A seismic refraction survey of the Western Approaches was completed in the autumn of 1973, totalling 28 stations. In addition to a few sonobuoys, an improved method of detection was used, consisting of geophones or hydrophones positioned on the sea floor. The seismic signal was transmitted by radio to a shipboard receiving and recording apparatus. This new technique considerably improved the signal noise ratio. The major structural trend in the Western Approaches to the Channel is NE-SW. Two large northeast-southwest fault systems border a large, downfaulted, elongated basin, floored by depressions and ridges. The seismic refraction data lead to the recognition of two distinct geological sequences. The first is associated with the tectonized, metamorphic floor with igneous intrusions and conformably layered Palaeozoic series. The second sequence is made up of nearly horizontal layers, consisting mainly of secondary and younger sediments. The two sequences seem to be separated by an unconformity. The relatively light sediments filling depressions of the Palaeozoic floor may partially cause the observed low free-air gravity anomalies. The basement has a block-faulted aspect, the blocks being tilted in a southeast and possibly also in a southwest direction.