Bulletin 54. Geophysical Methods of Exploration and their ...

GEOFHYSICAI_ METHODS OF EXPISORATION 3l
highly irregular, all of which results in the gravitational force
varying from place to place. Corrections can be made for such
factors as the effect of the general shape of the earth, its rotation,
and the variations in elevations, but the variations in mass distribution
due to geologic causes can only be approximated, since
they are for the most part not known. As a result, geologic investigations
can be made by noting the magnitude of the gravity
values left after correcting for all the Other known factors and
interpreting the residuals in terms of the known densities of rocks
and the geologic structure. These residual values are generally
referred to as anmnalles.
The gravlmeter is another instrument used for measuring
changes in the earth's gravitational field and though there are
numerous different types in use they are all essentially very sensitive
weighing devices which consist of a mass supported by a
single spring or spring system which permits variations in gravitational
attraction to be measured to one part in ten million of
the earth's gravitational field. As instruments of this Wpe require
only a few minutes for an observation as compared to several
hours with a pendulum, they are the ones eommonly used in field
work.
A third type of instrument that has been used in measuring
gravity is the tm'sion balance. This instrument is very sensitive
to horizontal forces, so much so in fact that corrections must be
made for the effect of trees and fence posts within a hundred feet
of the station. ]t consists essentially of two small gold weights,
either placed on an inclined boom, or attached to a horizontal
boom so that one weight is at a lower elevation than the other,
and the whole is suspended by a fine thread so as to be free to
rotate in a horizontal plane. Due to the differential attraction of
the gravitational field on the two weights, the system twists until
an equal opposing torque is established and this rotation is then
recorded. By taking observations in six azimuths, data are obtalued
which permit two gravitational quantities to be determined,
the gravitational gradient and the curvature or horizontal
directive force. These two quantities can perhaps best be visualized
if we consider the gravitational surface as being similar to
a topographic one and characterized by ridges and valleys, swells
and swsles, and peaks and basins. The gradient value obtained is
the degree of slope in the direction of maximum positive slope.
To use a topographic comparison, the value at any point might
be described thus: a five per cent grade up hill in a southwest
direction. The curvature value gives the direction of orientation
NEW JERSEY GEOLOGICAL SURVEY