|Syllabus reference (October 2002 version)|
2. Some physical phenomena such as gravitation and radiation provide information about the Earth at a distance from it
Students learn to:
Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
Prior learning: Preliminary modules 8.2, 8.3, 8.4.
Background: Most if not all, large mineral deposits now found in the world are found by the utilising processes of remote sensing. The ability of remote sensing techniques to provide baseline data for widespread environmental management programs is immense. Without remote sensing it is unlikely that many of the mineral deposits recently found would have been found. The techniques are relatively non invasive and hence cause little or no damage to the areas under study.
Note that the gravity difference around the world at different locations would result in differences in the calculation of the mass of the Earth.
Remember that gravity anomalies are measured in milliGals (mGal or mgal) or gravity units (g.u.)
|CLIENT: XYZ MINERALS|
|GRAVITY METER : Gxyz|
|Station||Field Reading||Time / Elapsed time (minutes)||Drift corrected reading||Meter scale constant||Relative gravity||Elevation
|Latitude correction||Bouguer Gravity|
The combined drift correction =
f b= final base reading, f i = initial base reading, T total = elapsed time for loop in minutes and
T elapsed = elapsed time in minutes for reading being processed.
From the above data: loop time = 120 minutes (09:42 to
Final base reading – initial base reading = 2360.72 – 2360.62 = 0.10
Therefore drift correction = -0.10/120 = -.000833 meter units / minute
The drift corrected value for station 1000E/0900N is then:
2368.00 – 08 x 57 = 2363.44 for reading taken at 1039
Elevation Correction = + (0.3086 –
where ρ is the mean density of the near surface rocks in g.cm-3 and h is the elevation of the gravity station in metres relative to a datum level for the survey.
In these data the elevation is relative to the base station and the local near surface rock density is close to 2.2g.cm-3. (Note. Local density can be estimated by a method called Nettledon Profiling where the density that results in the reduced data bearing least resemblance to the topographic profile, yields the most correct density)
The symbol for the latitude correction is gø and the correction is given by
where θ is the latitude. Note that the correction is a maximum at latitude 450 where it is a change of about 0.01mGal every 13m. Near Sydney the correction is about 0.07mGal/100m.Topographic corrections
There are three types: Free-air correction, Bouguer correction and Terrain correction. Only the free-air and Bouguer corrections are dealt with here but in areas of high relief, topographic corrections are needed to take into account the relative lateral attraction of hills and valleys.
Free-Air Correction. - Gravitational attraction decreases as elevation increases. To account for this, a correction needs to be made relative to a specific altitude. This altitude (the datum level) may be sea level or the height of a station chosen by the geophysicist. If g0 is the observed gravitational attraction at the datum level then at a height h above it the observed gravitational attraction g is given by
where R is the radius of the Earth.
For small changes the free-air correction (CF) is given by
where h is the elevation change from the datum level in metres. Note that this is a relatively large correction. It means that a geophysicist needs to know the elevation to within about 30cm if the gravity measurement is to be accurate to within 0.1mGal.
Bouguer Correction. - If a measurement is made above a datum level the effect of the rock material between the measuring station and the datum level must be taken into account. It makes a positive contribution to the gravity reading.
The Bouguer correction (CB) is given by
where h is the height above the datum level
in metres and ρ is the density of the rock
material in kg m-3. The correction is named
after Pierre Bouguer who first dealt with the effect in
his measurements in the Andes in 1735. To see how the
three corrections work together, consider the following
diagram showing a cross section along a gravity survey
Location 1 is our datum level and the traverse is from south (S) to north (N).
The latitude correction is added to take account of the increasing Earth radius as we move north.
The free-air correction is added at station 2 because it is above the datum level to take the increased elevation into account. The free-air correction is subtracted at station 3 because it is at a lower elevation.
The Bouguer correction(CB) is subtracted at station 2 to remove the effect of the rock between the station and the datum level. This correction is added at station 3 to compensate for the ‘missing’ mass of the valley.
The calculations in the diagram are called simple Bouguer anomalies (Bouguer anomalies contain a terrain correction). A free-air anomaly is one which involves only the free-air and latitude corrections. This type of correction is used at sea or on flat land.
As a final note it should be recognised that in gravity surveys a drift correction is also made to account for changes in the gravity meter and tidal effects.