Bouguer Gravity Anomaly Maps for Mining: How to Read Them Without a PhD

The first time someone handed me a Bouguer anomaly map, I stared at it for twenty minutes and pretended I understood. I didn't. It looked like a weather map drawn by someone having a bad day — blobs of red, blue, green, with numbers in milligals and no obvious answer to the only question I cared about: where do I drill?

That was 2019. I've spent the years since figuring it out the hard way, on real concessions, with real drilling budgets on the line. So here's what I wish someone had told me back then.

What a Bouguer Anomaly Actually Measures

Gravity isn't constant across the Earth. It changes depending on what's underneath you. A dense chromite body pulls slightly harder on a gravimeter than the granite next to it. We're talking tiny differences — fractions of a milligal — but modern instruments pick them up fine.

The raw gravity reading is messy though. It's affected by your elevation, the latitude you're standing at, the terrain around you, and the rocks between your sensor and sea level. The Bouguer correction strips all that noise out. What's left is the anomaly — the part caused by density variations in the subsurface itself.

That's the whole point. A Bouguer gravity anomaly map shows you, in milligals, where the rock underneath is denser or lighter than expected.

Dense stuff (positive anomaly, usually red or orange on the map): massive sulfides, chromite, iron ore, mafic intrusions, skarn bodies.

Light stuff (negative anomaly, usually blue): granite plutons, salt domes, sedimentary basins, weathered or altered zones, sometimes porphyry-related alteration halos.

That's it. That's the trick. Everything else is interpretation.

Reading the Map Like You Mean It

Honestly, most people make this harder than it is. When I open a gravity map at GeoMine AI, I look for four things in this order.

Gradient, not absolute value. A flat red zone tells me less than a sharp edge between red and blue. Sharp gradients = density contrast = a contact, a fault, or the edge of an intrusion. Most ore bodies sit on edges, not in the middle of homogeneous blobs. I missed this for almost a year. I kept drilling the centers of positive anomalies and getting boring rock.

Closed contours. A circular or oval anomaly with closed contour lines around it usually means a discrete body — a pluton, a sulfide lens, something with a defined shape. Open, smeared anomalies usually mean regional trends (a thick sedimentary basin, a deep crustal feature) that aren't going to make you rich.

Wavelength. Short, tight anomalies = shallow source. Broad, gentle anomalies = deep source. If you see a tight 2 km positive bullseye, that body is probably within the top 500–800 m. If the anomaly is 15 km across and gentle, the source might be 3 km deep and irrelevant to your drilling program.

Trends and lineaments. Faults show up as linear gradients. Where two fault trends intersect and you see a localized anomaly, that's where I start paying attention. Most of the copper-gold systems in the Chagai belt sit at exactly these intersections.

Where It Goes Wrong (And It Will)

Look, gravity is non-unique. Always. A small dense body near the surface gives you the same anomaly as a bigger dense body deeper down. The map alone can't tell you which is which. This is the thing junior explorers get burned on most often — they treat one milligal of positive anomaly as a guaranteed orebody and book a drill rig before doing anything else.

Don't do that.

What I do instead, and what we built into the platform, is overlay. Gravity anomaly interpretation only works when you stack it with other data. Sentinel-2 alteration signatures from Breeze geo mineral analysis. ASTER for iron oxide and clay mapping. SRTM DEM to make sure your "anomaly" isn't actually a topographic ghost the Bouguer correction didn't fully remove. SAR for structural lineaments.

When four datasets all point at the same 800-meter target — gravity high, clay alteration halo, iron oxide rim, structural intersection — that's when I'll spend $47,000 drilling a hole. Not before.

I've got a chromite prospect near Skardu where the Bouguer map shows a clean +3.2 mGal anomaly. Beautiful target on paper. But the ASTER alteration was wrong for chromite, and the structural setting didn't fit. We held off. Saved the drilling money for a less obvious target 6 km north where everything actually lined up. That one hit at 18 meters.

Why This Matters for Pakistan Specifically

Most of Pakistan's mineral belts — Chagai, Waziristan, Kohistan, the ophiolite zones in Balochistan and Gilgit-Baltistan — have partial or zero modern gravity coverage at the resolution that's useful for exploration. The GSP has regional data but the spacing is often too wide to catch deposit-scale anomalies.

This is the gap. Satellite-derived gravity products (GOCE, GRACE, and the newer combined models) now give us free Bouguer anomaly coverage at roughly 2 km resolution across the whole country. It's not enough to drill on, but it's absolutely enough to high-grade a 10,000 km² license down to three or four target zones worth ground-truthing.

Five years ago you needed a $400,000 ground gravity survey to even start this conversation. Now you can pull a regional Bouguer map for a Balochistan license in an afternoon, run it through a geomine platform alongside spectral data, and walk into a meeting with actual targets instead of vibes.

The gravity data has been sitting there the whole time. Free. Most people in Pakistani geo mining just never learned to read it — or got handed a map once and quietly gave up, like I almost did.

So if you've got one on your desk right now and it looks like a confused weather report, start with the edges. That's where the money is.