Chapter 3 RESULTS OF SURFACE AND SUBSURFACE SURVEY BY RADIATION DETECTING INSTRUMENTS — The in situ radiation intensity measurements described in Chapter 2 and the laboratory measurements of the radioactivity of water samples collected during the cruise afford two independent means for assessing the fallout. Water analyses were undertaken by NRDL; while the direct gamma measurements were evaluated at SIO with the aid of cali- bration data supplied by the U. S. Bureau of Standards for this purpose. The water analyses are to be discussed in Chapter 4, and in Chapter 5 results of both methods will be compared. This present chapter describes how the direct gamma measurements were resolved into a synoptic picture. 3.1 PROBLEMS INVOLVED IN COMPUTING A SYNOPTIC FALLOUT PICTURE A great many individual readings can be accumulated when a ship tows a gamma detector through water contaminated by fallout material. But to reduce these readings to any form of synoptic picture requires the introduction of information or assumption concerning the behavior of the fallout material after it arrived at the water surface. A slow ship sees the activity only after several agents have been acting for many hours; the de- bris has been moving downward and moving laterally, and it has undergone radioactive decay. Before a picture of what might have existed at any given time can be reconstructed, the time of arrival of fallout must be established, and also the rates of dispersal and of decay. Fortunately, there is available from other sources enough information to make rough estimates of the progress of the activity in the sea; some of it comes from auxiliary measurements made during the cruise, some comes from other oceanographic and radiological sources. In this chapter, the raw field data will first be presented; then these will be converted to consistent units (mr/hr) by application of correction and calibration data. The local data will then be used to compute a local dose rate which right have existed at 3 feet elevation if the fallout had been caught on a hypothetical fixed plane at the elevation of mean sea level. All the local dose rates will be reduced to the rate at synoptic time H + 1, and also at H + 12, and finally these synoptic dosages will be displayed in contour maps. 3.2 RAW MEASUREMENTS OF SURFACE GAMMAINTENSITY Figure 3.1 presents the running record of raw measurements made by towing the instruments Mark I, Mark H, and Mark Il behind the ship. Readings of the microammeters were made as frequently as every 5 minutes during a large part of the cruise. Two or more instruments were towed simultaneously, whenever possible, so as to give warning of instrument failure and to provide data of correcting for instrument contamination. Stations are identified on this graph by numbers and by asterisks. It should be noticed that roughly 2 hours cruise time were expended at stations where the deep hydrographic 26