Sage. eee t (or true crater) is of greater significance than the apparent crater. While-for small craters it is physically and economically feasible to determine the bouhdaries of both the "apparent"and tne "true craters,' for very large craters, the problem of excavation to determine the true crater becomes so extensive as tose 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 it was decided to .imit the crater surveys on CASTLE to the measurement of the apparent :raters formed by those detonations located at zero sites not used for prior detonations. Because of changes in shot Locations during the operation, the project effort was limited to Shots 1, 3, and 4, 1.4 BACKGROUND At an early stage in the planning «wo tecnniques were seriously considered in addition to those actually used. These were, first, the use of a high-power fathometer developed ty tne Navy Electronics Laboratory (NEL) which was considered to nave a reasonable probability of penetration of the layer of mud or aistarbed 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 or jetting tecnniques. Several methods of detecting the surface separating tne true crater from undisturbed earth were considered. The decision not to use either of these procedures was made on the bases, (1) that the drilling or jetting would add a large cost to the project, (2) that a penetrating fathometer would not be relieble without the supplementary information gained by the drilling or jetting, and finally, (3) that informat.on regarding the apparent crater would be very nearly as valuable for purposes of prediction of target damage as would measurements of tre tus crater. 1.5 THEORY The laws of similitude imply that the effects of an explosion of any (known) size in any medium are related 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 to some optimism that craters produced by such explosives car 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 extlosions on which crater measurements were made; namely, IVY Mike, JANGLE underground, end JANGLE surface. Second, the existing evidence leads to pessimism regarding the validity of scaling from conventional tc miclear explosion effects. om ES”, * ay me F 147 ~ t