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.
- 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.
- 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:
- station number
- reading (record 60 second average)
- comments (unusual meter behavior, local "geology," cultural
- Enter your data in an Excel (or other) spreadsheet and reduce the
- perform a linear drift correction
- remove a regional if appropriate
- 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.
- 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?
- 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:
- 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
- Assume the tunnel has a diameter of 2 meters. What density contrast
- 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.
- How would you improve on this survey?