face in -hallow water. In addition these data were to provide for comparisons with a shallow underwater burst (Crossroads) and a deep underwater burst (Wigwam). At the same time, this operation provided an opportunity to check out instrumentation and obtain experience in making underwater measurements that proved valuable in preparing for Operation Wigwam. 2.6.1 Underwater Pressures. Three laboratories jointly participated in this project, under the sponsorship of the Office of Naval Research. Some difficulty with instrumentation due to repeated delays was experienced by each agency during the operational phase; as a result, a lesser amount of reliable data was obtained than originally anticipated. However, sufficient measurements were recorded fromthe five events to allow some conclusions to he drawn. The major result of the recorded data indicated that except for the close-in region, the maximum, or peak, underwater pressures were of the same magnitude as the airblast peak overpresgures at the same range. The maximum underwater pressures recorded were probably not due to the air-coupled shock alone, but included some of the seismic and the direct water-borne shocks as well. However, this comparison breaks down for the region close in to surface zero. The exact range wherethe dissimilarity of pressures becomes significant appears to be a rather-involved function of yicld, water depth, and relative depth of the target. Figure 2.10 reproduces typical pressure-time recerds. Ali records of this type followed a similar pattern: an initial disturbance followed by several! positive and negative pulses. followed by a slow-rising signal caused by the air-blast wave passing over the surface. This iatter arrival was confirmed by air shock~-arrival times. The initial positive disturbance, with its succeeding pulses, travelled with average velocities faster than might be expected for transmission of underwater shock, and it is believed they were transmitted through the ground and reflected from various subsurface strata. The values of pressure and time after zero were measured at each point labeled A, B, C, etc., and entered in Table 2.4. Figure 2.11 shows a plot of data obtained with two types of gages: the ball-crusher (BC) and the pressure-time (Pt). These data are a composite of measurements made on all shots and at various depths, and have been normalized to 1 kt. The included curve is the 2-kt composite free-air pressure-distance function, approximating a surface burst of 1-kt yield. The measured (scaled) data show a fair fit to the free-air curve. It was concluded that a nuclear device detonated on the surface of a relatively shallow water laver produces underwater pressures which are probably of small military significance, because: (1) although they are of comparable magnitude to the air-blast pressures, typical underwater targets ace, by their very nature, of such strength that they require pressures which are at least one order of magnitude larger than air pressures normally considered as damaging; and (2) they are insignificant compared to pressures produced by underwater bursts such as Crossroads Baker or Wigwam. These conclusions must bo qualified, however, since they are based on results ob- tained under the specific environment as experienced in the Bikini and Eniwetok Lagoons. Different conditions will probably produce different results. 2.6.2 Acoustic Pressure Signals in Water (SOFAR). The presence of a low-velocity sound channel at a depth of 700 fathoms in the Atlantic and at 350 fathoms in the Pacific is well known. Low-frequency sound channeling into this layer will travel great distances. It is also possible for sound to travel long ranges through the water by reflecting successivley from top to bottom of the ocean—both boundaries being excellent reflectors 35