Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
267 
Absolute ceiling of the aircraft with all 
equipment aboard was 15,500 ft. Indicated 
cruising airspeed of the aircraft with the bird 
in tow at 10,000 ft was 80 miles per hr, and 
at 15,000 ft the indicated airspeed was 73 miles 
per hr. Full power and a high angle of attack 
were required in order to maintain that altitude. 
The equipment used proved adequate for all 
altitudes up to 15,000 ft. 
Methods Used in the Interpretation of 
Magnetic Anomalies 
Nearly all the magnetic anomalies observed 
over the Hawaiian Islands and the neighboring 
oceanic area can be divided into two groups: 
1. Local dipole anomalies related to centers 
of volcanism marked by surface caldera, vol- 
canic peaks, or geologic evidence defining a 
former vent area. 
2. Elongate, dipole anomalies related to dike 
complexes, observable, and probable, rift zones 
in the crust that appear to be occupied by in- 
trusives at depth. 
In the study of these anomalies, four factors 
were evaluated : ( 1 ) approximate size and shape 
of the anomalous geologic body, (2) orienta- 
tion in the earth’s magnetic field at the latitude 
of Hawaii, (3) depth to the top of the anom- 
alous body, (4) susceptibility contrast and the 
natural remanent magnetization contrast be- 
tween the surrounding rocks and the anomalous 
body. 
An approximation to the above parameters 
can be obtained by utilizing various analytical 
procedures based on the shape of the anomaly 
profile for the magnetic latitude. The three 
parameters can then be further defined through 
theoretical computations using two- or three- 
dimensional techniques with machine program- 
ming. These results are then matched with 
those observed. 
Most total force magnetic anomalies observed 
over the Hawaiian Ridge and the surrounding 
oceanic areas exhibit normally polarized mag- 
netic dipoles which remain as dipoles even 
after topographic corrections have been applied. 
These dipoles are of such intensity and wave- 
length that they can only be interpreted as 
vertical intrusive rocks intruding into the vol- 
canic domes along elongate rift zones. 
As volcanism in the Hawaiian Islands ap- 
pears in most cases to follow similar structural 
patterns, a scheme for estimating the horizontal 
cross-sectional size of the intrusive body, the 
depth to the top of the body from the level of 
observation, and the vertical length of the body 
would be useful for the rapid evaluation of the 
shapes of such intrusive bodies. The horizontal 
size of the body may be determined by the 
inspection of the relationship between theo- 
retical anomaly contours over theoretical bodies, 
as those computed by Vacquier et al. (1951), 
and by using this relationship to derive the 
horizontal size of the body giving rise to the 
observed magnetic anomaly. Depth to the top 
of the anomalous body may be readily deter- 
mined by using "depth indices” such as those 
in Vacquier et al. (1951). For dipole anomalies 
over the Hawaiian Islands, the G index was 
found to be the one giving the most consistent 
results and less likely to be affected by inter- 
ference from superposed smaller wavelength 
dipole anomalies. 
As is the case in the Hawaiian Islands, the 
susceptibility and the natural remanent mag- 
netization contrast between the extrusive and 
intrusive rocks of the volcanic domes may be 
obtained from measurements made on rock 
samples in the laboratory. Theoretical suscepti- 
bility contrast may be computed by using the 
formula 
ATcT 
where K is the minimum susceptibility contrast 
(Vacquier et al., 1951) ; ATc, the total ampli- 
tude of the intensity anomaly selected from 
the appropriate theoretical body, as computed 
in Vacquier et al. ; T, the intensity of the re- 
gional magnetic field at the point at which the 
anomaly is situated; and ATm, the observed 
amplitude of the actual anomaly. 
Having thus determined the above factors, 
another set of theoretical models, computed 
for the latitude of the Hawaiian Islands, may 
be used to determine the total vertical length of 
the intrusive geologic body. 
The amplitude of the magnetic anomaly asso- 
ciated with the geologic body depends on the 
magnetization of the body, on the length, and 
on the depth to the top of the body from the 
