Why Negative Gravity Anomalies Are Possible: 04-Geol-B10, December 2017, Q6

Question 6: Why Negative Gravity Anomalies Are Possible (Choose 6 of 10 – equal value)

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The resolution: an anomaly is a difference, not an absolute attraction

Newton's law is correct: any actual mass attracts the gravimeter, and that attraction is always directed toward the mass (always "positive" in that sense). But a gravity anomaly is never the raw attraction itself — it is the RESIDUAL left after subtracting a reference (or "normal") gravity value: the observed reading, corrected for latitude, elevation and the assumed Bouguer reduction density ρ₀ (conventionally 2.67 g/cm³), minus that theoretical reference. A negative anomaly therefore does not mean gravity itself is negative or that some mass is "repelling" the instrument; it means the LOCAL subsurface density is LOWER than the reference density used in the reduction, i.e., there is a local density deficit relative to the assumed background, not an absence of attraction. Every mass still attracts positively — the anomaly sign only reflects whether that mass attracts MORE or LESS strongly than the uniform reference slab the reduction subtracted.

Examples of situations producing negative anomalies

Voids and cavities — karst caves, old mine workings/adits, tunnels: essentially zero-density space (air) replacing rock, the most extreme possible density deficit.
Buried valleys / sediment-filled basins — unconsolidated sediment (ρ≈1.8–2.2 g/cm³) infilling a valley eroded into denser bedrock (ρ≈2.6–2.8 g/cm³).
Salt domes/diapirs — rock salt (ρ≈2.1–2.2 g/cm³) is markedly less dense than the surrounding sedimentary sequence it has intruded, giving a classic negative Bouguer anomaly used for salt exploration.
Felsic plutons in denser host rock — a granite batholith (ρ≈2.6 g/cm³) intruding mafic or metamorphic country rock (ρ≈2.8–3.0 g/cm³) produces a broad negative anomaly over the pluton.
Landfills and mine tailings — loose, poorly compacted fill (ρ≈1.0–1.5 g/cm³) is less dense than the natural ground it displaced.

Topic. Why a "negative gravity anomaly" is physically consistent with Newton's law of universal (always-attractive) gravitation.

Key relations. Bouguer anomaly Δg_B = g_obs (latitude-, free-air-, Bouguer- and terrain-corrected) − g_normal(φ); Δg_B < 0 ⇔ local ρ < ρ₀ (reduction density) on average beneath the station.

Why this works. Gravity reduction subtracts a THEORETICAL reference field computed as if the Earth were a uniform-density reference model (latitude formula plus a uniform Bouguer slab of ρ₀); any real density heterogeneity, whether an excess (positive anomaly) or a deficit (negative anomaly) relative to that assumed uniform model, shows up as a residual of the corresponding sign, even though the true underlying force is unconditionally attractive everywhere.

Common pitfall. Interpreting a negative anomaly as literal repulsion or "missing gravity" rather than as a lower-than-reference density; and forgetting that the reference/reduction density ρ₀ is a CHOICE (conventionally 2.67 g/cm³) — a survey area whose true average density differs substantially from 2.67 can show a broad, spurious regional anomaly of one sign purely from that choice, which is why the actual near-surface density should be used for the Bouguer reduction where it is well known.

Source. Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed., §2.7 (Bouguer anomaly interpretation, density contrast and anomaly sign).

Question 6 of 10: Why Negative Gravity Anomalies Are Possible

Question 6: Why Negative Gravity Anomalies Are Possible (Choose 6 of 10 – equal value)

The resolution: an anomaly is a difference, not an absolute attraction

Examples of situations producing negative anomalies