2.2 Gravity anomalies and their interpretation

Gravity anomalies reveal density differences in Earth's subsurface. They're crucial for understanding underground structures and composition. Geophysicists use these variations to explore for resources, study crustal thickness, and investigate geodynamic processes.

Interpreting gravity anomaly maps involves analyzing shapes, sizes, and orientations of anomalies. Positive anomalies suggest dense materials, while negative ones indicate lighter materials. Combining this data with other geophysical info gives a fuller picture of subsurface geology.

Gravity Anomalies in Geophysics

Definition and Significance

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• Gravity anomalies are the differences between the observed gravity at a location and the theoretical gravity calculated from a reference model (Earth's ellipsoid or geoid)
• Caused by lateral variations in the density of the Earth's subsurface materials (rocks, sediments, and fluids)
• Studying gravity anomalies helps geophysicists understand the subsurface structure, composition, and processes of the Earth
  • Crustal thickness variations
  • Sedimentary basins
  • Mineral deposits
  • Geodynamic processes
• Used in various applications
  • Oil and mineral exploration
  • Geotechnical engineering
  • Geodynamic studies

Applications and Techniques

• Gravity anomaly data are measured using gravimeters, which are highly sensitive instruments that detect minute changes in the Earth's gravitational acceleration
• Gravimeters can be deployed in different settings
  • Land-based surveys
  • Ship-based surveys
  • Airborne surveys
• The choice of survey method depends on the survey requirements and accessibility of the study area
• Advanced processing techniques can be applied to gravity anomaly data to extract more detailed information about the subsurface structure and density distribution
  • Spectral analysis
  • Wavelet transforms
  • Inversion modeling

Interpreting Gravity Anomaly Maps

Gravity Anomaly Representation

• Gravity anomaly maps display the spatial distribution of gravity anomalies over an area
• Contour lines or color gradients are used to represent the magnitude of the anomalies
  • Contour lines connect points of equal gravity anomaly values
  • Color gradients assign different colors to different ranges of gravity anomaly values
• Positive gravity anomalies indicate the presence of higher-density materials in the subsurface (igneous intrusions, dense basement rocks, or mineralized zones)
• Negative gravity anomalies suggest the presence of lower-density materials (sedimentary basins, salt domes, or cavities)

Interpretation Techniques

• The shape, size, and orientation of gravity anomalies provide insights into the geometry and depth of the causative subsurface structures
• Interpreting gravity anomaly maps often involves the use of filtering techniques to enhance specific features and remove unwanted signals
  • Regional-residual separation isolates the gravity anomalies caused by deep-seated regional structures from those caused by shallow local features
  • Upward/downward continuation transforms the gravity anomaly data to different elevations, helping to identify the depth of the causative bodies
• Gravity anomaly interpretation is often integrated with other geophysical data to develop a comprehensive understanding of the subsurface geology
  • Seismic data provide information about the velocity structure and layering of the subsurface
  • Magnetic data reveal the distribution of magnetic minerals and the presence of igneous or metamorphic rocks

Gravity Anomalies and Density Variations

Relationship between Gravity Anomalies and Density

• Gravity anomalies are directly related to lateral variations in the density of subsurface materials
• The magnitude of a gravity anomaly depends on three factors
  • Density contrast between the causative body and the surrounding rocks
  • Depth of the causative body
  • Size and shape of the causative body
• Higher-density materials (mafic and ultramafic rocks) typically produce positive gravity anomalies
• Lower-density materials (sediments and felsic rocks) generate negative anomalies

Factors Affecting Density Variations

• Density variations in the subsurface can be caused by various factors
  • Lithological changes: different rock types have different densities (basalt vs. granite)
  • Compaction: increasing depth leads to higher densities due to the compaction of sediments
  • Fluid content: the presence of fluids (water, oil, or gas) in porous rocks reduces their bulk density
  • Temperature gradients: higher temperatures can lead to thermal expansion and a decrease in density
• The relationship between gravity anomalies and density variations is described by the gravitational attraction formula
  • $F = G \frac{m_1 m_2}{r^2}$, where $F$ is the gravitational force, $G$ is the gravitational constant, $m_1$ and $m_2$ are the masses of the objects, and $r$ is the distance between them

Measuring and Processing Gravity Data

Data Acquisition

• The measured gravity data must be corrected for various factors to obtain the true gravity anomalies
  • Instrument drift: gravimeters may experience a gradual change in their readings over time, which needs to be corrected
  • Tidal effects: the gravitational pull of the Moon and the Sun causes periodic variations in the Earth's gravity field
  • Elevation: gravity decreases with increasing elevation, so measurements must be corrected to a common reference level
  • Latitude: the Earth's rotation causes a centrifugal force that counteracts gravity, leading to a latitude-dependent variation in gravity
• Free-air correction accounts for the variation in gravity due to elevation differences between the measurement points and the reference level
  • $\Delta g_{FA} = -0.3086 h$, where $\Delta g_{FA}$ is the free-air correction in mGal and $h$ is the elevation in meters
• Bouguer correction removes the effect of the mass between the measurement point and the reference level, assuming a constant density for the intervening material
  • $\Delta g_{B} = 0.04191 \rho h$, where $\Delta g_{B}$ is the Bouguer correction in mGal, $\rho$ is the density of the intervening material in g/cm³, and $h$ is the elevation in meters

Data Processing and Visualization

• Terrain corrections are applied to account for the gravitational effect of the surrounding topography, which can be significant in areas with rugged relief
  • The terrain correction is calculated by dividing the surrounding area into sectors and estimating the gravitational effect of each sector based on its average elevation and distance from the measurement point
• The corrected gravity anomaly data are typically gridded and interpolated to create continuous gravity anomaly maps for interpretation
  • Gridding involves creating a regular grid of gravity anomaly values from irregularly spaced measurement points
  • Interpolation methods (kriging, minimum curvature, or inverse distance weighting) are used to estimate the gravity anomaly values at unsampled locations