How to Conduct a Gravity Survey in Pakistan: Step-by-Step Field Guide for Mining Engineers
A gravity survey measures tiny changes in Earth's gravitational pull to find dense or light rock hidden underground. Denser bodies — like chromite, sulphide ore, or a buried intrusion — pull a bit harder. Lighter zones — salt, voids, sediment-filled basins — pull a bit less. Your job with a gravimeter is to record those differences across a grid, then correct the raw numbers until what's left points to actual geology.
Sounds simple. It isn't — the corrections are where most people mess it up.
Here's the thing about running a gravity survey in Pakistan specifically. The terrain fights you. Gilgit Baltistan, Chagai, the Sulaiman range — steep relief, weak roads, unreliable GPS elevation. That matters because gravity is exquisitely sensitive to how high your station sits. Get the elevation wrong by a metre and your correction is already off. So the field discipline you bring to a survey here decides whether the data is worth anything.
What a gravity survey actually tells you (and what it doesn't)
A gravity survey maps density contrasts. Full stop. It won't tell you "there's gold here." It tells you "there's a denser mass here, at roughly this depth and shape" — and then you use geology, drilling, and other data to figure out what that mass is.
Where it earns its keep in geo mining:
- Chromite and massive sulphide bodies (they're dense, they show up as positive anomalies)
- Mapping buried intrusions and contacts that host copper or gold
- Finding basin edges and sediment thickness
- Spotting voids or fault zones before you dig
Where it's weak: shallow, thin, or low-contrast targets. A gravity survey is a regional-to-district tool. For pinpointing a narrow vein, it's the wrong instrument.
Honestly, I'd never run a ground gravity survey cold. Look at satellite data first. At GeoMine AI we use Sentinel-2, ASTER, SAR, and SRTM DEM to narrow a licence area down to a few high-priority zones. Then you send a crew to those zones with a gravimeter — instead of dragging a $100k instrument across an entire lease. Cheaper. Faster. Fewer wasted station days.
Gravity survey procedure: the step-by-step field guide
This is the sequence a mining engineer actually follows on the ground.
1. Design the grid. Station spacing depends on target size and depth. Tighter spacing for small shallow bodies, wider for regional work. A common approach is closer stations over your anomaly and looser fill-in elsewhere. Decide before you leave the office — walking a grid twice wastes days.
2. Establish a base station. Pick a stable, easy-to-return-to point. Every reading ties back here so you can correct for instrument drift and tides. Read the base at the start, then re-read it every 1-2 hours through the day.
3. Survey each station's position and elevation precisely. This is the make-or-break step. You need centimetre-level elevation. Use differential GPS (RTK) — not a handheld unit. In deep Pakistani valleys where satellite lock is poor, plan for total-station backup. I can't stress this enough: elevation error is the number one killer of Pakistani gravity data.
4. Take the gravimeter reading. Level the instrument, let it settle, record the value and the exact time. Note temperature and any nearby disturbance (traffic, running water, wind on the tripod).
5. Loop back to base regularly. Drift is real. The spring inside the gravimeter creeps over hours. Re-occupying the base lets you correct it out.
6. Log everything. Station ID, coordinates, elevation, reading, time, operator notes. Digital and paper. Paper survives a dead battery in the Karakoram.
The corrections — where good data goes to die
Raw gravimeter readings are meaningless until you apply corrections. Here's what each one does, in plain terms.
| Correction | Fixes for | Why it matters in Pakistan |
|---|---|---|
| Drift | Instrument creep over time | Long field days, hot conditions |
| Tidal | Sun/moon pull changing through the day | Standard everywhere |
| Latitude | Gravity varies from equator to pole | North-south survey lines cover latitude range |
| Free-air | Station elevation above datum | Extreme relief makes this huge |
| Bouguer | Rock mass between station and datum | Needs a good density estimate |
| Terrain | Nearby mountains and valleys pulling sideways | Non-negotiable in GB and Chagai |
That last one — terrain correction — is the one people skip because it's tedious. In flat Punjab, maybe you get away with a rough one. In steep ground, skip it and your "anomaly" is just the mountain next door. Use a good DEM for the terrain correction. SRTM works; higher-resolution DEM is better where you can get it.
After corrections you get the Bouguer anomaly — the map that actually reflects subsurface density. That's your product.
Common mistakes I see on Pakistani surveys
- Trusting handheld GPS elevation. Don't. RTK or nothing for the vertical.
- Skipping or rushing terrain corrections in high relief.
- Reading the base too infrequently, so drift goes uncorrected.
- Guessing the Bouguer density instead of estimating it from local rock.
- Running a survey with no target model — you need a rough idea of what depth and size you're hunting, or you can't pick sensible station spacing.
- Treating gravity as a standalone answer. It's one layer. Pair it with magnetics, satellite spectral data, and drilling.
And a practical one: plan your logistics like the survey depends on it, because it does. Weather windows, permissions, fuel, a crew that can carry a gravimeter across scree without dropping it. A dropped gravimeter is a dead gravimeter.
How satellite data cuts your gravity survey cost
Ground gravity is slow and expensive per square kilometre. That's the trade-off — it's precise but it doesn't scale cheaply. So the smart move is to shrink the ground you have to cover.
Before you commit a crew, we can run a satellite pass over your licence. Sentinel-2 and ASTER pick up alteration and lithology signatures. SRTM DEM feeds structural and quarry-siting analysis. SAR handles surface and moisture. The output is a ranked set of zones worth walking with a gravimeter — and the DEM you'll need for your terrain corrections anyway.
That's the workflow I'd back for any serious exploration program in Pakistan. Satellite to target, ground gravity to confirm, drill to prove.
Want your licence area screened before you mobilise a gravity crew? Send us your coordinates or lease boundary at https://geomines.org and we'll tell you where the ground gravity is actually worth doing.