the ROV connector, dropping the towed array to a lower depth. Long seismic streamers have intermediate paravanes along their length which can be used to adjust the depth of the array in real time. Changing an ROV's depth allows a towed array to be deployed in different thermal layers, giving a surface anti-submarine warfare (ASW) vessel a view above and below the layer. This compensates for density and temperature differences, which conduct sound above or below a thermal layer by reflection. By dropping the array's 'tail' below the layer, a surface ASW platform can better detect a quiet, submerged contact hiding in cold water below a warm upper layer. A submarine can likewise monitor surface combatants by floating the tail of its array above a thermal layer while lurking below. The array's hydrophones can be used to detect sound sources, but the real value of the array is that the signal processing technique of beamforming and Fourier analysis can be used not only to calculate the distance and the direction of a sound source, but also to identify the type of ship by the distinctive, acoustic signature of its machinery noises. For this, the relative positions of the hydrophones need to be known, usually possible only when the cable is in a straight line (stable), or when a self-sensing system (see strain gauges) or GPS or other methods embedded in the cable, and reporting relative position of hydrophone elements, is used to monitor the shape of the array and correct for curvature. As an example, Thales Underwater Systems' CAPTAS-2 (passive and active sonar) claims a detection range up to 60 km and weighs 16t. The heavier CAPTAS-4 weighs 20-34t and claims a detection range up to 150 km. Use in geophysics Towed array systems are also used by the oil and gas industry for seismic exploration of geological formations under the sea bed. The systems used are similar in concept to the naval ones, but are typically longer and with more streamers in a given array (6 or more in some cases). Typical hydrophone spacing along each streamer is on the order of two meters, and each streamer may be up to 10 km long. Sometimes streamers are flown at different heights, to give a so-called 3D array. Limitations Effective use of the towed array system requires a vessel to maintain a straight, level course over a data sampling interval.