civilian uses. The Preparatory Commission has started the building and verification of the system of which as of April 2011 about 80% was operational. Consultation and Clarification (C&C) States Parties to the Treaty are encouraged to conduct a Consultation and Clarification process (C&C) before requesting an on-site inspection. The state that has concerns about an ambiguous event should, whenever possible, make any effort to clarify it through consultations with the state in whose territory this event occurred, either directly or through the Organization. On Site Inspection (OSI) 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 In addition to the IMS, C&C and OSI, the verification regime of the CTBT includes also the fourth element of Confidence-Building Measures. This requires States Parties to the treaty to notify the Organization, if possible in advance, of any chemical explosion using 300 tonnes or greater of TNT-equivalent blasting material to be detonated. This is required in order to contribute