268 
level of observation. A series of theoretical 
geologic bodies with variable depths to the top 
of the body and variable length-to-width ratios 
may be constructed. As the depth to the top of 
the geologic body increases, the amplitude of 
the magnetic anomaly profile decreases. 
Using arbitrary units for d, the depth to the 
top of the body, and corresponding arbitrary 
units for the length and width of the geologic 
body, a series of theoretical, two-dimensional, 
vertical geological bodies were constructed and 
their total force magnetic anomaly profiles com- 
puted. Each particular model (Fig. 3 4A to F) 
has a set of five of the profiles, computed for 
d r= 0.5, 1, 2, 5, 10, and 15 in arbitrary units. 
The profiles were calculated using a com- 
bined susceptibility-natural remanent magne- 
tization of 10.0 X 10~ 3 cgs units, a common 
magnetization contrast, observed from speci- 
mens and computed from anomaly profiles, be- 
tween the intrusive and extrusive basalt rocks 
of the Hawaiian Islands. The models are as- 
sumed to have an infinite horizontal length and 
a strike parallel to magnetic latitude. The 
models are magnetized in a regional total force 
magnetic field of 36,000 gammas, with a dip 
of 35° N. The total force magnetic profiles are 
assumed to strike parallel to magnetic longitude, 
i.e., perpendicular to the strike of the models. 
In practice it was found that by comparing com- 
puted total force magnetic profiles over models 
that had been computed using two- and three- 
dimensional techniques, end effect errors, in 
the case of two-dimensional assumptions, are 
likely to be less than 10% if the geologic body 
has in horizontal section a width of one unit 
and a length of four units. 
The total force magnetic anomaly profiles 
were computed by using machine programming 
in integrating the effects of horizontal and 
vertical magnetic fields of volume elements 
over the cross sections of the geologic models 
(Heirtzler et al., 1962). 
By comparing observed total force magnetic 
anomaly profiles with those computed in Figure 
34A to F, the figures become useful as a means 
of rapidly determining the vertical length of the 
highly magnetized, vertically or near-vertically 
dipping intrusives intruded within the Hawai- 
ian volcanoes of the Hawaiian Ridge, or within 
elongate seamounts or rift zones of the ocean 
PACIFIC SCIENCE, Vol. XX, July 1966 
floor, in the magnetic latitude where the mag- 
netic dip is between 30° and 40°. Similar re- 
lationships would also hold true for the above- 
mentioned geologic features within the Southern 
Hemisphere latitudes where the magnetic dip 
is also from 30° to 40°, though the anomaly 
dipoles would be reversed in sign. 
The curves in the figure may also be used in 
aeromagnetic surveys to measure the magnetic 
sensitivity with elevation. For instance, if zones 
of magnetization 1 km in width are to be exam- 
ined at an elevation of 3 km, extremely sensi- 
tive instrument techniques would have to be 
used in order to determine whether the zones 
of magnetization are 40 km in vertical length 
(Fig. 34£). On the other hand, too low a 
flight elevation will record anomalies that are 
great in amplitude with steep gradients — 
frequent crowding of anomalies — which may 
lead to difficulties in sorting. 
For the equatorial latitudes at least, the best 
flight elevation with respect to the wavelength 
of the geologic body to be examined is that 
with a ratio of 1:1 (Fig. 34D). That is to say, 
at a flight elevation of 2 km, reasonable total 
amplitudes of 700 gammas peak-to-peak to 120 
gammas peak-to-peak for the anomalies can be 
expected, depending upon the vertical length of 
the magnetized body. At this ratio, reasonable 
estimates of the vertical lengths of the mag- 
netized bodies can also be made, because the 
total wavelengths of the anomalies caused by 
relatively short geologic bodies are readily dis- 
tinguishable from those caused by relatively 
long geologic bodies. 
This procedure for determining the lengths 
of intrusive magnetized bodies and for selecting 
appropriate aeromagnetic flight elevations may 
be used at any magnetic latitude, providing an 
appropriate selection of models is computed for 
that latitude range. 
RESULTS, DISCUSSION, AND CONCLUSIONS 
Problems in Magnetic Surveying Over Magnetic 
T errain 
The primary advantage of an aeromagnetic 
survey method over those of ground surveys is 
the greater rate and density of coverage that can 
be achieved. An additional advantage is that the 
effect of changes in surficial geology and terrain 
