Gravity Anomalies





Dr. Tom Boyd of the Colorado School of Mines, has developed an interactive applet to introduce his students to gravity surveying.

There are a number of abandoned coal mine tunnels near Golden, Colorado.  Some of the tunnels are filled with air, some with water and some with water and air.  Can a gravity survey detect the negative gravity anomaly expected with a tunnel?

 You will need an Internet browser capable of displaying java script.  In general, Netscape 4.X or Explored 4.X should work just fine.

The workspace for this applet will look like the image shown below:


    
Read through the following and then try for yourself after you load the model.
    
The lower, green part of the applet shows the conditions of the model.  The tunnel is represented by the red circle.  When you click on the circle and it moves up and down.  The Model Parameters window can be moved out of the way.  Set Station Spacing, Number of Stations, and Std. Deviation of the Observations slider bars all the way to the left (this sets those conditions at the lowest possible values.  
    
Note that you can change the Tunnel Radius and the Density Contrast with the two remaining slider bars.  
    Assume that the density of the rock the tunnel is cut into is 2.65 g/cc (a sandstone).  The density of air is nearly 0.0 g/cc and that of water is 1.0 g/cc.
    Note that the tunnel modeled above must be filled with water as the density contrast is 1.65 g/cc.
    
As you change the model parameters, the gravity profile will appear on the top of the applet window (in blue).  The verticle axis is in miligals.  The greater the deflection of the gravity profile, the greater the strength of the gravity anomaly.  
    
Note that this anomaly is negative.  That means that materials between the gravimeter and the center of the Earth are less dense than to either side of where the tunnel is.

Load the  Gravity Anomaly Over A Tunnel applet.
    

  1. Set the three slider bars noted above all the way to the left.  Set the tunnel depth to 5 meters (about 16 feet) and radius to 1 meter.  Set the density contrast to 2.65 g/cc (filled with air).   Record the value at the tip of the anomaly (about -.023 mg).  A good instrument will let you make measurements down to about 0.001 mg.
    

  2. What do you think would happen to the magnitude of the anomaly if the tunnel radius were increased to 2 meters?  (it should increase because the volume of the low density material has increased).  Use the slider bar to increase the radius and confirm the predition.
    

  3. Keeping the radius 2 meters, what do you think will happen to the magnitude of the anomaly if the depth is increased to 10 meters?  Make the change an see what happens.  (the magnitude of the anomaly should decrease because its center of gravity is further from the recording instrument.
    

  4. What do you think will happen if the tunnel is filled with water?  Why?  Change the density contrast and see.
    

  5. What do you think would happen if Captain Kid had filled up the tunnel with gold?  (unfortunately, you can't try this but you should be able to predict what would happen)
    

    

Now, take the Potential Fields Quiz