Fiveable
Fiveable
Fiveable
Fiveable

Gravity is a fundamental force that shapes our universe, from keeping planets in orbit to forming galaxies. In geophysics, measuring gravity helps us understand Earth's structure and composition. This topic explores the principles of gravity and the tools used to measure it.

Gravimeters are essential instruments for measuring Earth's gravitational field. We'll learn about different types of gravimeters, including absolute and relative, and how they're used in geophysical surveys. We'll also explore how gravity data is interpreted to reveal hidden geological structures.

Gravity: Concept and Principles

Fundamental Force of Nature

Top images from around the web for Fundamental Force of Nature
Top images from around the web for Fundamental Force of Nature
  • Gravity is a fundamental force of nature that attracts objects with mass towards each other
  • One of the four fundamental forces, along with the electromagnetic force, the strong nuclear force, and the weak nuclear force

Newton's Law of Universal Gravitation

  • States that the force of gravity between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them
  • The gravitational constant, G, is used to calculate the force of gravity
  • The strength of the gravitational force depends on the masses of the objects and the distance between them
    • More massive objects and shorter distances result in stronger gravitational forces

Gravity's Role in the Universe

  • Responsible for keeping planets in orbit around the sun, the moon in orbit around Earth
  • Contributes to the formation of large-scale structures in the universe (galaxies, galaxy clusters)

Einstein's Theory of General Relativity

  • Describes gravity as a curvature of spacetime caused by the presence of mass and energy
  • Provides a more accurate description of gravity, especially in extreme conditions (near black holes, in the early universe)

Measuring Gravity

Gravimeters

  • Instruments used to measure the strength of the gravitational field at a specific location
  • Can be either absolute or relative gravimeters

Absolute Gravimeters

  • Measure the absolute value of gravity at a location by directly measuring the acceleration of a free-falling object in a vacuum
  • Examples include the FG5 and A10 absolute gravimeters
    • Use a falling mirror and laser interferometry to measure the acceleration due to gravity

Relative Gravimeters

  • Measure the difference in gravity between two locations
  • More portable and less expensive than absolute gravimeters
  • Examples include spring-based gravimeters and superconducting gravimeters

Gravity Gradiometers

  • Measure the spatial rate of change of the gravitational field
  • Provide information about subsurface density variations
  • Often used in mineral and oil exploration

Satellite-Based Methods

  • Gravity Recovery and Climate Experiment (GRACE) measures variations in Earth's gravity field
    • Tracks changes in the distance between two satellites as they orbit the Earth

Interpreting Gravity Data

Gravity Anomalies

  • Deviations from the expected value of gravity at a given location, based on factors such as latitude, elevation, and density of surrounding material
  • Positive anomalies indicate the presence of denser materials, while negative anomalies suggest less dense materials

Bouguer Anomalies

  • Gravity anomalies corrected for the effects of topography and the density of surface rocks
  • Used to investigate subsurface density variations
  • Can help identify geological structures (sedimentary basins, igneous intrusions, mineral deposits)

Isostatic Anomalies

  • Gravity anomalies corrected for the effects of isostatic compensation
    • Isostatic compensation is the process by which the Earth's crust and upper mantle adjust to maintain equilibrium
  • Provide insights into the thickness and density of the Earth's crust and upper mantle

Gravity Maps

  • Show the spatial distribution of gravity anomalies
  • Help identify geological structures (faults, folds, intrusions)
  • Used in mineral and oil exploration, as well as in studying the Earth's interior structure

Absolute vs Relative Gravity Measurements

Absolute Gravity Measurements

  • Determine the actual value of the gravitational acceleration at a specific location
  • Typically expressed in units of m/s^2 or Gal (1 Gal = 1 cm/s^2)
  • Independent of any reference point and provide the true value of gravity at that location
  • More accurate but require more expensive and complex instrumentation (FG5 or A10 absolute gravimeters)
    • Less portable and require a stable environment for operation

Relative Gravity Measurements

  • Determine the difference in the gravitational acceleration between two or more locations
  • Expressed in units of mGal (1 mGal = 10^-5 m/s^2)
  • Made relative to a reference point (base station with a known absolute gravity value)
  • Less accurate but can be made using more portable and less expensive instruments (spring-based gravimeters, superconducting gravimeters)
    • More suitable for field surveys and can cover larger areas more efficiently

Gravity Network Adjustment

  • Process of establishing a network of reference points with known gravity values using absolute gravity measurements
  • Used to tie relative gravity measurements to an absolute scale
  • Crucial for creating consistent and accurate gravity maps


© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary