Gravity and Magnetic Fields — R ose and Belshe 
377 
of Pacific Science ) centered over the island shelf 
at a depth of 500 rn„ 
The gravity measurements were made with 
the LaCoste-Romberg sea gravimeter SI 2. Mag- 
netic measurements were taken with a direct 
reading proton precession magnetometer, Varian 
model V4931DR. A narrow beam vertical 
sonar array produced by the Harris Company 
was used as the precision echo sounder for 
bathymetry. Navigation information relied 
heavily on Loran C for distant coverage and 
radar and visual observations when near the 
islands. 
The ship’s position was plotted every 15 
minutes during most of the cruise (Figure 1). 
Data (time, depth, gravity, and magnetic force) 
were logged every minute. The depth infor- 
mation was read in fathoms from a precision 
graphic recorder. Its accuracy is ±1 fm (±1.83 
m). The time was recorded digitally on the 
minute from a crystal-controlled clock and 
should be accurate to ±3 sec. The total force 
of the magnetic field was recorded to an ac- 
curacy of ±1 gamma from a digital frequency 
meter. 
The gravity measurements consisted of four 
record sets. The beam position of the heavily 
damped gravimeter was averaged over a 'SVi- 
minute interval and presented digitally by a 
computer. The average beam position and the 
average meter counter setting were displayed 
graphically on a potentiometer recorder. Two 
pairs of galvanometer recorders displayed the 
short-period and long-period ham records, 
showing the nature of the horizontal accelera- 
tions affecting the instrument. 
The digital data for time, magnetism, and 
gravity were recorded every minute, on the 
minute, by a Friden paper tape perforator in 
a B.C.D. form. This data tape could be printed 
out on a Flexowriter; an example is shown as 
Figure 3. The paper tape generated during the 
cruise was converted to a magnetic tape using 
the 1401 computer at the Statistical and Com- 
puting Center of the University of Hawaii, and 
the magnetic tape was then used directly in 
data processing with the IBM 7040 computer. 
Navigational control data, including the ship’s 
heading and speed, and depth data were pro- 
vided from a second magnetic tape which was 
generated from punched cards prepared at the 
end of the cruise. 
In the data processing program depths in 
fathoms were converted to meters and then cor- 
rected for sound velocity variations by an in- 
terpolation formula. The gravity value was an 
average value centered at 105 sec before it was 
recorded. The magnetic value represented a 
spatial point where the ship had been approxi- 
mately 30 sec earlier than the time of recording. 
Therefore, both the recorded gravity and mag- 
netic values were adjusted by simple averaging 
to correspond in real time to the depth record. 
The magnetic values were corrected for ship’s 
heading and diurnal variation (using records 
from the magnetic observatory operated by the 
U. S. Coast and Geodetic Survey at Ewa Beach, 
Oahu). The observed gravity values were com- 
puted after the instrument variations were ad- 
justed to the base value established by ties to 
the absolute gravity base site at Honolulu on 
September 21 and October 11. (The drift be- 
tween these ties was 0.7 mgal.) The Eotvos 
correction term, using the ship’s course and 
speed, was incorporated in the computation. 
The computed values for depth, gravity, free- 
air gravity anomaly, and magnetic force were 
printed out by the computer in one-minute in- 
tervals. These values were also plotted graphi- 
cally as profiles by a computer plotting program. 
Figure 4 is an example of such a plot for the 
g37ioio 
-> 050437 1060 106980093064 
050537 1 o 4 o 1 06976093064 
050637 1 020106977093064 
05073710101 06977093064 
050837 10 10 10697&93064 
57099^6973093064 
TIME'' 
MAG. FI ELD- 
TIME INTERVAL^ 
GRAV.BEAM POS. 
DATE 
J 1 
FRV/AI -I 
Fig. 3 . Perforated paper data tape from R/V "Sur- 
veyor.” 
