A Gravity Survey of the Island of Oahu, Hawaii 1 
William E. Strange, L. F. Machesky, and George P. Woollard 
Prior to the present study four pendulum 
bases had been established on the island by 
the U. S. Coast and Geodetic Survey (Duerk- 
sen, 1943): (1) a submarine pendulum ob- 
servation at the port of Honolulu by Vening 
Meinesz (1934), (2) submarine pendulum 
observations at various sites at Pearl Harbor by 
personnel of the Lamont Geological Survey 
( Worzel, in press ) , ( 3 ) pendulum observations 
at the Bishop Museum (Woollard and Rose, 
1963), and (4) a reconnaissance gravimeter 
survey at some 30 regionally distributed sites 
by Woollard (1951). Although personnel of the 
U. S. Geological Survey and the U. S. Air Force 
Air Photographic and Charting Survey had also 
established gravimeter stations in some areas, 
their data were not generally available. 
The present survey was a follow-up on the 
original work of Woollard (1951), who had 
shown that the gravity field was characterized 
by positive Bouguer anomalies ranging from 
+ 197 to +309 rngal, and that the maximum 
anomalies were correlated with two major vol- 
canic calderas over which there appeared to be 
a local effect of about +110 mgal. Although 
Vening Meinesz (1941) had concluded that 
the Hawaiian Islands represented an extra mass 
on the earth’s crust having a density of about 
2.94 g/cc, Woollard’s analysis of the gravity 
data suggested a mean density of only 2.3 g/cc 
and that only the volcanic pipes creating the 
local anomalies were characterized by a high 
density. 
Inasmuch as in the interim since Woollard’s 
original reconnaissance survey a considerable 
amount of crustal seismic information had been 
obtained as well as offshore gravity data, it was 
decided to make a more complete gravity sur- 
vey of Oahu which could be analyzed using 
modern computer techniques. Oahu also con- 
stituted the logical place for a meaningful inte- 
1 Hawaii Institute of Geophysics Contribution No. 
95. 
grated geophysical study of the subsurface mass 
distribution associated with volcanic calderas in 
that, through erosion and crustal subsidence, 
both of the calderas having gravity expression 
lay essentially at sea level. Thus, there was no 
significant surfkial mass contribution to the 
anomalies. Other favorable factors associated 
with Oahu were the road system giving easy 
access to most of the island, generally good ele- 
vation control, the ability to carry out gravity 
surveys over the center of volcanic pipes with- 
out the complications of major elevation changes, 
and local U. S. Marine Corps helicopter support 
for making observations in otherwise inaccessi- 
ble areas. In all, some 512 gravity stations were 
established; their loc?tions are indicated in Fig- 
ure 1. The table of principal facts is reported 
elsewhere (Hawaii Inst. Geoph., 1965, Table 
6). The observations were established by the 
writers with the assistance of A. S. Furumoto 
and L. W. Kroenke. 
BASE STATIONS 
The base value used was that for Hickam Air 
Force Base, which had been established orig- 
inally by Woollard in 1948 and subsequently 
reoccupied and tied to the Washington national 
gravity base many times (Woollard, 1950; and 
Woollard and Rose, 1963). The adopted value 
for this base on the Potsdam (Bad Harzburg) 
system is 978.9337 gal. Because Honolulu is 
used extensively as a control point for all gravity 
surveys in the Pacific area, and the Interna- 
tional Air Terminal is too far removed from 
Hickam Air Force Base to permit reoccupation 
of the Hickam base, Woollard had established 
a number of auxiliary bases (Woollard and 
Rose, 1963). However, because of new con- 
struction most of these base sites can no longer 
be recovered. The descriptions and values of 
these earlier bases and of new bases established 
by the writers are given in the following list. 
Figure 2 shows the base interconnections. 
350 
