Geophysics screens, borings confirm: sequencing a void investigation

Subsurface void investigations follow a sequence for a practical reason: drilling is accurate but blind, and geophysics sees widely but indirectly. In the right order the two complement each other — a survey covers ground quickly and identifies where the subsurface departs from its surroundings, and borings then determine what is actually there. Reported as though one substitutes for the other, the same tools produce conclusions that do not hold up. Most disputes over sinkhole investigations are disputes about this sequence rather than about the physics.

Why surface observation cannot close the question

The visible depression is the end of a process that occurred somewhere between the surface and the rock. Its shape carries some information, but not whether a bedrock void exists, whether the overburden raveled into it, or whether the material instead disappeared into a broken pipe.

Published karst-susceptibility mapping is a useful frame — it describes what a region does — but susceptibility is not occurrence. A conclusion resting on regional mapping plus a site walk is the kind of opinion that gets excluded.

Choosing the method before running one

ASTM D6429, the standard guide for selecting surface geophysical methods, exists because method selection is site-specific and consequential. The relevant inputs are the target's size and depth, its contrast in physical properties with the surrounding ground, the near-surface soil type, and the cultural noise present — reinforcement, utilities, pavement, traffic.

A survey whose method was never justified against site conditions is vulnerable regardless of how clean its output looks. The selection rationale, documented before mobilization, is part of the deliverable.

What ground-penetrating radar resolves, and where it stops

GPR transmits electromagnetic pulses and records reflections from interfaces where dielectric properties change. In dry sands it images raveled zones, disrupted stratigraphy and shallow voids with excellent lateral resolution, producing continuous coverage along a line rather than point measurements.

Its limitation is consistently underweighted: conductive material attenuates the signal. In clay-rich or saturated soils, useful penetration can fall to a small fraction of what the same equipment achieves in clean sand. When GPR returns nothing at a clay site, the reading is often that the method could not see the depth of interest.

Electrical resistivity and the conductive-soil problem

Resistivity imaging injects current through surface electrodes and inverts the measured potentials into a cross-section. It reaches deeper than GPR in conductive ground and suits mapping of the soil-rock interface, clay-filled solution features and raveled zones.

The output is a model, not a photograph. Inversion is non-unique: more than one subsurface arrangement can produce the same measured data, and an anomaly's apparent depth and size depend on the parameters chosen. Air-filled and water-filled voids also behave in opposite directions. A resistivity anomaly is a location to investigate, not a finding.

Seismic methods and microgravity

Seismic refraction under ASTM D5777 measures travel time of elastic waves refracted along velocity boundaries, and is conventional for mapping depth to rock. Its assumptions matter: refraction handles increasing velocity with depth well and can miss a low-velocity layer beneath a faster one — in karst, precisely the geometry of interest.

Microgravity instead measures small variations in the gravitational field caused by the mass deficiency a void represents. It is sensitive to the thing actually sought rather than to a proxy, but it is slow, demands precise elevation control, and requires correction for terrain and nearby structures.

Anomaly is not void

Every one of these methods measures a physical property, not a cavity. Each maps where the subsurface differs from its surroundings, and subsurfaces differ for ordinary reasons: a buried utility, a former excavation, a lens of different soil, a change in moisture, a variation in weathering.

This is why geophysics is properly described as screening. It converts an unbounded search area into a short list of locations worth the cost of drilling — a substantial contribution, and not the same as confirmation.

Borings are what confirm it

Confirmation means physically encountering the feature. Standard penetration testing under ASTM D1586 records blow counts and returns a sample; the signature sought is a zone of markedly reduced resistance — sometimes rod drops under the weight of the hammer or rods alone — indicating soil loosened by downward migration.

Cone penetration testing under ASTM D6067 gives a continuous profile of tip resistance and sleeve friction, though it recovers no sample. Coring into rock demonstrates dissolution directly: recovery loss, soft or clay-filled seams, and drilling-fluid loss are the record of an opening. Geophysics locates; borings adjudicate.

What a negative result does and does not mean

A survey that identifies no anomaly, and borings that encounter no void, are meaningful findings. They are not proof of absence, and describing them that way is where otherwise sound investigations overreach.

Each method has a resolution limit and a depth of investigation set by site conditions, not by equipment; a feature smaller than that limit, deeper than that depth, or lying between survey lines will not appear, and borings sample a very small column of ground. An honest negative reads: within the area surveyed, to the depth resolvable, no anomaly consistent with a subsurface void was identified.

Documentation that survives a second opinion

Sinkhole matters routinely produce competing reports, and the one that holds up is generally the one another engineer can reconstruct: recorded survey line locations and spacing, acquisition parameters, inversion settings, boring locations tied to the anomalies rather than to property corners, and raw records retained rather than only interpreted figures.

It also means stating what was not done. An investigation explaining why microgravity was not run, or why borings stopped where they did, is more defensible than one where those decisions surface first under questioning.

This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.