Magnetic & Geophysical Signatures
Rocks are not magnetically uniform. Some carry enough magnetic minerals to nudge the Earth's field measurably; others are nearly invisible to it. A magnetic anomaly map records those differences as a pattern of highs and lows draped across the landscape, and that pattern is a proxy for geology you often cannot see at the surface. The U.S. Geological Survey describes magnetic anomaly data as a way of effectively "seeing through" nonmagnetic cover to reveal rock types and structures, faults, folds, dikes, and contacts, that would otherwise stay hidden. For a prospector, geophysics is best understood as a regional filter: it narrows the ground worth examining, it does not pinpoint ore. This is geological background, not legal, financial, or land-access advice.
What magnetic anomalies actually show
A magnetic survey measures the strength of the Earth's field at many points. After the main planetary field is removed, what remains is the anomaly: the part produced by magnetic minerals in the crust, overwhelmingly magnetite. Because different rocks carry very different amounts of magnetite, the anomaly pattern maps onto lithology.
- Igneous rocks and many metamorphic rocks contain abundant magnetite and tend to produce strong anomalies. Sedimentary and metasedimentary rocks are usually weakly magnetic to nonmagnetic and read as quiet, low-relief ground.
- Mafic and ultramafic rocks, basalts, gabbros, and related dark, iron- and magnesium-rich rocks, are magnetite-rich and commonly stand out as pronounced magnetic highs. USGS notes that magnetic data are particularly effective at delineating mafic and ultramafic bodies.
- Intrusions exposed at the surface can be traced into the shallow subsurface by following their magnetic signature, which is valuable where bedrock exposure is poor.
- Faults and contacts often appear as steep gradients, linear breaks, or offsets in the anomaly pattern, because they juxtapose rocks of contrasting magnetization. USGS interpreters routinely map faults along linear aeromagnetic anomalies and breaks in the pattern.
Magnetic lows matter too. Hydrothermal alteration, the very process that can accompany mineralization, frequently destroys magnetite, so a "magnetic low" carved into otherwise magnetic rock can flag altered, potentially favorable ground. The signal cuts both ways, which is exactly why interpretation, not a single bright color, is what counts.
Interpretation, integration, and the limits of resolution
The amplitude and shape of an anomaly depend on more than rock type. They reflect the amount and kind of magnetic mineral, the size and geometry of the body, and crucially its depth. A small shallow source and a large deep one can produce similar-looking anomalies, so depth and source cannot be read off the color alone; they require modeling. This ambiguity is inherent to potential-field geophysics.
Resolution is governed mainly by how the data were collected. A detailed survey flown low, with tightly spaced flight lines, resolves small, near-surface features sharply. A survey flown high, with widely spaced lines, blurs them together and sees only broad, deep structure. USGS mineral-systems work in the southern Midcontinent stresses designing surveys with consistent flight-line spacing and flight height precisely because those parameters set what can be seen. As a rule, no anomaly can be resolved that is much smaller than the spacing and altitude of the measurements.
This is why magnetics is a regional first pass rather than a drill target. USGS uses airborne magnetic surveys to map concealed geology and rank mineral potential across whole districts, especially where deposits are buried and ordinary mapping fails. Geophysics is also rarely used alone: magnetic data are most powerful combined with gravity, radiometric data, and surface geology, so that several independent signals agree before ground is called prospective.
A continental-scale or global grid serves a different purpose entirely. NOAA's EMAG2 (Earth Magnetic Anomaly Grid) compiles satellite, marine, and airborne measurements into a 2-arc-minute global grid referenced to roughly 4 km above the geoid. That is excellent for crustal-scale trends and context, but at a couple of kilometers per pixel it cannot resolve an individual outcrop, vein, or small intrusion. It shows the regional fabric, not the deposit.
In Lodester
Lodester's magnetic anomaly layer is built from NOAA's EMAG2 grid. Treat it as regional context, not as a target generator, and always read it alongside the geology and structure layers (all in WGS84).
- Use the magnetic layer to see the big picture: broad highs that may mark magnetite-rich, mafic, or intrusive terrain, and quiet lows over weakly magnetic sedimentary cover.
- Cross-read with geology and structure. Where a magnetic gradient or linear break lines up with a mapped fault or a lithologic contact, you have two independent layers pointing at the same boundary.
- Remember the resolution honestly. EMAG2 is coarse, on the order of kilometers per cell and modeled at altitude, so it reflects regional crustal structure, not local detail. It will never show a single vein.
- Let it focus where you read further, not confirm what is there. A promising magnetic pattern is a reason to examine the geology, structure, and ground more closely, never a substitute for field checking, sampling, and confirming land access.