7.1 Gravity and magnetic potential fields

Gravity and magnetic fields are invisible forces shaping our planet. They're like Earth's fingerprints, revealing hidden structures beneath our feet. Understanding these fields helps geophysicists uncover secrets about our planet's composition and structure.

Potential field methods use gravity and magnetism to explore the Earth's subsurface. By measuring tiny variations in these fields, scientists can map out underground features like mineral deposits, oil reservoirs, and even ancient buried landscapes.

Gravity and Magnetic Fields: Fundamental Principles

Vector Fields and Potential Theory

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• Gravity and magnetic fields are both potential fields that vary in strength and direction in three-dimensional space
  • Represented by field lines indicating the direction of the force at any given point
• Gravitational and magnetic fields can be described mathematically using potential theory
  • The gravitational potential and magnetic potential are scalar fields that decrease with distance from the source

Gravitational Field Characteristics

• The gravitational field is a conservative field produced by the mass of the Earth
  • Always points towards the center of the Earth
  • Strength decreases with distance from the center according to the inverse square law
• The strength of the gravitational field is measured in milligals (mGal)

Magnetic Field Characteristics

• The magnetic field is produced by the motion of charges, such as electric currents in the Earth's outer core
  • Dipole field with north and south poles
  • Field lines connect the poles
• The strength of the magnetic field is measured in teslas (T) or nanoteslas (nT)

Sources and Characteristics of Anomalies

Gravitational Anomalies

• Gravitational anomalies are variations in the Earth's gravitational field caused by lateral variations in the density of subsurface rocks
  • Positive anomalies indicate the presence of high-density rocks (e.g., igneous intrusions)
  • Negative anomalies indicate low-density rocks (e.g., sedimentary basins)
• Gravitational anomalies can be caused by geological structures such as sedimentary basins, igneous intrusions, and ore bodies
• The shape and amplitude of gravitational anomalies depend on the geometry, depth, and physical properties of the subsurface sources
  • Shallow sources produce narrow, high-amplitude anomalies
  • Deep sources produce broad, low-amplitude anomalies

Magnetic Anomalies

• Magnetic anomalies are variations in the Earth's magnetic field caused by lateral variations in the magnetic susceptibility of subsurface rocks
  • Positive anomalies indicate the presence of highly magnetic rocks (e.g., igneous intrusions)
  • Negative anomalies indicate weakly magnetic or non-magnetic rocks (e.g., sedimentary rocks)
• Magnetic anomalies can be caused by geological structures such as igneous intrusions, metamorphic rocks, and mineralized zones
• The shape and amplitude of magnetic anomalies depend on the geometry, depth, and physical properties of the subsurface sources
  • Shallow sources produce narrow, high-amplitude anomalies
  • Deep sources produce broad, low-amplitude anomalies

Regional and Local Anomalies

• Gravitational and magnetic anomalies can be regional or local in scale
  • Regional anomalies reflect large-scale geological features (e.g., sedimentary basins)
  • Local anomalies reflect smaller-scale features (e.g., ore bodies)

Potential Fields and Subsurface Structures

Sedimentary Basins

• Sedimentary basins typically produce negative gravitational anomalies due to the low density of sedimentary rocks compared to the surrounding basement rocks
  • The shape of the anomaly reflects the geometry of the basin

Igneous Intrusions

• Igneous intrusions often produce positive gravitational and magnetic anomalies due to their high density and magnetic susceptibility
  • The shape of the anomalies can provide information about the geometry and depth of the intrusion

Faults and Folds

• Faults and folds can produce linear or curved gravitational and magnetic anomalies
  • Depends on the contrast in physical properties across the structure
  • Depends on the orientation of the structure relative to the potential field

Mineralized Zones and Ore Bodies

• Mineralized zones and ore bodies can produce local positive or negative anomalies
  • Depends on their density and magnetic susceptibility relative to the host rocks

Depth Estimation

• The amplitude and wavelength of potential field anomalies can be used to estimate the depth to the source
  • Deeper sources produce broader, lower-amplitude anomalies
  • Shallower sources produce narrower, higher-amplitude anomalies

Density vs Magnetic Susceptibility: Effects on Potential Fields

Density and Gravitational Fields

• Density is a measure of the mass per unit volume of a material
  • Controls the gravitational field and gravitational anomalies
  • Rocks with higher density produce positive gravitational anomalies (e.g., igneous and metamorphic rocks)
  • Rocks with lower density produce negative anomalies (e.g., sedimentary rocks)
• The density of rocks depends on their composition and porosity
• The density contrast between different rock types determines the amplitude of gravitational anomalies
  • A larger density contrast produces a larger anomaly

Magnetic Susceptibility and Magnetic Fields

• Magnetic susceptibility is a measure of the extent to which a material can be magnetized in the presence of an external magnetic field
  • Controls the magnetic field and magnetic anomalies
  • Rocks with higher magnetic susceptibility produce positive magnetic anomalies (e.g., igneous and metamorphic rocks)
  • Rocks with lower susceptibility produce negative anomalies or no anomalies (e.g., sedimentary rocks)
• The magnetic susceptibility of rocks depends on their content of magnetic minerals, such as magnetite, pyrrhotite, and ilmenite
• The magnetic susceptibility contrast between different rock types determines the amplitude of magnetic anomalies
  • A larger susceptibility contrast produces a larger anomaly

Comparison of Gravitational and Magnetic Anomalies

• Gravitational and magnetic anomalies do not always coincide, as they are controlled by different physical properties
  • A rock unit may have a high density but low magnetic susceptibility, or vice versa
  • Results in different patterns of gravitational and magnetic anomalies