Patent ID: 7109718

Claim:
An underground exploration apparatus comprising: a first oscillating means for generating an alternating voltage signal of a first frequency out of two different frequencies selected on the basis of frequency characteristics of a dielectric constant of a substance to be probed; a second oscillating means for generating an alternating voltage signal of a second frequency out of the two frequencies; adding means for calculating an added value of output signals supplied from the first oscillating means and the second oscillating means; amplifying means for amplifying an added alternating current signal supplied from the adding means to supply signals to two current application electrodes each intended for applying a current between two different points on a ground; differential amplifying means for measuring a potential difference between voltage signals generated on the ground by a current supplied by the amplifying means to the ground by means of signals supplied from voltage measurement electrodes placed at two different positions; subtracting means for calculating a subtracted value between the output signals supplied from the first oscillating means and the second oscillating means; multiplying means for performing synchronous detection by means of signals supplied from the differential amplifying means and the subtracting means; and low pass filtering means for removing a high-frequency component from a signal supplied from the multiplying means.