are outlined in this chapter. Chapter 3 discusses the research activities necessary to enhance U.S. CTBT monitoring capabilities.

Nuclear explosions can occur in space, the atmosphere, underwater, and underground. For monitoring sources in each medium the basic problem can be posed in terms of source excitation of the signals of interest, propagation of those signals through the various media, recording the signals, and ensuing signal analysis to detect, locate, and identify the source.

There have been 514 nuclear tests in the atmosphere and space conducted by five different nations (Adushkin, 1996). In an atmospheric nuclear explosion, a fireball is produced in the first fraction of a second after detonation as a result of the interaction of the atmosphere and the initial x-ray emission from the device. The thermal energy is reemitted in the visual and infrared spectrum, with the fireball light history involving a double flash (a rapid-duration flash followed by a longer-duration flash) that forms the basis for distinguishing nuclear explosions from chemical explosions and lightning. The pattern of light can be detected by satellite optical sensors (so-called bhangmeters) and is highly diagnostic of the source type. The atmosphere absorbs most other types of radiation (e.g. x rays, gamma rays, neutrons, beta particles) from low-altitude or below-surface tests, preventing them from being observed by remote sensors (DOE, 1993).

High-altitude explosions between 15 and 80 km produce large fireballs due to the thin atmosphere, with high visible light intensity. Above 80 km, the fireball is largely the result of ionized debris (DOE, 1993). As the atmosphere thins in approaching deep space (altitude > 110 km), the optical effects of the fireball diminish, but there is a corresponding increase in x-ray and gamma-ray emissions that can be monitored by upward-looking sensors on orbiting satellites. Explosions in the atmosphere produce ionized plasma in the source region that has a strong electromagnetic pulse (EMP) capable of disrupting communications over a large area and being monitored by EMP detectors on satellite systems. The DOE is currently developing a new generation of EMP detector for deployment as part of the U.S. NTM for atmospheric monitoring. However, numerous large lightning bolts as well as human-generated broadcasting activities also produce EMP signals that must be differentiated from explosion signals.

A nuclear explosion in the atmosphere will also produce acoustic signals (sound waves) that travel through the air at about 300 m/s, with energy concentrated in the 20 to 500 mHz range. Only a tiny fraction of the explosion energy is transmitted as sound waves, but testing experience has empirically been able to characterize the sound level as a function of explosion yield. This relationship, together with the background noise, provides a basis for defining infrasonic detection thresholds. Sounds transmitted through the atmosphere have predictable propagation properties influenced mainly by the vertical thermal and density structure of the atmosphere, which is quite well known. Secondary effects of wind patterns on the sound intensity are predictable if the wind patterns at various altitudes are known. IMS infrasound stations are arrays of surface atmospheric pressure sensors designed to record these signals.

In this period range, several natural phenomena can produce a background noise level. These include long-duration signals with a frequency of about 200 mHz, which are ocean wave-generated ''microbaroms." Short-duration impulsive signals more likely to be confused with explosion signals are generated by volcanic blasts, meteor falls, sonic booms, and auroral infrasonic waves propagating beneath supersonic auroral electrojet arcs (Figure 2.1).¹⁷ Historical experience with atmospheric monitoring suggests that at most a few events per day will be recorded at more than one of the IMS infrasound stations, whereas a 1-kiloton (kt) explosion will typically be observed at a large number of stations. Relative to the seismic problem, for which on the order of 100 events per day (or even more in the case of large earthquake aftershock sequences) are large enough for the IMS

Front Matter (R1-R12)

Executive Summary (1-6)

1 Introduction: The Comprehensive Nuclear Test Ban Treaty (7-22)

2 CTBT Monitoring Technical Challenges That Drive Research (23-48)

3 Monitoring Technologies: Research Priorities (49-82)

4 U.S. Research Infrastructure (83-88)

5 Conclusions and Recommendations (89-96)

References (97-100)

Appendix A: Statement of Task (101-102)

Appendix B: Research Support History (103-106)

Appendix C: Seismic Event Location (107-112)

Appendix D: Seismic Magnitudes and Source Strengths (113-120)

Appendix E: Hydroacoustics (121-124)

Appendix F: Infrasonics (125-130)

Appendix G: Radionuclide Source Term Ranges for Different Test Scenarios (131-136)

Appendix H: Acronyms (137-138)