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From FY 1986 through FY 1988, we monitored extremely low-frequency (ELF) electromagnetic (EM) fields to determine whether these signals could be detected from underground nuclear explosions. Signals clearly related to underground tests were detected only when the ELF field sensors were located within 10 km of surface ground zero. Theoretical analysis, based on the results of these measurements, shows that the ELF impulse signals from underground nuclear tests are of longer duration than those from lightning sources and are, therefore, less efficient in exciting resonances in the earth-ionosphere cavity, even though the source strength for each may be the same. Thus, ELF signals from underground nuclear tests with yields of < 150 kT are generally lower than the background signals caused by worldwide lightning activity. Our conclusion is that ELF monitoring probably will not be useful for detecting underground nuclear tests at distances >10 km from the tests.

[1] We present experimental results of a study of electromagnetic field generation during underground detonation of high explosive charges in holes bored in sandy loam and granite. Three components of electric field (vertical component in air and two horizontal components in the soil) and three components of the magnetic induction were recorded during the field experiments. Test conditions and physicomechanical properties of the soil exert significant influence on the parameters of electromagnetic signals generated by underground explosions with masses of 2-200 kg. The electric and magnetic field experimental data are satisfactorily described by an electric dipole model with the source embedded in layered media. We used the solution for a field produced by stationary vertical and horizontal electric dipoles placed near the interface between two layers with different conductivity. The magnitude of the field source was estimated on the basis of the records of electromagnetic signals obtained at different distances from the borehole. For an underground explosion of a TNT charge with a mass of 2 kg carried out in granite the maximum estimated value of the electric dipole component is about 10 À7 C m. This estimate is more than an order of magnitude greater than that obtained for an explosion of the same mass carried out in sandy loam.

The ability to calibrate seismic stations to improve the monitoring of the Comprehensive Test Ban Treaty is partially limited by the availability of seismic events with known locations and source properties. To confidently extrapolate from these events to aseismic regions, and to properly account for discontinuities in seismic properties requires accurate geophysical models. This paper lays out a preliminary, first-order, regionalization of the Middle East and North African (MENA) region. The model specifies boundaries and velocity structures based on the geology and tectonics of the region, previously published studies, and empirical data observations by the LLNL group. This model is a starting point and is expected to be improved and refined by comparisons with ongoing tomography efforts and the collection of new data. We anticipate that this model and its successors will prove useful as a background model in the process of forming station calibration maps based on intelligent interpolation techniques such as kriging. We also hope the model, as it improves and demonstrates some predictive power, will provide a reference model for broader CTBT research efforts in detection, location and discrimination as well as other aspects of earth science.