# 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

• 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

Sheet1

G1_2Base Pend12:4348.22926.53232926.532926.4924333333del_gmetersfeet

G1_3Top Pend1:0148.22922.32412922.322922.25303333334.239413.737524303345.0590797148

G2_2Base Pend1:4548.22926.565852926.5652926.499

G2_3Top Pend2:0848.22922.1871082922.1872922.2134.28613.888528839945.5543745949

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

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

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

Sheet1

G1_2Base Pend12:4348.22926.53232926.532926.4924333333del_gmetersfeet

G1_3Top Pend1:0148.22922.32412922.322922.25303333334.239413.737524303345.0590797148

G2_2Base Pend1:4548.22926.565852926.5652926.499

G2_3Top Pend2:0848.22922.1871082922.1872922.2134.28613.888528839945.5543745949

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

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

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.