Testing seismic aftershock measurements
at "Ore Mountain" in Sweden
It's all in the name. Located above the Arctic Circle, the Swedish town of Malmberget - literally "ore mountain" - sits atop several big iron ore deposits. About ten of these deposits are currently being mined at an average depth of 1000 metres.
These mining activities at Malmberget by the company LKAB - Luossavaara-Kiirunavaara Aktiebolag - recently provided a perfect setting for an on-site inspection exercise by the CTBTO, the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization.
The CTBTO develops means to monitor compliance with the Treaty banning all nuclear explosions, the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Integrated into the CTBT verification regime, these means are meant to ensure that no nuclear explosion goes undetected.
Once the Treaty enters into force, on-site inspections are the ultimate verification measure to clarify whether or not a nuclear explosion has occurred. In a number of exercises, CTBTO experts, along with specialists from Member States, test the various techniques, methods and equipment that will potentially be used in an on-site inspection.
During the exercise at Malmberget in August 2007, a group of 15 experts from Member States and the CTBTO looked at seismic aftershock measurements as one of the on-site inspection techniques. Project leader Ruth Murdie of the CTBTO said that the organization had just received new equipment, including computers and software. "All of this equipment needs to be tested in a realistic setting", she added.
But why is it necessary to venture into an underground mine to do that? It is because some of the geological effects of mining blasts are strikingly similar to a potential clandestine underground nuclear test. The recent exercise in Sweden was built on these similarities.
The iron ore at Malmberget is deposited in almost vertical seams with diameters of up to several hundred metres. Blasting therefore is set up in a vertical fashion in tunnels at various depths. A blast not only creates big cavities but also destabilizes the surrounding geological structures.
Readjustment of the stresses in the roofs of these cavities results in a continued cracking of the rock which may cause these cavities to implode. "This is what we listen to. The cracking and the falling rocks create distinctive seismic signatures", Rainer Arndt, a senior scientist working at the CTBTO, said.
It is understood that a potential underground nuclear explosion would create very similar results. The blast would create a big underground void, and smaller pieces of rock would continue falling from the roof of the cavity. On-site inspection aftershock measurements concentrate on detecting the signals emanating from these typical effects.
LKAB continuously monitors the aftereffects of its blasting activities, using around 30 seismometers in the area of Malmberget to detect changes in the geological stresses. During the exercise, LKAB set off a number of small-calibration blasts and recorded them with their own monitoring system.
CTBTO experts listened in with their seismometers and were able to compare the results. "We knew exactly where the calibration shots were", Ruth Murdie said, "so we should be able to verify whether the equipment is working." The comparison of results will allow for a good evaluation of the equipment, its functioning, its sensitivity and its accuracy.
A few months after the exercise, Ruth Murdie and Rainier Arndt are still occupied with the analysis of the findings. Much of the work now focuses on the respective software that is being used to analyze the recordings. "The testing of hardware and software helps us to identify error margins and gives us the chance to optimize our software", Rainier Arndt explained.
What sounds easy and straightforward can create quite a few challenges for the scientists, the varied provenance of the equipment certainly being one of them. The CTBTO is currently using equipment of three different systems for on-site inspection aftershock measurements. Hence, there is a need to render all this equipment and the respective software compatible.
The various elements that are being tested by the CTBTO's On-Site Inspection Division are all part of the preparations for an all-encompassing exercise next year, the Integrated Field Exercise in Kazakhstan. A complete on-site inspection will then be simulated. It will help to identify remaining work to be completed before the Treaty's entry into force.