Gravity Modelling
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Transcript of 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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