Void Detection Survey: Coal Mine Areas and Haul Roads

Can GPR find voids in coal mine areas?

Yes for shallow voids, no for deep old workings – and a good void detection survey says clearly which is which. The radar sends electromagnetic waves into the ground, and an air- or water-filled cavity changes the return signal sharply. It picks up loosened ground and collapsed zones in the top few metres, where most damage to roads and buildings begins. It cannot see galleries of an old mine lying tens of metres down.

That honesty matters in a coalfield. A radar scan over a haul road or colony can find the soft patch that will become tomorrow's pothole subsidence. A bord-and-pillar working at 40 metres is a different problem. It needs a borehole programme or a deeper geophysical survey by a specialist, with radar checking only the surface layer. We say this at the enquiry stage, before any crew is booked.

For the method itself, see more on GPR scanning.

Where is a void detection survey needed in a coalfield?

Wherever people, vehicles or services sit on ground that may have been undermined, burnt or poorly backfilled. The void detection survey coal mine managers ask for most often covers one of these:

• Haul roads and dumper routes – heavy trucks over a hidden cavity are the classic failure; a scan along the road shows soft zones before they open up.
• Colonies, offices and workshops built over or near old workings, especially in long-mined fields such as Jharia and Raniganj, where subsidence and underground fires have affected settlements for decades.
• Rail sidings and coal handling plants, where settlement under a track or conveyor foundation causes costly downtime.
• Backfilled opencast areas being reclaimed or reused, where loosely placed overburden can settle unevenly.
• Pipelines and cable routes crossing mining land, where a sudden collapse could rupture water pipes or cut power to a pit.

Jharia has had a government master plan for fire control, subsidence and rehabilitation since 2009, which shows how seriously the problem is treated. Our role in such areas is narrow and practical: mapping shallow ground conditions at specific sites before something is built or dug.

Record surface warning signs before the crew arrives. Look for cracked boundary walls, doors that stop closing, puddles that drain overnight, a road that keeps needing patches, or heat and smoke from the ground. Each sign helps us place the grid and set its spacing. Dated phone photographs are useful evidence for the interpretation.

What can GPR see, and to what depth?

Range depends on the ground: dry sandy soil and sandstone let radar go deeper, while wet clay, shale and ash cut it short. The table gives a fair picture for coalfield conditions.

If services must also be graded by how certain their position is, the survey can follow subsurface utility engineering practice – our page on SUE survey explains how.

How is the survey grid laid out on site?

As a set of parallel and cross lines over the area of concern, close enough that no cavity of interest can fit between them. Higher frequencies give high resolution near the surface, while lower ones reach further down, so the antenna is chosen for the depth that matters.

1. Define the question: the structure or road at risk, the size of void that matters and any history of cracks or sinking.
2. Set the grid: line spacing is chosen from the smallest void you need to find; tighter spacing costs more but misses less.
3. Scan in two directions: crossing lines confirm that an anomaly is a real subsurface object and not a one-off reflection.
4. Mark and photograph: suspect zones are pegged on the ground the same day so your team can fence them off if needed.
5. Process and interpret: anomalies are graded by confidence and plotted on the site plan with approximate depth.

Mine sites bring their own rules. Work follows the mine manager's permission and the safety system in force under DGMS, the regulator for mine safety in India. We do not enter any area without that clearance.

As a rough guide, a colony block or workshop yard takes one day, while a long haul road is scanned in stages so traffic keeps moving. Blasting schedules, dumper movement and shift changes decide when the crew can use the road. The programme is therefore planned with the mine's own transport team.

When should boreholes or other methods be added?

Whenever a radar anomaly sits under something important, or when the target is deeper than radar can reach. We recommend a check borehole or a trial pit on the strongest anomalies under foundations, rail tracks and haul road crossings. For deep workings, a geotechnical or geophysical specialist should design the investigation; our radar results then help them place boreholes where they matter. This combination is more cost-effective than a blind drilling grid, because each hole is placed on evidence instead of a fixed pattern. After the boreholes, a short repeat scan can also confirm whether grouting or backfilling has filled a cavity as planned. Some of the practical cautions we share with site teams before any digging are written up in our engineers' notes.

What does the report and drawing show?

A plan of every suspect zone, graded high, medium or low confidence, with approximate depth and extent. The drawing comes in DWG and PDF, with the scan lines shown so anyone can see where we looked and where we did not. Utility lines found along the way are drawn on a separate layer, so the same visit also gives basic underground utility mapping. To see how layers and coordinates are handled, read about CAD and GIS utility mapping. Annotated radargram extracts are included for the main anomalies, so your engineers can judge the evidence themselves.

What does a void detection survey cost in a coal mine area?

Scanning is charged per metre of radar line; boreholes and trial pits are outside our rate and arranged separately.

• ₹24 per metre for a standard scan, ₹32 per metre for an L-section profile.
• Minimum order ₹1,00,000 for sites outside Delhi NCR (₹50,000 within NCR).
• Whole-order discounts of 25% above 100 km, 40% above 500 km and 50% above 1,000 km.
• GST extra.

A haul road scanned in four parallel lines is charged on all four lines, so a 3 km road becomes 12 km of scan. The GPR survey price list has the full card. To get a rate for your site, share a plan of the area and what is at risk.

Coal India's subsidiaries, SCCL and NLC India are the main operators in this sector. Mezux Touch has run void and utility surveys for these miners under direct work orders, as well as for the contractors they engage.

Frequently asked questions

How deep can GPR detect voids in coalfield ground?

A few metres in most cases. Dry sand and sandstone allow more; wet clay, shale and fly ash allow less. The report states the depth reached in the data collected.

Can you find old bord-and-pillar workings?

Not at normal working depths, which are well beyond radar range. For those, a borehole or deeper geophysical investigation by a specialist is needed.

Do you drill check boreholes?

No. We recommend where boreholes or trial pits should go, and your geotechnical contractor carries them out.

How quickly are dangerous zones reported?

Suspect zones are pegged on the ground on the same day, before processing, so they can be barricaded at once.

Can the same visit map pipes and cables?

Yes. Services crossing the void zone appear in the same radar data, so we detect and map them on a separate layer.

What should we send for a quote?

A plan or Google Earth outline of the area, what is at risk, any history of cracks or sinking, and the access rules for the mine.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026