Void and sinkhole detection

Why one method is rarely enough

A void is a hole in the ground's physical properties. It has no density, no conductivity and no moisture, and it sits inside material that has all three. That sounds like an easy target and sometimes it is. More often the ground above it defeats whichever instrument you brought.

Each method fails in a different way, and that is the useful part:

• Radar gives the highest resolution available for the first few feet — the roof of a shallow void, the ragged throat of a developing sinkhole, the disturbed column above a collapse. It is defeated by clay, by saturation and by depth, and in karst country the clay is usually exactly where the problem is.
• Microgravity responds to the missing mass itself. It does not care whether the ground is conductive, wet, reinforced or paved, which makes it the method that still works where everything else has failed. It is slow, it is station work rather than walking work, and the elevation of every station has to be surveyed to a fraction of an inch.
• Resistivity sections the ground. An air-filled void is extremely resistive; a clay-filled one is extremely conductive. Both stand out against sound limestone, and a resistivity line tells you which of the two you are dealing with — a distinction that decides whether the problem is stable or active.
• VLF and EM respond to steeply dipping conductive features: the fractures and fissures that carry the water that makes the voids in the first place. Often the feeder is more useful to find than the cavity.

Agreement between two methods measuring independent physical properties is real evidence. A single anomaly from a single instrument is a hypothesis, and on this subject the cost of confusing the two is a collapse.

The conversation that should happen before you commission

Detectability falls away sharply with depth and scales with the size of the thing you are looking for. A six-foot cavity at ten feet is a straightforward target. The same cavity at sixty feet may be genuinely undetectable by any method, and no amount of care in the field changes that.

So before quoting we work out what is detectable on your site — for your target size, at your target depth, in your ground — and tell you. Sometimes the answer is that the survey is worth doing. Sometimes it is that the method cannot deliver what the question needs and you should spend the money on a borehole programme instead. We would rather say that at the start than produce a clean report that means nothing.

Karst

Limestone and dolomite dissolve, and the result is a landscape that looks ordinary at surface and is not. Dissolution features, throats, soil piping and the slow upward migration of a cavity toward a slab or a pavement are all invisible until the day they are not.

This is where the pairing of radar and microgravity earns its keep, with resistivity across the features that matter. Radar maps the shallow disturbance in detail; gravity sees the deep mass deficit the radar cannot reach; resistivity says whether a feature is open or choked with clay.

On a wartime runway in the Caribbean we surveyed exactly this problem around live cargo traffic — no closure, no coring — and it was VLF, not radar, that found the deep dissolution features feeding the system.

Mine workings and man-made voids

Abandoned room-and-pillar workings, bell pits, shafts, culverts, cisterns, forgotten basements and backfilled excavations. These are often shallower and more regular than natural voids, which helps, and frequently absent from any record, which does not.

Where a plan exists it is a starting point rather than an answer — mine plans were drawn to the standards of their day and the workings were not always driven where the plan says. Geophysics is how you find out which parts of the plan are true.

What it will not tell you

A survey can locate an anomaly consistent with a void and bound its probable extent. It cannot measure the thickness of the roof over it, confirm whether it is stable, or tell you when it will fail. Those are geotechnical and structural questions, and they are answered by drilling into the thing we found.

Nor is a clear result a guarantee. On a difficult site it means nothing above the detection threshold was found, and the threshold is the number that matters. Every report we issue states it.

Who commissions it

Developers and their engineers on karst terrain; owners with a hole that has appeared and a need to know whether more are coming; highway and rail authorities after a pavement collapse; dam, levee and reservoir operators; insurers and loss adjusters establishing cause; and anybody building over ground that was mined before the records were kept.

Equipment and capability

• Radar for shallow voids and the disturbed column above a collapse
• Microgravity for deep mass deficit, with survey-grade elevation control
• Resistivity imaging to separate air-filled from clay-filled features
• VLF and EM for the fractures and conduits that feed the system
• Stated detection threshold for your target size and depth
• Karst, mine working, shaft and man-made void investigation
• Target lists positioned for a drilling programme to confirm

What you receive

Each anomaly located and bounded, with the methods that saw it and the methods that did not — because a feature detected by two independent properties and a feature detected by one are different findings and should never be drawn the same way. A stated detection threshold. And where the answer has to be certain, the positions where a drill will settle it, rather than another pass with the same instrument.

Related methods

← All geophysical services