304 
PACIFIC SCIENCE, Vol. XIX, July 1965 
DISTANCE FROM SHOTPOINT (KILOMETERS) 
Fig. 7. Structure deduced from the observed refrac- 
tion data. 
does not penetrate the 4.74 zone. As can be 
seen from the sample record at 7.25 km, the 
first arrival is especially strong and probably 
corresponds to the intersection of the direct 
and the refracted waves. 
Using the analysis corresponding to Figure 6 
and assuming a depth of 2 km, we obtain an 
offset for the refracted data of about W 2 km. 
An estimate of the width of the plug may be 
obtained from the lead of the early arrival seg- 
ment occurring after 11.7 km. This segment 
leads the extension of the previous first arrival 
curve by 0.410 sec. The meaning of this time 
lead can be seen from Figure 8. 
Two rays are considered traveling from the 
source, S, to the observer, O, with one passing 
just above the plug and the other passing 
through the top of the plug. If we write the 
time of transit for the upper path as T u and 
the time in the lower path as Tj, then 
where SO is the over-all path distance, L the 
width of the plug, V p is the velocity in the 
plug, and V 0 is the velocity in the matrix rock. 
The difference in arrival time will, therefore, be 
This approximation assumes that the depth j 
of the plug is negligible. If the top of the plug 
is moved downward, then the width of the plug 
must be increased. 
Using this equation, the observed velocities, 
and the observed travel-time lead, we obtain a 
width for the plug of 5.4 km, which agrees 
closely with the surface geological indications 
of the size of the caldera and the gravity solu- 
tion (Strange, Woodard, and Rose, p. 381 in 
this issue). 
Some control on the horizontal position of 
the plug exists in the present data. The pre- 
viously mentioned reflections occurring at about 
2 sec after the shot time in the distance range 
3.8-6 km correspond to reflections from a depth 
of about 3 km outside the boundary of the 
plug, as shown on Figure 7. This, incidentally, 
is of the same order as the depth extrapolated i 
for the magma chamber under Kilauea by Eaton 
( 1962 ) . Therefore, we tentatively identify these 
as being from an existing or former magma 
chamber. 
Assuming a geometrical ray path, one can 
obtain an inner bound from the onset of the 
effect of the plug. The plug effect would then 
persist along the line for about 5 or 6 km. 
There is some additional constraint on the 
outer limit from the critical angle at which 
the refraction from the plug occurs. The critical 
angle to the plug is about 41° for the velocities 
used here, so the refractions from the high 
velocity layer extend beyond the outer edge of 
the dome to a distance roughly equal to the 
case to a point about 11.7 km from the shot 
point. For a model depth of 2 km, the top of 
the plug begins to curve downward at about 
10 km from the shot point along the record- 
ing line. 
Fig. 8. Relation of transit times for paths inside and 
outside the plug. 
