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

Exploration and Environmental Geophysics

Free-Air Gravity Exercise

Jeffrey A. NunnLouisiana State University

Page 2: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana 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

Page 3: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

Context

• Audience:– Graduate/Undergraduate;

Some PE students– 20 Students– Weekly Laboratory

• Prior Skills– Instrument Drift/Earth Tides– Free-Air Correction

Page 4: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

Instrument Drift and Earth Tides

Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940. Graph from galitzin.mines.edu/INTROGP

Page 5: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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).

Page 6: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 7: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 8: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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.

Page 9: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

Measurements

Above: Base of PendulumOpposite: Top of Attrium

Page 10: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 11: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 12: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

Conversion to mGals

Page 13: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 14: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 15: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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

Page 16: Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

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.


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