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Questions and Answers
Modern gravimeters can detect a change of 1/100,000,000th g.
Modern gravimeters can detect a change of 1/100,000,000th g.
True
Gravimeters must be carefully leveled before a reading is made, which is a time-consuming process.
Gravimeters must be carefully leveled before a reading is made, which is a time-consuming process.
True
The data from a gravimeter is directly useful without any further processing.
The data from a gravimeter is directly useful without any further processing.
False
Gravity data can be corrected by applying several corrections including latitude correction, drift correction, and topographic correction.
Gravity data can be corrected by applying several corrections including latitude correction, drift correction, and topographic correction.
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Which of the following must be applied to raw gravity data in order for anomalies to be identified?
Which of the following must be applied to raw gravity data in order for anomalies to be identified?
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The spring inside the gravimeter may slowly creep or stretch over time, resulting in a change of readings. What is this called?
The spring inside the gravimeter may slowly creep or stretch over time, resulting in a change of readings. What is this called?
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Drift can be corrected by periodically returning to a base station to measure the temporal variation and subtracting it from the rest of the data.
Drift can be corrected by periodically returning to a base station to measure the temporal variation and subtracting it from the rest of the data.
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Why do we need to perform a latitude correction in gravity measurements?
Why do we need to perform a latitude correction in gravity measurements?
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A good gravimeter can detect a 0.01 mGal change, making it sensitive enough to notice a difference in gravity due to a movement of 12 meters north or south.
A good gravimeter can detect a 0.01 mGal change, making it sensitive enough to notice a difference in gravity due to a movement of 12 meters north or south.
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The effect of latitude is removed by subtracting the theoretical gravity from the observed gravity, a process known as the theoretical gravity formula.
The effect of latitude is removed by subtracting the theoretical gravity from the observed gravity, a process known as the theoretical gravity formula.
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The Eötvös correction is essential if gravimeter readings are made on moving objects like airplanes.
The Eötvös correction is essential if gravimeter readings are made on moving objects like airplanes.
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What is the main limiting factor in aerial gravity surveys?
What is the main limiting factor in aerial gravity surveys?
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Up to three further corrections are needed when gravity measurements are taken at different elevations.
Up to three further corrections are needed when gravity measurements are taken at different elevations.
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The free-air correction accounts for the decrease in gravity when moving farther from the Earth's center.
The free-air correction accounts for the decrease in gravity when moving farther from the Earth's center.
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The Bouguer correction accounts for the extra gravity felt due to the mass of rock below the position of the gravimeter.
The Bouguer correction accounts for the extra gravity felt due to the mass of rock below the position of the gravimeter.
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What is the formula used to calculate the Bouguer correction?
What is the formula used to calculate the Bouguer correction?
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The Combined Elevation Correction combines both the free-air and Bouguer corrections into a single correction, which is more efficient for analyzing gravity data.
The Combined Elevation Correction combines both the free-air and Bouguer corrections into a single correction, which is more efficient for analyzing gravity data.
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The standard Bouguer density used to perform Bouguer correction is 2.67 g/cm³.
The standard Bouguer density used to perform Bouguer correction is 2.67 g/cm³.
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Terrain Correction plays a significant role in accounting for the effects of topography on gravity measurements.
Terrain Correction plays a significant role in accounting for the effects of topography on gravity measurements.
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The pull of a nearby mountain has the same effect as the pull of a valley on gravity.
The pull of a nearby mountain has the same effect as the pull of a valley on gravity.
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The Terrain Correction is only applied to the nearby topographic features, and distant features are not considered.
The Terrain Correction is only applied to the nearby topographic features, and distant features are not considered.
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Terrain correction can be implemented with the help of digital elevation data and knowledge of local rock density.
Terrain correction can be implemented with the help of digital elevation data and knowledge of local rock density.
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The rule of thumb for Terrain correction is that it needs to be performed if the location of the gravimeter is less than 200 meters away from steep topography.
The rule of thumb for Terrain correction is that it needs to be performed if the location of the gravimeter is less than 200 meters away from steep topography.
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The Bouguer Anomaly is the result of applying all the necessary corrections, including drift, free-air, latitude, Eötvös and terrain corrections to gravity measurements.
The Bouguer Anomaly is the result of applying all the necessary corrections, including drift, free-air, latitude, Eötvös and terrain corrections to gravity measurements.
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The Bouguer Anomaly is the same as the Bouguer correction.
The Bouguer Anomaly is the same as the Bouguer correction.
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The Simple Bouguer Anomaly is the result of the Bouguer Anomaly without the terrain correction.
The Simple Bouguer Anomaly is the result of the Bouguer Anomaly without the terrain correction.
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The purpose of the Bouguer Anomaly is to give the anomaly due to the density variations below the datum, without the effects of topography and latitude.
The purpose of the Bouguer Anomaly is to give the anomaly due to the density variations below the datum, without the effects of topography and latitude.
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The Free-Air Anomaly is calculated by subtracting the International Gravity Formula (IGF) from the observed gravity and applying both the latitude and free-air corrections.
The Free-Air Anomaly is calculated by subtracting the International Gravity Formula (IGF) from the observed gravity and applying both the latitude and free-air corrections.
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The Standard Bouguer Anomaly uses a standard Bouguer density of 2.67 g/cm³.
The Standard Bouguer Anomaly uses a standard Bouguer density of 2.67 g/cm³.
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The Complete Bouguer Anomaly is derived by applying both the Bouguer correction and Terrain Correction.
The Complete Bouguer Anomaly is derived by applying both the Bouguer correction and Terrain Correction.
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The Isostatic Anomaly accounts for the effects of isostasy, which is the balance between the gravitational pull and the buoyant force of the Earth's crust.
The Isostatic Anomaly accounts for the effects of isostasy, which is the balance between the gravitational pull and the buoyant force of the Earth's crust.
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The Bouguer Anomaly map of Egypt is a visual representation of gravity anomalies after applying all the necessary corrections to gravity data.
The Bouguer Anomaly map of Egypt is a visual representation of gravity anomalies after applying all the necessary corrections to gravity data.
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Study Notes
Modern Gravimeters
- Detect changes of 1/100,000,000th g or 0.01 mGal.
- Rely on a pivoted mass on a beam attached to a spring.
- Require a thermostat to prevent thermal contraction or expansion.
- Must be carefully leveled before measurements (time-consuming).
- Some models have automatic leveling.
- Require protective transport boxes.
- Data is not directly useful; requires reduction to correct for various effects.
- Gravity anomalies are small, often comparable to corrections.
Gravity Corrections
- Measurements taken at different elevations need correction due to distance from Earth's center.
- Several corrections are applied to raw gravity data to identify anomalies.
- Drift: Change in readings even without device movement (daily). Corrected by returning to base station.
- Latitude: Correction due to Earth's shape; least at Equator, most at Poles, total variation ~0.5%.
- Eötvös: Correction for centrifugal force on moving gravity instruments (e.g., airplanes, ships). Significant correction (~2.5 mGal per km/hr).
-
Topographic: Correction for elevation changes:
- Free-air correction: Correction for the elevation difference.
- Bouguer correction: Correction for the mass of rock beneath the measurement point.
- Terrain correction: Correction for the irregular shape of the terrain.
- Time-dependent: Instrumental drift, Tidal affects.
- Place dependent: Spatial variations, Latitude variations, Elevation variations, Slab effects, Terrain Effects
Drift
- Change in readings even if device is stationary during a measurement.
- Spring inside gravimeter can slowly creep or stretch.
- Diurnal variations/tides can affect drift.
- Corrected by periodically returning to a base station to determine and subtract the temporal variation.
Drift Example
- Temporal drift variations must be corrected.
- Base station readings are normalized and subtracted from the raw data to correct for drift.
- Data should take into account all other corrections besides drift.
Field Procedure
- Establish base station location for repeated measurements.
- Establish gravity stations' locations.
- Record relative gravity and time at base station initially.
- Measure relative gravity at each station and record time at each.
- Return to base station periodically.
- Final reading taken at base station after recording gravity at last station.
Latitude Corrections
- Earth's rotation causes centrifugal force, impacting weight.
- Least at equator, most at poles (0.5% difference).
Theoretical Gravity
- Gravity based on a mathematical earth model (considering the ellipsoid shape instead of a sphere).
- Used to correct for latitude variations.
- Specific formulas are used depending on datum (e.g., POTSDAM, IGSN71, WGS84).
Eötvös Correction
- Needed for moving gravimeters (boats, airplanes).
- Earth's rotation introduces centrifugal force; weight is reduced at equator (~0.34% due to 465 m/s rotation).
- Correction is needed for eastward/westward travel to account for Earth's rotation.
Topographic Corrections
- Essential because gravity measurements are not taken at the same elevation. -Free-air corrections,
- Bouger corrections for mass of rock beneath.
- Terrain corrections adjust for irregular topography
- Water considerations require accounting for mass differences
Free-Air Correction
- Corrects for elevation variations by accounting for the decreasing gravitational force with increasing elevation.
- Roughly 0.3086 mGal/meter of elevation change.
Bouguer Correction
- Takes into account the mass of the material between the measurement point and the datum.
- 0.04192ph mGal (where p is the density and h is the elevation).
Combined Elevation Correction
- Combines free-air and Bouguer corrections for both positive and negative elevation variations.
- This is important as corrections for both need to be considered.
Selecting Reduction Density
- Standard Bouguer density (2670 kg/m³), suitable in most cases, assumes average crustal density.
- Alternative methods like direct measurement (core samples) or geological map readings to estimate rock types are also used.
- Density profile: (Nettleton method) used to collect closely-spaced readings, generate latitude, free-air, and Bouguer corrections simultaneously to determine the density that correlates least with topography
Bouguer Terrain Correction
- As mentioned earlier, the Bouguer terrain correction needed for irregular topography, making the Bouguer anomaly.
Finally...The Bouguer Anomaly
- Result of applying all corrections (free-air, Bouguer, latitude, terrain, and Eötvös).
- It isolates density variations below the datum from the effects of topography and latitude.
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