Geophysical Techniques For Karst Features


 

 

Detecting sinkholes or other karst-related features using geophysics (nonintrusive subsurface testing) is a proven geotechnical application in which Enviroscan specializes; however, it is our experience that there may be confusion about the use of karst-related terminology – which may lead to expectations that geophysics cannot meet. In particular, the word “sinkhole” is often applied to features including:

  • Bedrock solution openings
  • Depressions in the top-of-bedrock
  • Stoping voids in the soil mantle
  • Zones of wet soupy soils (i.e. mud-filled voids in the soil mantle)
  • Clay seams (i.e. mud-filled voids in bedrock), and
  • Actual surficial collapse features or true sinkholes.

The interrelationships of these features are described on the Karst Processes And Terminology page. While any or all of these features may be present in a karst terrane, and may be involved in sinkhole collapse, they have very different physical properties, and therefore cannot all be detected by a single-technique geophysical survey.  In other words, there is a geophysical technique capable of detecting any of these features, and there are techniques capable of detecting more than one feature, but there is no “magic bullet”, single-technique geophysical panacea, that can simultaneously detect all karst or sinkhole-related features.

The geophysical techniques commonly applied to detection of karst features can be classified as follows:

  • Electromagnetics (EM) and DC Resistivity - detect variations in subsurface electrical properties related to anomalously thick or wet soils (electrical conductivity highs), voids in the electrically conductive clay soil mantle (producing electrical conductivity lows), or clay-filled seams or cavities within bedrock (electrical conductivity highs).
     
  • Spontaneous Potential (SP) - detects naturally occurring minute electrical currents or potentials commonly associated with concentrated infiltration, or other movement, of water (often called streaming potentials).
     
  • Microgravity - detects minute variations in gravity, which in karst terranes are often due to subsurface voids or solution cavities where “missing” subsurface mass results in measurably lower gravity.
     
  • Seismic Methods - can provide profiles of the top-of-rock which may display conical depressions of the type associated with subsidence sinks, or deep gouges or cutters which may represent sinkhole-prone lineaments.  Some seismic methods may also be able to detect low velocity zones or bodies  Seismic refraction is also used to calibrate microgravity data in the absence of available boring data, and is also often used to discriminate between EM/DC resistivity anomalies caused by bedrock deeps versus wet, soupy soils.

Schematic Karst Features Combinations, or cartoons depicting the various karst features which commonly precede sinkhole formation are depicted on the linked page.  These cartoons depict the features to which a survey intended to identify potential incipient sinkhole locations or sinkhole-prone lineaments must be sensitive.

Since there is no single geophysical technique capable of detecting all sinkhole features. Complete geophysical detection would require performance of numerous coincident surveys (e.g. EM, SP, seismic and microgravity) to identify all of the cases depicted in the cartoons.  This necessity leads to the classic conflict between confidence and budget.  To solve this dilemma or achieve the proper cost/confidence balance, Enviroscan recommends geophysics not as a replacement for borings or other direct testing, but as a reconnaissance tool to identify areas that might require additional borings, and areas where fewer borings may be necessary, thereby optimizing the number and location of borings, and minimizing the overall geotechnical investigation cost.

The most common reconnaissance geophysical techniques are EM, SP and seismic methods since data can be collected, over potentially large areas, relatively rapidly. In contrast, microgravity surveys are labor-intensive and typically cost two to three times what an EM, SP, or seismic survey would cost for similar data coverage.  Therefore, microgravity surveys are rarely used for reconnaissance. Instead, they are a cost-effective way of providing detailed mapping and dimensions of a cavity or cavity system in a specific area (e.g. where cavities are encountered in a boring,  where sinkhole activity suggests their presence, or within the footprint of a critical structure).

In the cartoons, note that Case 1-A (a bedrock void with no overlying soil void, bedrock depression nor anomalous hydraulic activity) represents the most difficult hypothetical target for a geophysical “sinkhole” survey. Luckily, however, the difficulty in detecting a Case 1-A cavity also implies that it represents the lowest imminent sinkhole hazard – i.e. anomalous hydraulic activity of the type which triggers and drives sinkhole development (and is detectable using EM or SP) is not present. The general hazard of imminent sinkhole collapse for each case is listed with the cartoons.

In summary, when Enviroscan is asked to perform a geophysical “sinkhole” survey, the term sinkhole may connote a wide range of features - and possibly different features to different persons. As we have increasingly encountered this source of confusion, we have realized that it is incumbent upon us as the geophysics experts to be clear about exactly which karst features we can (and cannot) detect based on the techniques we intend to employ in any survey.  This will ensure that clients know ahead of time exactly what results to expect.  In this way we hope to prevent skepticism about the utility of geophysics for detecting sinkholes and other karst-related features.

 

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