The Koolau Volcanic Plug— -Adams and Furumoto 303 
TABLE 3 
Values Obtained from Computer Program Run on First Arrivals, 
Bubble Pulses, and Early First Arrivals 
computer 
code 
PARAMETER 
UNITS 
FIRST 
ARRIVALS 
BUBBLE 
PULSES 
EARLY 
FIRST 
ARRIVALS 
K 
1 
3 
4 
N 
5 
9 
6 
X BAR 
km 
5.38 
9.49 
14.95 
T BAR 
sec 
1.16 
2.49 
2.83 
T RECIP 
km 
42.25 
43.55 
42.37 
T NULL 
sec 
0.025 
0.453 
-0.363 
SLOPE 
sec/km 
0.211 
0.216 
0.214 
APP VEL 
km/ sec 
4.74 
4.64 
4.68 
VAR SLP 
(km/sec) 2 
0 . 
0 . 
0 . 
DEV SLP 
km/ sec 
0 . 
0 . 
0 . 
XDELTA 
km 
0 . 
582.25 
-448.66 
zontal at the point where the critically refracted 
ray enters. To be conservative in the following 
developments, we will use the value of 2 km. 
Actual drilling, however, should intersect the 
high velocity rock at less than 1630 m. 
An estimate of the structure to which the 
observed travel times correspond is given in 
Figure 7. This should be compared with the 
h 
Fig. 6. Relation of refraction arrival section to posi- 
tion of the plug. 
preliminary interpretation of line ABLE ( Furu- 
moto, Thompson, and Woollard, p. 306 in 
this issue), northwest from Kailua School, as 
the velocities obtained in that study have been 
used to guide this work. Note that the high 
velocity layer of about 6 km/sec was not ob- 
served to the southeast of the pipe in this pres- 
ent study. Because of this, one might be in- 
clined to interpret the data as normal faulting,, 
with the downthrown block on the southeast 
side. This is impossible because of the high 
energy arrivals observed in the 3- to 6-km 
range and arriving after the bubble pulse. 
These are interpreted as reflections off a possible 
former magma chamber at a depth of 3-4 km. 
An alternative interpretation might attempt 
to fit the strong reflections arriving at short 
range with the bubble pulses in the intermedi- 
ate range and make claim for a high-speed layer. 
Fortunately, the present survey was continued 
out to a distance from the shot point sufficient 
to preclude that possibility. Such a high-speed 
layer is not possible. 
Since secondary arrivals still occur after the 
diffraction range, corresponding to direct ar- 
rivals through the 4.74 km/sec zone, the plug 
