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
GUTs: The Unification of Forces · Physics View original
Is this image relevant?
1 of 3
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