Geology Reference
In-Depth Information
In areas subject to heavy seasonal rainfall or snow melt, the level of
groundwater in the soil or in surface water may change significantly. The
effect is small and dependent on porosity, but is unlikely to exceed 0.01 mGal
for a metre change in water table (which is not likely to occur during a single
day's surveying).
2.4.7 Elevation control
The ways in which the elevations of gravity survey points are determined
depend on the purpose of the survey. An elevation error of 5 cm produces a
0.01 mGal error in Bouguer gravity, so 0.01 mGal contours, which are at the
limit of what is achievable with modern meters, require at least 1 cm relative
accuracies. This is still best achieved using optical levelling. Contours at the
0.1 mGal level require elevation control to
±
10 cm, which can be obtained
with real-time kinetic or differential GPS (see Chapter 15). Barometric
levelling or direct reference to sea-level and tide tables may be adequate for
the 5 or 10 mGal contours common in regional surveys.
Measuring elevation is an important and often time-consuming part of a
gravity survey, and advantage should be taken of any 'free' levelling that
has been done for other purposes, e.g. surveyed seismic or resistivity lines.
2.4.8 Error summary
An understanding of the environmental and other influences on reading
accuracy is essential, especially in microgravity surveys. The listing in
Table 2.1 of typical effect magnitudes also summarises the actions that
can be taken to minimise those effects. The impressive reading accuracies
quoted by instrument manufacturers (
2 µGal for the CG-5 and CG-3) are
almost irrelevant unless all other effects are taken into account. In micro-
gravity surveys the time between base station readings should not exceed
1-2 hours.
∼
2.4.9 Field notebooks
At each station, the number, time and reading must be recorded, and au-
tomatic meters incorporate data loggers that do this at a keystroke. Other
information, for example positional information (unless there is a suffi-
ciently accurate built-in GPS receiver) and elevation data from barometers,
must be recorded in field notebooks. Any factors that might affect readings,
such as heavy vibrations from machinery, traffic, livestock or people, un-
stable ground or the possible presence of underground cavities, should be
noted in a
Remarks
column. Comments on weather conditions may also be
useful, even if only as indicating the observer's likely state of mind. Where
local terrain corrections are only occasionally significant, estimates may
also be entered as 'Remarks', but individual terrain-correction sheets may
