What Is a Gravity Survey in Mineral Exploration? A Guide for Pakistani Mining Engineers

By Sufyan · 2026-07-12 · 5 min read

Last March I was standing on a chromite prospect in Muslim Bagh, Balochistan, watching a crew set up a gravimeter on a limestone outcrop. The lead engineer, a guy who'd worked for GSP for 22 years, turned to me and said something I still think about: "Half the mining engineers in this country have never seen one of these machines running in the field."

He's probably right. And that's a problem.

Because a gravity survey, done properly, is one of the most useful tools we have for finding dense ore bodies buried under Pakistani soil. Chromite. Iron. Copper sulfides. Massive gold-bearing sulfide zones. All of these are heavier than the rock around them. Gravity picks that up.

So let me walk through what a gravity survey actually is, how it works, and when you should be commissioning one on your license area.

The Physics, Explained Like You're Standing on a Drill Pad

Every rock has a density. Granite sits around 2.65 g/cm³. Limestone is close to 2.71. Chromite ore? That's 4.5 to 4.8. Massive sulfides can push past 4.2. Gold-hosting quartz veins are lighter than surrounding mafic rock, so they show up as negative anomalies.

A gravimeter measures the pull of gravity at a single point on the ground. Not the general 9.81 m/s² we all learned in school — the tiny variations. We're talking microgals. One microgal is one billionth of Earth's average gravitational pull. That's the resolution.

When you set the instrument down over a dense buried body, gravity pulls slightly harder. The needle moves. You record the value, move 50 or 100 meters, and repeat. Do this across a grid and you've got a map.

That map — after corrections for elevation, latitude, terrain, and tidal effects — is called a Bouguer anomaly map. It's the working document for any serious gravity survey mineral exploration program.

What gets corrected out (and why it matters)

Raw gravity readings are noisy. Here's what a proper reduction removes:

Latitude correction — gravity is stronger at the poles than the equator. Islamabad reads different from Karachi.
Free-air correction — accounts for your elevation above sea level. Critical in Gilgit-Baltistan where survey stations can sit at 3,400m.
Bouguer correction — subtracts the mass of rock between your station and sea level.
Terrain correction — this one's brutal in Pakistan's mountains. Nearby valleys and peaks distort readings and you have to model them out.
Tidal correction — yes, the moon affects your gravimeter. About 0.3 milligals over a day.

Skip any of these and your "anomaly" is just noise wearing a costume.

When a Gravity Survey Is Actually Worth Running

Honestly, I used to think gravity was overkill for most exploration budgets in Pakistan. Then I watched a 12 km² survey in Chagai identify a buried porphyry target that surface geochemistry had completely missed. The dense sulfide core showed up as a 2.4 milligal positive anomaly. Drilling confirmed it at 340m depth.

So here's when I now recommend it:

Run gravity when you're chasing: chromite pods in ophiolite belts (Muslim Bagh, Waziristan, Kharan), massive sulfide bodies, iron ore (Kalabagh, Chiniot), buried porphyry copper systems in the Chagai arc, or salt-hosted mineralization in the Salt Range.

Don't bother when you're chasing: disseminated low-grade gold in placers, shallow marble and granite quarries (you can see those from the road), lithium brines (you want different geophysics), or emerald pockets in Swat schists — the density contrast is too small.

A gravity survey Pakistan operators can realistically afford runs anywhere from 8 to 25 lakh PKR for a 5-10 km² block, depending on station spacing and terrain. Compare that to a single failed drill hole at 40-60 lakh and the math gets obvious fast.

How We Combine Gravity With Satellite Data at GeoMine AI

Here's the thing — gravity tells you where the mass is. Satellite spectral analysis tells you what minerals are exposed at the surface. Neither is complete on its own.

At geomines, we take Sentinel-2 and ASTER data to map surface alteration mineralogy. Iron oxides. Clay caps. Silica caps. Serpentinization halos. Then we overlay that against gravity data (ours or the client's) to see if surface signatures line up with buried density anomalies.

When they do — when a hydrothermal alteration halo sits directly over a gravity high — that's a high-confidence target. That's a drill site.

When they don't line up, you've got questions to answer before spending a single rupee on drilling.

Our breeze geo mineral analysis workflow was built specifically for this kind of data fusion. Satellite alteration mapping first (cheap, fast, covers hundreds of km²), then targeted gravity on the top 3-4 anomalies, then drilling. That sequence has saved clients real money.

I got the order wrong on my first two projects in Skardu back in 2022. Drilled first, geophysics second. Wasted about 18 lakh doing it. Won't make that mistake again.

Practical Advice for Engineers Running Their First Survey

A few things nobody tells you until you've done it:

Station spacing matters more than instrument grade. A 50m spacing with a decent CG-5 will find shallow targets better than 200m spacing with a lab-grade absolute gravimeter.
Base station discipline. Return to your base every 2-3 hours to check instrument drift. Skip this and your data reduction becomes guesswork.
GPS elevation isn't good enough. You need differential GPS or total station survey for your station elevations. A 30cm elevation error becomes a 0.06 mGal error in your reduced data — enough to hide a small target.
Terrain corrections in Pakistan need SRTM plus local DEM. The 30m SRTM alone won't cut it near cliffs and steep valleys. We supplement with drone-derived DEM for the inner correction zones.
Reprocess old data before commissioning new surveys. GSP has gravity data from the 1970s and 80s sitting in archives for parts of Balochistan and KP. Sometimes it's already been collected — you just need someone to reduce it properly with modern methods.

That last point saves people money more often than they'd expect. Before you spend on a new geo mining geophysics campaign, ask what's already been done on your block or nearby. The answer might surprise you.

And if you're not sure where to start on your license area — what to survey, what to skip, how to sequence the work — that's honestly the conversation I'd rather have with you than another abstract debate about which mineral is hottest this quarter.