BACKGROUND Weaponseffects studies concerned with damage to structures include observations of ground motion, to certain aspects of which underground structures are sensitive. Limitations imposed by instrument characteristics have usually restricted the measured parameter to acceleration. Velocity and displacement information has been derived by iterated integration of acceleration-time data. Direct measurementof velocities or displacements is feasible for small-charge experiments wherein durations of phenomena can be resolved by practical gauges. Energy from an explosion is coupled to the earth either directly or as a secondaryeffect through ground incidence of an air shock. Direct coupling is most obvious in underground bursts in which secondary coupling is usually negligible because energy in the air shock is relatively smaller. Surface bursts involve both direct and secondary coupling, the first being probably predominant close to Ground Zero, where the two effects are essentially undifferentiable. At more remote distances from Ground Zero, the directly coupled energy effects outrun those derived from local incidence of air shock and become separated in time. Secondary air-coupled effects are propagated at velocities characteristic of air-shock velocity, which decreases with distance, approaching acoustic air speeds. Transmission of directly coupled effects through the ground is at seismic velocities, which are generally several times acoustic air velocities and tend to increase with ground range because of refraction through deeper, higher velocity strata. Effects from directly coupled energy are attenuated by angular dispersion as well as by frequency dispersion. Ground-acceleration frequencies are consequently relatively low and probably decrease with ground range. The lower frequency portion of the motion becomes stronger in the particle velocity data and often predominates in the displacements. Secondary coupling effects, on the other hand, result from a shock wave which retains a steep front and is attenuated less rapidly than the ground-transmitted motion. These effects are characterized in general by relatively higher frequencies and peak accelerations than the directly coupled motion, but resultant particle velocities and displacements may be comparatively small because of the short periods of the frequencies invo)]ved. Ground motion from explosions centered well above the ground involves only air-shock coupling. Effects caused by incidence of air shock in the immediate vicinity of a measurement Station predominate at ali ground ranges. At stations remote from Ground Zero, effects derived from incidence of the air shock at or near Ground Zero may be distinguishable from those caused by local incidence, but the ground-transmitted signals are usually negligible because attenuation due to coupling, dispersion, and transmission over large distances within the earth greatly exceeds that to which the locally incident air shock has been subjected. Measurements of ground motion produced by high explosives, usually represented by accelerations, have been included in the Office of Scientific Research and Development Underground Explosion Effects program,' the Underground Explosion Test Program at Dugway 11