The Koolau Volcanic Plug — A dams and Furumoto 
297 
and usually at shorter distances, which corre- 
spond to head waves that are refracted upward 
from the top of the plug. It is these head waves 
which permit an estimate of the velocity in 
the plug. 
Note that this program was initiated with 
the expectation that the data desired would 
occur as second arrivals. It should also be noted, 
however, that if the plug extends upward to 
a sufficiently shallow depth, then the refraction 
segment will move downward across the 6.2 
km/sec leg and actually be composed of the 
first arrivals. In this case, the end of the refrac- 
tion segment would still taper off into the dif- 
fraction zone. Another point of interest in con- 
nection with this model is that if the plug does 
05 10 15 
km /sec 
Fig. 1. Structural model used for planning seismic 
field effort. 
not completely penetrate the 6.2 km/sec layer, 
double arrivals at distances greater than the dif- 
fraction distance are to be expected. If the plug 
does completely penetrate this layer, then the 
6.2 km/sec leg would not be extended. 
FIELD PROGRAM 
A location map of the area of interest is 
shown in Figure 2. The other seismic lines 
established by the Hawaii Institute of Geo- 
physics — ABLE, BRAVO, and DELTA — are also 
shown. The area of interest is denoted by the 
block oriented NW-SE. This is shown in de- 
tail in Figure 3. 
The gravity high defining the Koolau caldera 
lies on the present coastline and adjoins Kane- 
ohe Bay. Because the caldera is now a residen- 
tial area, Kaneohe Bay provided the only feasible 
area for shooting charges large enough to ob- 
tain data over the ranges required. The initial 
field effort was confined to the caldera, but it 
soon became apparent that operations restricted 
to this area would not yield satisfactory data 
because of the slope-effect of the buried caldera 
walls, which on some spreads resulted in infinite 
apparent velocity values. The shot line finally 
established based on the model analysis extends 
across the center of the gravity high with rela- 
tively slight deviations of stations from a 
straight line, except in the vicinity of Kawainui 
Swamp where transportation was restricted. An 
effort to use U. S. Marine personnel carriers to 
work straight across the swamp ended in failure 
when the vehicles became hopelessly bogged 
down. 
All the shots that were conducted for record- 
ing along this line are listed in Table 1. Each 
shot was recorded with 250-ft spreads at two 
positions. Each shot consisted of 50 lb of Nitra- 
mon detonated on bottom at a depth of 50 ft 
at the location of Buoy 27 in Kaneohe Bay. 
It was hoped that shooting on bottom would 
give maximum seismic energy coupling and 
minimize the bubble pulses ( Worzel and Ewing, 
1948: 18). Larger charges could not be shot due 
to cultural restrictions. 
The usual field difficulties due to imperfec- 
tions of men and machines were encountered. 
Sample records are shown in Figure 4. The 
paper speeds used were 2 inches/sec or 8 
