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

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  • Exploration and Environmental GeophysicsFree-Air Gravity Exercise

    Jeffrey A. NunnLouisiana State University

  • Laboratory ExercisesGravity AcquisitionGravity/Magnetics InterpretationRay PathsRefraction AcquisitionLandmark/Refraction InterpretationReflection AcquisitionReflection ProcessingReflection InterpretationResistivity SurveyResistivity InterpretationWell Logs (BakerHughes)GPR AcquisitionGPR Interpretation

  • Context

    Audience:Graduate/Undergraduate; Some PE students20 StudentsWeekly LaboratoryPrior SkillsInstrument Drift/Earth TidesFree-Air Correction

  • Instrument Drift and Earth TidesData are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940. Graph from galitzin.mines.edu/INTROGP

  • Free-Air CorrectionTo correct for variations in elevation, the vertical gradient of gravity (0.3086 mGalm-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 + (g/z) hwhere: go = observed gravity (mGal) gt = theoretical gravity (mGal) g/z = vertical gradient of gravity (0.3086 mGalm-1) h = elevation above mean sea level (m).

  • GoalsContent/ConceptsEstimate Height of a Platform in Howe-Russell Atrium using gravity measurementsInstrument Drift/Earth Tides CorrectionsFree-Air CorrectionNoisy DataPrecise Corrections to raw data

  • InstructionsWe 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 GravimeterAppreciate the precision of a GravimeterLearn about measurement error first handAppreciate the effort involved in data acquisitionWork in a group

    Things I learned the hard way 20-30 minutes a measurement if they spin the dial.

  • MeasurementsAbove: Base of PendulumOpposite: Top of Attrium

  • Data Acquisition

  • Drift Correction (linear)BaseBaseTopTop

    Chart1

    2926.4052926.405

    2926.532926.4924333333

    2922.322922.2530333333

    2926.5032926.405

    2926.5032926.405

    2926.5652926.499

    2922.1872922.213

    2926.3352926.405

    Meter

    Corrected

    Time, minutes

    Dial Reading

    Sheet1

    MeasurementTimeTemp., FReadingTimeMeterGradientsCorrected

    G1_1Loading Dock12:2048.22926.40502926.4052926.405MeasuredEstimated Height

    G1_2Base Pend12:4348.22926.53232926.532926.4924333333del_gmetersfeet

    G1_3Top Pend1:0148.22922.32412922.322922.25303333334.239413.737524303345.0590797148

    G1_4Loading Dock1:2048.22926.503602926.5030.00163333332926.405

    G2_1Loading Dock1:3748.32926.715772926.5030.0982926.405

    G2_2Base Pend1:4548.22926.565852926.5652926.499

    G2_3Top Pend2:0848.22922.1871082922.1872922.2134.28613.888528839945.5543745949

    G2_4Loading Dock2:1948.22926.6851192926.335-0.0042926.405

    InferredObserved Hieght

    TimeMeterCorrectedmgalsdel_gMetersfeet

    0.0002926.4052926.4053010.6414.563113914.786585365948.5

    23.0002926.5302926.4923010.731

    41.0002922.3202922.2533006.367

    60.0002926.5032926.4053010.641

    77.0002926.5032926.4053010.641

    85.0002926.5652926.4993010.738

    108.0002922.1872922.2133006.326

    119.0002926.3352926.4053010.641

    TimeMeterGradientsCorrectedMeasuredEstimated Height

    02926.4052926.405del_gmetersfeet

    232926.532926.4923010.731

    412922.322922.2533006.3674.23913.7445.06

    602926.5030.0022926.405

    772926.5030.0982926.405

    852926.5652926.4993010.738

    1082922.1872922.2133006.3264.28613.8945.55

    1192926.335-0.0042926.405

    InferredObserved Height

    del_gMetersfeet

    4.56314.7948.5

    Sheet1

    00

    00

    00

    00

    00

    00

    00

    00

    Meter

    Corrected

    Sheet2

    00

    00

    00

    00

    00

    00

    00

    00

    Meter

    Corrected

    Time, minutes

    Dial Reading

    Drift Correction

    Sheet3

  • Conversion to mGals

  • Drift Correction

  • Sensitivity of Gravity Interpretation (4 mGal)BaseBaseTopTop

    Chart1

    2926.4052926.405

    2926.532926.4924333333

    2922.322922.2530333333

    2926.5032926.405

    2926.5032926.405

    2926.5652926.499

    2922.1872922.213

    2926.3352926.405

    Meter

    Corrected

    Time, minutes

    Dial Reading

    Sheet1

    MeasurementTimeTemp., FReadingTimeMeterGradientsCorrected

    G1_1Loading Dock12:2048.22926.40502926.4052926.405MeasuredEstimated Height

    G1_2Base Pend12:4348.22926.53232926.532926.4924333333del_gmetersfeet

    G1_3Top Pend1:0148.22922.32412922.322922.25303333334.239413.737524303345.0590797148

    G1_4Loading Dock1:2048.22926.503602926.5030.00163333332926.405

    G2_1Loading Dock1:3748.32926.715772926.5030.0982926.405

    G2_2Base Pend1:4548.22926.565852926.5652926.499

    G2_3Top Pend2:0848.22922.1871082922.1872922.2134.28613.888528839945.5543745949

    G2_4Loading Dock2:1948.22926.6851192926.335-0.0042926.405

    InferredObserved Hieght

    TimeMeterCorrectedmgalsdel_gMetersfeet

    0.0002926.4052926.4053010.6414.563113914.786585365948.5

    23.0002926.5302926.4923010.731

    41.0002922.3202922.2533006.367

    60.0002926.5032926.4053010.641

    77.0002926.5032926.4053010.641

    85.0002926.5652926.4993010.738

    108.0002922.1872922.2133006.326

    119.0002926.3352926.4053010.641

    TimeMeterGradientsCorrectedMeasuredEstimated Height

    02926.4052926.405del_gmetersfeet

    232926.532926.4923010.731

    412922.322922.2533006.3674.23913.7445.06

    602926.5030.0022926.405

    772926.5030.0982926.405

    852926.5652926.4993010.738

    1082922.1872922.2133006.3264.28613.8945.55

    1192926.335-0.0042926.405

    InferredObserved Height

    del_gMetersfeet

    4.56314.7948.5

    Sheet1

    00

    00

    00

    00

    00

    00

    00

    00

    Meter

    Corrected

    Sheet2

    00

    00

    00

    00

    00

    00

    00

    00

    Meter

    Corrected

    Time, minutes

    Dial Reading

    Drift Correction

    Sheet3

  • Estimate of Height from Free-Air Correction

  • Laboratory ReportTurn in a copy of data sheet plus an explanation of how you converted dial measurements to mgalsplot of instrument drift/Earth tides correctionestimate of elevation of the walkway using a Free-Air Correctiona brief (paragraph or two) discussion of potential errors in the survey.