(B) To measure variations in Earth's gravitational field Explanation: Gravity methods are used to measure small variations in Earth's gravitational field to study subsurface structures, such as ore bodies, voids, and sedimentary basins. These variations are caused by differences in the density of underlying materials.

(B) Gravimeter Explanation: A gravimeter is a highly sensitive instrument used to measure gravitational acceleration. It detects minute variations in the gravitational field that provide insights into subsurface structures.

(B) Milligal Explanation: Gravity variations are typically measured in milligals (mGal), a smaller unit of gravitational acceleration suitable for detecting minute changes.

(C) Higher density increases gravitational acceleration Explanation: Dense materials exert a stronger gravitational pull, causing a positive gravity anomaly in measurements.

(B) Presence of voids or low-density material Explanation: Negative gravity anomalies suggest regions with lower density, such as sedimentary basins or voids, which reduce gravitational acceleration.

(B) Terrain correction Explanation: Terrain correction adjusts gravity measurements to account for the influence of nearby topographical features on gravitational readings.

(C) A dense igneous intrusion Explanation: Dense materials, such as igneous intrusions, cause a positive gravity anomaly because they increase gravitational acceleration in the region.

(B) A corrected gravity value accounting for terrain and elevation Explanation: The Bouguer anomaly is the gravitational measurement corrected for the effects of terrain and elevation. It helps isolate the gravitational effect of subsurface density variations.

(B) Cost-effectiveness and passive nature Explanation: Gravity methods are relatively low-cost and passive (do not require active sources like seismic surveys), making them suitable for preliminary geological studies.

(B) The rate of change of gravity with distance Explanation: A gravity gradient refers to the spatial rate of change of gravitational acceleration, which can provide detailed information about subsurface density variations.

(C) Latitude correction Explanation: The latitude correction accounts for variations in gravitational acceleration caused by the Earth's oblate shape and rotation, which result in slightly different gravity values at different latitudes.

(A) Limited resolution for small-scale features Explanation: Gravity methods have low resolution for small-scale or shallow features, as they are better suited for detecting broader density variations in the subsurface.

(B) It accounts for the elevation of the measurement point above sea level Explanation: Free-air correction adjusts gravity measurements for the effect of altitude, assuming the absence of any intervening material, to normalize readings to sea level.

(B) A sediment-filled basin Explanation: Sediment-filled basins have lower densities than surrounding rocks, leading to a strong negative Bouguer anomaly.

(B) An anomaly caused by deep-seated geological structures Explanation: Regional anomalies are broad, low-frequency variations in gravity data that arise from deep-seated geological structures, as opposed to local anomalies caused by shallow features.

(B) In Bouguer anomaly maps Explanation: Gravity data are typically processed and displayed as Bouguer anomaly maps, which help geophysicists identify variations in subsurface density.

(D) Magnetic field intensity Explanation: Gravity measurements are influenced by factors like density, rotation, and topography but are independent of Earth's magnetic field intensity.

(B) Negative anomaly Explanation: Salt domes have lower densities than the surrounding rocks, resulting in a negative gravity anomaly in measurements.

(C) Earth's mass and density distribution Explanation: The Earth's gravitational field is generated by its mass and the density distribution within the planet. Variations in subsurface density cause measurable changes in the gravitational field.

Important MCQ on Gravity Methods in Geophysics

21. What is the main purpose of terrain correction in gravity methods?

(A) To eliminate the effect of Earth’s rotation
(B) To account for topographic irregularities near the measurement site
(C) To adjust for variations in Earth’s magnetic field
(D) To correct for latitude-related gravity variations

(B) To account for topographic irregularities near the measurement site
Explanation: Terrain correction adjusts for the gravitational influence of nearby hills and valleys, ensuring that the gravity data accurately reflect subsurface density variations.

22. What does a residual anomaly represent in gravity methods?

(A) A gravity variation caused by Earth’s rotation
(B) A localized gravity anomaly after removing regional trends
(C) An uncorrected gravity measurement
(D) A magnetic anomaly in the gravitational data

(B) A localized gravity anomaly after removing regional trends
Explanation: Residual anomalies are localized variations in gravity data after subtracting regional trends, often indicating near-surface features.

23. Which geological structure would most likely cause a positive gravity anomaly?

(A) A sediment-filled basin
(B) A salt dome
(C) A metallic ore body
(D) A void or cavity

(C) A metallic ore body
Explanation: Dense materials, such as metallic ore bodies, increase the gravitational acceleration, resulting in a positive gravity anomaly.

24. Why are latitude corrections necessary in gravity surveys?

(A) To adjust for instrument drift over time
(B) To compensate for variations due to Earth’s shape and rotation
(C) To remove effects of nearby terrain
(D) To account for atmospheric pressure changes

(B) To compensate for variations due to Earth’s shape and rotation
Explanation: Gravity varies with latitude because the Earth is not a perfect sphere (it is an oblate spheroid), and its rotation slightly reduces gravity at the equator compared to the poles.

25. What is the relationship between gravity and distance from the Earth’s centre?

(A) Gravity increases with distance
(B) Gravity decreases with distance
(C) Gravity remains constant
(D) Gravity fluctuates unpredictably

(B) Gravity decreases with distance
Explanation: Gravity decreases with the square of the distance from the Earth’s center, as described by Newton’s law of gravitation.

26. What type of data is collected in gravity surveys?

(A) Gravitational acceleration values
(B) Magnetic field intensity
(C) Seismic wave velocities
(D) Electrical resistivity

(A) Gravitational acceleration values
Explanation: Gravity surveys collect data on variations in gravitational acceleration, which are used to infer subsurface density differences.

27. How is gravity data used in petroleum exploration?

(A) To locate magnetic anomalies
(B) To identify low-density sedimentary basins
(C) To detect seismic activity
(D) To measure groundwater levels

(B) To identify low-density sedimentary basins
Explanation: Gravity methods are used to detect low-density sedimentary basins that may contain petroleum deposits.

28. What is the expected gravity anomaly of a high-density subsurface feature?

(A) No anomaly
(B) Negative anomaly
(C) Positive anomaly
(D) Fluctuating anomaly

(C) Positive anomaly
Explanation: High-density subsurface features, such as igneous intrusions, create positive gravity anomalies due to their stronger gravitational pull.

29. Which correction in gravity methods adjusts for instrument-related variations over time?

(A) Drift correction
(B) Bouguer correction
(C) Free-air correction
(D) Latitude correction

(A) Drift correction
Explanation: Drift correction is applied to gravity data to remove errors caused by gradual changes in the sensitivity of the gravimeter over time.

30. What is the Bouguer correction primarily used for in gravity surveys?

(A) To account for latitude variations
(B) To correct for the mass of the topography above the measurement point
(C) To eliminate the effect of Earth’s rotation
(D) To adjust for changes in atmospheric pressure

(B) To correct for the mass of the topography above the measurement point
Explanation: The Bouguer correction removes the gravitational effect of the mass between the measurement point and sea level, ensuring that gravity readings reflect subsurface features.

31. In which geological setting would you expect a gravity low?

(A) Over a dense mafic intrusion
(B) Over a high-density ore deposit
(C) Over a sedimentary basin
(D) Over a volcanic dyke

(C) Over a sedimentary basin
Explanation: Sedimentary basins typically have lower densities compared to surrounding rocks, resulting in gravity lows.

32. Which instrument is commonly used to measure gravitational acceleration in geophysical surveys?

(A) Gravimeter
(B) Magnetometer
(C) Seismometer
(D) Barometer

(A) Gravimeter
Explanation: A gravimeter is a sensitive instrument used to measure variations in gravitational acceleration at different locations.

33. Why are gravity anomalies typically expressed in milli gals (mgal)?

(A) To correct for latitude and elevation differences
(B) Because gravity variations on Earth are small and require finer units
(C) To standardize measurements across instruments
(D) To account for atmospheric effects

(B) Because gravity variations on Earth are small and require finer units
Explanation: Earth’s gravity variations are typically in the range of milligals, necessitating the use of smaller units for accurate representation.

34. What is the primary purpose of a gravity survey in mineral exploration?

(A) To identify magnetic anomalies
(B) To locate subsurface density variations indicating ore bodies
(C) To map water-bearing formations
(D) To study seismic wave propagation

(B) To locate subsurface density variations indicating ore bodies
Explanation: Gravity surveys help identify density contrasts in the subsurface, which may indicate the presence of mineral deposits.

35. What is a free-air anomaly?

(A) A gravity reading without any corrections applied
(B) A corrected value accounting for elevation but not density
(C) An anomaly caused by nearby bodies of water
(D) A regional gravitational effect

(B) A corrected value accounting for elevation but not density
Explanation: A free-air anomaly is the gravity measurement corrected for elevation differences, assuming there is no material between the measurement point and sea level.

36. Which region of Earth is likely to exhibit the highest gravity values?

(A) The equator
(B) The poles
(C) Mid-ocean ridges
(D) Deep sedimentary basins

(B) The poles
Explanation: Gravity is strongest at the poles due to Earth’s oblate shape and centrifugal force reduction from rotation.

37. What is the main advantage of using airborne gravity surveys?

(A) High resolution for small-scale features
(B) Rapid coverage of large areas
(C) Elimination of topographic corrections
(D) Ability to detect magnetic anomalies

(B) Rapid coverage of large areas
Explanation: Airborne gravity surveys are efficient for covering large and inaccessible areas quickly, although they provide less resolution compared to ground-based surveys.

38. How does the Earth’s crustal thickness affect gravity measurements?

(A) Thicker crust causes higher gravity values
(B) Thicker crust causes lower gravity values
(C) Crustal thickness does not affect gravity
(D) Crustal thickness only affects magnetic data

(B) Thicker crust causes lower gravity values
Explanation: Thicker crust generally has lower density compared to the mantle, resulting in lower gravity values over regions with thicker crust.

39. What is a “microgravity survey” primarily used for?

(A) Exploring deep crustal features
(B) Detecting small-scale density variations like voids or tunnels
(C) Mapping large-scale tectonic structures
(D) Studying variations in magnetic susceptibility

(B) Detecting small-scale density variations like voids or tunnels
Explanation: Microgravity surveys are highly sensitive and used for detecting small-scale subsurface features such as voids, cavities, or archaeological structures.

40. What is the relationship between gravity and distance from the Earth’s center?

(A) Gravity increases with distance
(B) Gravity decreases with distance
(C) Gravity remains constant
(D) Gravity fluctuates unpredictably

Detecting small-scale density variations like voids or tunnels
Explanation: Microgravity surveys are highly sensitive and used for detecting small-scale subsurface features such as voids, cavities, or archaeological structures.