Monitoring of nuclear explosions and natural seismicity using the CTBTO seismic (PS24) and Infrasound (IS32) stations
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The Comprehensive Nuclear Test-Ban Treaty (CTBT) prohibits all nuclear weapon test explosions in any environment, and is the culmination of over forty years of effort. The first nuclear explosive test was conducted by the United States of America on 16th July 1945. In subsequent years, over fifty nuclear explosions were registered worldwide until 31st December 1953. In 1954, Prime Minister Nehru of India first expressed the vision of treaty banning all nuclear explosions reflecting the rising international concern over radioactive fallout and the escalating arms race. However, within the context of the cold war, skepticism over the technical capacities then available to verify compliance with a comprehensive nuclear test ban posed a major obstacle to any agreement. No adequate technology existed then to detect underground explosions.

Kenya being a state party to CTBT hosts two IMS stations that are operated by the Department of Geology in the University of Nairobi. The IMS stations acquire and transmit data to IDC in Vienna on a 24 hours basis. These stations include a primary seismic station (Treaty code PS24, site code KMBO) located at Kilimambogo Hill in Ol Donyo Sambuk National park and an Infrasound station (Treaty code IS32, site code I32KE) located at Karura forest.

The primary IMS seismic station was established in 2001 and became operational in 2002. The seismic station is installed with Broadband (0.01 Hz to 50 Hz), three channels (i.e. measures vertical, north-south and east-west ground movements) seismic sensor. The seismic station is located about 70 km to the north east of the city of Nairobi in Ol Donyo Sambuk National park. All the seismic data acquisition and power back-up equipment are located in a 40 meters tunnel which is subdivided into four compartments. A UPS system with 20 batteries for 24 hours power backup is located in the outermost compartment. The second compartment houses Global Communication Infrastructure (GCI) equipment, a workstation computer and a monitor. The fourth compartment houses the IMS seismic sensor and a Digitizer. This compartment of the tunnel also houses a humidity sensor and dehumidifier and the door is equipped with tamper switches. The seismic data is transmitted to the IDC from the workstation computer via GCI VSAT antenna which is located outside the tunnel. All the seismic data is time stamped and the times are in Universal Coordinated time (UTC). The sensor for this seismic station are meant for detecting underground nuclear explosions by detecting the seismic waves generated by such underground nuclear explosions. In addition, the sensor is also used to detect natural seismicity due to earthquakes. Owing to the very large number of naturally occurring seismic disturbances due to earthquakes and man-made explosions detonated during mining and quarrying operations, the seismic data needs to be supplemented with data from other technologies (e.g. hydroacoustic, infrasound and radionuclide).

The second IMS station in Kenya, the Infrasound station, is located about 10 km to the NNE of the city of Nairobi in Karura forest. Construction works for this station commenced in 2002. The station was completed and became operational in 2004. The infrasound station consists of seven array elements (sites). Each array element is installed with broadband infrasound detectors/sensors (microphones, microbarographs, microbarometers) which detect the sound wave originating from the initial shock wave generated by a nuclear explosion. The sensors acquire 2 channels of infrasonic data. However, at one array element, in addition to the two channels of infrasonic data, is also installed with meteorological sensors which acquire three channels of meteorological data which includes temperature, wind speed and wind direction. The significance of using an array of infrasound detectors/sensors is such that the direction of approach of the sound wave can be determined. Detection at two such stations enables a location of the source to be made by the intersection of the two back-bearings.  The equipment at each array site are powered by 12V DC power, which is converted from 240V AC mains power supply and charges batteries when the mains power is available. During times of mains power outage, the batteries then power the equipment at all array sites. The infrasound station is also equipped with a standby power supply generator which is syncronised with mains power supply and goes on after the mains power outage.

Infrasonic data from the seven array sites is transmitted via Fibre Optic Cables to a local centre, the central facility (CF). At the central facility, the IMS and GCI equipment are equipped with UPS which supply back up power. The IMS equipment is also equipped with a power stabilizer which regulates the incoming voltage of the mains power supply to the equipment. The data is then forwarded from a workstation computer at CF to a VSAT satellite circuit and then transmitted via satellite to IDC. All times are in Coordinated Universal Time (UTC).

Research Description by Dr. J.K. Mulwa


CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization)

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