GRAVITY METHOD - Field Geophysics - page 51

Figure 2.5 Bouguer and terrain corrections. Even after the application of
the Bouguer and free-air corrections, the gravity effects of the masses M
and m will appear on the maps as they are measured at the station point P,
and not as they would be measured at P on the reference surface. Terrain
corrections, because they are made for the deviations of the topography
from a surface through the gravity station and parallel to sea level, and not
from sea level itself, are always positive (see discussion in text).
correction, it is simpler to calculate the Bouguer gravity and then correct for
deviations from the Bouguer plate.
A peculiarity of the two-stage approach is that the second-stage correc-
tions are always positive. In Figure 2.5, the topographic mass (A) above
the gravity station exerts an upward pull on the gravity meter, the effect is
negative and the correction is positive. The valley (B), on the other hand,
occupies a region that the Bouguer correction assumed to be filled with rock
that would exert a downward gravitational pull. This rock does not exist. The
terrain correction must compensate for an overcorrection by the Bouguer
plate and is again positive.
Terrain corrections can be extremely tedious. To make them manually,
a transparent Hammer chart is centred on the gravity station on the topo-
graphic map (Figure 2.6) and the difference between the average height of
the terrain and the station height is estimated for each compartment. The
corresponding corrections are then obtained from tables (see Appendix).
Computers can simplify this process but require terrain data in digital form
and may be equally time-consuming unless a digital terrain model ( DTM )
already exists. The SRTM and ASTER topographic grids discussed in Sec-
tion 1.3.2 are extremely valuable in this respect, but are not reliable where
slopes are very steep. Corrections for the very near topography have to be