The purpose and benefits of geophysics are illustrated nowhere better than in karst terranes (i.e. areas underlain by solution-prone limestone or dolomite in temperate climates).  In these areas, rock surfaces can be highly irregular (pinnacled); the bedrock may contain joints, fractures, or open bedding planes called cutters or clay seams; bedrock may also contain networks of convoluted solution channels or scenic caves; and there may be soil pipes or air-, water-, or mud-filled voids in the soil mantle. Click here for a description of typical karst feature development and terminology.

Karst areas are of particular geotechnical concern (due to their propensity for ground subsidence and occasional catastrophic sinkhole collapse), and hydrogeologic concern (due to the potential for rapid and unexpected migration of contaminants). In addition, due to tremendous lateral variations in subsurface conditions, correlation of information between even closely spaced borings is highly speculative in karst terranes. For instance, rock depth may be 5 feet in one boring, and 50 feet in a boring only yards away. Geophysics can provide data both to locate potential “problem areas” where boring programs should be focused, and to allow accurate extension of data between borings.

Detecting incipient sinkholes, bedrock cavities, rock pinnacles, and other karst-related features using geophysics is a proven geophysical application in which Enviroscan specializes. We perform electromagnetics (EM) and electrical imaging (EI) to detect variations in subsurface electrical properties related to anomalously thick or wet soils (electrical conductivity highs), or voids in the electrically conductive clay soil mantle (producing electrical conductivity lows). We do spontaneous potential (SP) surveys to detect naturally occurring minute electrical currents/potentials commonly associated with concentrated infiltration or subsurface movement of water. We do microgravity surveys to map minute variations in gravity that, in karst terranes, may be due to soil voids or bedrock solution cavities where “missing” subsurface mass results in measurably lower gravity.  We do seismic refraction, reflection, and surface wave analysis to provide profiles of the top-of-rock which may display conical depressions of the type associated with ancient sinks, or linear deeps which may represent sinkhole-prone faults, fractures, bedding planes, or other lineaments. Seismic rock depths are also used to calibrate microgravity results where no boring data are available.  The latter two methods (microgravity and seismic profiling) are also often used to discriminate between EM or EI conductive anomalies (which could represent either a bedrock deep or wet, soupy soils), or between EM or EI resistive anomalies (which could be caused by either dry, competent rock or air-filled voids) .

The complexity of karst terranes makes it impossible to detect all subsurface features that might be of concern using only a single geophysical technique. [Click here to see a description of the karst features detectable using various geophysical techniques]. Therefore, Enviroscan typically recommends a two (or more) technique geophysical survey.  In addition, we always recommend that geophysics in karst terranes not be used as a replacement for a boring program, but as a means for ensuring a cost-effective, high confidence study - cost-effective because geophysical data can minimize the number of required borings and optimize their placement; and high confidence because geophysical data can guide accurate extension of direct data between borings.