Wide-ranging surveys of the sea-bed and of its fauna may be made either from sledges or similar vehicles towed along the bottom, or from free-floating manned or unmanned submersibles. In general, towed sledges involve less sophisticated equipment because the needs for units for forward propulsion and for maintenance of a floating body at a specified distance above bottom are absent. However, sledges lack the manoeuvrability of submersibles, and are more likely to be fouled by obstructions on the sea-bed.