CHAPTER 1 INTRODUCTION La“emer The! immediate objective of the crater survey, was to determine the dimensions ofthe apparent craters formed by Shots 1, 3, and 4. The long-range objectives of the work were to obtain data to assist in the prediction, for military purposes, of the crater produced by any large nuclear weapon fired under any circumstances. ‘Two situations were of particular interest in this regard in Operation CASTLE. These were (a) a surface burston land, and (b) a surface burst in relatively shal- low water. Le, 1.2 MILITARY SIGNIFICANCE, The major military.‘interest in craters stems from the observation that the limiting distance of important damage to well constructed under- ground fortifications lies only a relatively short distance outside the crater. For the prediction of.Jbych damage it is clear that the shape of the crater near the rim is”‘more.Amportant than its shape, or depth, near the center. Of somewhat less military inPeas t is the crater produced by a surface shot in shallow water. Both iolimiting distance of damage to tunnels and the possibility of damming harbor by the formation of a crater with a shallow, or above-water 1 Bp, are matters of some concern. gE 1.3 LIMITATIONS ON THE OBJECTIVES In the investigation of craters formed pybmaller explosions it has been recognized that while the crater surfaceapparent to the eye was relatively easy to measure, there was neverthelessa disturbance in the earth, caused by the explosion, to some depth beldw this upper surface. The lower boundary of this volume of disturbed earth has become known as the “true crater" in contredistinction to the upper surface, which has been called the “apparent crater." While the term’“true crater" may be slightly misleading in its implications, it seems reasonably clear that for the purposes of determining the limitations of.damage to under- ground fortifications the lower surface of the volume of disturbed earth 13