 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.