Geology Reference
In-Depth Information
Figure 2.7
Gravity base station sketches. Two sketches, at different scales,
together with a short written description, are usually needed to ensure the
station can be reoccupied quickly and accurately.
topographic map can be gravitationally significant. Corrections can be es-
timated in the field using a truncated graticule such as that in Figure 2.8,
which covers the Hammer Zones B and C only. Height differences of less
than 30 cm in Zone B and 130 cm in Zone C can be ignored since they pro-
duce effects of less than a microGal per compartment. The charts can also be
used qualitatively, to select reading points where overall terrain corrections
will be small. The CG-5 gravity meter can calculate corrections for the B, C
and D zones, provided that the necessary topographic information is entered
before the reading sequence is begun.
The effect of a normal survey vehicle is detectable only if the observer
actually crawls underneath it, and most modern buildings produce similarly
small effects. Old, thick-walled structures may need to be treated with more
respect (Figure 2.9). Subsurface cavities, whether cellars, mine-workings or
natural caverns, can produce anomalies of well over 0.1 mGal. The gravity
method is sometimes used in cavity detection but where this is not the object
of the survey it is obviously important that stations are not sited where such
effects may occur.
2.4.4 Tidal effects
Before meter drift can be estimated (but after conversion of the reading from
meter scale divisions to mGal) allowance must be made for
Earth tides.
These are background variations due to changes in the relative positions
