Abstract: A method of locating near-surface underground chambers, rooms, caves, pits, tubes, faults in surface rocks, and the like, using acoustic techniques is disclosed. For mapping large areas, a survey grid for transmitter transducer placement first is laid out over the area of interest.A transmitting transducer is coupled to the bedrock at one of the grid sites, and acoustic waves are generated in the earth by operation thereof. Portable acoustic receiving means, including a receiving transducer, are used to detect acoustic waves from the transmitter at a plurality of receiving sites surrounding the transmitting station. Transmitted acoustic waves from the transmitting site received by the receiving transducer are converted to electrical signals by the receiving transducer and recorded for subsequent use. Following operation of the receiving means at each receiving site surrounding the transmitter transducer, the received signals are compared to determine any differences in their magnitudes.