Gravity Survey Methods Explained: Bouguer, Free-Air, and Isostatic Corrections for Mining Projects

Last month I was sitting with a geophysicist in Skardu, looking at a gravity map of one of my chromite blocks. He pointed at a subtle low and said, "That's your ophiolite pod." I asked him how he knew. He said, "Because I corrected for everything that isn't the ophiolite pod."

That's gravity surveying in one sentence.

Here's the thing most mine owners in Pakistan don't get told: a raw gravity reading is almost useless. What matters is what you subtract from it. And the three corrections that decide whether your survey is worth the 3.2 million rupees you paid for it are Bouguer, Free-Air, and Isostatic.

Let me walk through them the way I wish someone had walked through them with me back in 2019, when I first commissioned a ground survey on a copper prospect near Chagai and honestly didn't understand half the report I got back.

Why Gravity Even Works for Mining

Rocks have different densities. Chromite sits around 4.5 g/cm³. Granite is closer to 2.65. Sedimentary cover can be as light as 2.1. When a gravimeter measures the pull of gravity at a station, it's picking up the combined density of everything beneath it — from the topsoil down to the mantle.

So if you have a dense chromite pod hiding under 40 meters of alluvium, gravity will feel it. Barely. We're talking anomalies of 0.5 to 2 milligals, which is why the instrument needs to be sensitive to microgals.

But — and this is the part people skip — the gravimeter also feels the mountain next to you. The valley below you. The bulge of the earth. The tides. Even the moon. All of that noise has to come out before you can see the signal.

That's what corrections do.

The Three Corrections That Actually Matter

Free-Air Correction. This one's the simplest. Gravity gets weaker as you go up in elevation (roughly 0.3086 mGal per meter). If your station is at 2,400 meters in Gilgit and your reference is at sea level, you add back the gravity you "lost" by being higher up. Free-Air pretends there's nothing between your station and sea level — just air. Hence the name.

Useful. But incomplete. Because there IS rock between you and sea level. A lot of it, if you're surveying in Karakoram terrain.

Bouguer Correction. This is where it gets interesting. The Bouguer correction accounts for the mass of rock between your station and the reference datum. You subtract the gravitational pull of that rock slab, assuming an average crustal density (usually 2.67 g/cm³ for continental crust, though in Pakistan's ophiolite belts we often use 2.85–2.90).

Get the density wrong and your Bouguer correction mining interpretation goes sideways. I've seen consultants use 2.67 across a Baluchistan survey where the actual near-surface density was closer to 2.9. The anomaly map looked like there was a huge low where there wasn't one. Real target got buried in the noise.

So Bouguer is powerful, but only as honest as your density assumption.

Isostatic Correction. Now the deep one. Mountains have roots. The Himalayas aren't just sitting on the crust — they're floating in it, with a thick low-density root pushing down into the mantle. That root creates a big regional gravity low that has nothing to do with your ore body.

Isostatic correction removes that regional effect. It's essential for any survey in Gilgit-Baltistan, Kashmir, or KPK, where crustal thickness varies wildly. Skip it and your "mineral anomaly" might just be the shadow of a mountain root 40 km down.

Honestly, I used to think isostatic was overkill for a mine-scale survey. I was wrong. On one of my blocks near Bunji, the isostatic-corrected map showed a completely different anomaly pattern than the Bouguer map. Drilling confirmed the isostatic version.

Where Satellites Come Into This

Okay, founder plug incoming, but stay with me because it's practical.

Ground gravity surveys are expensive. A proper 2,000-station survey in rough terrain can run 4 to 8 million rupees, plus 3 to 6 months of field time. For a lot of prospects, that spend only makes sense AFTER you've narrowed the target area.

That's the gap GeoMine AI fills. We use Sentinel-2, ASTER, SAR, and SRTM DEM data to do the first pass — the breeze geo mineral analysis layer that tells you WHERE to spend the gravity money. Alteration halos, structural lineaments, lithological contacts, all mapped before a single gravimeter leaves the office.

Then the ground gravity survey techniques mineral exploration teams use — Bouguer, Free-Air, Isostatic, the works — get deployed on the 5% of your license area that actually matters. Not the whole 100 sq km.

One client of ours in Waziristan cut their planned gravity budget by 62% because our satellite work eliminated 78% of their license area as low-priority. The remaining ground survey found two drill targets. Both hit.

A Small Warning About Contractors

I'll say this bluntly. There are gravity survey contractors operating in Pakistan who deliver reports with Free-Air and Bouguer maps but no isostatic correction, no density justification, and no terrain correction (which I didn't even cover here — that's the fourth one, and in mountainous terrain it's non-negotiable).

Ask. Every. Time.

• What crustal density did you use for the Bouguer correction, and why?
• Did you apply a terrain correction out to at least 22 km (Hammer zone M)?
• Was isostatic correction applied? Which model — Airy-Heiskanen or Pratt-Hayford?
• What's the station spacing, and does it match the target depth?

If the contractor can't answer those in one breath, walk. I've paid for bad surveys before and it's not a mistake you make twice.

Gravity is one of the most powerful geo mining tools we have. But it's also one of the easiest to fake with pretty colored maps. The corrections are where the truth lives — or doesn't.

Anyway. If you're staring at a gravity report right now and you're not sure what you're looking at, send it over. I'd rather you know before you drill than after.