Explosive model: MRI mathematics pinpoints bombs

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  • Published: Sep 1, 2013
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Explosive model: MRI mathematics pinpoints bombs

Saucy model

Sandia National Laboratories researcher Sandy Ballard and colleagues from Sandia and Los Alamos National Laboratory have developed SALSA3D, a 3-D model of the Earth’s mantle and crust designed to help pinpoint the location of all types of explosions (Photo by Randy Montoya)

A three-dimensional model of the Earth's crust and the mantle beneath it can be analysed using the same equations as are used to analyse and interpret magnetic resonance data for display of an MRI. Instead of revealing the presence of a brain tumour or damaged tissues following injury the equations can be used to illuminate the material properties of the Earth's interior which can help to reveal the precise locations of explosions across the globe.

The system - SALSA3D, or Sandia-Los Alamos 3D - developed with support from the US National Nuclear Security Administration's Office of Defense Nuclear Nonproliferation R&D, Sandia National Laboratories and Los Alamos National Laboratory will assist the US Air Force and the international Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in Vienna, Austria.

During the so-called Cold War and beyond international monitoring agencies were well versed in detecting the nuclear tests being carried out by nation states and perhaps others. Seismic activity unaccounted for my earth tremors and earthquakes as well as spikes in atmospheric radiation levels allowed them to home in on the site and measure the scale of any underground test detonation. Throughout that period of modern history there were just such spikes revealed quite frequently, it added to the frisson of mutually assured destruction (MAD), which was thought to be the only thing that kept the peace and held off World War III. Today, international monitors are not expecting to see large-scale nuclear weapons testing, but still have a significant need to pinpoint much smaller explosives tests.

Constructing a model

Sandia's Sandy Ballard and colleagues explain that the model uses a scalable triangular tessellation and seismic tomography to map the Earth's compressional wave seismic velocity. This property of the rocks and other materials comprising the crust and mantle gives scientists an indication of how quickly compression waves travel through the material and so can be used to accurately locate seismic events. Compression waves - measured first after seismic events - move the particles in rocks and other materials minute distances backward and forward between the location of the event and the station detecting it. The team adds that SALSA3D also reduces the uncertainty in the model's predictions, which is important to those in a position of power where suspicious activity by "rogue" states or terrorists may have been detected and decisions regarding an appropriate response are to be made.

Uncertainty

"When you have an earthquake or nuclear explosion, not only do you need to know where it happened, but also how well you know that. That's a difficult problem for these big 3D models," Ballard says. "It's mainly a computational problem. The mathematics is not so tough, just getting it done is hard, and we've accomplished that." The Sandia team has been working on and refining the necessary computer code for the model since 2007 and has recently demonstrated its much greater precision than possible with other current models. Indeed, in recent tests, SALSA3D was able to predict the source of seismic events over a geographical area that was 26 percent smaller than the traditional one-dimensional model and 9 percent smaller than the recently developed Regional Seismic Travel Time (RSTT) model.

The Sandia lab recently released the framework for SALSA3D - the triangular tessellated grid on which the model is built - allowing other Earth scientists, seismologists and the public direct access to the tools. By standardizing the framework, the seismological research community will be able to more easily share models of the Earth's structure for scientific research. Moreover, global monitoring agencies will now be able to test the many different models produced by academic and government research institutions more effectively. The team's GeoTess framework makes all the different models and their output compatible with each other and standardizes everything Ballard says. "This will really facilitate sharing of different models, if everyone agreed on it," he adds. GeoTess is not specific to any particular data, so users have considerable flexibility in how they store information in the model.

"We hope to include surface waves and gravity constraints to our model in the coming year," Ballard told SpectroscopyNOW. "Our current model is based only on compressional body waves. While compressional body waves are the attribute that is of primary interest, there are many parts of the globe that are not well sampled by available body wave data. Surface waves and gravity are indirectly dependent on compressional wavespeed, and sample parts of the Earth not sampled by body waves, and hence can be useful for constraining global models."

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GeoTess

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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