Preparation of Gravity Anomaly Maps
and their Interpretations in terms of Shape, Size and Depth
The Bouguer anomaly map is computed from a free-air anomaly map by computationally
removing from it the attraction of the terrain (the Bouguer reduction). The equation for
the simple Bouguer reduction is δG_f = 2Hρπγ, where H is the thickness of the plate, γ is
the constant of gravitation and ρ is the density of the material.
In case the terrain is approximated by a flat plate of thickness H, the height of the gravity
measurement location above sea level, we speak of a simple Bouguer reduction. In case
the terrain attraction is removed precisely, we speak of a refined Bouguer reduction. The
difference between these two maps is the differential gravitational effect due to
unevenness of the terrain (called the terrain effect). This is always negative.
The Bouguer anomalies are usually negative in mountains because of isostasy, since the
density of mountain ‘roots’ is lower, compared with the surrounding mantle. Typical
anomalies beneath the Himalaya are 100-150 milligals.
Anomalies of local extent are used in applied geophysics: if they are positive, this may
indicate metallic ores. At scales between entire mountain ranges and ore bodies, Bouguer
anomalies may indicate rock types. For example, the continuation of two ENE-WSW
trending anomalies seen north of the Narmada-Son Lineament may be due to the
extension of the Bundelkhand Craton beneath the alluvials in a ENE direction.
Variations in Bouguer anomalies are produced by rock bodies of various shapes and sizes
with densities different from the average density of the surroundings.
Causes of anomalies in the crust:
General:
$ boundaries between different rock formations
Specific:
$ sedimentary basins
$ faults (sometimes)
$ volcanic intrusions
$ volcanic plugs
$ changes in crustal thickness
$ salt domes
$ cavities (near surface - microgravity)
$ many, many other geological formations
1. Sphere
The most simple model to calculate.
Parameters:
$ gz: gravity anomaly caused by the spherical body
$ Density: ρ = ρ1 - ρ0. This is the density contrast between the spherical body and
surroundings
$ Radius of sphere: b
Interpretation of Gravity Anomalies
S. Farooq, Dept of Geology, AMU
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$ Depth to centre of sphere, h
$ Distance from point on surface directly above sphere, x
Anomaly caused by buried sphere:
Can be applied to:
$ Volcanic intrusions
$ Salt domes
$ near surface cavities
$ ... basically all situations where a near-spherical shape can be assumed for a geological
feature
2. Infinite plate
This is the same model as used in the Bouguer correction, with the anomaly calculated
by:
gz = 2πGρz
$ gz gravity anomaly caused by plate
$ ρ: density contrast between plate and surroundings
$ z: thickness of plate
Interpretation of Gravity Anomalies
S. Farooq, Dept of Geology, AMU
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Note that because the plate is infinite, the depth to its top does not affect gz.
This model works surprisingly well for all bodies that are very wide compared to their
thickness and depth to their top.
Can be applied to:
$ sedimentary basins
$ almost all flat + wide bodies
$ gives crude results
Regional - residual anomaly separation
Bouguer anomaly maps - the net sum of
anomalies caused by all sources underlying
the area which the map covers
Total Bouguer anomaly:
$ main trend of gravity field
+
$ one or several fluctuations from the main
trend
Main trend
$ regional anomaly
Variations
$ local anomalies
Figure shows example of separation of
regional and local gravity
field
$ local = total - regional
$ regional field: deep sources
$ local field: shallow-local source
Effect of depth of source on anomaly
As the depth to the source gets greater, the more ambiguous the interpretation gets
Any of the three bodies in c) is a potential source for the anomaly
Interpretation of Gravity Anomalies
S. Farooq, Dept of Geology, AMU
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Interpretation of Gravity Anomalies
S. Farooq, Dept of Geology, AMU
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