Gravity vs. Magnetics: What Are You Really Measuring?
Decoding the Subsurface Through Density and Susceptibility
Introduction
Gravity and magnetic surveys are two of the most widely used geophysical methods in mineral and energy exploration; yet they’re often misunderstood, incorrectly compared, or applied without fully appreciating their differences. While both produce maps of the subsurface based on physical properties; there is fundamental difference in what they measure and how they respond to the geology.
The Fundamentals
Gravity: Mapping Mass
Gravity surveys detect variations in the Earth's gravitational field caused by changes in mass density beneath the surface. A dense body produces a local increase in gravitational acceleration, a gravity "high.", convesely low-density features result in gravity lows.
Magnetics: Mapping Magnetism
Magnetic surveys detect variations in the Earth's magnetic field due to changes in magnetic susceptibility or magnetic memory. They are especially sensitive to minerals like magnetite or pyrrhotite. Magnetics are ideal for mapping igneous and metamorphic terrains.
What Actually Causes the Anomalies?
Practical Differences in the Field
Resolution and Target Size
- Gravity detects broader, smoother anomalies. In large scale geophysics it’s suited to regional-scale mapping, or detecting deep, dense structures like basin-bounding faults or buried intrusions.
- Magnetics, especially from UAVs or high-res airborne platforms, delivers fine spatial resolution, ideal for mapping shallow lithology, fracture systems, or narrow dykes.
Response to Cover and Weathering
- Magnetic signals can be suppressed by weathering or overprinted by later magnetisation.
- Gravity is unaffected by magnetic mineralogy or remanence (the phenomenon of remaining magnetic fields) it is only affected by a body’s density. All this makes gravity a reliable measurement in non-magnetic terrains or where magnetics is ambiguous.
Environmental and Cultural Noise
- Magnetics can be sensitive to cultural noise (metal fences, powerlines, vehicles).
- Gravity is insulated from most man-made noise, though it requires precise leveling, stable platforms, and terrain corrections for best accuracy. It is sensitive to vibrations be that seismic activity or local events (cars, construction, wind).
When Should You Use Gravity?
Use gravity surveys when:
- You're targeting buried structures beneath sedimentary cover
- Mapping basin geometry, salt bodies, or density-driven traps
- Working in magnetically quiet environments, such as felsic terrains, weathered profiles, or lateritic surfaces or areas with hematite rich iron
- You need to validate structural interpretation from EM or seismic data
When Should You Use Magnetics?
Use magnetics when:
- You're exploring greenstone belts, BIFs, or mafic-ultramafic complexes
- You want high-resolution maps of dykes, faults, or lithological contacts
- You're working in areas with strong magnetic contrast
- UAV or airborne deployment enables efficient wide-area surveys
They’re Not Competitors — They’re Complements
Some of the most successful exploration campaigns integrate gravity and magnetics as complementary datasets:
- Gravity gives the structural framework: faults, basin depth, buried density anomalies
- Magnetics gives the lithological and fabric detail: intrusive edges, fault splays, alteration zones
When processed and interpreted together, these datasets offer a more complete physical model of the subsurface. For example, a dense but non-magnetic intrusive may show up in gravity but be invisible in magnetics, while a highly magnetic but low-density dyke does the opposite.
Integration in Practice
Best practices when using gravity and magnetics together:
- Co-locate data: Use consistent grids, projection, and resolution
- Apply terrain corrections and filters to isolate relevant signals
- Cross-validate anomalies: Look for coincident or offset responses that reveal structural relationships
- Joint inversion or constrained modelling: Improves robustness and geological plausibility of interpretations
Conclusion
Gravity and magnetic surveys don’t compete! What they do is see the different aspects of the subsurface, governed by different physical properties. Understanding the “what” and “why” behind each method is critical for designing effective geophysical programs.
Whether you're mapping buried lithium-bearing pegmatites beneath cover, tracing BIF-hosted iron deposits, or building a basin model for sediment-hosted critical minerals, the best results often come not from choosing between gravity and magnetics, but from using both, in the right context.
SMG Gravity is actively working in this field. Working with SBQuantum Canada we have recently field tested the first combined quantum magnetometer and MEMS gravity instrument.
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