Fourthly, once certified, operation and maintenance agreements are established between the CTBTO and a station operator. Long-term quality monitoring is then undertaken to maintain the high standards of data quality, data availability and station performance.
Seismic monitoring networks
The Treaty calls for two global seismic monitoring networks: a primary seismic network with 50 stations and an auxiliary one with 120 stations. The stations of the primary seismic network send data continuously in real time to the IDC and will be utilized most extensively. The auxiliary seismic network often takes advantage of existing seismic stations which are being upgraded to meet the IMS technical standards. These stations do not send data in real time but upon request only.
There are two global seismic monitoring networks. The primary seismic network of 50 stations sends data continuously. The auxiliary seismic network consists of 120 stations that send data on request only.
There are basically two different types of seismic stations: seismic arrays and three-component stations. About 60% of the primary seismic network will consist of seismic array stations, which are essentially sets of nine to 25 seismic sensors geometrically arranged over a wide area. Most of the stations in the auxiliary seismic network are three-component stations.
Seismic arrays consist of several seismic sensors that are laid out across a large area. Information from seismic arrays is used to identify the source of a seismic event.
Groups of individual sensors deployed in a specific geometric pattern across an area ranging from a few to several hundred square kilometres are called “seismic arrays”. New seismic arrays built by the IMS usually have a distribution diameter of three to four kilometres. Older array stations, which have been upgraded by the IMS and incorporated into its seismic networks, may cover an area of up to 500 square kilometres.
Seismic arrays enhance monitoring capability for several reasons. They improve the signal to noise ratio. This means that it is easier to distinguish the actual signal against the background noise since it is filtered out. Array stations allow for a better estimation of the azimuth of incoming signals, i.e. they identify the direction from which the signal arrived. The spatial distribution of the sensors also permits an estimation of the seismic waves’ speed. Information on both the direction and speed of the incoming seismic waves is crucial when identifying the source of a particular event.