struments after corrections were made for instrument contamination in the mannerjust described and followed by application of the instrument calibration data discussed in Chapter 2 with reference to Figures 2.2 through 2.5. The relative local gammaintensity in milliroentgen per houris plotted normal to the ship’s track as base line in Figure 3.3. This is the actual ship’s track; and it is the relative local gammaintensity the ship intercepted. This is what was seen from the ship; it has little in common with the synoptic method of summarizing fallout. The values of the intensities which are shown graphically in Figure 3.3 are proportional to those tabulated in column 5 of Table 3.5, which will be discussed later. 3.3.3 Computation of Absolute Magnitude of In Situ Intensity. When instruments must be used to measure absolute gamma intensity inside a mass of water which contains radioactive sources emitting photons of several energies, an elaborate calibration procedure must be undertaken. A full calibration of Mark Hf has been made from data obtained in. the field, and data obtained by testing the instruments later against known radioactive sources, and from estimates of the spectral nature of the radioactive material in the fallout. Details of this calibration study have been put in Appendix C. With the aid of factors derived in Appendix B and the calibration curves of Figures 2.2 through 2.5, the value of absolute gammaintensity in milliroentgens per hour has been computed corresponding to each field measurement, and these local in situ values are plotted in Column 5 of Table 3.5. This quantity has been called $4 and has the units of milliroentgens per hour. 3.4 COMPUTATION OF SYNOPTIC PICTURE 3.4.1 Vertical Extent of Activity. In preparation for computations of a synoptic pic- ture, an estimate of the extent of penetration of the activity into the sea at any given time will now be undertaken. Numerous bathythermograph measurements taken in the area establish that the thermocline lay at about 100 meters depth during this period. The temperature discontinuity at the thermocline indicates that the water had been recently stirred to this depth, presumably because of forces originating in the winds. Such mixing presumably would force fallout material to progress downward ultimately to 100 meters even if the material had neutral buoyancy. There is considerable evidence that the upper layers of the sea mix to a state of homo- geneity, and it is known that transport by mixing becomes exceedingly small below the depth of the thermocline. However, the mechanism behind this surface mixingis still not well understood, and it has not been possible to predict the progress of mixing by oceanographic considerations alone. Fortunately, during this particular field operation, some actual measurements of the rate of penetration were acquired. These experimental data along with an estimate, made by NRDL, ofthe time at which fallout arrived at the sea surface permit computation of the progress downward of the contaminant. Table 3.1 illustrates some actual penetration measurements; it lists the readings from the Mark II instrument as it was lowered at Station Y- 1. Identical readings were made as the instrument was again raised slowly. The samedata is plotted (at the left with circled points) in Figure 3.4. It is believed that all the depths are accurate in Figure 3.4, except at those points indicated by crosses at the left and relating to a preliminary cast made by hand. On later casts a winch was used and a 150-pound weight was used to assure that the wire remained vertical. Figure 3.4 indicates an abrupt ¢ crease in activity at about 60 mcters depth at Station Y-1, and at the time the station +s occupied. Table 3.2 and the middle curve . Figure 3.4 summarize the results of lowering the Mark I instrument at Station Y ~ 2 three hours later. 32 Both stations were roughly the