Key Concepts of Gravity Anomalies

Why This Matters

Gravity anomalies are your window into Earth's hidden architecture. When you measure gravity at the surface, you're detecting density variations that extend kilometers below your feet—from shallow ore bodies to the base of mountain roots. Understanding these anomalies connects directly to isostasy, plate tectonics, and crustal structure, all core concepts you'll encounter repeatedly on exams. Every gravity survey, whether for oil exploration or studying subduction zones, relies on the corrections and interpretations covered here.

You're being tested on more than definitions—examiners want to see that you understand why different corrections exist and when each anomaly type reveals something meaningful about Earth's interior. Don't just memorize that Bouguer anomalies correct for topography; know why that correction matters for isolating subsurface density contrasts. Master the logic behind these concepts, and you'll handle any FRQ that asks you to interpret gravity data.

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Correction Types: Isolating the Signal

Before interpreting gravity data, geophysicists must strip away predictable effects to reveal the anomalies that matter. Each correction removes a specific "noise" source, and the order of corrections determines what geological signal remains. The goal is always the same: separate what you expect from what's geologically interesting.

Free-Air Anomaly

• Corrects only for elevation—removes the effect of distance from Earth's center without accounting for intervening mass
• Reveals large-scale tectonic features like mid-ocean ridges and subduction zones where crustal thickness varies dramatically
• First step in most correction sequences, making it foundational for understanding how subsequent corrections build on each other

Bouguer Anomaly

• Removes the gravitational pull of topographic mass—treats everything above sea level as if it were an infinite slab of standard density (typically 2670 kg/m³)
• Isolates subsurface density variations by eliminating the obvious signal from mountains and valleys themselves
• Strongly negative over mountain ranges because removing topographic mass reveals the low-density crustal roots beneath—a direct test of isostatic compensation

Terrain Effect

• Accounts for irregular topography that the simple Bouguer slab approximation misses—valleys below and peaks above the measurement point
• Critical in rugged terrain where nearby mountains or gorges can introduce errors of several milligals
• Applied as a terrain correction to refine Bouguer anomalies, especially important in mining surveys and mountainous regions

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Isostatic Principles: Earth's Balancing Act

Isostasy describes how Earth's lithosphere floats on the denser asthenosphere below. Gravity anomalies reveal whether regions are in isostatic equilibrium or actively adjusting—key for understanding post-glacial rebound, mountain building, and basin subsidence.

Isostatic Anomaly

• Measures departure from isostatic equilibrium—compares observed gravity to what you'd expect if the crust were perfectly compensated
• Near-zero values indicate equilibrium, while significant anomalies suggest ongoing adjustment or dynamic support from mantle processes
• Positive anomalies over Scandinavia and Canada reveal incomplete post-glacial rebound—the crust is still rising to compensate for removed ice mass

Mass Deficiency

• Indicates less mass than expected—produces negative gravity anomalies relative to surrounding regions
• Associated with sedimentary basins, salt domes, and crustal roots—anywhere low-density material replaces denser rock
• Key exploration target because sedimentary basins with mass deficiencies often host petroleum reserves

Mass Excess

• Signals more mass than expected—creates positive gravity anomalies that stand out from regional trends
• Found over dense intrusions, ore bodies, and oceanic crust—anywhere high-density material concentrates
• Direct exploration application for locating metallic mineral deposits and mapping mafic/ultramafic intrusions

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Scale Separation: Regional vs. Local Signals

Gravity data contains overlapping signals from features at different depths and scales. Separating these signals is essential for targeting specific geological questions—deep crustal structure versus shallow ore bodies require different analytical approaches.

Regional Gravity Anomaly

• Captures long-wavelength variations reflecting deep, broad structures like crustal thickness changes and mantle density variations
• Typically spans tens to hundreds of kilometers, smoothing over local features to reveal tectonic-scale patterns
• Extracted through filtering or polynomial fitting, providing the baseline against which local anomalies are measured

Residual Gravity Anomaly

• Isolates short-wavelength signals by subtracting the regional trend from observed data
• Highlights shallow, localized structures like faults, ore bodies, cavities, and small intrusions
• Primary tool for mineral exploration because economic deposits create distinct local anomalies against the regional background

Gravity Gradient

• Measures how quickly gravity changes with distance—expressed as the derivative of the gravity field (units: Eötvös, where $1 \text{ E} = 10^{-9} \text{ s}^{-2}$)
• Enhances edges and boundaries of subsurface structures, making it superior to gravity magnitude for mapping contacts
• Modern gradiometry uses multiple measurements to capture the full gravity gradient tensor, dramatically improving resolution

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Reference Surfaces: Defining "Normal" Gravity

To identify anomalies, you need a reference for what gravity should be. The choice of reference surface determines what your anomaly actually measures—this is where geodesy meets geophysics.

Geoid Undulation

• Quantifies how the geoid departs from the reference ellipsoid—ranges from about $-100$ m to $+85$ m globally
• Reflects internal mass distribution because the geoid is an equipotential surface shaped by all density variations within Earth
• Essential for converting GPS heights (measured from ellipsoid) to meaningful elevations (measured from geoid/sea level)

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Quick Reference Table

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Self-Check Questions

1. Why does a mountain range typically show a positive free-air anomaly but a negative Bouguer anomaly? What does this combination reveal about isostatic compensation?

2. You're exploring for a dense sulfide ore body in a sedimentary basin. Would you focus on regional or residual gravity anomalies, and why?

3. Compare and contrast mass deficiency and mass excess: what geological features produce each, and how would their gravity signatures differ on a profile?

4. A gravity survey shows near-zero isostatic anomalies across an ancient mountain belt but significant positive isostatic anomalies over a recently deglaciated region. Explain what each observation indicates about crustal equilibrium.

5. How does the gravity gradient provide different information than the gravity anomaly magnitude, and why is this distinction important for mapping geological boundaries?