Estimated Model Parameters
Gravimetry in Geophysical Exploration
Gravimetry is a geophysical method that measures minute variations in the Earth's gravitational field to detect subsurface features based on density contrasts.
Physical Principles
The gravitational acceleration (g) at a point on the Earth's surface is influenced by the distribution of mass beneath it. Newton's law of universal gravitation states that the force between two masses is proportional to the product of the masses and inversely proportional to the square of the distance between them:
F = G(m₁m₂/r²)
Where G is the gravitational constant (6.674×10⁻¹¹ m³kg⁻¹s⁻²).
Gravity Anomalies
Subsurface features with density contrasts relative to surrounding rock create measurable gravity anomalies:
- Negative anomalies: Caused by mass deficiencies (cavities, voids, salt domes)
- Positive anomalies: Caused by mass excesses (ore bodies, intrusions)
Mathematical Models
This simulator implements rigorous mathematical models for various geometries:
- Sphere: g = (4/3)πGΔρR³/z² where R is radius and z is depth
- Horizontal Cylinder: g = 2πGΔρR²/z where R is radius and z is depth
- Vertical Cylinder: g = 2πGΔρh(1-z/√(z²+R²)) where h is height
- Rectangular Prism: Calculated using Nagy's formula for gravitational potential
- Irregular Shape: Modeled using multiple point masses or polyhedra
Typical Density Values
| Material | Density (g/cm³) |
|---|---|
| Air (cavity) | 0.0 |
| Water | 1.0 |
| Soil/Sediment | 1.7-2.2 |
| Sandstone | 2.0-2.6 |
| Limestone | 2.2-2.8 |
| Granite | 2.5-2.8 |
| Basalt | 2.7-3.1 |
| Iron Ore | 4.5-5.2 |
Units
Gravity is measured in Gal (1 Gal = 1 cm/s²) or more commonly in milliGal (mGal) and microGal (μGal):
- 1 mGal = 10⁻³ Gal = 10⁻⁵ m/s²
- 1 μGal = 10⁻⁶ Gal = 10⁻⁸ m/s²
Modern gravimeters can detect changes as small as a few μGal, equivalent to the gravitational effect of a coin at a distance of about 1 meter.
Data Processing
Raw gravity data requires several corrections to isolate anomalies of interest:
- Latitude correction: Accounts for Earth's elliptical shape
- Free-air correction: Accounts for elevation differences
- Bouguer correction: Accounts for mass between measurement point and reference level
- Terrain correction: Accounts for topographic effects
- Regional-residual separation: Isolates local anomalies from regional trends