GPS and Gravity Field Assignment - an evolving assignment due April 17th

The objective here is to initiate and complete a gravity survey of your own using the CG3 gravity meter and our Trimble GPS equipment which will yield the submeter elevation control we require for the gravity survey.

0. One of our graduate students, Shandin Pete, is working on a geohydrology project on the Jocko River, near Arlee. Shandin needs a good estimate of depth to bedrock across the valley, which would make a great small-scale gravity study. Recommended, I'd like to see this as a group project and we could cover gas money.

1. The Lewis and Clark fault system strikes NW along the north edge of the Missoula Valley. The system is comprised of several strands. Casey Evans (1997) found an interesting offset of the main strand northeast of the airport. Andy Hennes (PPT, 2002) determined the configuration of the fault systems that bound the northeast corner of the Missoula Valley, near the mouth of Hellgate Canyon. Currently the gravity data, and thus constraints on the fault's shape, are sparse between where Andy and Casey worked. This assignment is to provide and interpret at least two profiles of complete Bouguer anomalies across the north edge of the valley as shown on the linked map.

2. Recall our little experiment with measuring the height of the table and desk in my lab. You could follow up on that experiment and, with multiple observations, determine the gradient of gravity in the Science Complex. Compare that with the gradient as measured outside and away from the building and near and far from Mount Sentinel. Because I also want you to learn the GPS equipment, you should also complete an experiment that lets you assess the uncertainty of repeat measurements with our PRO-XRS system.

3. I'd be really interested in a detailed gravity survey, using a pretty tight grid, over a UM steam tunnel just to see what you can come up with. This sort of scale is applicable to archeology as well as environmental geoscience.

4. Think of something interesting, challenging and rewarding on your own and let me know what it is.

Teams of two to three can work together to collect the data. However, each individual is responsible for their own data and the processing and interpretation of those data. I want a well-written report on your experiment from each participant.

The complete Gravity Flow Chart has all the details of completing a gravity survey, we've gone over most of the steps in class, will go over some more on a rainy day, and I am available for clarification. A short list of tasks goes like this:

• collect a sufficient number of GPS and gravity observations that are Correctly spaced for your experiment
• drift correct those points
• calculate the free air anomaly, simple, and complete Bouguer anomalies
• compare your results with others in your group
• you'll probably find Surfer and GravCadW handy for presentation and interpretation of your data.
• Turn in a well-presented, nicely written and illustrated report detailing your experimental method and interpretation with no more than three pages of precise text.