Method and apparatus for interface location determination

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. As the tool is raised and lowered in the well bore, changes in the sum and delta signals will be observed when an interface is passed. A logging tool for use in petroleum production and exploration wells is also disclosed.