Column
9
Is the probe reading from Column 7 corrected for radioactive decay toH+1 hour.
The
factors used for this correction are shown in Figure 2.34.
Column
10
Lists the time of fallout arrival for each point of observation.
The derivation of this
column involves the use of several of the figures already presented and is derived at the same time the
ship's tracks are corrected for currentdrift.
‘
The geographic distribution of arrival time is shown in Figures 2.21 through 2.24. The deduced ocean
currents are shown in Figures 2.25 through 2.28. For each shot, these two figures are overlaid and on
top of them, the appropriate figure of the ships’ original tracks, Figures 2.6 through 2.9.
Any point ona
ship's tracks is then moved back along the current streamline, a distance corresponding to the current
velocity and the number of hours between time of observation and time of fallout arrival. If this shift results in a corrected position that does not correspond to the time of arrival that was used in determining
the amount of shift, a new time of arriva) is used.
position and time of arrival correspond.
This tria] and error is continued until] the corrected
This process results in an arrival time, as listed in Column 10,
and a corrected ship’s track, shown in Figures 2.29 through 2.32, which corresponds to the path the ships
would have followed had there been no ocean currents in the fallout area.
Column 11 Is the difference between Columns 8 and 10. This is the number of hours thefallout has
been in the water, up to the time of measurement. It is used for correcting the ships’ tracks as described
above and for determining the depth of penetration at the time of measurement.
Column
12
Is the depth of penetration of fallout (mixing depth) at the time of measurement.
It is
derived by applying the times listed in Column 1] to the fallout penetration curves shown in Figures 2.17
through 2.20
Column 13
Is the dose rate in roentgens per hour that would be received at an elevation of 3 feet
had this same fallout occurred on an infinite hypothetical plane at H+1.
This is derived by multiplying
Column 9 by the depth of penetration (Column 12) and by the conversion factor of 0.01 derived in Appendix A.
The values listed in this column are plotted along the corrected ships’ tracks.
then outlined, resulting iso-dose-rate contour lines at H+1 hour.
Column
14
Areas of equal intensity are
Results from the application of the times listed in Column 10 to the conversion factors are
shown in Figure 2.35. These are used to calculate the total dose accumulated during the first two days
after detonation.
Column 15 Lists the accumulated dose, in roentgens, between time of fallout and H+50 hours. This
is derived from the products of Columns 13 and 14 and is a fairly realistic presentation of the actual hazard
from fallout, since it takes into consideration the fact that fallout does not occur simultaneously over the
entire area. These values are also plotted on the corrected ships’ tracks and presented as accumulated
dose contours.
2.6.16 Fallout Surveys, General. The final presentation of the oceanographic fallout surveys
is in the form of contours of H+1 hour iso-dose-rates and accumulated total dose (time of arrival
to H+50 hours) that would be received at 3 feet above an infinite hypothetical plane. The methods
used to derive these contours has just been presented.
The reliability of the final contours is
dependent upon the measurements and factors that were used in the data reduction. Therefore,
these factors will be reviewed before the presentation of the individual survey results:
1.
Probe
surface
measurements.
These measurements have been carefully cross
checked between the three survey ships and are in agreement within 5 percent.
In addition, the
probe contamination has been determined for all measurements and subtracted from the probe
readings.
The accuracy of the probe measurements is estimated to be +15 percent of the cali-
bration value.
2. Radiation background of ocean water. As the operation progressed, the
background of the surface water increased. A background survey prior to Shot Navajo showed
that a wide expanse of ocean had retained a measurable amount of radicactivity from previous
shots.
This was a small increase and amounted to no more than 30 percent of the Navajo read-
ings, even in the extreme case of boundary measurements late in the survey.
In most cases,
the background amounted to a very-small fraction of the actual measurement and is neglected
in this report.
3. Penetration depth. The penetration depths used for data reduction of each shot
represent the average of the probe measurements taken during the corresponding survey. These
values are within a probable error of 10 percent or less for all surveys.
The probability that
these figures are representative of the entire quantity of fallout depends on the amount of fallout
28