(or true crater) is of greater significance than the apparent crater. While for small craters it is physically and economically feasible to determine the boundaries of both the “apparent” and the "true craters,” for very large craters, the problem of excavation to determine the true , crater becomes so extensive as to be impractical. The difficulty of measuring the true crater becomes even more severe under circumstances where the crater is water-filled and where the level of radioactivity remains for some time high enough to prohibit extensive work. Both of these situations existed in CASTLE. Because of these difficulties and other considerations 1t was decided to limit the crater surveys on CASTLE to the measurement of the apparent craters formed by those detona- tions logated at zero sites not used for prior detonations. Because of changes\, m shot locations during the operation, the project effort was limited +o,Shots 1, 3, and 4. 1.4 BACKGROUND At an early. stage in the planning two techniques were seriously considered in addition to those actually used. These were, first, the use of a high-power fathometer developed by the Navy Electronics Labora- tory (NEL) whichwas considered to have a reasonable probability of penetration of the layer of mud or disturbed earth separating the apparent from the true crater. The second technique was designed to supplement the penetrating. fathometer as a means of determining the true crater. This technique‘involved the production of holes through the crater either by drilling orjetting techniques. Several methods of detecting the surface separating the true crater from undisturbed earth were considered. The decision not to use either of these procedures was made on the bases, (1) that the: d@piliing or jetting would add a large cost to the project, (2) that apenetrating fathometer would not be reliable without the supplementarf¥,in ormation gained by the drilling or jetting, and finally, (3) that ormation regarding the apparent crater would be very nearly as valuable for ipurposes of prediction of target damage as would measurements of the pe crater. 1.5 THEORY i The laws of similitude imply that the effects of an explosion of any (known) size in any medium are releted-precisely to the effects of an explosion of any other size in the same medium, provided the medium fulfills certain rather stringent conditions. Experimental measurements using conventional explosives such as TNT lead tp some optimism that craters produced by such explosives can be predicted with an accuracy almost entirely adequate for military purposes, even though it is clear that some properties of the medium (earth) in which the explosive is fired are. very sensitive parameters in affecting the”‘crater. The situation regarding craters produced by nuclear explosives is less satisfactory. First, the evidence is meager, since,:‘prior to CASTLE there have been only three such explosions on which crater measure-~ ments were made; namely, IVY Mike, JANGLE underground, andJANGLE sur-~ face. Second, the existing evidence leads to pessimism regarding the validity of scaling from conventional to nuclear explosion effects. 14