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Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics II

tom.h.wilson

tom.wilson@mail.wvu.edu

Department of Geology and GeographyWest Virginia University

Morgantown, WV

Gravity L ab R eview

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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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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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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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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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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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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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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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Tom Wilson, Department of Geology and Geography

In GMSYS >Explore density contrast issues. Use DC shift = 0

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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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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.