Geology Reference
In-Depth Information
Table 2.1
Typical magnitudes of sources of error, and corresponding
mitigating steps
Source of errors
Typical magnitude
Mitigating steps
Seismic (earthquakes)
Possibly hundreds of
mGal at stations near
earthquake epicentres,
but typically 0.1 mGal
for remote
earthquakes
Survey work may have to
be suspended for
periods of minutes to
several hours
Microseismicity
related to wave
action on shorelines,
tree root movements,
vehicle movement,
on-site drilling or
construction
activities
0.01 to 0.1 mGal
Digitally stack readings
and use a statistical
measure (e.g.
convergence of means)
to decide when a
reading is
representative. It may be
necessary to occupy a
station for 5 or more
minutes
Wind induced
vibration
0.01 to 0.1 mGal
Use a wind break
Station elevation (the
use of inappropriate
technology to
measure elevations
can lead to errors
that are significant
for any given survey
type)
Total effect about
0.2 mGal m
−
1
. Errors
proportional to errors
in elevation
Use optical levelling for
<
1-cm accuracies,
RTK-GPS for 10-cm
accuracies and
GPS/barometers for
typical regional surveys
Instrument height
(poor field technique
can easily result in
2-3 cm errors)
Up to 10 µGal (implied
by free-air effect of
0.3 mGal m
−
1
)
Take great care to
accurately measure to a
standard point on the
meter
Atmospheric pressure
changes
∼−
0.35 µGal mbar
−
1
(hPa) of atmospheric
loading
Monitor pressure changes.
Increase frequency of
repeats at base stations
Soft footing
Can cause 5 µGal or
more reading error
due to time-variant
off-levelling
Use an automatic meter
and stand well away
when it is accumulating
readings on soft ground
