The Microbarographic Detection of Nuclear Explosions

In order to study the atmospheric pressure perturbations produced by the passages of gravity and acoustic-gravity waves through the atmosphere, a sensitive rate of change instrument (microbarovariograph) has been developed at Lamont. Similar instruments of different design have also been in successful opertion at several other institutions here and abroad. In general, this instrumentation has also proven capable of recording the pressure variations from nuclear explosions in the megaton range for distances which depend on the magnitude and elevation of the initial explosion. The Lamont pressure transducer is a U-tube manometer using a stable liquid plasticizer, dioctyl sebacate, of very low thermal expansion, vapor pressure and creep. One side of the system is connected to a fixed insulated air reservoir or compliant volume. An adjustable leak in parallel controls the long period time constants. Fluctuations of the liquid level in the open end are sensed by a moving coil in a differential transformer whose 60-cycle output is directly proportional in magnitude to the level changes, provided such changes are small. The signal, amplified and demodulated, gives a sensitivity of about 0.030 millibars (30 dynes per cm2) per cm of scale deflection for a stop displacement. A DC post amplifier can give a sensitivity of about 0.001 millibars (1 dyne per cm2) per cm of scale deflection. At the higher sensitivity, special hydraulic filtering procedures must be employed on the transducer in order to reduce atmospheric noise. Broad band recording is best accomplished at the lower sensitivity.