that the gauges should be placed within the relatively homogeneous coral sand at least 5 ft below zones of cemented sand or fragmented coral blocks. These conditions were more likely to be realized on the various islands and were expected to provide reasonably consistent data. Nine borings for exploration and gauge installation were made during August 1952. Three were for exploration purposes at sites where no previous sutsurface data were available, and six Were for later use during gauge installation. Exploratory borings were extended about 5 ft below elevations chosen for gauges on the basis of conditions observed during exploration. This procedure prevented placing the gauges directly above a soil discontinuity. Sand samples and cores were taken in each boring, and logs of all borings were kept (Fig. 1.2). Instrument borings were cased with 10-in. pipe to the chosen gauge elevation, and the upper endag of the casings, projecting about | ft above the surrounding sand, were plugged and identified by station numbers (Fig. 1.3). Suitable conditions for instrumentation were encountered at depths between 16.5 and 17.5 ft at all stations. Surveys, made after all gauge borings were completed, established the coordinates and ground range of each 650-station. 1.4 INSTRUMENTATION End instruments used for ground-motion measurements were Wiancko accelerometers.' Three of these gauges were mounted with mutually perpendicular response axes in a watertight case (Fig. 1.4). The volume and weight of the assembly were adjusted to give it a density approximately equal to that of the coral rock at Eniwetok. This assembly was about 25 per cent more dense than the surrounding material as a result of the change from rock to sand. Carrier power and accelerometer signals were transmitted between end instruments and recording shelter by buried four-conductor shielded cables. The output of the accelerometers was amplified and recorded on magnetic tapes. A description of the recording equipment and its performance is included in reference 2. 1.4.1 Set Range and Calibration of Accelerometers Accelerometer set ranges were derived with the aid of Los Alamos Scientific Laboratory (LASL) memorandum J-9122, which included an estimate of the ground acceleration as a function of ground range, This estimate was based on scanty data from _ushot of Operation Greenhouse’ scaled to the anticipated Mike yield. Energies were assumed tto scale as the square root of the yield, and the very low burst height of Mike was assumed to make the fraction of energy coupled to the ground 50 times greater than it was for The magnitude of this factor (50) was questioned, but, since no data were available for verifying the scaling procedure, the LASL estimate was used. Individual accelerometers were chosen in response ranges slightly above the estimated set range. All were damped approximately to 0.65 critical, and response over their full range was within 2 per cent of linearity. Each accelerometer was calibrated to set range in the field on its assigned recording channel shortly before installation of the gauge assembly in the ground. Details of the calibration procedure and calibration-signal circuitry are given in reference 2. 1.4.2 Installation of Instruments Tidal fluctuation of the ground water caused invasion of loose sand through open bottoms of the cased instrument borings, essentially filling all holes to the water line during the three, month period between drilling and gaugeinstallation. It was necessary to clean out all holes preliminary to placing the accelerometers. Gauge assemblies, as shown beside the casing in Fig. 1.3, were placed and oriented with respect to Ground Zero by meansof positioning pipes and were bonded to the surrounding sand by cement grout. Casings were removed from each hole after the grout had set, special care being taken not to disturb orientation of the gauges.” Water seeped into two of the gauge units before Mike shot. One of these units, at Station 650.02, was removed, rehabilitated, and replaced in satisfactory operating condition. The other, at Station 650.04, could not be similarly reconditioned becauseall drilling equipment had been 15