Geology Reference
In-Depth Information
Figure 2.6
Hammer chart (Zones E to I) overlaid on a topographic map.
The difficulties in estimating average heights in the larger compartments
are easily appreciated. The letters identifying the zones are difficult to see
in this example but are clear when the overlay is removed from the map and
viewed on its own.
estimated in the field (see Section 2.4.3) but can often be minimised by
careful selection of station location.
Adding terrain corrections to the simple Bouguer gravity produces a quan-
tity often known as the
extended
or
complete Bouguer gravity
. Topographic
densities are sometimes varied with geology in attempts to reduce terrain
dependence still further.
2.4 Gravity Surveys
A gravity survey is a basically simple operation but few are completed wholly
without problems, and in some cases the outcomes can only be described
as disastrous. Most of the difficulties arise because gravity meters measure
only differences in gravity field, so that readings have to be interrelated by
links to a common reference system.
