Part 3: Chart Data with a GIS

In any step, click the Show me link to reveal extra information. If you prefer a printout of the full set of instructions for this part, choose Print from the File menu.

Step 1-
Select Data for Graphing

In this step, you'll select the data necessary to create line graphs of shear wave velocities across North America. Each line graph will represent a vertical profile that shows how shear wave velocites change across the continent.
  1. If necessary, click the Zoom to Full Extent button to ensure that you are at the full world view. Turn on Plat_lin.shp, Country.shp, and the Depth_028_grid.shp themes and turn all other themes off.

    2. The Depth_028_grid.shp file shows the seismic shear wave velocities measured at a depth of 28 km below Earth's surface. What do the brown areas of this theme represent?

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    The brown areas represent continental crust, which transitions to the denser upper mantle beneath the oceans. Reminder: Shear waves move more slowly through continental crust and faster in the denser upper mantle.

    Do a screen capture of View1 and save it as Depth_028_grid.shp.

  2. Now you'll examine shear wave velocities at a deeper depth of 100 km. Turn off the Depth_028_grid.shp theme and turn on the Depth_100_grid.shp theme.

    3. At 100 km depth, has the velocity of seismic waves under the continents increased or decreased compared to measurements at 28 km depth?

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    Increased. This increase in velocity under the continents indicates that measurements come from the upper mantle material that is denser than the shallower continental material.

    Do a screen capture of View1 and save it as Depth_100_grid.shp.

  3. Turn on the Depth_028_grid.shp theme and make both the Depth_028_grid.shp and Depth_100_grid.shp themes active.
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    Make both themes active making one of them active, then hold down the Shift key and click the other one; a raised box will surround the name of each theme.
  4. Click-and-hold on the Select Feature button to select the line tool . You'll use it to drag a line across North America. On the map view, place your cursor in the Pacific Ocean off the coast of western North America. Click and drag a line across the continent and beyond into the mid-Atlantic, then double-click to end the line. A yellow line will appear.
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    The yellow line represents the selected data that will be graphed.
  5. With both the Depth_028_grid.shp and Depth_100_grid.shp themes active, click on the Open Theme Table button. Both theme tables will open. Drag them by their titles to position them so you can view both tables at once.
  6. Activate one of the tables by clicking it, then click the Promote button to bring the records you selected (the ones that your yellow line crosses) to the top of the listing. Do the same for the other table.

    7. Each record in these themes represents a measurement of shear wave velocity at the indicated depth. Out of the total 15,454 records in each theme, how many did you select with your line?

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    In the example below, 49 of 15454 total records in the Depth_028_grid.shp table have been selected and will be used to create a line graph. The table for Depth_100_grid.shp has a similar number of selected records. Yours may have more or fewer than 49, depending on the length of your line.

Step 2-
Create Two Graphs

  1. In the pull-down menu under Window, click 1 voyager.aprto open the Project window. Click once on the Charts icon then click on New. Select Attributes of Depth_028_grid.shp then click on OK to create a graph of the Depth_028_grid.shp table.
  2. Change the chart name to Depth 28 km, and add Velocity to groups and for series labeling.
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    Make the following changes in the Chart Properties dialog box.

    • Change the name to Depth 28 km.
    • Using the pull-down menu, select Velocity under Label Series Using.
    • Select the Velocity in the Fields box.
    • Click on the Add button.
    • Velocity will be added under Groups. Click on OK.

  3. Reverse the chart series so velocity values are plotted along the x-axis.
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    Change the chart series by clicking on the Series from Records/Fields button.

  4. Change the graph type to a Line Chart by clicking on the Line Chart Gallery button. Select the upper left chart in the Line Chart Gallery dialog box, then click OK.
  5. Now create a line graph for Depth_100_grid.shp by performing the above steps again, substituting Depth_100_grid.shp for Depth_028_grid.shp.

Your ArcVoyager project should now have 2 line graphs: Depth 28 km and Depth 100 km. In the next step, you'll modify the x- and y-axes and add a title.

Step 3-
Modify the Graph Elements

  1. Activate the Depth 28 km chart.
  2. Click the Chart Elements Properies tool to bring up the Chart Elements Properties dialog box.
  3. Use the dialog box to make the following modifications to the chart.
    • Remove the Tick Mark Labels along the x-axis and change the axis label to "Location."
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      Click on the X-axis. The following dialog box will appear.

      Remove the numbers along the x-axis and change the label by:
      • Unchecking the box beside Tick Mark Labels.
      • Checking the box beside Axis Label.
      • Changing the name in the Axis Label to Location.
      • Clicking on OK.
    • Change the y-axis to display a scale minimum of 3.5, scale maximum of 5.0, and major unit of 0.5. Also, display the major grid and label the axis "Velocity (km/sec)".
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      Click on the y-axis. The following dialog box will appear.

      Change the scale and the label by:
      • Checking the box beside Axis Label.
      • Checking the box beside Major grid.
      • Changing the name in Axis Label to Velocity (km/sec).
      • Changing Scale min to 3.5.
      • Changing Scale max to 5.0.
      • Changing Major unit to 0.5.
      • Clicking on OK.

    • Change the graph title to "Shear Wave Velocity at depth of 28 km" and select Chart > Hide Legend to hide the legend.
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      Click on the title. The following dialog box will appear.

      • Change the title to 'Velocity at depth of 28 km', then click on OK.
      • Pull down the menu under Chart and select Hide Legend
  4. Resize the chart box so you can view the entire graph. Do a screen capture, and save it as depth_28_chart.
  5. Activate the Depth 100 km chart. Perform the above steps above to produce to modify the graph for the 100 km data. When completed, do a screen capture of Depth 100 km, and save it as depth_100_chart.

Step 4-
Compare Graphs to Map

  1. Resize and move the Map View (View1) and Depth 28 km windows so that the line graph appears directly below the map. The yellow line in the View1 window corresponds to the line graph, which represents the change in shear wave velocities along the line.

    2. Examine corresponding sections of the map and graph together to understand how shear wave velocity at 28 km depth differs under the ocean and within the continental crust.


    Do a screen capture of View1 and save it as Depth_28_and_chart.

  2. Click and drag the Depth 100 km chart below the Map View.

    3. Examine corresponding sections of the map and graph together to understand how shear wave velocity at 100 km depth differs under the ocean and within the continental crust.


    Do a screen capture of View1 and save it as Depth_100_and_chart.

  3. Optional: Using a paint program or Layout capabilities within ArcVoyager, you can add a variety of graphics and annotations to the images.
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    Velocity at depth of 28 km


    Velocity at depth of 100 km.
  4. After examining your line graphs, answer the following questions.
    • What general statment can you make about shear wave velocities in the crust and in the mantle? (Do seismic waves travel at the same speed through both?)
    • Based on your observations, what can you say about the depth of the transition from crust to upper mantle under ocean basins and under continents?
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    Seismic waves travel more slowly through Earth's crust than through the mantle. The relative low density of crustal material cannot pass the shear wave energy along as efficiently as the higher density material of the mantle can, so the shear waves travel more quickly whenever they move through mantle material.

    Under ocean basins, denser upper mantle material is present at a depth of 28 km or less. Under continents, the transition from crustal material to mantle material appears to occur at a depth between 28 km and 100 km.


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