Gravity Survey of Oahu — STRANGE, MACHESKY, and Woollard 
353 
ern North American calibration line and inter- 
mediate stations established with other gravity 
meters and comparisons against other meters on 
the r;nd- Pacific calibration line indicate that 
gravimeter No. 607 has a screw effect which 
causes the calibration constant to vary from 
0.1140 to 0.1150 mgal per meter unit for dif- 
ferent ranges of the gravimeter. The exact na- 
ture of this screw effect has not been definitely 
established, but since the error to be expected 
from this source would generally be 0.2 mgal 
or less no attempt was made to take it into 
account in the reductions. The screw effect in 
gravimeter No. 366 was small and did not ex- 
ceed 0.1 mgal* 
Terrain corrections were carried out where it 
was believed that the effect would be larger 
than 1-2 mgal. Two different methods were 
used— the normal circular template method and 
a profile angle method. The profile angle method 
is only approximate and the complete Bouguer 
anomalies obtained by this method may be in 
error by 1-2 mgal. The simple Bouguer anoma- 
lies for most of the stations are probably ac- 
curate to better than 1 mgal. A few of the sta- 
tions where elevation control is uncertain may 
be in error by as much as 5 mgal 
No detailed interpretation of results will be 
given here, but a few brief comments can be 
made. The Bouguer gravity anomaly on the 
island of Oahu varies from a low of about 
+ 190 mgal to a high of about +310 mgal. 
The lowest value occurs in the Pearl Harbor 
area and perhaps is caused partly by the thick 
section of sediments present. The highest values 
are associated with the Koolau and Waianae 
volcanic calderas as defined geologically. Posi- 
tive anomalies of 115 mgal magnitude are asso- 
ciated with each of these calderas. The major 
northwest rift zone of the Koolau volcano and 
the south and northwest rift zones of the Waia- 
nae volcano have lower gravity positive anoma- 
lies associated with them and average +50 mgal 
above the general level. As seen from the Bou- 
guer anomaly map (Fig. 1), the topographic 
effect of the island is not pronounced because 
the gravity effect of the two major volcanic 
pipes and their associated rifts dominate the 
gravity field. Also, it is to be noted that al- 
though Salt Lake Crater, where inclusions of 
eclogite are common, has a small gravity effect 
of about 5 mgal, other late-stage volcanic cen- 
ters, such as Diamond Head, Koko Head, and 
Punch Bowl, have no discernible gravity effect. 
This implies no density-significant contrast be- 
tween the pipe filling and the surrounding lavas. 
ACKNOWLEDGMENTS 
We would like to acknowledge the excellent 
cooperation of the U. S. Army, Navy, Air Force, 
and Marine Corps, who provided permission for 
establishment of gravity stations on their in- 
stallations and often provided special escorts 
and transportation. The Marine Corps also pro- 
vided helicopters for establishment of gravity 
stations in some of the more inaccessible areas 
in the Koolau and Waianae ranges. The work 
was carried out under nsf Grant No. GP2256 
under the over-all supervision of Dr. G. P. 
Woollard. 
REFERENCES 
Duerksen, J. A. 1943. Gravity anomalies and 
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Hawaii Institute of Geophysics. 1965. 
Data from gravity surveys over the Hawaiian 
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waii Inst. Geoph. Rept. 65-4, March 1965. 
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Vening Meinesz, F. A. 1941. Gravity over the 
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pp. 
— J. H. F. Ampgrove, and Ph. H. Kyn. 
1934. Gravity Expeditions at Sea, vol. II. 
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Woollard, G. P. 1950. The gravity meter as 
a geodetic instrument. Geophysics 15:1-29. 
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