1. Exploration and Environmental Geophysics
Free-Air Gravity Exercise
Jeffrey A. Nunn
Louisiana State University

2. Laboratory Exercises Gravity Acquisition
Gravity/Magnetics Interpretation
Ray Paths
Refraction Acquisition
Landmark/Refraction Interpretation
Reflection Acquisition
Reflection Processing
Reflection Interpretation
Resistivity Survey
Resistivity Interpretation
Well Logs (BakerHughes)
GPR Acquisition
GPR Interpretation

3. Context Audience: Prior Skills
Graduate/Undergraduate; Some PE students
20 Students
Weekly Laboratory
Prior Skills
Instrument Drift/Earth Tides
Free-Air Correction

4. Instrument Drift and Earth Tides
Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, , 1940.
Graph from galitzin.mines.edu/INTROGP

5. Free-Air Correction
To correct for variations in elevation, the vertical gradient of gravity ( mGal·m-1) is multiplied by the elevation of the station and the result is added, producing the free-air anomaly. The free-air gravity anomaly is given by the formula:
FA = go - gt + (δg/δz) h
where: go = observed gravity (mGal) gt = theoretical gravity (mGal) δg/δz = vertical gradient of gravity ( mGal·m-1) h = elevation above mean sea level (m).

6. Goals Content/Concepts
Estimate Height of a Platform in Howe-Russell Atrium using gravity measurements
Instrument Drift/Earth Tides Corrections
Free-Air Correction
Noisy Data
Precise Corrections to raw data

7. Instructions
We will take a series of gravity measurements to estimate the height of the walkway near the top of the atrium in Howe-Russell.
After taking a measurement at a base station, we will take turns taking measurements around the pendulum.
At each station, we will record the time, location, temperature, and instrument reading.
Periodically we will return to an earlier station to document any instrument drift and/or earth tide effect.
We will then return to the base station in Howe-Russell to make a repeat measurement.
The instrument readings must be corrected for instrument drift/Earth Tides and converted to mGals. We will not make a latitude correction.
Station
Location
Time
Temp.
Dial
Gravity
Drift

8. LaCoste Romberg Gravimeter
Appreciate the precision of a Gravimeter
Learn about measurement error first hand
Appreciate the effort involved in data acquisition
Work in a group
Things I learned the hard way – minutes a measurement if they spin the dial.

9. Measurements
Above: Base of Pendulum
Opposite: Top of Attrium

10. Data Acquisition Measurement Location Time Temp., F Reading G1_1
Loading Dock
12:20
48.2
G1_2
Base Pend
12:43
G1_3
Top Pend
1:01
G1_4
1:20
G2_1
1:37
48.3
G2_2
1:45
G2_3
2:08
G2_4
2:19

11. Drift Correction (linear)
Base
Base
Top
Top

12. Conversion to mGals

13. Drift Correction Time Location Meter Corrected mGals 0.000
Loading Dock
23.000
Base Pend
41.000
Top Pend
60.000
77.000
85.000

14. Sensitivity of Gravity Interpretation (4 mGal)
Base
Base
Top
Top

15. Estimate of Height from Free-Air Correction
Location
mGals
Loading Dock
Measured
Height
Base Pend
del_g
meters
feet
Top Pend
4.364
14.141
46.384
4.412
14.297
46.894
 Observed
Meters
4.563
14.787
48.500

16. Laboratory Report Turn in a copy of data sheet plus
an explanation of how you converted dial measurements to mgals
plot of instrument drift/Earth tides correction
estimate of elevation of the walkway using a Free-Air Correction
a brief (paragraph or two) discussion of potential errors in the survey.