If an event detected by the IMS (or by other means) raises concerns about violation of the basic obligations of the CTBT, an OSI may be conducted to clarify whether a nuclear explosion has taken place. Such an inspection could take place only after entry into force of the Treaty, and would require agreement by at least 30 of the 51 members of the CTBTO's Executive Council. An inspection area of up to 1000 square kilometres would be searched by a team of inspectors (up to 40). Only State Parties to the Treaty may submit a request for an OSI. When conducting an OSI, a number of detection techniques can be used. These techniques include position finding, visual observation, passive seismic measurements and radioactivity measurements including gamma radiation and radioactive noble gases such as
argon-37 and
isotopes of xenon for an initial period of up to 25 days. Further, for a continuation period of up to 60 days, more intrusive measurements can be used on-site including active and resonance seismic measurements as well as
ground penetrating radar, gravity, and electric and magnetic field mappings. Argon-37 field measurement is a unique technology specially developed for the purpose of OSI. Drilling to obtain radioactive samples from a suspected underground explosion site is also allowed. Data collected from various methods have to be fused and interpreted for decision making purposes. An important task of the CTBTO is to explore how recent scientific and technical advances in these technologies can be applied to an OSI. ==Confidence-building measures==