Gravity Method in Geology

Gravity Method

• Gravity prospecting uses Earth's passive potential fields.
• Subsurface geology is mapped based on density variations in subsurface rocks.
• A causative subsurface mass creates a localized gravity anomaly.
• Small-scale anomalies indicate buried landforms like valleys.
• Larger-scale anomalies are linked to salt domes, granite plutons, or sedimentary basins.
• Micro-gravity surveys pinpoint very small features like cavities.

Gravity Method Applications

• Shape of the Earth is determined.
• Hydrocarbon exploration is enhanced.
• Regional geology is studied.
• Isostatic compensation is determined.
• Subsurface cavities (microgravity) are detected.
• Buried valleys are localized.
• Glacier thickness is measured.
• Tidal oscillations are studied.
• Basin geometry is analyzed.

Basic Theory

• Newton's Law of Gravitation is fundamental.
• The gravitational force between two masses is inversely proportional to the square of the distance between their centers.
• The universal gravitational constant (G) is a key factor.
• G's experimentally determined value dictates the strength of the attraction.

Units of Gravity

• The average surface gravity is approximately 9.8 m/s².
• Variations due to density differences are around 100 μms⁻².
• Gravity units (gu) are used in surveys.
• High accuracy (±0.1 gu) is achievable on land.
• Sea surveys have lower accuracy (±10 gu).
• The milligal (mgal) is the c.g.s. unit; 1 mgal = 10⁻³gal = 10⁻³cm/s².

Basic Theory (Additional Factors)

• Earth's gravitational field changes across space.
• Numerous factors impact regional gravity.
• Measured gravity represents all components' vector sum at the observation point.
• Earth's non-uniformity stems from shape, rotation, and composition.

Gravity Variation

• Earth's rotation and flattening cause global gravity variations.
• The equatorial radius is greater than the polar radius.
• This results in a difference in these two values.
• There are other types of local variations too.

Normal Gravity

• Normal gravity (gn) changes based on latitude (°).
• It's a mathematical function expressing both of the Earth's flattening and rotation effects.
• The function displays a curve when plotted against latitude.

The Earth Gravity Variation

• Global and local gravity variations are categorized.
• Specific materials have density ranges and approximate average densities.
• Space and geographic features are influential.
• Time-varying factors, like tidal effects and instruments, impact gravity measurements.