3.2 Gravity anomalies and their interpretation

Gravity anomalies in geophysics

Gravity anomalies reveal what's hidden beneath Earth's surface by measuring how the actual gravitational pull at a location differs from what we'd theoretically expect. These differences arise from lateral density variations in the subsurface, and interpreting them is one of the core skills in exploration geophysics and geodesy.

Definition and significance

A gravity anomaly is the difference between the observed gravitational acceleration at a point and the theoretical value predicted by a reference model (typically a reference ellipsoid). Lateral density contrasts within the Earth's interior drive these variations.

• A positive anomaly means gravity is stronger than expected, pointing to denser-than-average material below (e.g., a mafic intrusion).
• A negative anomaly means gravity is weaker than expected, pointing to less dense material (e.g., a thick sedimentary basin or salt body).

Gravity anomalies are central to geophysics because they provide direct information about subsurface density structure. That makes them useful for mapping geological features you can't see at the surface, from crustal thickness variations down to individual ore bodies.

Applications in geosciences

• Sedimentary basin delineation: Low-density sedimentary fill (shales, sandstones) produces broad negative anomalies, making gravity surveys a first-pass tool for oil and gas exploration.
• Subsurface structural mapping: Faults, folds, and unconformities create density contrasts that show up as gradients or offsets in gravity data. This helps reconstruct the tectonic history of a region.
• Mineral exploration: Dense ore bodies like iron ore, chromite, or massive sulfide deposits generate localized positive anomalies, guiding exploration targeting.
• Crustal and upper mantle studies: Regional gravity data constrain the depth to the Moho (crust-mantle boundary) and can indicate features like mantle upwellings or lithospheric downwellings.
• Integrated interpretation: Gravity data are routinely combined with magnetic anomalies, seismic reflection/refraction data, and borehole logs. This multi-method approach reduces the inherent non-uniqueness of potential field interpretation, where multiple subsurface models can produce the same surface anomaly.

Calculating gravity anomalies

Measurement and data acquisition

The basic idea: measure gravity at a location, then subtract what theory predicts for that location. The remainder is your anomaly.

• Observed gravity is measured with a gravimeter, an instrument that records the local acceleration due to gravity ($g_{obs}$) with precision on the order of 0.01 mGal or better.
• Theoretical (normal) gravity is computed from a reference ellipsoid using the International Gravity Formula or its modern equivalent (e.g., GRS80). This accounts for Earth's shape, mass distribution, and rotation, and it varies primarily with latitude:

$g_{theoretical} = g_e \frac{1 + k \sin^2\phi}{\sqrt{1 - e^2 \sin^2\phi}}$

where $\phi$ is latitude, $g_e$ is gravity at the equator, $k$ is a derived constant, and $e$ is the ellipsoid's eccentricity.

The raw anomaly is then:

$\Delta g = g_{obs} - g_{theoretical}$

But this raw difference isn't very useful yet. Several corrections must be applied first.

Corrections and processing

Each correction removes a known, non-geological effect so that the final anomaly reflects only subsurface density variations.

1. Free-air correction: Accounts for the station's elevation above the reference ellipsoid. Gravity decreases with height, so this correction adds approximately $0.3086 \text{ mGal/m}$ of elevation. The result is the free-air anomaly:

$\Delta g_{FA} = g_{obs} - g_{theoretical} + 0.3086 \cdot h$

where $h$ is the station elevation in meters.

1. Bouguer correction: Removes the gravitational attraction of the rock mass between the station and the ellipsoid (modeled as an infinite horizontal slab). For a slab of density $\rho$:

$\Delta g_{Bouguer\,slab} = 2\pi G \rho h$

A standard crustal density of $2670 \text{ kg/m}^3$ is commonly used. Subtracting this from the free-air anomaly gives the simple Bouguer anomaly.

1. Terrain correction: The Bouguer slab approximation assumes flat topography. Where hills rise above or valleys dip below the station, a terrain correction compensates for the actual shape of the surrounding landscape. Both hills and valleys cause the simple Bouguer anomaly to underestimate the true value, so terrain corrections are always positive.

2. Complete Bouguer anomaly: After applying all three corrections, you get the complete Bouguer anomaly, which is the standard product for geological interpretation.

Gravity anomalies are expressed in milliGals (mGal), where $1 \text{ mGal} = 10^{-5} \text{ m/s}^2$. Typical crustal anomalies range from a few mGal to several hundred mGal.

Interpreting gravity anomaly maps

Visualizing gravity anomalies

Gravity anomaly maps display the spatial distribution of anomaly values across a survey area, using either color scales or contour lines.

• Warm colors (reds) typically represent positive anomalies, suggesting relatively dense subsurface material such as igneous intrusions or uplifted basement rock.
• Cool colors (blues) represent negative anomalies, suggesting low-density features like sedimentary basins, salt domes, or voids.
• Steep gradients (closely spaced contours) often mark the edges of density contrasts, such as fault contacts or basin margins.

Analyzing gravity anomaly profiles

A gravity anomaly profile is a cross-section extracted along a line across the map. Profiles are powerful because the shape of the anomaly curve encodes information about the source body:

• Amplitude relates to the density contrast ($\Delta\rho$) and the size of the body. A large, dense intrusion produces a high-amplitude anomaly.
• Wavelength (the horizontal breadth of the anomaly) relates to the depth of the source. Deeper bodies produce broader, smoother anomalies; shallow bodies produce sharper, narrower ones. This is a direct consequence of potential field theory: the signal attenuates and spreads with distance.
• Shape and symmetry give clues about geometry. A symmetric bell-shaped anomaly suggests a roughly equidimensional body (like a sphere or vertical cylinder), while an asymmetric anomaly might indicate a dipping contact or fault.

Interpretation typically involves forward modeling (assuming a subsurface geometry, computing its predicted anomaly, and comparing to the observed data) or inversion (mathematically deriving a density model that fits the observed anomaly). Both approaches must be constrained by geological knowledge because gravity inversion is inherently non-unique.

Gravity anomalies for geological problems

Subsurface structure and tectonics

Gravity data are especially valuable for mapping large-scale crustal structure:

• Faults and basin margins appear as linear gravity gradients. The magnitude of the gradient relates to the throw and density contrast across the fault.
• Crustal thickness variations produce long-wavelength regional anomalies. Thicker crust (e.g., beneath mountain belts) tends to show negative Bouguer anomalies because of the deep, low-density crustal root, consistent with isostatic compensation.
• Moho depth can be estimated from gravity data using isostatic models or spectral analysis techniques. For example, the Bouguer anomaly over the Tibetan Plateau reaches roughly $-500$ mGal, reflecting a crustal root extending to ~70 km depth.
• Mantle dynamics: Broad positive anomalies over oceanic ridges or hotspots can indicate mantle upwellings, while negative anomalies in subduction zones may reflect the downgoing slab geometry.

Combining gravity with seismic and magnetic data produces more tightly constrained models and is standard practice in both academic research and industry.

Resource exploration

Gravity surveys remain a cost-effective reconnaissance tool for resource exploration:

• Oil and gas: Sedimentary basins filled with low-density shale and sandstone generate regional negative anomalies. Mapping basin geometry with gravity helps prioritize areas for more expensive seismic surveys. Salt domes, which trap hydrocarbons, also produce distinctive anomalies because salt ($\rho \approx 2160 \text{ kg/m}^3$) is less dense than the surrounding compacted sediments.
• Mineral deposits: Dense ore bodies produce localized positive anomalies. For instance, a massive sulfide deposit with $\rho \approx 4000\text{–}4500 \text{ kg/m}^3$ embedded in country rock at $\rho \approx 2700 \text{ kg/m}^3$ creates a strong, compact anomaly that can be modeled to estimate the deposit's depth, size, and tonnage.
• Drill site selection: Gravity modeling helps optimize where to drill confirmation boreholes, reducing exploration costs by narrowing the target area before committing to expensive drilling programs.