Geoscience Reference
In-Depth Information
With the assumed numerical values, we have
gravity measured at
P
g
minus Bouguer plate
−
0
.
1119
H
(3-30)
plus free-air reduction
+ 0
.
3086
H
Bouguer gravity at
P
0
g
B
=
g
+0
.
1967
H.
Since
g
B
now refers to the geoid, we obtain genuine gravity anomalies in the
sense of Sect. 2.12 by subtracting normal gravity
γ
referred to the ellipsoid:
∆
g
B
=
g
B
−
γ.
(3-31)
They are called
Bouguer anomalies
.
Terrain correction
This simple procedure is refined by taking into account the deviation of the
actual topography from the Bouguer plate of
P
(Fig. 3.6). This is called
terrain correction
or
topographic correction
.At
A
the mass surplus ∆
m
+
,
which attracts upward, is removed, causing
g
at
P
to increase. At
B
the
mass deficiency ∆
m
−
is made up, causing
g
at
P
to increase again.
The
terrain correction is always positive
.
The practical determination of the terrain correction
A
t
is carried out
by means of a template, similar to that shown in Fig. 2.20, using (3-22) and
adding the effects of the individual compartments:
A
t
=
∆
A.
(3-32)
earth's surface
A
H
H
B
P
Bouguer plate
H
H
P
geoid
P
0
Fig. 3.6. Terrain correction
