Abstract: A method and apparatus for determining the interface between two materials having different dielectric properties, especially in determining the thickness of a hydrocarbon layer on ground water, is disclosed. The apparatus consists of an air-filled, cased surveillance well in which a logging tool is raised and lowered. The logging tool contains an electromagnetic receiving antenna and two transmitting antennas located above and below the receiving antenna. A first wave and a second wave are transmitted from the transmitting antennas. The first and second waves are offset 180.degree. to produce "sum" and "delta" signals using a 1/2.degree. hybrid. Alternatively, a single wave can be transmitted from a central transmitting antenna to receiving antennas located above and below the transmitting antenna. The received signals are then phase shifted 180.degree. using a 180.degree. hybrid to produce sum and delta signals.