Nuclear Explosion Test Detection: The Seismic Technique

The nuclear explosive is emplaced in its test configuration inside a horizontal shaft excavated deep inside a mountain. The purpose of this test is to validate a new design concept which reduces the quantity of fissile material used while maintaining the original device yield of 20 kilotons equivalent of conventional explosives. The test director, upon receiving the numerous confirmations from both man and machine that all is properly configured and no anomalies are identified, initiates the firing signal to the computers, which relay the properly coded message to the device and initiate the test explosion and data recorders.

 

In a matter of microseconds, more than 95 percent of the fissile elements in the device have undergone their fission, releasing energy that a few microseconds before consisted of a small fraction of matter within trillions upon trillions of atomic nuclei. The energy heats the materials within the device to millions of degrees, generating enormous pressures and prompt gamma and x-ray fluxes which interact with the surrounding environment. The plasma thus produced expands spherically, engulfing all matter it encounters, transferring energy to the surroundings in an ever-increasing bubble of superheated material. As the bubble expands it vaporizes the nearby rock, cooling rapidly as it transfers its heat to the ever increasing volume of the surroundings.  As the cooling progresses, the surrounding rock begins to support a shock wave expanding in all directions and the volume begins to condense from vapor to liquid. At a distance determined by device yield and surrounding material mechanical properties, the shock front begins crushing the surrounding rock to powder, gravel and severely fractured rock as it slows to the speed of sound, dividing its energy according to the ever-increasing volume of material, the pressure dropping rapidly. At a certain radius from the center of the explosion, the pressure is no longer sufficient to crush and fracture the surrounding rock, and the shock now induces an elastic compressional wave spreading in all directions. This elastic compressional wave propagates at the speed of sound in the surrounding rock, and is the beginning of the seismic energy propagation which can be detected at enormous distances from the explosion, up to and including halfway around the world, or 180 degrees’ distance.

As an elastic wave, the wave front passes through surrounding solid material in much the same manner as sound in air. The compression moves the material radially away from the source then, as it passes, the material rebounds and falls back toward the source in a rarefactional motion. This compressional-rarefactional wave continues to propagate outward in all directions, until it encounters a medium of different density or incompressibility, often referred to as a discontinuity. Much like light through a water glass, the elastic wave will change direction, bending or refracting toward the slower propagating media, as well as reflecting away, splitting the energy into different directions. In seismic terms, the elastic wave we are describing is known as a ‘primary’, or ‘P,’ wave which can be detected and measured by a very sensitive seismometer at great distances from the source. 

The seismic ‘P’ waves are generated by energy releases in the rocks of the Earth, and the sources include natural earthquakes, mining activity, physical impacts, or nuclear tests. Their velocity in rocks of the earth varies with types of rock and depth of the rocks. At the surface, the wave front will propagate at approximately 4 km per second. As the wave reaches deeper into the earth, the velocity increases to approximately 20 km per second at the base of the earth’s mantle. Then, suddenly, the velocity drops to about 8 km per second as the wave front transits from the mantle to the outer core of the earth. This causes a strong refraction and reflection of the wave, which makes it change direction into the core, and away from the core back toward the surface of the earth. There are many effects caused by the wave propagation and discontinuities that can be used to accurately detect and measure the origin time and location of the source.

Modern seismometers are sensitive to and routinely record signals much smaller than a fraction of a nanometer, and are accurately timed to fractions of a second. With three components of motion measured, vertical, north/south, and east/west, an azimuth of arrival can be readily calculated, giving a direction to the source from each station. A network of these seismometers is in place around the globe, forwarding the recorded signals in near real time to central processing facilities, where the difference in arrival times between sensors and stations is measured and location, distance to source, and origin time are routinely calculated. With a maximum transit time of only 20 minutes for a ‘P’ wave to travel from origin through the center of the earth and be recorded at the antipodal point 180 degrees away, detection and location is rapidly accomplished. Three or more globally dispersed seismometer stations are required to accomplish the location anywhere on the globe; the more stations recording the event, the more accurate the location.  

For our hypothetical test described above, 37 stations around the globe automatically record the ‘P’ wave, submit the recorded waveforms to a central processing facility which make the signal detections and measurements for each station and assigns their attributes to a computer file.  A batch processing algorithm starting approximately every 10 minutes  recalls these attributes and uses them to calculate a 3-dimentional location on the globe, thus estimating the source origin. Yet another algorithm then provides the list of sources detected and measured, and publishes them on the internet within 2 hours.  The world thus will have been notified that an event, which may be, but not necessarily is, of nuclear origin has occurred, complete with origin time, location and magnitude, and a list of detecting stations. 

As an example of the sensitivity of the various networks that are currently in operation, the International Data Centre of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO IDC), located in Vienna Austria, detected and measured the North Korean event of 2006 with 22 of its global international monitoring system stations, including LaPaz Bolivia, some 16,000 km distant. The event, one of almost 100 other ‘natural’ sources recorded that day, was estimated by several other organizations as being of merely 400 to 600 tons of TNT equivalent (less than one kiloton), very small for a nuclear test device. CTBTOs first automatic computer analysis report was sent to member states less than 2 hours after origin of the event, followed by a final human analyst report about 2 days’ post event. These reports contain a lot of information estimates, such as origin time to a fraction of a second, latitude, longitude, magnitudes, estimated depth in the Earth as well as calculation error estimates. The character of the event source, nuclear or non-nuclear, is almost impossible with only the seismic technique. It requires association to a detection of radionuclides by a complementary technique, which will be discussed in a following article.

 

 

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