A facility for Research in Seismology and Nuclear Explosion Monitoring.

Introduction

The seismological array located near Yellowknife in the Northwest Territories was installed in 1962. Since then it has recorded tens of thousands of seismic events worldwide, including most of the underground nuclear explosions detonated by the nuclear weapons states. The array is Canada's flagship installation for research on Canadian and global earthquakes, and is this nation's principal contribution to the international monitoring system that will be used to verify compliance with a Comprehensive Test Ban Treaty (CTBT).

History

The first multilateral negotiations on a CTBT took place in Geneva in 1958. These negotiations failed because the experts could not agree on the monitoring system to verify compliance with a ban on underground testing. The United States, the UK and a number of other countries immediately began research programs on the seismological detection and identification of underground nuclear explosions.

The UK research program was devoted to the development of seismological arrays, which act as seismic antennas with the capability to improve detection and to independently determine the location of a seismic event. Four such arrays with apertures of 25 km, designed for optimum detection of seismic events at distances from 3 000 to 10 000 km from the sensors, were installed by the UK in Scotland, Australia, India and Canada (Yellowknife). The predecessor of NRCan received funding and Cabinet approval to operate the Yellowknife array as a research facility for seismological verification. This work has now continued for more than three decades, and is currently carried out under the Nuclear Explosion Monitoring project in the Geological Survey of Canada.

An experimental version of the seismic monitoring system was put into operation in January 1995. The Yellowknife seismological array is one of the most sensitive of the 115 global seismic installations contributing to this system. In August 1995, the Yellowknife array was chosen by the expert group advising the negotiating committee as one of the primary components of this system.

Brief history of the Yellowknife Seismological Array.

The Array

A seismograph or a seismological array is most sensitive to distant seismic events if it is sited far from ocean coast lines and as far as possible from human sources of seismic noise, such as vehicle traffic and heavy equipment. An ideal site was found in 1962 on the outskirts of the town of Yellowknife.

The array has 19 short period seismic sensors in the form of a cross with 2.5 km between sensors, plus four broadband sites with special sensors which detect a wide range of seismic wave frequencies. Continuous measurement of the ground motion by the sensors is digitized at the site and transmitted by radio links to the control centre. At the control centre, all of the data are both stored on permanent optical disk archives and transmitted by dedicated ANIK satellite link to the Seismological Laboratory in Ottawa. In Ottawa, computers continuously monitor the incoming data stream searching for signals indicative of distant seismic events. When such signals are detected, processing of the entire array data set allows a time of occurrence, a seismic magnitude and a geographical location to be assigned to the event. With very small time delays due to data transmission and computing, staff in the Nuclear Explosion Monitoring project know these details of distant seismic events within minutes of their detection by the array. To support the development of the CTBT International Monitoring System (IMS), continuous data from the Yellowknife Array (YKA) and 4 other stations are forwarded in near real-time to the International Data Centre (IDC) in Vienna.