HYDROPHONE STATION
The hydrophone station of the IMS is a fixed cable station. It consists of a triplet of hydrophones separated horizontally (1-2km), anchored to the sea floor and floated to the axis of the SOFAR channel, with a cable to a nearby land station for data and power transmission.
Cable stations enjoy lifetime, power and communication advantages. Lifetimes are longer for fixed cable stations than moored buoy stations, since there are no surface components to be battered by wind and waves. Power and communications services are available on land through the shore cable, and the data transmission is continuous via satellite.
There are cable stations that have operating for more than twenty years with little or no maintenance, and they are considered to be the most reliable stations for long-term hydroacoustic monitoring. The installation of a fixed cable station requires a high capital investment. In the case of a station located at an island, two cables are required on opposite sides of the island to ensure unblocked reception from all directions. The costs of site survey of the area, procurement of equipment (length of cables from 50km to more than 100km) and installation make the fixed cable hydroacoustic stations the most expensive of the IMS. On the other hand, each of these stations covers huge areas of the ocean. The hydroacoustic network can adequately cover all ocean areas with few stations.
The principal threats to long and reliable operation of these stations are the destruction of the cables at the land-sea interface and cable breakage when anchors or trawls snag the cable. The solutions to these problems are to choose heavily armoured cables for deployment on the continental shelf locations, and to bury the cables in the surf zone. In these areas, the cable can also be protected with a split pipe around it.
The characteristics and minimum requirements of a hydrophone station of the International Monitoring System have been fixed by a group of experts from around the world. A set of minimum specifications for the equipment and communications has been approved, as well as a set of recommendations for better performance of the station. The fixed cabled station characteristics may be summarised as follows:
･ Omnidirectional hydrophones with 120dB of dynamic range, with sensitivity down to ambient noise levels.
･ Continuous data transmission at 240 samples per second in order to resolve frequencies up to 100Hz.
･ Hydrophone located at the axis of the SOFAR channel, with a wet-end digitiser and with the communication system located at the shore station.
･ One or two cables at each station with a triplet of hydrophones horizontally separated.
･ Cables on two sides of an island for maximum coverage of broad ocean areas and minimisation of local blockage by the island.
A prime characteristic of the hydrophone based stations is a very good capability at discrimination between explosions and other transient events in the ocean. These other events include such sources as earthquakes and underwater volcanoes. This high capability is associated with the higher frequencies (up to 100Hz) that are available to these stations. This is in contrast to the seismic monitoring, which operates at significantly lower frequencies.
