Magnetic Data Acquisition Exercise

Abbreviated 2000 Version

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

1. Collect magnetic readings along a north-south profile perpendicular to the east-west tunnel which crosses the North Oval. Make your station spacing 60 cm and extend the survey 6 meters on north and south of the tunnel center, for a total of 21 stations. Begin and end the profile by taking a reading at a base station. Put the bottle (sensor) on the pole, and be sure the bottle is directly above the station location. 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:
   • station number
   • location
   • time
   • dial readings (Take 3 readings at each station occupation; if one reding is quite different from the other 3, discard it and read again. Near the tunnel/sidewalk, readings will likely be less consistent.)
   • comments (unusual meter behavior, local "geology," nearby cultural objects, etc.)
3. Enter your data in an Excel (or other) spreadsheet and reduce the data:
   • average the dial readings
   • perform a linear drift correction
4. Make a graph of magnetic field strength versus distance along the profile. On the horizontal axis, show station number, distance, and mark significant nearby features (location of sidewalk/tunnel, lamp post, etc.)
5. Make an 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? 

STOP HERE

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

1. Collect magnetic readings along 3 north-south profiles perpendicular to the east-west tunnel which crosses the North Oval. Profiles should be spaced 10 feet apart. Establish a base station somewhere in this area that all survey teams use (so we can tie the two data sets together). For each profile, make your station spacing 2 feet and extend the survey 20 feet on north and south of the tunnel center, for a total of 21 stations per profile, or 63 stations altogether. Begin and end each profile by taking a reading at the base station. Put the bottle (sensor) on the aluminum pole, and be sure the bottle is directly above the station location. 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:
   • station number
   • location
   • time
   • dial readings (Take 3 readings at each station occupation; if one reding is quite different from the other 3, discard it and read again. Near the tunnel/sidewalk, readings will likely be less consistent.)
   • comments (unusual meter behavior, local "geology," cultural objects, etc.)
3. Enter your data in an Excel (or other) spreadsheet and reduce the data:
   • average the dial readings
   • perform a linear drift correction
4. Make a graph of magnetic field strength versus distance along each profile. On the horizontal axis, show station number, distance, and mark significant features (location of sidewalk/tunnel, lamp post, etc.)
5. Make an interpretation of each of the profiles. 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? How do the 3 profiles compare to each other? Are the anomalies consistent?
6. Contour the data using Surfer (in the Golden Software group on the machines in M220). You will need to create an x-y data set. This can be done in Excel, but you will need to save the file in Excel 4.0 format. You can have a header line. The first few lines of the spreadsheet will look something like this:

x (ft north) y(ft east) mag (gammas)
0            0          57809
2            0          5765

Experiment with the different display types in Surfer (simple contour lines, filled contours, shaded relief, "image", etc.)