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Exploration and Environmental Geophysics. Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University. Gravity Acquisition Gravity/Magnetics Interpretation Ray Paths Refraction Acquisition Landmark/Refraction Interpretation Reflection Acquisition Reflection Processing.
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Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University
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
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) 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).
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.
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 Pendulum Opposite: Top of Attrium
Drift Correction (linear) Base Base Top Top
Sensitivity of Gravity Interpretation (4 mGal) Base Base Top Top
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.