Geology Reference
In-Depth Information
drift and tidal corrections are correctly applied. Accuracy is thus limited,
but not determined, by instrument sensitivity. Precision, which is concerned
only with the numerical presentation of results (e.g. the number of decimal
places used), should always be appropriate to accuracy (see Example 1.1).
Not only does superfluous precision waste time but false conclusions may
be drawn from a high implied accuracy.
Example 1.1
Gravity reading
=
858.3 scale units
Calibration constant
=
0.10245 mGal per scale division (see Section 2.2.6)
Converted reading
=
87.932835 mGal
But reading accuracy is only 0.01 mGal (approximately), and therefore:
Converted reading
=
87.93 mGal
(Note that five decimal place precision is needed in the calibration constant,
because 858.3 multiplied by 0.00001 is equal to almost 0.01 mGal)
Geophysical measurements can sometimes be made with more accuracy
than the interpreters need or can use. However, the highest possible accuracy
should always be sought, as later advances may allow the data to be analysed
more effectively.
1.5.4 Drift
A geophysical instrument will usually not record the same result if read
repeatedly at the same place.
Drift
may be due to changes in background field
but can also be caused by changes in the instrument itself. Drift correction is
often the essential first stage in data analysis, and is usually based on repeat
readings at
base stations
(Section 1.6).
Drift is often related to temperature and is unlikely to be linear between
two readings taken in the relative cool at the beginning and end of a day
if temperatures are 10 or 20 degrees higher at noon. Survey
loops
may
therefore have to be limited to periods of only 1 or 2 hours.
Changes in background field are sometimes treated as drift but in most
cases the variations can either be monitored directly (as in magnetics) or
calculated (as in gravity). Where such alternatives exist, it is preferable they
be used, since poor instrument performance may otherwise be overlooked.
Drift calculations should be made whilst the field crew is still in the survey
area, so that readings can be repeated if the drift-corrected results appear
suspect.
