Field Assignment using the Cesium Vapor Magnetometer

Due December 4th, 2006

We have gone over the operation of the magnetometer in the field on two occasions, I suspect most of you are sufficiently familiar with it to complete your field assignment. however, I want each of you to take a careful look at the manual.

A couple of choices:

• Take the magnetometer to an open area (no cultural artifacts like buildings, power lines, etc.) and determine the magnetic anomaly on a grid, around a car, a barrel of toxic waste, or a small pile of mountain bikes - any known source will do. Make sure you sufficiently sample the anomaly, particularly close to source.
• Instead of a small static source as suggested above, you might want to investigate the influence of various power lines, develop a method to delineate a causative source near a building like the Science Complex, measure the effects from a dike or alteration zone, or come up with some idea of your own. The only constraint is that you learn something about magnetic sources and how to measure them with a proton precession magnetometer.

In any of the above cases, you need to determine the background magnetic field and the appropriate sample spacing to best characterize the magnetic anomaly from your source. You want to get your sample spacing small enough near-to-source to nicely characterize your anomaly. Further from source (since it is known) you can use a larger sample spacing to contour the anomaly. Remember, magnetic anomalies go as 1/r3; you want to collect just enough data to adequately and accurately characterize the anomaly. This will require some experimenting in the field. My objective is to get you familiar with the size and nature of various magnetic anomalies pertinent to environmental site assessments - we'll share and compare results.

The most common problem people make on their first survey is to collect too much data too far from the source and not enough data within the area of the anomaly. So, go through something like this when you are in the field:

• Measure B very close to your causative source (say within a meter of a car) and note the value
• Now step away from the source a meter or two at a time and measure B at each step. When B quits changing by several percent at each step, you are beyond the influence of the causative source
• Do the previous step in a perpendicular direction.
• You should now have a working knowledge of your peak anomaly value and the way it falls off with distance. Get sufficient points, within the area of the anomaly, to adequately recreate it on a contour map. You want to see the highs, lows, and local topography of the surface.
• Collect your data.

After collecting data you'll have to:

• download those data from the magnetometer to the lab computer.
• get them in cartesian coordinates (x, y, z; maybe you'll just type them in) in an Excel spreadsheet
• import them into Surfer (we'll look at Surfer in lab one day)
• grid them using kriging as the default gridding method
• make an annotated contour map
• write a short report using the same outline as suggested for your seismic report.
  • Introduction/Problem
  • Experimental Design
  • Results
  • Interpretation & Conclusion

Everybody (each individual) turns in a report, no more than three pages, by the first week of December, 2005. Include a description of your source, sample spacing, data acquisition and processing, maps of the total field anomaly and field-gradient anomalies using Surfer, and some thoughts on magnetic survey design in areas with cultural artifacts.

Web links on Proton Precession Magnetometers:

Build your own Proton Precession Magnetometer

Magnetometer Description and Design