In-situ Density from Gravity by Nettleton's Method

Scott White
,
University of South Carolina, Dept Geological Sciences
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Summary

This activity develops students' understanding of gravity data corrections and density estimates by using a gravimeter to collect data and then interpret this data to find an estimate for the local bulk density.

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Context

Audience

Designed for undergraduate/graduate course in geophysics, used in environmental geophysics course for upper-division science/engineering majors
Designed for a geophysics course

Skills and concepts that students must have mastered

Mastered the concepts of gravity data collection, the concept of density, basic surveying or use of GPS

How the activity is situated in the course

This is a stand-alone field exercise that takes place in the context of lectures on the gravity method. The activity takes places during a 3-hour lab period plus homework.

Goals

Content/concepts goals for this activity

derive an estimate of the local bulk density by interpreting gravity data; master the reduction of raw gravity to anomaly

Higher order thinking skills goals for this activity

evaluating assumptions (such as density) and competing models, analysis of noisy data

Other skills goals for this activity

learning to work in the field, working in groups, operating gravimeter

Description of the activity/assignment

This lab exercise involves small teams of 3-4 students working to collect a profile of gravity measurements over a topographic feature. There is a slope break on our campus where this exercise is done, but any fairly prominent topographic feature will suffice. The students make several measurements with the gravimeter, and collect their own topographic data using meter-tapes and hand-levels although a GPS unit could also be used. The students then convert the meter readings into relative gravity measurements, and make graphs using several different assumed densities. This exercise gives the students practice in collecting field data, analyzing numerically and graphically, and understanding theoretical concepts.

Determining whether students have met the goals

Primarily, look for a reasonable density value obtained at the end of the exercise. Secondarily, check for proper data reduction. This second part is not too hard because the students will use the spreadsheet. Field notes are also checked but not much emphasis is placed on making highly accurate measurements with the gravimeter.

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