No phone can find a void. A microgravity survey detects one at five to ten microgals; this phone's accelerometer is about ten thousand times coarser than that, and averaging cannot close the gap because the drift is larger than the signal. Ground-penetrating radar, microgravity, resistivity and a drill are what find cavities.
What this instrument does is triage and monitoring: whether the ground here can dissolve at all, what kind of hole you are looking at, how far back to stand, and whether it is still moving. Those are answerable. Where a void is, and when it will go, are not.
Karst needs rock that dissolves. Everything else on this page is irrelevant if the answer here is no — and a hole in ground that cannot dissolve has a different cause, which usually means a different and cheaper remedy.
| on the ground | what it tells you |
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Not every hole is a sinkhole, and the commonest false alarm is a leaking pipe. Answer what you can see; the key names the likeliest cause and what would confirm it.
A collapse does not stop at the hole you can see. The cover ravels upward and outward at about its own angle of repose, so the eventual opening is wider than the throat by roughly 2 × depth × tan(90° − φ). Stand outside that, not outside the rim.
Do not walk the rim — the ground that fails next is the ground you are standing on to look. Keep vehicles and plant outside the same circle; a loaded truck is the commonest trigger for the second collapse. Do not fill it, and do not let anyone else: filling a throat without knowing what is beneath makes the next hole bigger and hides the evidence of what caused this one.
This is the part a phone is genuinely good at. Mark a point, measure across a crack against a fixed gauge, photograph from the same spot, and come back. What matters is not the width but whether the rate is rising.
Cover collapses are usually triggered, not spontaneous. Heavy rain loads the cover and washes fines down; falling water tables take away the buoyancy that was holding it up. That second one is arithmetic: every metre of drawdown adds 9.81 kPa to the load the soil arch has to carry.
| trigger | how it works |
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Ground over a shallow cavity rings differently under a hammer: lower, and for longer. This records a tap and measures it, so you can compare suspect ground against ground you trust. It cannot tell you a void is there — a difference tells you where to bring an instrument that can.
Strike the same way each time, with the same tool, on ground of the same cover. Damp ground rings differently from dry, and a buried slab rings differently from soil — which is why the control tap has to be a few metres away on the same material, not across the road.
Whether the water is aggressive enough to still be dissolving the rock — a calcite saturation index below zero means the passage is being enlarged now — is Water chemistry. Surveying a cave you can get into is Cave survey, and the ground's strength and the arch above a cavity belong to Foundations and tunnels.