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Exploration and Environmental Geophysics

This laboratory exercise at Louisiana State University covers gravity acquisition, interpretation, refraction, reflection, resistivity surveys, GPR acquisition, and interpretation. Students will learn how to estimate height using gravity measurements, correct for instrument drift, and analyze noisy data. The goal is to appreciate the precision of a Gravimeter, understand measurement errors, and work in groups to interpret gravity data accurately. The exercise involves data acquisition, instrument corrections, conversion to mGals, and estimation of elevation through Free-Air Correction.

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Exploration and Environmental Geophysics

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

  2. 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 Laboratory Exercises

  3. Context • Audience: • 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, 317-320, 1940. Graph from galitzin.mines.edu/INTROGP

  5. 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) h where: 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).

  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.

  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 – 20-30 minutes a measurement if they spin the dial.

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

  10. Data Acquisition

  11. Drift Correction (linear) Base Base Top Top

  12. Conversion to mGals

  13. Drift Correction

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

  15. Estimate of Height from Free-Air Correction

  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.

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