A Preliminary Gravity Survey of the Island of Lanai, Hawaii 1 
Harold L Krivoy and Michael P. Lane 2 
The GRAVITY VALUE of the base station at 
Kahului Airport, Maui, was measured at 
978.88927 gal by R. R. MacDonald and W. T. 
Kinoshita in 1961 and 1962 (written commu- 
nication). Gravity differences measured be- 
tween Maui and Lanai during this survey con- 
firm the base value of 978.84757 gal established 
by R. R. MacDonald at Lanai Airport in 1961. 
LaCoste and Romberg gravity meter G-8 was 
used to establish both of these base stations. 
Figure 1 shows the location of both gravity 
bases used on Lanai; the Lanai Inn base was 
used for control before and after each day’s 
survey. Table 1 lists the date, time, and read- 
ing in milligals at the base station (Lanai Inn) 
from which all of the loops on Lanai were orig- 
inated. These data are introduced to demon- 
strate the drift-free behavior of the meter. 
Gravity meter readings were not corrected for 
instrumental drift because the drift would in- 
volve errors of the order of tenths of a mgal, 
whereas uncertainties in elevation and in sta- 
tion location could introduce errors of the order 
of 1 mgal. The reconnaissance nature of the 
survey, as well as the size of the gravity anom- 
alies anticipated, obviated the necessity for the 
drift and tidal corrections which would have 
been needed for a more detailed gravity study 
where elevation, location, and bulk density 
were better known. 
The table of principal facts for the gravity 
survey on Lanai is reported elsewhere (Hawaii 
Inst. Geoph., 1965, Table 4). A few U. S. Geo- 
logical Survey and U. S. Coast and Geodetic 
Survey benchmarks and triangulation stations 
were located and occupied. Other stations were 
at rain gauge sites maintained and surveyed by 
the Lanai Plantation, Dole Corporation. The 
1 Publication authorized by the Director, U. S. Geo- 
logical Survey. 
2 U. S. Geological Survey, Denver, Colorado. 
Plantation field maps were the most recent 
charts available for this gravity survey, and the 
rain gauge sites shown on them were convenient 
in number and distribution for use as gravity 
stations. The present engineering staff of Dole 
Corporation is not certain how the rain gauges 
were located or how their elevations were deter- 
mined. It seems probable that the majority of 
the rain gauges were located by horizontal tri- 
angulation and that their elevations were then 
read from existing (1921, 1940) U. S. Geologi- 
cal Survey topographic maps of the island. Un- 
certainties lead to the possibility that any station 
may be in error by as much as 10 ft of elevation. 
Adjacent stations could have a relative elevation 
error as large as 20 ft. An error of 20 ft in 
elevation would be the equivalent of about 
1.3 mgal. 
At the latitude of Lanai a change of V in 
latitude is roughly equal to 1 mgal in theoreti- 
cal gravity. Gravity errors due to mislocation, 
therefore, would probably be less than a few 
tenths of a mgal. Gravity errors resulting from 
errors in both elevation and location could be 
as large as 2 mgal. 
Woollard (1951) used a density of 2.3 g/cc 
to reduce gravity data for Oahu to a sea level 
datum. He based that choice on gravity profiling 
computations using Vening Meinesz’ submarine 
data. Gravity data on the island of Hawaii were 
reduced on the same basis of 2.3 g/cc (Krivoy 
and Eaton, 1961). Recent density determina- 
tions appear to confirm that choice. On Lanai 
the choice of bulk density to sea level is com- 
plicated by a rather thick weathering zone 
which seems to cover the island with varying 
depths of low density soil, talus, and detrital fill. 
THE EFFECT OF VARIOUS DENSITIES 
ON THE BOUGUER GRAVITY MAP 
Thus far, other Hawaiian volcanic units stud- 
ied with gravity methods have been mapped on 
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