The Treaty lists specific techniques that can be applied during an on-site inspection. In following the principle that the inspection team should move from using less intrusive to more intrusive techniques, the various techniques are assigned to the different phases of an on-site inspection.

The Treaty lists specific techniques that can be applied
during an on-site inspection, starting with less intrusive
and moving to more intrusive techniques.

Initial Period Techniques

In the initial period until the first report is submitted, i.e. 25 days after the inspection’s  approval by the Executive Council, the inspection team can apply the following techniques: position finding, overflights, visual observation, video and still photography, multi-spectral imaging (including infrared measurements), gamma radiation monitoring, environmental sampling and analysis, and passive seismological monitoring of aftershocks.

Position finding
Position finding activities are particularly important at the outset of an on-site inspection as they help confirm the location and boundaries of the inspection area. They are also conducted at various times during an inspection since they facilitate the orientation and navigation of the inspection team within the inspection area.

Position finding activities help confirm the location and
boundaries of the inspection area. They facilitate the
orientation and navigation of the inspection team
within this area.

The equipment to be used is listed in the inspection mandate and may include satellite based positioning devices, such as GPS, and surveying equipment. When carrying out position finding activities, the inspection team will use some reference system based either on a geographical projection (using latitude and longitude projection) or a grid. Position finding may also include the mapping of landmarks or man-made changes on the surface, in order to help with the general orientation in the inspection area.


The inspection team has the right to conduct an initial overflight of the inspection area as soon as possible. The purpose of this overflight is to help the team get oriented in the area. It helps to narrow down the number of locations on the ground that need further inspection and to facilitate the collection of factual evidence. The initial overflight is limited to a maximum of 12 hours. The Treaty identifies the equipment that can be used during the initial overflight including field glasses, passive location-finding equipment (f.i. maps and theodolites), video cameras and hand-held digital photo cameras.

The initial overflight helps the inspection team get oriented
in the inspection area and identify locations on the ground
that need further inspection. Additional overflights can
only be conducted with the inspected State Party’s approval.

Additional overflights can only be conducted with the inspected State Party’s approval. The inspected State Party has the right to impose restrictions on the parameters of an overflight, such as flight altitude, in-flight procedures like circling and hovering, the number of inspectors on board or the type of measurements and observations. Subject to approval by the State Party, the inspection team may perform different activities during the additional overflights, including multi-spectral imaging, gamma spectroscopy and magnetic field mapping.

Visual observation

To attentively observe the natural environment in the inspection area with one’s own eyes is the starting point for visual observation. A skilled observer can detect anomalies in geological features or disturbances of vegetation that may point to a possible nuclear explosion in the underground. Visual observation can help the inspection team narrow down the inspection area or identify specific inspection activities that may be warranted.

Visual observation detects anomalies in the geology
and the surface, and helps the inspection team narrow
down the inspection area or identify techniques
necessary for the inspection.

The inspection team can use digital still and video photography to record their observations and findings. Ground based visual observation should be carried out in close coordination with overflights and should start immediately at the beginning of an on-site inspection. Visual observation and photographic recording will continue throughout the entire inspection.

There are very strict procedures for securing photographic evidence and records.  Both the inspection team and the inspected State Party are responsible for meeting these requirements. All related information - such as date, time, location and subject of each photograph or video recording - needs to be recorded in logs  Any further handling of images, including their processing and examination, is also strictly regulated.

In case additional overflights are agreed upon, the inspection team can proceed to perform multi-spectral imaging including infrared measurements and gamma spectroscopy.

Multi-spectral imaging (including infrared measurements)

This technology measures light at wavelengths around the spectra of visible light. The resulting images are then combined and may provide additional information about changes in surface, near-surface and sub-surface features.

Air-borne gamma spectroscopy
Used during additional overflights as well as during ground based surveys, this technology helps identifying gamma radiation emitting substances by measuring their energy and intensity. Elevated radiation can be caused by geological phenomena but also by man-made activities and may thus point to a possible nuclear explosion.

Gamma radiation monitoring, environmental sampling and analysis
In conjunction with visual observations, initial gamma radiation surveys help select regions within the inspection area for detailed investigation.  These surveys help identify gamma radiation that appears to be anomalous against the natural radiation background. During this phase, the inspection team uses predominantly vehicle mounted equipment, but may also use handheld devices for the detection of gamma radiation.

The inspection team conducts gamma radiation surveys
to identify anomalous radiation in contrast to
natural background radiation.

The inspection team collects environmental samples, such as soil, vegetation and water, for later analysis in a specialized laboratory. A very important on-site inspection activity is air sampling to detect radioactive particulates and radioactive gases. Air sampling is not only restricted to the surface air, but also extends to the shallow layers of the underground in order to sample gases that may have been trapped in the upper layers of the soil. This sampling has to be done as soon as possible to ensure that elements with a short half-life can still be detected.

Preferably all analysis of samples is performed in an on-site inspection field laboratory at the base of operations. When necessary, because equipment for some analysis activities is not available in the field, samples may be sent to laboratories that have been designated prior to the inspection.

Environmental samples from gamma radiation surveys
are preferably analysed using mobile equipment which
gives the inspectors more flexibility.

In some cases, the presence of radionuclides relevant for an on-site inspection can be identified immediately in-situ using a portable instrument.  The use of mobile equipment gives the inspectors more flexibility in the field, and measurements can be taken immediately and at several locations.

At a later stage and once additional overflights are approved, gamma radiation monitoring can also be done from the air.

As with photographic material, the inspection team must be meticulous when recording its findings and storing samples. As with other procedures, representatives of the inspected State Party can be present during all activities and have the right to receive copies of all reports and retain parts of the samples.

Seismological monitoring of aftershocks

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 monitoring is conducted at the early stages
of an on-site inspection to identify cavities and changes
in the geological structures caused by a possible nuclear

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.

Geophysical inspection techniques are used when
findings from the initial inspection period indicate
the need for further examinations.

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

Some techniques used during the continuation period
are aimed at identifying infrastructure elements of an
underground nuclear explosion such as pipes or cables.
Others help point to changes in the geological structures
caused by such an explosion.

Gravitational field mapping

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.

Ground penetrating radar

Ground penetrating radar uses electromagnetic waves to locate objects in the ground.  The radar emits electromagnetic waves into the ground where they may reflect on certain objects.  The reflected waves then allow for the identification of these objects and their location. Higher frequencies correspond to shorter waves.  And the shorter the wave, the smaller the buried objects that can be detected.  This technology is used to identify parts of an explosions infrastructure in the ground.

Electrical conductivity measurements

There are two distinct purposes of electrical conductivity measurements during an on-site inspection.  Shallow measurements, up to five metres depth, of underground electrical properties can help identify metallic objects belonging to the infrastructure of a possible nuclear explosion.  Deeper measurements will lead the inspectors to identify disturbances in the underground geological structures, such as cavities or changes in the water table, which, in both cases, may result from an underground nuclear explosion.

Active seismic survey

High resolution seismic surveys are conducted to identify changes and disturbances in the underground geological structures. During passive seismic monitoring, the inspectors “listen” to the ground, in hopes of detecting minute seismic events created by the movement of the rock adjusting to the stresses caused by the explosion.

Alternatively, inspectors may use active seismic sources to create shock waves.  Such a seismic source could be a hammer on a metal plate or a large vibroseis truck.  The latter is a specialized vehicle that vibrates a large plate on the ground to generate shock waves.  The reflected waves are then measured by a series of geophones, enabling the inspectors to establish an image of the underground geology.

During passive seismic monitoring, inspectors listen
to the sound of movements in the rock; during active
seismic monitoring, they use artificially created seismic
waves to listen to the waves’ reflections off structures
in the ground.

This technology could be used to identify and measure the effects of an underground nuclear explosion in the surrounding geological structures. Disturbances can relate to increased fracturing in the rock and its changing porosity.

Resonance seismometry

Resonance seismometry measures changes of seismic tremours in the underground. This so-called micro seismic background noise changes its properties when going through different kinds of material in the ground.  Such a change may be noticeable when these tremours move from rock to a void in the underground.  Hence, this technique could be used to locate cavities caused by a nuclear explosion.


Drilling is the final technique employed to ascertain whether or not a nuclear explosion has taken place.  This method is applied when other techniques have located a potential explosion cavity which needs to be examined for its nuclear nature.

As drilling is about obtaining samples from the site of the actual explosion and therefore carries potentially great health risks, the procedure to allow the use of this technique is rather complex.

Based on findings derived from already conducted inspection activities, the inspection team may propose drilling. The Treaty spells out how a request for drilling is dealt with.  The inspection team passes the request to the Director-General who in turn submits the request to the Executive Council.

Once a potential explosion cavity has been located,
drilling is conducted to obtain samples from the
site of the actual explosion.

A request for drilling has to contain information to support the request, such as information about disturbances on the surface of the proposed drilling location; analysis results from radiation measurement and environmental sampling; data from geophysical techniques; and any other information derived from the various inspection techniques and activities.

Drilling takes place exactly at the location of the cavity that was created by the nuclear explosion.  Hence, the geophysical methods already applied have to provide information about the exact location of this underground cavity.

Drilling takes place exactly at the location of the cavity
that was created by the nuclear explosion. Since samples
are expected to be radioactive, strict precautions must be

Drilling is used to sample the cavity that was created by a supposed nuclear explosion.  Material is extracted, solid or molten, from the actual location of the explosion.  It has to be assumed that the extracted material is contaminated, i.e. radioactive, and possibly still very hot.  Hence, stringent health and safety precautions are taken.  The obtained samples are then forwarded for further analysis.

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