentrations will outright block the signal. This means in areas of wet clay the depth might be limited to as low as 4 feet but in very dry sandy conditions may reach as low as 12 feet. This property can make water leaks very easy to locate with GRP. If you are having trouble locating the source of your massive water bill having a GPR tech come out might be your best option.