What Is a Gravity Survey? A Field-Tested Guide for Pakistani Mining Engineers

By Sufyan · 2026-08-01 · 4 min read

Last month I was standing on a ridge in Skardu with a geophysicist who charged me PKR 380,000 for four days of work. He was carrying a gravimeter that looked like a metal thermos. And he was telling me the ground beneath my chromite prospect was 0.12 milligals denser than the surrounding rock.

That single number ended up saving me from drilling in the wrong spot.

So let me explain what a gravity survey actually is — not from a textbook, but from someone who's paid for them, argued about them, and used the results to decide where to spend real drilling money.

The Physics, Without the Boring Part

Every rock has a density. Chromite sits around 4.5 g/cm³. Granite averages 2.7. Copper sulphide ore bodies can hit 4.2. Sedimentary cover? Often below 2.4.

A gravity survey measures tiny changes in Earth's gravitational pull caused by these density differences. Denser rock underground = slightly stronger gravity above it. We're talking measurements so small they're recorded in milligals (one thousandth of a gal, which itself is 1 cm/s²).

To put that in scale — normal gravity at sea level is about 980,000 milligals. A good ore body might create a variation of 0.5 to 2 milligals. That's roughly one part in a million. And yet modern gravimeters catch it easily.

Here's the thing most engineering graduates in Pakistan don't get taught properly: gravity surveys don't tell you what mineral is down there. They tell you where the dense stuff is. Big difference. You still need geochemistry, satellite spectral analysis, and eventually drilling to confirm what the density anomaly actually contains.

What the Survey Actually Looks Like on the Ground

A field crew shows up with a gravimeter, a differential GPS (this part matters a lot — I'll explain why), and usually 2-3 people. They set up stations in a grid. For a typical chromite or copper prospect in Balochistan or GB, station spacing runs 25 to 100 meters depending on target depth.

At each station they do three things:

That elevation accuracy? Non-negotiable. A 10 cm error in height translates to roughly 0.03 milligals of error in your reading. Which is bigger than the anomaly you're hunting. I learned this the hard way on my second survey in Chilas — the crew used a regular handheld GPS and the whole dataset was basically noise. PKR 240,000 wasted.

After fieldwork comes the corrections. And there are many:

What you get at the end is called a Bouguer anomaly map. Peaks and lows on this map are where the interesting geology lives.

When Gravity Surveys Actually Make Sense in Pakistan

Honestly, I used to think gravity surveys were mostly overkill for small operators. Then I ran the numbers on three of my mines and changed my mind — partially.

Gravity works best when:

You're chasing dense targets in less-dense host rock. Chromite pods inside serpentinite (Muslim Bagh, Waziristan ophiolite belts) — perfect target. The density contrast is huge, sometimes 1.5-2.0 g/cm³. Massive sulphide copper-gold at Reko Diq style deposits — also great. Barite, iron ore, manganese — all excellent.

You already have a defined area. Gravity is not a regional reconnaissance tool. Not really. It's expensive per square kilometer. You use it after satellite data (this is where breeze geo mineral analysis and platforms like geomines come in) has narrowed things down to a few hundred hectares of high-priority ground.

Depth of investigation matters. Surface mapping and Sentinel-2 imagery show you outcrops. Gravity sees 50 to 500 meters down. If your target is buried under alluvium or colluvium — common across the Chagai belt and much of Balochistan — you need something that penetrates cover.

Where gravity struggles: lithium pegmatites (density contrast is tiny, sometimes negative), gold in quartz veins (veins are too thin to detect), and shallow marble/granite deposits where you can just look at the outcrop.

The Cost Reality and How I Sequence It Now

A ground gravity survey in Pakistan runs anywhere from PKR 8,000 to PKR 25,000 per station depending on terrain access, crew reputation, and how many corrections they apply. A serious survey has 200-800 stations. So you're looking at PKR 15 lakh to PKR 1.5 crore for a proper job.

Compare that to a satellite-based geological analysis on geomines which runs a small fraction of that and covers 10x the area. But — and this is important — they answer different questions. Satellite data tells you where to focus. A gravity survey mineral exploration Pakistan program tells you what shape and depth the target has once you've focused.

My sequence for any new prospect now:

  1. Satellite spectral + structural analysis via geo mining tools (2-3 weeks, low cost)
  2. Field verification with a geologist walking the priority zones (1 week)
  3. Geochemical soil sampling on the top 2-3 targets
  4. Then gravity survey — only on the target that survived steps 1-3
  5. Drilling

Skipping steps 1-3 and jumping straight to a gravity survey is how people burn 40 lakhs and end up with a beautiful anomaly map they can't afford to drill.

And look — if a consultant tries to sell you a gravity survey as the first thing you do on a raw exploration license, ask him why. There might be a good reason. Or he might just want the invoice.

What's the density contrast on your target?

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