A Seismic Refraction Study of the Koolau Volcanic Plug 1 
Wm. Mansfield Adams and Augustine S. Furumoto 
ABSTRACT: The seismic data from the GASHOUSE line support the gravity and 
magnetic data as indicating a plutonic body occurring beneath the center of the 
Koolau caldera in the vicinity of Kailua, Oahu. This plug has a velocity greater 
than 7 km/sec and adjoins material with a velocity of about 4.6 km/sec at the top 
and to the southeast. The width of the plug is estimated to be about 6 km down 
to 3-4 km. Reflections from a horizon greater than 3 km deep may indicate an 
underlying magma chamber. The top of the plug is about 1600 m deep. 
Drilling of this plug would have considerable scientific value both geologi- 
cally and geophysically. An appropriate drilling site would be at the southwest 
corner of Kaelepulu Pond, which lies close to the center of the plug as now defined 
geophysically. Seismic reflection work directly above the dome is also recommended 
to test the present estimate of the depth based on refraction results. 
Gravity measurements on the windward 
side of Oahu define a local anomalous high of 
about +110 mgal. This was first reported by 
Woollard (1951), and subsequently has been 
surveyed in more detail by the Institute of Geo- 
physics. Woollard interpreted this anomaly as 
being caused by high density (3.2-3. 3 g/cc) 
pipe material extending to within 1-2 km of 
the surface. A more recent analysis (Strange, 
Woollard, and Rose, p. 381 in this issue) arrives 
at much the same conclusion. Although the ini- 
tial intention of the seismic work reported here 
was to "fan shoot” the plug, as well as to deter- 
mine its velocity and depth, the over-all diffi- 
culty of the experiment and higher-than-antici- 
pated costs dictated that part of the operation 
be curtailed. It was decided that the depth and 
velocity of the plug — as possible mantle mate- 
rial — was of greater interest than the extent. 
Therefore, all work has consisted of in-line re- 
fraction shooting in order to obtain estimates 
of the seismic velocity in the plug and the 
depth to the top of the plug. 
PREDICTION TECHNIQUE EMPLOYED 
Previous seismic work on plugs has been con- 
cerned mostly with exploration for oil around 
salt domes. The objective in such work is to 
1 Hawaii Institute of Geophysics Contribution No. 
89 . 
define the flanks of the dome so that explora- 
tory drilling can be conducted. There is very 
little interest in the velocity or shape of the top 
surface of the dome, and a satisfactory estimate 
of the depth of the dome can usually be ob- 
tained from reflections or from potential meth- 
ods since the density of salt is quite uniform 
from dome to dome. Investigations of buried 
volcanic plugs have been few and mostly unsuc- 
cessful. Thus, the present investigation is rela- 
tively unique in that it appears to have been a 
success. This can be attributed in large part to 
planning of the field work. 
A theoretical model was first constructed 
based on the gravity estimates of the depth and 
size of the plug and seismic velocities deter- 
mined from short exploratory refraction spreads. 
The model used is shown in Figure 1. The 
velocities at depth were taken from preliminary 
estimates on the ABLE line running to the north- 
west along the northern shore of Oahu (Furu- 
moto, Thompson, and Woollard, p. 306 in this 
issue). The features in the resulting theoretical 
travel-time curves are the usual crossovers, but 
with the 6.2 km/sec leg being shifted down- 
ward at greater distance, similar to that which 
occurs on the down-thrown side of a normal 
fault (see, for example, Nettleton, 1940:272). 
In addition, there are later arrivals in the region 
of the start of the downward diffraction distance 
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