Geology Reference
In-Depth Information
Distance
anomaly can be computed, and the shape of the model is
altered until the computed anomaly closely matches the
observed anomaly. Because of the inverse problem this
model will not be a unique interpretation, but ambiguity
can be decreased by using other constraints on the nature
and form of the anomalous body.
A simple approach to indirect interpretation is the
comparison of the observed anomaly with the anomaly
computed for certain standard geometrical shapes
whose size, position, form and density contrast are
altered to improve the fit. Two-dimensional anomalies
may be compared with anomalies computed for
horizontal cylinders or half-cylinders, and three-
dimensional anomalies compared with those of spheres,
vertical cylinders or right rectangular prisms. Com-
binations of such shapes may also be used to simulate an
observed anomaly.
Figure 6.20(a) shows a large, circular gravity anomaly
situated near Darnley Bay, NWT, Canada.The anomaly
is radially symmetrical and a profile across the anomaly
(Fig. 6.20(b)) can be simulated by a model constructed
from a suite of coaxial cylinders whose diameters de-
crease with depth so that the anomalous body has the
overall form of an inverted cone. This study illustrates
well the non-uniqueness of gravity interpretation.
The nature of the causative body is unknown and so no
information is available on its density. An alternative
interpretation, again in the form of an inverted cone,
but with an increased density contrast, is presented in
Fig. 6.20(b). Both models provide adequate simulations
of the observed anomaly, and cannot be distinguished
using the information available.
The computation of anomalies over a model of
irregular form is accomplished by dividing the model
into a series of regularly-shaped compartments and
calculating the combined effect of these compart-
ments at each observation point. At one time this
operation was performed by the use of graticules,
but nowadays the calculations are invariably performed
by computers.
A two-dimensional gravity anomaly may be repre-
sented by a profile normal to the direction of elongation.
This profile can be interpreted in terms of a model
which maintains a constant cross-section to infinity in
the horizontal directions perpendicular to the profile.
The basic unit for constructing the anomaly of a
two-dimensional model is the semi-infinite slab with a
sloping edge shown in Fig. 6.21, which extends to in-
finity into and out of the plane of the figure.The gravity
anomaly of this slab
D
g
is given by
d
g
d
x
Distance
d
g
d
x
Fig. 6.19
Bouguer anomaly profiles across (a) a granite body, and
(b) a sedimentary basin.The inflection points are marked with an
arrow.The broken lines represent the horizontal derivative (rate of
change of gradient) of the gravity anomaly, which is at a maximum
at the inflection points.
This thickness will always be an underestimate for a body
of restricted horizontal extent.The method is common-
ly used in estimating the throw of a fault from the dif-
ference in the gravity fields of the upthrown and
downthrown sides.
The technique of source depth determination by
Euler deconvolution, described in Section 7.10.2, is also
applicable to gravity anomalies (Keating 1998).
6.10.4 Indirect interpretation
In indirect interpretation, the causative body of a gravity
anomaly is simulated by a model whose theoretical
