Geology Reference
In-Depth Information
subtracted from the observed data due to the mathemat-
ical procedures employed.
It is necessary before carrying out interpretation to
differentiate between two-dimensional and three-
dimensional anomalies.Two-dimensional anomalies are
elongated in one horizontal direction so that the anom-
aly length in this direction is at least twice the anomaly
width. Such anomalies may be interpreted in terms of
structures which theoretically extend to infinity in the
elongate direction by using profiles at right angles to
the strike. Three-dimensional anomalies may have any
shape and are considerably more difficult to interpret
quantitatively.
Gravity interpretation proceeds via the methods of
direct and indirect interpretation.
tude and greater extent than those caused by shallow
sources. This wavenumber-amplitude relationship to
depth may be quantified to compute the maximum
depth (or limiting depth) at which the top of the anoma-
lous body could be situated.
(a) Half-width method . The half-width of an anomaly
( x 1/2 ) is the horizontal distance from the anomaly maxi-
mum to the point at which the anomaly has reduced to
half of its maximum value (Fig. 6.18(a)).
If the anomaly is three-dimensional, the initial
assumption is made that it results from a point mass.
Manipulation of the point mass formula (equation
(6.6)) allows its depth to be determined in terms of the
half-width
x
12
z
=
6.10.3 Direct interpretation
Direct interpretation provides, directly from the gravity
anomalies, information on the anomalous body which is
largely independent of the true shape of the body.Vari-
ous methods are discussed below.
3
41
-
Here, z represents the actual depth of the point mass
or the centre of a sphere with the same mass. It is an over-
estimate of the depth to the top of the sphere, that is,
the limiting depth. Consequently, the limiting depth for
any three-dimensional body is given by
Limiting depth
Limiting depth refers to the maximum depth at which
the top of a body could lie and still produce an observed
gravity anomaly. Gravity anomalies decay with the
inverse square of the distance from their source so that
anomalies caused by deep structures are of lower ampli-
x
12
z
<
(6.15)
3
41
-
A similar approach is adopted for a two-dimensional
anomaly, with the initial assumption that the anomaly
A
A max
A max
2
x
1/2
A' max
Fig. 6.18 Limiting depth calculations
using (a) the half-width method and
(b) the gradient-amplitude ratio.
x
(a)
(b)
 
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