Gravity Data Acquisition Exercise

Do this exercise individually, except step 1. You will need to make a copy of your field notes for your survey partner.

  1. Collect gravity readings along a north-south profile perpendicular to the east-west tunnel which crosses the North Oval. Establish an easily identifiable base station on concrete somewhere in this area. The profile should consist of stations spaced 1 meter apart, extending 8 meters north and south of the sidewalk (tunnel) center, for a total of 17 stations. Begin and end the profile by taking a reading at the base station. See below for data you should collect in your survey.
  2. Prepare a neat "field log," including a description of the profile location and orientation (include a simple sketch) and individual station locations. Describe your procedures for locating stations, reading the meter, etc. For each station, show:
  3. Enter your data in an Excel (or other) spreadsheet and reduce the data:
  4. Make a graph of gravity (vertical axis) versus distance (horizontal axis) along the profile. Indicate north and south ends. On the horizontal axis, show station number, distance, and mark significant features (location of sidewalk/tunnel, lamp post, etc.). Since these are relative, not absolute, gravity readings, if the numbers to the left of the decimal point do not change, you can just use the "microgal portion" on the horizontal axis. For example, a station might have a reading of "199 microgals". Indicate the north and south edges of the sidewalk on the graph. Plot the data as points, which you can connect with straight lines if you like.
  5. Make a qualitative interpretation of the profile. What does the profile look like? What is the cause of any "anomalies" you see? Are they real, or the result of error, lack of precision, failure to make certain corrections? Which ones? What would the effect of the correction be?
  6. Make a quantitative interpretation of the anomaly. What simple geometric shape would best approximate a tunnel? Use the techniques we discussed in class to model the tunnel as this simple geometric shape (if you aren't sure what shape I'm talking about, check with me first). How deep is the tunnel? Make two models:
    1. Assume a density contrast appropriate for air versus the material which makes up the upper few meters of the North Oval. Explain where you got your "soil" density. Find the diameter of the tunnel
    2. Assume the tunnel has a diameter of 2 meters. What density contrast results?
  7. To the graph you made in step 4 add a curve showing the calculated gravity for the simple geometric shape you used in the previous step. Plot the calculated gravity as a line (curve), not as points; you will probably want to calculate the model field more frequently than every meter.
  8. How would you improve on this survey?