The coupling of seismic energy under vacuum conditions, such as the moon, using an untamped surface charge is different from coupling in air. In vacuum, the explosive gas blast and the detonation products continuously expand outward and interact with the solid surface. A series of model experiments was performed to investigate the effect of vacuum on coupling seismic energy. HNS charges of 0.2 gm each were detonated in contact with a plate and block of acrylic plastic in vacuum and in air. The amplitudes of the first and second arrivals (longitudinal and shear plate wave) are about 50 percent greater in vacuum than in air because the plate velocity (∼2.4 km/sec) more closely matches the gas‐blast velocity (∼3.5 to 7.5 km/sec) than the sound‐wave velocity (∼0.35 km/sec). When the charges are detonated in contact with the block to generate direct body waves, little difference is noted in the first arrival amplitudes in air and vacuum; suspending the charge one charge‐diameter above the surface produces about 25 percent lower first amplitudes in a vacuum. Large scale experiments were also performed in air to examine the effect of the detonation configuration on seismic coupling.
EXPLOSIVE SEISMIC SOURCES FOR THE MOON