Geology Reference
In-Depth Information
(Mg m
-3
)
+
Bulk density,
ρ
1.8
2.0
2.2
2.4
2.6 2.8 3.0
Estimated
regional field
9.0
0.9
7.5
Limestone
0.8
6.0
Dolomite
Observed
gravity
0.7
Rock salt
4.5
Anhydrite
0.6
0
3.5
Time average
(sandstone)
Distance
0.5
3.0
Residual gravity
anomaly
-
0.4
2.5
Sandstone
Shale
Fig. 6.17
The separation of regional and residual gravity anom-
alies from the observed Bouguer anomaly.
0.3
1.8
0.2
1.5
= 1.74
V
0.25
sphere. This ambiguity represents the
inverse problem
of
potential field interpretation, which states that, although
the anomaly of a given body may be calculated uniquely,
there are an infinite number of bodies that could give rise
to any specified anomaly. An important task in inter-
pretation is to decrease this ambiguity by using all
available external constraints on the nature and form of
the anomalous body. Such constraints include geological
information derived from surface outcrops, boreholes
and mines, and from other, complementary, geophysical
techniques (see e.g. Lines
et al.
1988).
ρ
0.1
0.2
0.3
0.4
0.5
Log
ρ
Fig. 6.16
Graphs of the logarithm of P-wave velocity against
density for various rock types. Also shown is the best-fitting linear
relationship between density and log velocity (after Gardner
et al.
1974).
workers (e.g. Birch 1960, 1961, Christensen & Fountain
1975) have derived similar relationships. The empirical
velocity-density curve of Nafe and Drake (1963) indi-
cates that densities estimated from seismic velocities are
probably no more accurate than about ±0.10 Mg m
-3
.
This, however, is the only method available for the esti-
mation of densities of deeply buried rock units that
cannot be sampled directly.
6.10.2 Regional fields and residual anomalies
Bouguer anomaly fields are often characterized by a
broad, gently varying, regional anomaly on which may
be superimposed shorter wavelength local anomalies
(Fig. 6.17). Usually in gravity surveying it is the local
anomalies that are of prime interest and the first step in
interpretation is the removal of the
regional field
to isolate
the
residual anomalies
.This may be performed graphically
by sketching in a linear or curvilinear field by eye. Such a
method is biased by the interpreter, but this is not neces-
sarily disadvantageous as geological knowledge can be
incorporated into the selection of the regional field.
Several analytical methods of regional field analysis
are available and include trend surface analysis (fitting
a polynomial to the observed data, see Beltrão
et al
.
(1991)) and low-pass filtering (Section 6.12). Such
procedures must be used critically as fictitious residual
anomalies can sometimes arise when the regional field is
6.10 Interpretation of gravity anomalies
6.10.1 The inverse problem
The interpretation of potential field anomalies (gravity,
magnetic and electrical) is inherently ambiguous. The
ambiguity arises because any given anomaly could be
caused by an infinite number of possible sources. For ex-
ample, concentric spheres of constant mass but differing
density and radius would all produce the same anomaly,
since their mass acts as though located at the centre of the
