An apparatus for three-dimensional single-channel seismic measurements, wherein at least one seismic source and hydrophone devices are towed behind a vessel, and a pair of submerged deflectors are used and which, as the vessel moves, seek to move in a direction transverse to the vessel's direction of travel. Fastened between the deflectors is a wire which limits the spacing between the deflectors. Along the wire there are also mounted thereon hydrophone devices which in relation to the spacing of the devices have a short lengthwise extent. The devices are connected together by a hydrophone signal cable. An additional signal cable connects the cable to the signal processing equipment on the vessel. The seismic source may be connected to signal equipment on the vessel and located in an area between the vessel and the wire, or at least one of the deflectors may be equipped with a seismic source.