Gravity data reduction

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Jessie Harper
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1 Gravity data reduction REDUCTION: raw data à gravity anomaly data Temporal corrections tides and instrument drift Spatial corrections latitude and elevation GRS67 = gravity variation with latitude at sea level:

2 Elevation corrections after removing the effects of latitude, any variation in gravity is due to local topography (elevation) and local geology EFFECT OF ELEVATION: g depends on the distance from the centre of the Earth (r): g = GM r à further from centre = smaller g (pg 5 in notes): change in gravity with elevation change is: 2 E Δg = Δh (Δg in mgal and Δh in m)

3 Going from Site A to Site B: increase in elevation of 100 m predicted decrease in gravity is: Δg = = 30.86mgal This is not a small change!

4 Free air correction In order to compare the gravity data from Sites A, B and C, it is necessary to correct for the differences in elevation. à Free Air Correction (C FA ) First, define a reference level sea level, Prairies, elevation of Site A.

5 Free-air correction: C FA = Δh Δh is the difference in elevation between site and reference level for a site above the reference level (SITE B), C FA is added to observed gravity value ( moving site closer to centre of Earth) for a site below the reference level (SITE C), C FA is subtracted Resulting gravity value is called the Free Air Anomaly (g FA ) à this is gravity variation if you were to take measurements at a constant distance from the centre of the Earth

6 Free Air Anomaly from GRACE satellite data: gravity field measured at the altitude of the satellites, after correction for latitude effects (GRS67) (

7 Bouguer Correction Site B: observed gravity will be due to elevation AND all the mass below the surface elevation corrected with C FA but the gravity value will still be larger than at Site A because of the mass of the hill

8 Infinite layer Approximate the gravity effect of the hill by an infinite slab of uniform density and thickness. Remember: gravity of an infinite slab is: g = 2πGρΔh Δh is the thickness ρ is the density (assumed to be constant)

9 Infinite layer Correction for the difference in mass due to elevation is the Bouguer correction: C B = 2πGρΔ h = ρΔ h (mgal) Often assumed that ρ = 2670 kg m -3 (average density of crustal rocks). Other methods: local geology, borehole data, seismic velocities, Nettleton s method With ρ = 2670 kg m -3 : C B = Δh

10 For site B: C B = Δh = = 11.19mgal Not small! Site B: Above reference level à Subtract C B (removing the gravitational effect of the hill) Site C: Below reference level - therefore missing mass à Add C B Resulting gravity value is the Bouguer Anomaly (g B )

11 Summary of Elevation Corrections à Choose a reference level C FA = Δh C B = Δh Site elevation Above reference level: Below reference level: C FA C B Add Subtract Subtract Add

12 Terrain correction In areas of rugged topography, there is a non-negligible effect of topographic features surrounding the measurement site. At stations A and C: the presence of the hill will cause a small upward pull à observed gravity will be slightly too low At site B: the observed gravity will be slightly too low due to the missing mass in the adjacent valleys In both cases: need to ADD a correction to account for topographic effects

Terrain correction: Hammer chart divide surrounding region (to 22 km away) into compartments determine average elevation in each compartment calculate gravitational effect of each

13 Terrain correction: Hammer chart divide surrounding region (to 22 km away) into compartments determine average elevation in each compartment calculate gravitational effect of each compartment (empirical equation) sum of compartments = terrain correction à add to Bouguer anomaly Tedious and can have large uncertainties only used if there is significant topography

14 Gravity Data Reduction - removing known effects from observed gravity values: 1. Temporal corrections tides instrument drift 2. Latitude correction GRS67 equation 3. Elevation corrections Free air Bouguer terrain correction (if needed) If the resulting gravity value (Bouguer anomaly) is not zero: à gravity anomaly due to local density structure

15 Example of data reduction Using a relative gravimeter, these gravity values were recorded: outside CCIS building: mgal - elevation is 605 m above sea level at airport: mgal Assume data has already been corrected for instrument drift and tidal effects. The absolute value of gravity at the airport is: 981, mgal. Using sea level as the reference, what is the Bouguer gravity anomaly outside CCIS? The topography is fairly flat no terrain correction is needed

16 1. Find absolute gravity at CCIS outside CCIS building: mgal at airport: mgal à CCIS gravity is mgal larger Airport absolute gravity = 981, mgal Therefore CCIS absolute gravity is: 981, = 981, mgal

17 2. Gravity anomaly (w.r.t. GRS67) at CCIS Observed CCIS absolute gravity = 981, mgal Latitude is N à GRS67 equation gives a gravity value of 981, mgal (page 4 in notes) Observed is less than expected: à gravity anomaly is mgal Gravity (m/s2) GRS Latitude (degrees)

18 3. Free air anomaly at CCIS Gravity anomaly is mgal after latitude correction. Elevation of CCIS is 605 m above sea level. Taking sea level as a reference Δh=605 m Free air correction is: C FA = Δh = mgal ADD C FA (CCIS is above sea level) Free air anomaly is: g FA = = mgal

19 4. Bouguer anomaly at CCIS Free air gravity anomaly is mgal Need to correct for the 605 m thickness of material between CCIS and the reference level (sea level) using a density of 2670 kg/m 3 Bouguer correction is: C B = Δh = mgal (SUBTRACT THIS removing rocks to get to the reference level) Bouguer anomaly (sea level reference) is: g B = = mgal

20 Bouguer gravity data for Alberta (Pilkington et al., 2000)

21 Gravity survey procedures (on land) Survey design: station spacing depends on what you are studying - small (near-surface) features require closely-spaced measurements (Q#1 on assignment) conduct study on a 2D grid if possible helps with interpretation (sphere vs. cylinder) allows you to examine repeatability of measurements establish a base station and visit several times (at least once every 1-2 hours) one station should be at a location where absolute gravity measurements have been made

22 Gravity survey procedures At each station: measure latitude and elevation (GPS, leveling) level the gravimeter record the time at which the measurement is made observe surrounding topography à terrain correction needed? Terrain correction (Hammer) chart

23 Data reduction steps 1. Remove tidal effects (use tide model or base station readings) 2. Correct for instrument drift using base station readings 3. Calculate absolute gravity 4. Subtract GRS67 gravity value (theoretical gravity) latitude correction 5. Choose reference level and correct for station elevation free air and Bouguer corrections 6. Make any additional terrain corrections à the result is a map (or profile) of the Bouguer gravity anomaly in the study area

24 Observed gravity for Canada (

25 Bouguer gravity for Canada (

26 Bouguer gravity in Canada (

Note that gravity exploration is different to seismic exploration in the following way:

224B3 Other factors that cause changes in g and need to be corrected Note that gravity exploration is different to seismic exploration in the following way: In a seismic survey, the travel time depends