Ocean bottom mapping is carried out by projecting a sonic pulse in the formf a fan-shaped beam from a vessel toward the ocean floor, receiving echoes by an array of sonic detectors extending athwartship, repeatedly taking instantaneous, simultaneous, snapshot-like samples of all of the detectors in the array, and, by means of a digital computer, applying a beam-forming transform, such as a Fast Fourier Transform, to the samples, thereby producing a spectrum of amplitudes versus angles. The directions of the floor areas from which the snapshot data emanate are determined from refinement of the peaks in the spectrum of amplitudes, and the distances of the same floor areas from the vessel are determined by the time interval between the pulse and the sample. A bottom profile is constructed from the directions and distances so determined. Two alternative techniques for refining and interpreting spectrum data are described.