Andrew A. Piacsek - piacsek@s62.es.llnl.gov
Shock Physics Group
Lawrence Livermore National Laboratory
University of California
P.O. Box 808, Livermore, CA, 94551
Summary of Paper 3aUW13
Presented Wednesday morning, May 15 1996
ASA Spring Meeting, Indianapolis, Indiana
Embargoed until 15 May 1996
The Comprehensive Test Ban Treaty (CTBT), currently being negotiated in Geneva, Switzerland, provides a framework by which signatory countries agree not to conduct nuclear tests. Part of that framework is an effort to monitor the earth to detect nuclear explosions. [1]
Underground explosions generate seismic waves that can be detected on other continents. Seismologists match signals from different receivers to backtrack and locate their source. The same principle applies for explosions at sea, since sound waves in the ocean can also travel great distances. Because locating the source in this case would not be sufficient to identify those responsible for the explosion, a remote ocean location may be attractive to a group wishing to conduct a clandestine test. However, ocean monitoring can be combined with other sources of information to reliably identify and attribute (lay blame for) nuclear explosions.
The CTBT ocean monitoring research program, coordinated and funded in the U.S. by the DOE, seeks to provide an understanding of how underwater signals can be interpreted to answer the following questions:
- Is the signal due to an explosion or an earthquake?
- If an explosion occurred, how large was it? Was it nuclear?
- Where and when did the explosion occur?
The program also provides technical advice for negotiators regarding the choice of monitoring equipment and locations.
Our group's effort at the Lawrence Livermore National Laboratory (LLNL) is focused on the near-source region (within 10-50 km, depending on the environment). We are using computers to model an explosion and the shock wave that it generates in the ocean. We propagate the wave for as long as nonlinear or boundary effects are important, then turn it over to other programs that will model propagation across an entire ocean basin.
One important result of our work is a description of the effect on the shock wave when the height or depth of burst is varied. If the explosion takes place underwater, the energy is well coupled to the water and a shock wave of very large amplitude is radiated away. In most ocean areas, this pulse will reach a special layer (about 1000 meters deep) that effectively traps acoustic energy; within this layer, called the SOFAR channel (for Sound Fixing And Ranging), acoustic waves can propagate thousands of kilometers with relatively little attenuation. Acoustic energy from an underwater explosion will also penetrate into the bottom, where it propagates seismically.
If the explosion is fairly deep (more than 50 meters), then an extra signal is generated by the giant gas bubble produced by the explosion. The bubble will expand and collapse several times, each cycle producing another strong wave. The presence of this extra signal is a clear indication that the source was an explosion. If the source is not sufficiently deep, however, the bubble breaks through the ocean surface, preventing an acoustically efficient bubble collapse.
If the source is above the water's surface, then much of the energy contained in the explosion is reflected; only a fraction is transmitted to the water. Even so, the amplitude of the water- borne shock wave is considerable. A somewhat surprising result of our calculations is that the amount of energy that reaches the SOFAR channel is fairly constant beyond a certain height of burst. Thus, even very high bursts will put enough energy into the water to be detectable at long distances.
We are also studying explosions in and above shallow water. Because the sound wave continuously interacts with the ocean surface and bottom, and because the water depth and bottom composition can change significantly within tens of kilometers, sound propagation in shallow water is quite complicated. It is our immediate goal to assess whether bursts in shallow water environments are likely to be detected with ocean monitoring.
Reference
1. DOE publication #NN-96005281, ''Nuclear Weapons Test Detection: Ensuring a Verifiable Treaty,'' 1995 Progress Report on the Comprehensive Test Ban Treaty Research and Development Program.
Related WWW pages
1. CTBT page at LLNL
http://www-ep.es.llnl.gov/www-ep/esd/seismic/ctbt-pub.html
2. CTBT R&D home page at DOE
http://www.ctbt.rnd.doe.gov/