CHAPTER 1 GROUND MOTION FROM MIKE SHOT OF OPERATION IVY 11 PURPOSE Observation of the ground motion produced by Mike shot of Operation Ivy was undertaken primarily to furnish information concerning these effects for super bomb detonation, but its most useful purpose was expected’to Le the prediction of motion of instrumentation shelters and other structures for future tests in the Pacific Proving Grounds. Except for possible comparison with data from Easy and i __ Shots of Operation Greenhouse, it was not anticipated that ground motion of a coral atoll would correlate particularly well with or be significant to weapons effects in likely target areas. 1.2 a PLAN OF THE EXPERIMENT Project 6.5 of Operation Ivy was planned to furnish data concerning vertical, radial, and tangential components of ground acceleration at six stations ranging from about 8000 to 114,000 ft from Ground Zero of Mike shot. The depth of this instrumentation was to be determined by local subsurface conditions with the restrictions that gauges should be below the water table and, if feasible, well within a massive layer of coral rock or sand. These requirements were included to minimize effects of discontinuities close to the instruments. 1.3. CHOICE OF INSTRUMENT STATIONS Genera!location of instrument stations (i.e., the 650-stations) for ground-motion observations was dictated by location of Sandia Laboratory recording shelters (i.e., the 600-stations). Specific instrument locations were selected on the basis of subsurface conditions as determined by exploratory borings. The site map, Fig. 1.1, includes enlarged mapsof the islands on which ground- motion instrumentation was located. Ideally, end instruments for ground-acceleration measurement should be placed along a single radial line at such depth that they could be considered to be within a continuous homogeneous medium. Pertinent information on subsurface conditions at Eniwetok was limited to logs of several borings made on Engebi, Muzin, and Aomon during Operation Greenhouse. The original expectation that gauge assemblies could be placed within coral rock strata at least 10 ft thick on the reef side of the islands was abandoned after discussion with Holmes and Narver engineers and with geophysicists from the U. S. Geological Survey. Consequently it was decided 13 |