Abstract: Pulses of monochromatic coherent sound waves are emitted at an underground test station into a coal seam from several closely juxtaposed sources to form a beam whose reflection at a discontinuity in the seam is detected by a group of receivers. The time lapse between the outgoing and incoming sound pulses is a measure of the distance of the seam boundary from the test station and, upon a controlled shifting of the phases of the outgoing and/or incoming beam components, can be displayed on an oscilloscope screen as a function of sweep angle in an azimuthal and/or an elevational plane.