Geoscience Reference
In-Depth Information
where
A
C
is the attraction of the compensation which is actually negative,
so that its removal is equivalent to the term +
A
C
.Thequantity
A
T
is the
attraction of topography, to be computed as the effect of a Bouguer plate
combined with terrain correction, Eq. (3-36), or in one step, as described in
Sect. 3.4;
F
is the free-air reduction approximated by (3-26).
−
Oceanic stations
Here the terms
A
T
and
F
of (3-72) are zero, since the station is situated on
the geoid, but the term
A
C
is more complicated.
In the Pratt-Hayford model, the procedure is as follows. The mass sur-
plus (3-53) of a suboceanic column of height
D
H
(Fig. 3.9) is removed
and used to fill the corresponding oceanic column of height
H
to the proper
density
0
. In mathematical terms, this is
−
A
C
=
−
A
1
+
A
2
,
(3-73)
where both
A
1
and
A
2
are of the form (3-32), ∆
A
is given by (3-22). For
A
1
we have
H
,
b
=
D
−
c
=
D,
(3-74)
and density
−
0
;for
A
2
we have
b
=
c
=
H
(3-75)
and density
0
w
.
In the Airy-Heiskanen model, the mass surplus of the antiroot,
1
−
−
0
,
is used to fill the oceans to the proper density
0
. The corresponding value
is again given by (3-73), where for
A
1
we now have
b
=
t
,
c
=
T,
(3-76)
and density
1
−
0
;andfor
A
2
we have, as before,
b
=
c
=
H
(3-77)
and density
0
−
w
.
In both models, Eq. (3-72) reduces for oceanic stations to
g
TI
,
ocean
=
g
+
A
C
.
(3-78)
