CHAPTER 2
GROUND MOTION FROM
SHOT
OF OPERATION CASTLE
2.1
PURPOSE
Study of the ground motion produced by burst of megaton-yield weapons near the ground
surface was planned for Operation Castle to extend and supplement the incomplete results of
Project 6.5 of Operation Ivy. Primarily interest was focused on ground motion closer to
Ground Zero than during the previous test and on data to fill the vacancy left by incomplete
records from Station 650.01.
2.2
PLAN OF THE EXPERIMENT
The general plan for Project 1.7 included observation of vertical, radial, and tangential
components of acceleration in the ground below the water table at ground ranges corresponding
to 200-, 100-, and 40-psi air overpressure.
Specific plans required that instrumentation be split between.
fired on Eninman Island of Bikini Atoll and the
__Jghot to be
sshot to befiredEberiru Island of
Eniwetok Atoll. This split was dictated partly byavailable information channels but more
critically by the fact that
_the larger yield burst, was scheduled for an atoll
whose subsurface conditionsdiffered to an unknown degree from those which had affected the
ground-motion data from Mike shot of Operation Ivy. Since the data from the new project were
not expected specifically to overlap those from Mike shot, but were to replace an important
partially recorded set of information, it was considered advisable to include check measurements involving insofar as possible subsurface conditions similar to.those which prevailed for
the previous operation. Such a check test was feasible on the
Shot, although the estimated yield was in the fractional megaton range.
These considerations were the basis forchoice of three accelerometer stations (Fig. 2.1)
on Bikini Atoll at a ground range of 2600 {t on Eninman (Station 170.01) and on Reere at ground
ranges of 3650 ft (Station 170.03) and 5600 ft (Station 170.02), corresponding to approximately
200-, 100-, and 36-psi air overpressure for the estimated 1-Mt yield. Similar considerations
were involved in locating two stations at Eniwetok Atoll on Rujoru at ground ranges of 2450ft
(Station 170.05) and 3000 ft (Station 170.04), corresponding to estimated overpressures of 70
and 40 psi for the predicted Ramrod-shot yield of 0.2 Mt.
Boring logs for islands of the Eninman-Airukiiji complex at Bikini indicated considerable
difference between subsurface conditions there and at Eniwetok. However, soil and rock con-
ditions reasonably consistent with those prescribed for Operation Ivy ground-motion stations
existed at depths of about 15 ft on Eninman and Reere. Consequently accelerometers at
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