Seismic technology is used preferably in the beginning of an on-site inspection as the number of seismic aftershocks after a nuclear explosion decreases rapidly. An underground nuclear explosion would create a cavity and changes in the geological surroundings. Following a nuclear explosion, geological structures at the site of the explosion will settle, causing minute seismic events with distinct seismic signatures that can be detected by what is called passive seismological monitoring.

Seismic field equipment is placed at several locations throughout the inspection area, creating a seismic network. The network is comprised of approximately 40 mini-arrays which consist of four seismometers and one digitizer each.  For a rapid analysis of the incoming seismic data, an on-site inspection will have a seismological centre at the base of operations.

Continuation Period Techniques
Continuation period techniques are mainly geophysical techniques, which will be used in those areas where findings indicate the need for further examinations.  The term “continuation period techniques” indicates that these techniques can only be used when suggested by the first inspection report.  The inspection team has to provide this report, which the Treaty terms the progress inspection report, no later than 25 days after the inspection was approved by the Executive Council.

The use of geophysical techniques is meant to detect, from the surface, changes in the geological structures and may reveal forensic findings.  These techniques include: magnetic and gravitational field mapping, ground penetrating radar, electrical conductivity measurements and active seismic surveys.

Magnetic field mapping
Magnetic field mapping measures deviations in the Earth’s magnetic fields that can be caused by different iron-containing objects in the ground.  The presence of such objects can point to infrastructure elements of an underground nuclear explosion, such as pipes and cables. Magnetic detectors can also help identifying man-made structures in the ground, such as foundations or shafts

Gravitational field mapping looks for changes in the density of the rock and can help locate the cavity created by an underground nuclear explosion. A cavity, or a void, represents a change in the density of the rock compared with surrounding rocks. The size of the cavity depends on various factors, such as the yield of the explosion, the depth and the surrounding geology. Depending on the rock type, a one kiloton explosion at a depth of 200 metres could cause a cavity of approximately 17 metres in diametre.