Gravity Measurement and Corrections
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Questions and Answers

What does a modern gravimeter rely on?

  • A mass pivoted on a beam attached to a spring (correct)
  • A thermostat to prevent thermal contraction or extension (correct)
  • Careful leveling before a reading is made (correct)
  • All of the above (correct)
  • The data from a gravimeter is directly useful.

    False

    Which of these is NOT a type of gravity correction?

  • Drift
  • Eötvös correction
  • Topographic correction
  • Longitude correction (correct)
  • Tidal affects are an example of a time dependent variation.

    <p>True</p> Signup and view all the answers

    Terrain effects are an example of a place dependent variation.

    <p>True</p> Signup and view all the answers

    What is the correction that must be applied to a gravity measurement due to the difference in Earth's radius at different elevations?

    <p>Free-air correction</p> Signup and view all the answers

    What does drift refer to in gravity measurements?

    <p>Changes in readings caused by instrument changes over time</p> Signup and view all the answers

    Which of the following is NOT a method to correct drift in gravity measurements?

    <p>None of the above</p> Signup and view all the answers

    What is the Eötvös correction used for?

    <p>Correcting gravity measurements made on moving objects due to centrifugal acceleration.</p> Signup and view all the answers

    A gravity measurement made on a westward moving object will have a higher measured gravity.

    <p>True</p> Signup and view all the answers

    What is the most limiting factor in aerial gravity surveys?

    <p>Determining the accurate velocity of the airplane</p> Signup and view all the answers

    Topographic corrections are necessary when gravity measurements are taken at different elevations.

    <p>True</p> Signup and view all the answers

    What is the basic idea behind the free-air correction?

    <p>Correcting for the decrease in gravity due to increasing distance from Earth's center.</p> Signup and view all the answers

    What does the free-air correction typically amount to?

    <p>Approximately 0.3086 mGal per meter of elevation change.</p> Signup and view all the answers

    What does the Bouguer correction take into account?

    <p>The gravitational pull of rock mass beneath the location where the measurement is taken.</p> Signup and view all the answers

    What is the Bouguer correction typically based on?

    <p>An assumed infinite sheet or slab of uniform density.</p> Signup and view all the answers

    What is the combined elevation correction?

    <p>A combined correction that combines the free-air and Bouguer corrections to account for both elevation and rock density.</p> Signup and view all the answers

    The combined elevation correction is dependent on both elevation and rock density.

    <p>True</p> Signup and view all the answers

    Which of the following densities is typically used in the standard Bouguer anomaly calculation?

    <p>2.67 g/cm³</p> Signup and view all the answers

    What is the primary purpose of the Bouguer anomaly?

    <p>To isolate the gravitational anomalies due to density variations below the datum, without the influence of topography or latitude.</p> Signup and view all the answers

    The terrain correction is a simple correction that can be easily applied using a single formula.

    <p>False</p> Signup and view all the answers

    What is the rule of thumb regarding terrain corrections?

    <p>Terrain corrections are generally necessary if the observation point is less than 200 meters from steep topography.</p> Signup and view all the answers

    What is the final result of applying all the gravity corrections?

    <p>The Bouguer anomaly</p> Signup and view all the answers

    The Bouguer anomaly is essentially the same as the Bouguer correction.

    <p>False</p> Signup and view all the answers

    Study Notes

    Modern Gravimeters

    • Can detect a change of 1/100,000,000th g or 0.01 mGal.
    • Rely on a mass pivoted on a beam attached to a spring.
    • A thermostat prevents thermal contraction or expansion.
    • Must be carefully leveled before a reading (time-consuming).
    • Some models have automatic leveling.
    • Are delicate and need protective transport.
    • Data is not directly useful; needs reduction to correct for various effects.
    • Gravity anomalies are small, so corrections are similarly sized and important.

    Gravity Corrections

    • Measurements at different elevations need correction because distance to Earth's center affects gravity.
    • Corrections needed to analyze raw data for anomalies
    • Drift: Changes in readings even without movement, often due to spring creep or diurnal variations that are corrected using base station readings.
    • Latitude correction accounts for Earth's shape (oblateness) and latitude.
    • Eötvös Correction: Accounts for centrifugal force due to Earth's rotation, important for moving objects (vehicles).
    • Topographic corrections adjust for variations in altitude and terrain features (free-air and Bouguer corrections).
      • Free-air correction: Accounts for changes in gravity with elevation.
      • Bouguer correction: Accounts for the mass of the terrain above the measurement point.
    • Temporal variations:
      • Instrumental drift
      • Tidal effects
      • Latitude variations
      • Elevation variations
      • Slab effects
      • Terrain effects

    Drift

    • Drift is a change in readings that can occur even with a stationary device.
    • The spring inside the gravimeter can slowly creep or stretch.
    • Drift is corrected by periodically returning to a base station to measure the drift rate.
    • The drift is subtracted from the rest of the data.

    Drift Example

    • Temporal drift variations must be corrected for land-based surveys.
    • Base station readings are used to normalize and subtract the temporal variations to account for drift.

    Field Procedure

    • Establish the location of one or more base stations (e.g., station 9625).
    • Establish locations (numbered 158-163) for gravity stations to collect readings over the survey area.
    • Record base station relative gravity readings and time.
    • Collect relative gravity readings at each gravity station.
    • Return to the base station every hour, taking final readings after the last station.

    Causes of Tides

    • The variations of gravity due to the motion of the Earth and the moon are corrected for and accounted for in the Gravity calculations.

    Effects of Earth's Shape: Latitudinal Gravity Variations

    • Gravity varies with latitude due to Earth's oblate spheroid shape.
    • Least at the Equator, most at the Poles.
    • Variation is about 0.5%.
    • Earth's rotation causes centrifugal force larger at the equator than the poles and causes Earth to bulge outward.
    • This produces a difference in the observed gravity between the equator and the poles.

    Theoretical Gravity: Normal Ellipsoid, Latitude Correction

    • Gravity varies with latitude, this correction is determined using the International Gravity Formula (usually called a theoretical gravity formula).
    • The observed gravity must have the theoretical gravity subtracted from it to obtain the anomaly.
    • The formula accounts for the Earth's shape and latitude.
    • Corrections are ~0.8 mGal/km.

    Eötvös Correction

    • Necessary when gravity readings are taken from a moving object.
    • Gravity measurements are affected by centrifugal acceleration induced by movement.
    • Your weight reduction due to centrifugal force is about 0.34% at the equator because of rotation

    Topographic Corrections

    • Measure gravity at the same elevation for accuracy.
    • Free-air and Bouguer corrections needed when elevations differ; these correct for variation due to Elevation changes or Mass variations.

    Free-Air Correction

    • Accounts for change in gravity with altitude.
    • Gravimeter readings are corrected for variations due to elevation using the Free-Air correction.
    • The correction is approximately 0.3086 mGal/meter of elevation change.
    • Positive h = Mountain range, negative h = Valley.

    Bouguer Correction

    • Accounts for the mass of the terrain above the measurement point.
    • The difference between the observed gravity and the calculated gravity based on a uniform density slab is the Bouguer Anomaly..

    Combined Elevation Correction

    • The free-air and Bouguer corrections can be combined into one.
    • It calculates the total change in observed gravity due to both elevation and the mass of the terrain (using density).

    Selecting Reduction Density

    • Standard Bouguer density used for consistency across surveys.
    • Field measurements or geologic maps (data tables) can also be used for selecting appropriate density.

    Bouguer Terrain Correction

    • Bouguer correction assumes an infinite slab, reasonable around C. The Bouguer correction should be used.
    • Necessary when topography is present to account for the localized effect of mass.
    • Terrain and density are relevant for corrections.

    Finally... The Bouguer Anomaly

    • After applying all previous corrections to the observed gravity readings, the result is called a "Bouguer Anomaly."
    • The Bouguer Anomaly reflects density variations below a datum level, aside from the effects from topography and latitude.

    Bouguer Anomaly of Egypt

    • Example map of the Bouguer Anomaly of Egypt.

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    Gravity L3 PDF

    Description

    Explore the intricacies of modern gravimeters and the necessary gravity corrections for precise measurements. This quiz covers the mechanisms of gravimeters, the importance of leveling, and the various corrections needed for accurate gravity anomaly analysis.

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