Grav Lab Rev
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Transcript of Grav Lab Rev
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7/30/2019 Grav Lab Rev
1/12
Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics II
tom.h.wilson
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Gravity L ab R eview
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7/30/2019 Grav Lab Rev
2/12
Tom Wilson, Department of Geology and Geography
time dial
reading
Converted to
milliGals
relative
difference
Tide &
Drift
Drift
corrected
Base Station 0 762.71 66.279499 0 0 0
1 22 774.16
2 54 759.72
3 77 768.95 66.821755 0.542256 -0.09307 0.635326
4 99 771.01
Base Station 110 761.18 66.146542 -0.132957 -0.13296 0
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7/30/2019 Grav Lab Rev
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Whats the tide and drift rate
Tom Wilson, Department of Geology and Geography
time dial
reading
Converted to
milliGals
relative
difference
Tide &
Drift
Drift
corrected
Base Station 0 762.71 66.279499 0 0 0
1 22 774.16
2 54 759.72
3 77 768.95 66.821755 0.542256 -0.09307 0.635326
4 99 771.01
Base Station 110 761.18 66.146542 -0.132957 -0.13296 0
66.147 66.28 0.1330.0012
110 110 min
g mG
t
= = =
Carry calculations out to 3 decimalplaces
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7/30/2019 Grav Lab Rev
4/12
Prepare a drift curve. Values are shown relative
to a base station value of 0
Tom Wilson, Department of Geology and Geography
Time (minutes)
0 20 40 60 80 100 120
g
(mGals)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Base Station
Drift Line
Initial
Base Station
Reference
Initial
Base Station
Measurement
Station 3 measurementis 66.822 mG. Its valuerelative to the base is
+0.542
3
With tide and drift rateof -0.0012 mG/min thedifference of station 3relative to the base is
increased.
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7/30/2019 Grav Lab Rev
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The fol lowing equation can be used to
compute the tide and drift correction:
( )( ) Relative Difference (mG) 0.0012mG/min . (min)g t t =
Relative Difference =obs base
g g
this equation works whether the drift is positive ornegative
( ) Relative difference (mG) calculated tide and driftg t =
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7/30/2019 Grav Lab Rev
6/12
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7/30/2019 Grav Lab Rev
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Tom Wilson, Department of Geology and Geography
Items on the list .
Turn in your gravity paper summaries Today
Finish reading Chapter 6
Questions on problems from the text? Problems6.1 and 6.3 are due this Thursday.
Questions about the Gravity Lab?
Bring up GMSYS & open your word document
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7/30/2019 Grav Lab Rev
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Tom Wilson, Department of Geology and Geography
Back to the gravity labStewarts simplification of the model and
the impact of edge effects
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7/30/2019 Grav Lab Rev
9/12
Tom Wilson, Department of Geology and Geography
Stewart uses different conversionfactors to convert inputs in
different units to obtain
gt 130=
This expression comes directly from tGgplate 2=
Stewart has solved it using a density =-0.6 gm/cm3. He has also included thefactor which transforms centimeters to
feet so that the user can input t in units offeet. g is in units of milligals.
where t is in feet130
tg = or
What are the implications of the negative densitycontrast on the resulting gravity anomalies?
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7/30/2019 Grav Lab Rev
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Tom Wilson, Department of Geology and Geography
In GMSYS >Explore density contrast issues. Use DC shift = 0
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7/30/2019 Grav Lab Rev
11/12
Tom Wilson, Department of Geology and Geography
Open up the upper pane, set the planview depth; specify a pattern. Note how the upper pane fills out. Do the same for the bedrock. Adjust plan view depth again Change range on Plan View scale.
Extending your model into the 3rddimension
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7/30/2019 Grav Lab Rev
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Tom Wilson, Department of Geology and Geography
Items on the list .
Turn in your gravity paper summaries Today. Keep reading Chapter 6 review remainder of thechapter (past page 378).
Any questions about problems 6.1 6.3. Theywill be due this Thursday, Nov. 11th.
Gravity Lab will be due next Tuesday, Nov. 16th.Well spend the rest of the day today and a little
time on Thursday to make sure your questions areanswered.