Patent US 4,577,153

Instrumentation and procedures for detecting geological anomalies occurring in layered coal formations. The instrumentation comprises a medium frequency continuous wave narrowband FM transmitter and receiver pair. Two instrument configurations are disclosed with one being a portable instrument for use within an existing mined area and another being a downhole instrument for insertion into boreholes in unmined areas. Survey procedures are provided to detect anomolies through signal attenuation, path attenuation and signal phase shift. Continuity measurements provide data to determine the existance of anomalies. Tomographic techniques are employed to provide a visual image of the anomaly. Computer aided reconstruction techniques provide such visual images from the generated data.