4.7
SUMMARY
The EPG operations from which cratering data are available are listed in Table 4.1.
These data have been used to construct cratering curves for washed craters in saturated
soil (Figures 4.1 and 4.2).
In Figure 4.1, the lower line is the dry-soil curve from TM 23-200 representing NTS
conditions; the dotted line is the TM curve displaced by a factor of two in order to adjust
for washing; the upper line is a curve drawn through data available from EPG. The EPG
points are plotted from the data in Table 4.1, with the exception of dotted lines indicating
possible limits of Seminole when adjusted as an underground burst and as an underground
burst with washing. The Seminole adjustments have been made so that a trend line could
be drawn for underground detonations.
The shots in which the crater radii do not fit the adjusted TM curve may be explained
by the extreme difficulty in obtaining crater dimensions at EPG, especially for tower
bursts or for large craters; an unwashed crater such as Lacrosse; different degrees of
washing as in the case of Seminole, Mohawk, and Castle Shot 3 (the radius of these shots
should be adjusted upward by at least 10 percent); or unusual environmental conditions,
as in the case of Seminole, Zuni, or Castle Shot 3.
The Seminole device was placed in
a tank of water, part of the energy from Zuni vented into the lagoon, and Castle Shot 3
had radii listed at 380, 400, and in excess of 600 feet (the true radius is probably closer
to 500 feet than the 400 feet commonly listed).
If, for near-surface bursts, a soil factor
should be derived for comparing the radius of the EPG washed craters in saturated coral
to the Nevada cratering curve given in TM 23-200, a factor of 1.8 to 2.0 should be used.
This factor is substantiated by a series of high-explosive surface charges fired on Eniwetok during Operation Ivy and compared to similar data gathered at NTS during the
“Mole” experiment and Operation Jangle (References 3 and 9).
As more Pacific operations are conducted and additional cratering data is obtained,
there is a trend apparent in the data from surface explosions.
This trend is the increase
in the ratio of the crater radius to crater depth as the yield increases into the multimegaton range. One would expect that as the yield and, correspondingly, the duration
of the positive phase of the air blast gets very large, lower pressure would be required
to produce measurable displacement at ground surface. This is because the impulse required to produce ground displacement sufficient to be measured as part of the crater
can be obtained from lower pressures as the duration increases.
The crater depth data is not as consistent as the radius data and does not lend itself
to soil factors, but should be accepted as a curve for approximate crater depths for air
and surface bursts over saturated coral. As mentioned above, as the yield increases so
does the value of R/D. This is due to the fact that depth does not increase at the same
rate as radius. It is believed that this is caused by the increasing lack of similitude of
the soil with increasing real depth as the yield increases.
Simply stated, the hydrostatic
forces at a point in soil 300 feet below the surface are much greater and different from
those at a point 30 feet below the surface. Scale-wise, these are the same cube root
scaled depths for a 1-kt and a 1-Mt device.
As can be realized from the above discussion, the larger the yield of weapon the
greater the depth and, thus, the greater the deviation from cube-root scaling.
This lack
of scaling should apply to all soils and indicates the dangers in extrapolating present data
on crater depth to extremely high yields. The use of higher root scaling will give closer
approximations over a wider range; however, no scaling for depth will be absolutely accurate.
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