Abstract: The instrument and method locate anomalies and the interface between two materials having little difference in their density in a mining or storage well. More particularly, the method comprehends the generation of gamma rays and the detection of scattered rays at a point spaced from the point of generation of the gamma rays. By moving the points of generation and detection through the well, there will be observed a pronounced change in the detection rate of the scattered gamma rays when various interfaces and anomalies are traversed.The focus interface instrument includes a decentralizer, collar locator, electronics assembly, detector, and radiation generator, all enclosed in a nonmagnetic stainless steel housing except for the electronics assembly whose housing is made of tubular steel. The radiation generator has a radioactive source which emits gamma rays, and the detector detects those rays reflected back to the detector.