Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
269 
is not only minimized, but the terrain correc- 
tions are simplified. In ground surveying meth- 
ods, such as those described by Nettleton 
(1940), terrain corrections are not considered 
because only relations found in petroleum 
provinces are treated. In such areas, the sedi- 
mentary rocks encountered at the surface are 
relatively non-magnetic as compared with the 
buried crystalline rock complex at depth, and 
the terrain effect is negligible. Ground magnetic 
surveys in such areas, therefore, define geologic 
boundaries beneath the sediments in the crystal- 
line rock complex. 
Surveying over a surficial magnetic rock sur- 
face presents an entirely different problem. In 
such areas, a magnetic dipole is induced over 
each topographic rise as well as subsurface 
bodies having an abnormal magnetic suscepti- 
bility. Because the magnetic intensities of any 
two-pole magnetic body vary inversely as the 
square of the distance between the sensing head 
of the magnetometer and each of the induced 
poles on the geologic or topographic body creat- 
ing the anomaly, near-ground magnetic surveys 
over highly magnetic country rock, such as 
basalt, reflect local terrain irregularities as well 
as the effect of buried geologic bodies. Which 
effect will be dominant depends upon the rel- 
ative susceptibility contrast associated with each 
body, the size of the respective bodies, their 
geometry, orientation in the earth’s field, and 
the distance from the magnetometer sensing 
head to each body. To illustrate this relation- 
ship, the apparent susceptibility contrasts be- 
tween air and normal basalt is of the order of 
15.0 X 10 -3 cgs units, which is similar to the 
contrast between normal basalt and intrusive 
gabbroic dike rock. If all other factors are equal, 
but the distance between the sensing head and 
a local basaltic terrain feature (such as the wall 
of a caldera) is of the order of 20 ft, and the 
distance between the sensing head and the top 
of an intrusive mass in the underlying volcanic 
feeder pipe is of the order of thousands of feet, 
it is obvious that the terrain effect will be 
dominant. To assess the magnetic effect of basal- 
tic terrain in Hawaii, a total force ground mag- 
netic survey as well as an airborne survey was 
carried out across the crater of Kilauea Iki on 
the island of Hawaii. 
Kilauea Iki is a small side crater merging 
with Kilauea Crater along the northeastern 
portion of the latter. The floor of the small 
crater lies 650 ft below the rim. The total mag- 
netic intensity, as observed with the polarizing 
head 4 ft above ground level, varied from a 
reading of 39,400 gammas at the rim of the 
caldera to a reading of 34,300 gammas at the 
floor of the caldera. This change would nor- 
mally be interpreted as indicating a magnetic 
anomaly of — 5,100 gammas due to anomalous 
geology located within the confines of the 
Kilauea Iki crater. However, if the aeromag- 
netic anomaly above Kilauea Iki is examined, 
the maximum residual anomaly that can be 
assigned to an anomalous body within the 
crater is only 60 gammas. Also, it was noted 
that, if a magnetic reading is taken on basaltic 
terrain at 5 ft above ground level, a difference 
of up to 300 gammas can be obtained from the 
effects of local irregularities in terrain. Because 
of this pronounced ground-level terrain effect 
and the occurrence of highly ferromagnetic sec- 
ondary minerals in basaltic soils, no surface 
magnetic surveys were attempted for the study 
of subsurface geologic structure. All observa- 
tions were made using an airborne system. 
The flight elevations over the islands varied 
between 15,000 ft above sea level for flights 
above the peaks of Mauna Loa and Mauna Kea 
on the island of Hawaii to 8,000-10,000 ft for 
the remaining islands. These elevations were 
chosen on the basis of theoretical studies of the 
magnetic effects to be expected for topography. 
At a flight elevation of 10,000 ft, the magnetic 
effect to be expected for an 8,000-ft peak built 
of material with a magnetic susceptibility of 
1.0 X 10 -3 cgs units and a natural remanent 
magnetization of 10.0 X 10~ 3 cgs units should 
be of the order of -[-100 gammas. It was on the 
basis of both theoretical and actual profiles 
across the topographic features of Maui, Molo- 
kai, and Oahu that a standard flight elevation 
of 8,000-10,000 ft above sea level was selected 
for use everywhere except where this elevation 
would not permit clearance of the land surface 
by at least 2,000 ft. 
Comparison of Ship and Airborne Magnetic 
Survey Data at Sea 
The total force magnetic intensity survey re- 
sults obtained with the airborne magnetometer 
