The Structure of Koolau Volcano from Seismic Refraction Studies 1 
Augustine S. Furumoto, Noel J. Thompson, and George P. Woollard 
The ISLAND OF OAHU in the Hawaiian Archi- 
pelago was formed by the coalescing of two 
volcanoes, Waianae Volcano on the west and 
Koolau Volcano on the east. Both volcanoes 
have been dormant for tens of thousands of 
years. Erosion and weathering have cut deeply 
into them so that present topography does not 
clearly indicate the former shapes and sizes of 
the volcanoes. 
A gravity survey by Woollard (1951) showed 
unusually high Bouguer anomalies over the cal- 
dera sections of both volcanoes. These high 
anomalies have whetted the interests of geo- 
physicists, and recently projects were planned 
to make a detailed survey of the Koolau Vol- 
cano, using whatever geophysical and geological 
methods were available. In the fall of 1963 and 
the winter of 1964 intensive gravity surveys 
(Strange, Machesky, and Woollard, p. 350 in 
this issue) were carried out over the volcano. 
An aerial magnetic survey (Malahoff and Wool- 
lard, 1965) was carried out during the spring 
and summer of 1964. After information and 
data from the gravity and magnetic surveys be- 
came available, seismic refraction profiles were 
planned and carried out during the fall of 1964. 
The prominent features of the Koolau Vol- 
cano are the caldera area, the northwest rift 
zone, and the southeast rift zone. From surface 
geology the existence of the northwest rift zone 
and the caldera was known (Stearns and Vaks- 
vik, 1935; Stearns, 1946). Gravity surveys 
(Strange, Woollard, and Rose, p. 381 in this 
issue) later confirmed the findings by surface 
geology for the caldera area, but showed that 
the northwest rift zone was farther to the east 
than had been indicated by geology and topog- 
raphy. The southeast rift zone became known 
only after the gravity surveys. 
1 Hawaii Institute of Geophysics Contribution No. 
90 . 
INSTRUMENTATION 
Because mobility was necessary for the planned 
seismic refraction program, the recording units 
were housed in trucks. The trucks contained 
recording cameras, amplifier banks, portable 
box-type darkrooms, communications gear, geo- 
phones, and other accessories to set up a re- 
cording station. 
The geophones were Hall-Sears geophones, 
with a natural frequency of 4.5 cps. Most of 
them were vertical components, but a few hori- 
zontal components were used. A typical spread j 
consisted of six geophones in line, with 30- ft 
spacings. 
The amplifiers were model type T-l, manu- 
factured by Fortune Electronics. These com- 
pletely transistorized amplifiers with low power 
comsumption were found to be very satisfac- 
tory in field operations. 
The recording cameras were manufactured by 
Southwest Industrial Electronics, Ltd. These 
cameras contained sufficient channels to record i 
the signals from the six geophones at two dif- 
ferent dynamic levels. 
Communication between the recording tracks 
and the shooting party was done usually through 
Citizen’s Band transceivers. The all-important 
shot instant was also transmitted by Citizens 
Band transceivers by an audio tone beginning 
at the shot instant. 
The firing of the explosives was done by a 
device, familiarly called the "tricky ticker,” 
which in essence is a break-circuit chronometer 
operating a fixed sequence of audio tones and 
switches for firing. Details of the firing equip- 
ment may be found in Steinhart and Meyer 
(1961:158-163). 
FIELD WORK AND DATA PROCESSING 
The seismic refraction traverses carried out 
for the project are shown in Figures 1, 2, and 3. 
Figure 1 shows the traverses designated by code 
306 
