—_ 218 resulted from energy propagated with a mean velocity of about 13 km/min. The first effect has veen postulated as due to a compressional wave and the second to a hydromagnetic wave. The fact that the first effect was seen ap- proaching but not receding is indicative of the shape of the ion-densit; variation associated with the disturbance. Two projects participeted during shot Umbrella to obtain effect data or feasabiiity of using nuclear weapons as a naval-mine counteremeasure, On? of them, sponsored by The Naval Ordnance Laboratory wes to determine the ranges at which typieal stockpile bottom mines would be neutralized by a shallow-veter burst. To set np the experiment, a total or 120 :nines,consisting of lines MKe25=2, 1K-39-0, 1K=-50-0, iKe52-1, 1K-5202, iK=-52-3, and 1K~52-6 were Ladd on the Eniwetck Lagoon, north of ground zero for shot Umbrella, at distances from ground zero from 1,400 to &,7.0O feet. Tuc oneration of 23 of the mines (planted at distances srecter than those at which damage wis expected) were monitored during the shot or means of a system cf internal recorders designed to beg*n recording when the mines were armed and to continue recording until the mines were recovered. between 120 and 150 fect. The depth of water at the mine field varied The results of the test indicate that: <p FV Rd The Navy Hine Defense Laboratory was the sponsoring agency of the other project participating in underwater experimentction. The overall objective of the project was to determine the feasibility of the employment of nuclear : LANL res CGFIED ‘LO | PGE . 77 / os EC