Geology Reference
In-Depth Information
obtained at the two levels, then, applying free-air and
Bouguer corrections, one obtains
location and an upward attraction at the underground
location. The density
r
of the medium separating the
two observations can then be found from the difference
in gravity. Density may also be measured in boreholes
using a density (gamma-gamma) logger as discussed in
Section 11.7.2.
Nettleton's method of density determination involves
taking gravity observations over a small isolated topo-
graphic prominence. Field data are reduced using a series
of different densities for the Bouguer and terrain correc-
tions (Fig. 6.15).The density value that yields a Bouguer
anomaly with the least correlation (positive or negative)
with the topography is taken to represent the density of
the prominence. The method is useful in that no bore-
hole or mineshaft is required, and a mean density of the
material forming the prominence is provided. A dis-
advantage of the method is that isolated relief features
may be formed of anomalous materials which are not
representative of the area in general.
Density information is also provided from the P-wave
velocities of rocks obtained in seismic surveys.
Figure 6.16 shows graphs of the logarithm of P-wave
velocity against density for various rock types (Gardner
et
al.
1974), and the best-fitting linear relationship. Other
(6.14)
gg
-=
3 086
.
h Gh
-
p
4
r
1
2
The Bouguer correction is double that employed on
the surface as the slab of rock between the observation
levels exerts both a downward attraction at the surface
g
1
h
ρ
g
2
Fig. 6.14
Density determination by subsurface gravity
measurements.The measured gravity difference
g
1
-
g
2
over a
height difference
h
can be used to determine the mean density
r
of
the rock separating the measurements.
50
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
40
2.8
30
20
1.8
10
Fig. 6.15
Nettleton's method of density
determination over an isolated
topographic feature. Gravity reductions
have been performed using densities
ranging from 1.8 to 2.8 Mg m
-3
for both
Bouguer and terrain corrections.The
profile corresponding to a value of
2.3 Mg m
-3
shows least correlation with
topography so this density is taken to
represent the density of the feature.
(After Dobrin & Savit 1988.)
120
100
80
Elevation datum
60
