exploration and environmental geophysics free-air gravity exercise jeffrey a. nunn louisiana state...
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Exploration and Environmental Geophysics
Free-Air Gravity Exercise
Jeffrey A. NunnLouisiana State University
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
Context
• Audience:– Graduate/Undergraduate;
Some PE students– 20 Students– Weekly Laboratory
• Prior Skills– Instrument Drift/Earth Tides– Free-Air Correction
Instrument Drift and Earth Tides
Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940. Graph from galitzin.mines.edu/INTROGP
Free-Air Correction
To correct for variations in elevation, the vertical gradient of gravity (0.3086 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) hwhere:
go = observed gravity (mGal)gt = theoretical gravity (mGal)δg/δz = vertical gradient of gravity (0.3086 mGal·m-
1)h = elevation above mean sea level (m).
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
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
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 – 20-30 minutes a measurement if they spin the dial.
Measurements
Above: Base of PendulumOpposite: Top of Attrium
Data Acquisition
Measurement Location Time Temp., F Reading
G1_1 Loading Dock 12:20 48.2 2926.405
G1_2 Base Pend 12:43 48.2 2926.53
G1_3 Top Pend 1:01 48.2 2922.32
G1_4 Loading Dock 1:20 48.2 2926.503
G2_1 Loading Dock 1:37 48.3 2926.715
G2_2 Base Pend 1:45 48.2 2926.565
G2_3 Top Pend 2:08 48.2 2922.187
G2_4 Loading Dock 2:19 48.2 2926.685
Drift Correction (linear)
2922
2922
2923
2923
2924
2924
2925
2925
2926
2926
2927
2927
0 20 40 60 80 100 120
Time, minutes
Dia
l Re
ad
ing
Meter Corrected
Base Base
Top Top
Conversion to mGals
Drift Correction
Time Location Meter Corrected mGals
0.000 Loading Dock 2926.405 2926.405 3010.641
23.000 Base Pend 2926.530 2926.492 3010.731
41.000 Top Pend 2922.320 2922.253 3006.367
60.000 Loading Dock 2926.503 2926.405 3010.641
77.000 Loading Dock 2926.503 2926.405 3010.641
85.000 Base Pend 2926.565 2926.499 3010.738
108.000 Top Pend 2922.187 2922.213 3006.326
119.000 Loading Dock 2926.335 2926.405 3010.641
Sensitivity of Gravity Interpretation (4 mGal)
2922
2922
2923
2923
2924
2924
2925
2925
2926
2926
2927
2927
0 20 40 60 80 100 120
Time, minutes
Dia
l Re
ad
ing
Meter Corrected
Base Base
Top Top
Estimate of Height from Free-Air Correction
Location mGals
Loading Dock 3010.641 Measured Height
Base Pend 3010.731 del_g meters feet
Top Pend 3006.367 4.364 14.141 46.384
Loading Dock 3010.641
Loading Dock 3010.641
Base Pend 3010.738
Top Pend 3006.326 4.412 14.297 46.894
Loading Dock 3010.641
Observed Height
del_g Meters feet
4.563 14.787 48.500
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.