Gravity Modelling

5/27/2018 Gravity Modelling

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Gravity Modelling


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Basic Shapes 1

Spheres and Cylinders spreadsheet Adjust densities and depths and look at the

corresponding anomalies produced

What can you say about the steepness of thecurves and depth of burial?

What can you say about depth of burial versusdensity?

What can you say about sphere versus cylinder(Hint - match the peak anomalies and look at theanomaly shapes)?


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Basic Shapes 2

The Fault / edge of a slab anomaly the Slabspreadsheet

Try adjusting the angle of the fault and throwacross the fault?

What is the basic shape of the gravity anomalyacross a sharp edge (fault or intrusive contact)and where would you pick the contact?


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Basic Shapes 3

Layer equivalenceGrav2dColumn

spreadsheet

Open One profile try adjusting the densities

and thickness of the columns (btw do not

touch the cells below !)

Open two profiles and compare !!!!


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Basic shapesbasic lessons

The nearer the surface the higher the gradient

Elongate bodies have a contribution to their

anomalies from the distant part of the body

Layer equivalence points to what is called the

non-unique nature of gravity anomalies

they are the product of density contrast and

volumevery different shapes can give the

same answer ! Be aware, very aware !!!!


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Now for something real

The Cornubian Batholithaka the granites of SWEngland.

The granites of SW England have been of long interestas the gravity over them was first measured &

modelled back in the 50 and 60s and they helped forma significant stepping stone in our understanding ofmodelling sub-surface bodies. More recently there hasbeen renewed interest due to the fact that they arehot i.e. radioactive. This means that if they are thick

enough then they might act as a source of geothermalenergy and indeed they are being exploited for thisnow (e.g. Redruth, St Austell)


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Forward Modelling

Suck it and see !

Compare the anomaly of a known shape withthe field data, adjust the shape of the body

and recalculate until you get a satisfactory fit.

Relies upon geological knowledge and

plausability of the final model


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GRAVCADW

available at

http://www.cas.umt.edu/geosciences//faculty

/sheriff/Sheriff_Vita_abstracts/Sheriff_softwar

e.htm

You need to download and install the

software. (unfortunately I cannot get this on

to University machines)


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Try adjusting the corners of the bodies, the densities and the

bodies positions. Also use the view tab to adjust what you can see.

BE AWARE do not use the full screen button to expand the

modelling windowit fails. Equally you can expand the window by

dragging the edges BUT only so far ! You will (like me) cause it tofail!


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Switch View > anomalies to each and sum

Drag bodies


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Adjust densities


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Dartmoor

Run GRAVCADW from fresh

File

Gravity or Elevations

Open file dartmoor2.dat

When it says draw a new Model make sure you

switch the scale to kilometres and allow 70 km for

th horzontal and 15 km for the vertical scales!!!!!


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Bodmin

Run GRAVCADW from fresh

File

Gravity or Elevations

Open file Bodmin2.dat

When it says draw a new Model make sure you

switch the scale to kilometres and allow 70 km for

th horzontal and 15 km for the vertical scales!!!!!


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Density contrast for our models

For our purposes -0.13 gm.cc-1


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Some questions ?

How deep do you think the sheets of granite

are and how realistic is this ?

What factors do you need to control?

In the end can we get a real answer?


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Gravity Modelling Summary

Gravity is monopole and relatively easy tomodel regular and hence irregular shapes in 2,2.5 (strike corrected for cylinders) and 3D

It suffers from the non-unique solution as do allpotential fields (anomaly is the product of shape* density)

With geological and geophysical controls you cancome up with sensible geological models


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Inversion Modelling

Essentially flat topped or flat bottomed

Doesnt rely on pre-determined input from

the userhas some independence from theuser bias

Does not necessarily have any geologicalmeaning! Relies on basic shapes/controls e.g.flat top, flat bottom.


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Gravity Modelling