PHYS2403 Geophysics Simulation Lab

Experiment 2: Gravity Corrections & the Bouguer Anomaly

Raw gravity readings reflect elevation, nearby topography, and latitude, in addition to subsurface density. To discover geological structures (like the Malawi Rift Valley), geophysicists apply corrections to isolate the Bouguer anomaly. Use the controls to observe each correction step on the synthetic W-E traverse.

Parameters & Corrections

Profile Latitude (°S)11.5°S

Alters standard normal gravity reference (IGF). At equator, g ≈ 9.7803 m/s².

Bouguer Slab Density (ρ)2670 kg/m³

Average crust density for correction. Standard crust is 2670 kg/m³.

Active Corrections

1. Latitude Correction (IGF)Adjusts reference for Earth's shape & rotation

2. Free-Air Correction (+0.3086h)Corrects for station distance above sea level

3. Bouguer Slab Correction (−0.04193ρh)Removes the gravity contribution of rock above sea level

Elevation & Gravity Stations (Traverse Profile)Lake Centre depth = 700m

Correction sizes at Lake Centre (h=0m, depth=700m)

At station 5, the elevations are zero, but latitude is 11.5°S and density is 2670kg/m³. Shows magnitude of each contribution in mGal.

Gravity Anomaly Profile (mGal)

Black dashed: Raw gravity anomaly (Observed − Reference). Solid red: Anomaly after applying selected corrections.

Final LkCentre: 367.3 mGalMax: 777.3 mGal

Key Corrections & Formulas

\Delta g_B = g_{\text{obs}} - g_{\text{IGF}} + \delta g_{\text{FA}} - \delta g_B - \delta g_T

1. Latitude Correction

Earth's rotation creates centrifugal forces, and polar flattening places poles closer to center. Normal gravity increases from $9.7803\text{ m/s}^2$ at the equator to $9.8322\text{ m/s}^2$ at the poles.

g_{\text{IGF}}(\phi) = 9.780327 \cdot (1 + 0.0053024 \sin^2\phi - 0.0000058 \sin^2 2\phi)

2. Free-Air Correction

Gravity decreases with distance from Earth's center. Elevation $h$ places the station further away, reducing gravity. The free-air correction adds back this loss.

\delta g_{\text{FA}} = +0.3086 \cdot h \text{ (mGal)}

3. Bouguer Slab Correction

The rock slab of thickness $h$ between station and datum adds excess mass, increasing local gravity. The Bouguer correction models this as an infinite slab of density $\rho$ and subtracts its attraction.

\delta g_B = -0.04193 \cdot \rho \cdot h \text{ (mGal)}

Observation Guide

Use the controls above to observe how latitude, elevation, and density change the corrected gravity profile. All calculation prompts now live in the lab manual.