Lab 9: Gravity SurveyingKyle Basler-Reeder - 04/08/2014

GEOL333 Structure, Tectonics, and Earth Physics II Lab

Part 1: Gravity Preliminaries

Question 1 - 3 points

State Newton’s law of universal gravitation.

Question 2 - 3 points

State Newton’s law of universal gravitation in terms of density.

Question 3 - 3 points

What is the difference between density and mass?

Question 4 - 3 points

Which gravitational force has greater magnitude: the Earth’s mass acting on a gravimeterspring or a gravimeter spring’s mass acting on the Earth?

Question 5 - 3 points

You are on a boat on the ocean. When the moon is on the opposite side of the Earth, whichmeans that gravity is pulling you down harder, is the ocean deeper or shallower beneath youcompared to when the moon is near the horizon?

Question 6 - 3 points

What is the minimum wavelength of bedrock undulations that a gravity survey conductedwith a station spacing of 200 m could discern? Think about the Nyquist criteria.

Question 7 - 3 points

Could a gravity survey be used to discern the thickness of a planar sheet of gold in sand-stone? If no, what would we need to figure this out?

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Question 8 - 3 points

What is the advantage of satellite-based gravity survey systems over other survey systems?

Question 9 - 7 points

Give the average density for the following geologic materials:

Water:Soil:Sandstone:Limestone:Granite:Basalt:

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Part 2: Introduction to Oasis Montaj GM-SYS

(1) Start -> All Programs -> Geosoft Oasis montaj - Edu. -> Oasis montaj - Edu.

(2) Click “Ok” when prompted that software support has ended

(3) Click “No” when prompted to open the test project

(4) File -> Project -> New -> name it “GEOL333 Lab 10” -> Click “Ok”

(5) GX -> Load Menu -> gmsys.omn -> Click “Open”

Notice: The GM-SYS menu has now been added to the menu bar.

(6) GM-SYS -> New Model -> Run GM-SYS

Collapse the following regions: Time (km), Magnetics (nT), Plan View (km). Your screenshould look like the above image.

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We will now create a new density model.

(7) File -> New Model (Depth)

(8) Enter the following parameters:

Geologic Strike Angle: 180Flat surface at elevation: 0 mOrigin X, Y, and Z: 0 mCoordSys -> Projected (x,y) -> Projection method -> UTM zone 11 NAzimuth: 90Profile X Min/Max: 0/10,000 mProfile Z Min/Max: 0/4,000 m

(9) Make sure that “Add Anomaly Data to new model?” is checked and click “Create”

(10) Manage Anomaly Data window -> “Gravity Survey” -> Add New/Replace Existing

(11) Click “Create equally-spaced stations”

(12) Enter the following parameters:

Starting X coordinate: 0Spacing: 200 mNumber of stations: 40Last X coordinate: 7800 mReference elevation: 0 mElevation relative to: sea level

(13) Click “OK”, then enter a background density of 2.67 cgs

(14) Once again, collapse Time (km), Magnetics (nT), and Plan View (km).

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(15) Click the “Examine” tool; the symbol is an eyeball

(16) The upside-down triangles are the positions of our stations. Click below them.

(17) You should see this screen. Change the bedrock to look like the above image.

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(18) Now it is time to add some blocks. Click “Add Point” and add two points near thecenter of the line of survey stations.

(19) Click “Split Block” and click on one of the points, then trace a box. See above figure.

(20) Click “Accept new block”.

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(21) Now repeat the block-splitting process, attaching another block to the bottom ofthe previous block.

(22) Click “Examine” and modify the lowermost block to the following parameters:

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(23) You will now see a lot of different lines in the mGal (Eotvos) window. For thislab, we just want the gravity anomaly. Right click on the mGal window -> “Show Grav.Gradients” -> “Switch to Show SBRs.”

(24) If at any time you want to readjust the scale of the plot, right click on the mGalwindow and select “Autoscale”

(25) Store the anomaly produced by the buried basalt block by right clicking on the mGalwindow and selecting “Manage Anomaly Data”.

(26) Select the “Calc. + D.C. (mGal)” column header

(27) Edit -> Copy Cells

(28) Select the “Obs. (mGal) column header

(29) Edit -> Paste Cells

(30) Press OK

(31) The observed gravity at each station is now indicated by black boxes. Select theleftmost of these to make that station the station that all others are computed relative to.

Your window should look like the following:

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Part 3: Exploring the Non-Uniqueness of Gravity

OK, now it’s time to get an intuitive feel for how gravity works. Click “Move Group”,then select the radio button “Mark Block” and click on the buried block. Now select theradio button “Move Marks”. You can now move the buried block.

You can modify the geometry of the block using the “Move Point” option.

Recall that you can modify density using the “Examine” tool.

Your objective is to approximately match the observed gravity response of the originalburied block with different configurations of block depth, block aspect ratio, and/or blockdensity. You may choose to complete any of the following configurations that I was ableto obtain or be creative. Each unique configuration is worth 3 points of 10 total for thequestion. Each configuration over the point total of 10 will be worth extra credit of 1 pointeach. I will choose the top 3 creative configurations and award additional extra credit. Makesure to label the block name using the “Examine” tool with the corresponding rock (or min-eral) type or lose 1 point. Take a screen capture of each configuration and attach it to the lab.

Example Configurations:

Vertically-elongate rectangular block of syenite

Smaller, horizontally-elongate pyrite (density 5 g/cc) block

Large block of dolomite with small eclogite block on top

Isosceles triangle block of gneiss

Octagon block of slate with square of amphibolite on top

Three vertically-elongate blocks of varying density and height

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Part 4: Modeling Common Geologic Structures

You may wish to save your current model for reference use. Next, go back to page 4 andstart a new model from step (7), but this time at step (13) use a background density of 1.8g/cc and at step (17) make the background density look like this:

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The objective for this part of the lab is to use the tools as your disposal to create gravitymodels of the following common geologic structures. Use steps (24) through (30) to storethe gravity anomaly of the high angle normal fault to get started. That will provide abasis of comparison for the other structures. Take a screen grab of the following commonly-encountered structures, each of which is worth 4 points:

-High angle normal fault-Very Low angle normal fault-Reverse fault-Buried valley

Your first screen grab should look like this:

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