Karst is a terrain characterized by sinkholes, sinking streams, springs and caves associated with highly soluble carbonate rock, providing both physical hazards and natural beauty. Geophysical methods, including seismic, microgravity, self-potential, electrical resistivity, electromagnetic, and ground-penetrating radar, have been applied in karst areas for water resource and geohazard investigations for decades. The main advantages of these geophysical methods are that they are nonintrusive and less expensive than direct drilling. Additionally, the probability of a random borehole will intersect a karst feature is very low and may require many holes at a great expense.
Fractures serve as major pathways for groundwater flow in karst aquifers. In most karst areas, the majority of groundwater is carried through a fracture system before discharging to the groundwater or a spring. For this reason, geophysical methods are very useful for studying karst terrains because of the intrinsic heterogeneity of the medium, between the water filled fracture and host carbonate rock. If a highly detailed geophysical study is conducted, the geometry and structure of a karst system (i.e. epikarst, karst conduits and cavities) can lead to a greater understanding of the different processes taking place (infiltration from surface to groundwater level, dissolution, suffusion, the movement of contaminants).
Because the main heterogeneities in karst terrains are due to the existence of voids (minor and major cavities, conduits, fractures and fault zones) any geophysical method must involve void detection as its main objective. Furthermore, the geophysical mapping of karst heterogeneities needs to encompass the many difficulties involved: (1) target depths may vary within one to hundreds of meters; (2) void sizes may range from centimeters to tens of meters; (3) the surfaces of karst areas are often rough and consist of a combination of soil and compact rock; (4) investigation sites often belong to protected environmental areas; and (5) groundwater and its seasonal variations plays an important role in that it changes physical parameters at different times. Although all these points might sometimes be disadvantageous, the vadose zone affords the advantage of not being a fully saturated medium and thus provides a sharp contrast between the geophysical properties of the target and its host. Consequently, it is important to choose the best of the methods available for detecting the difference between compact rock and karstic voids.
Kartchner Caverns State Park, Benson, AZ
-Detecting air filled voids in desert terrains
In 2007, hydroGeophysics, Inc. performed a karst investigation at Kartchner Caverns State Park in Arizona. The geophysical survey was designed over the area where the known limestone caverns lie beneath with the resistivity line extending to the east and west over unexplored terrain. The survey was initially conducted to test the capabilities of detecting air filled voids in desert terrains using electrical resistivity. Using limestone as a host is particularly difficult because you are sensing a resistive target (void) within a resistive matrix (limestone). What HGI found was that the known explored portions of the caverns showed up as very resistive bodies, as expected. Unexplored regions of the subsurface also appeared to have voids, and possibly larger than anything that has already been mapped. This has implications for Paleozoic limestone blocks across Arizona, as it continues to provide evidence that these blocks will likely contain karstic features even though there are no direct expressions at the surface. HGI plans to return to Kartchner Caverns State Park in the future to map more of the caverns, in both the southern and northern limestone blocks.
The below figures show the location and results of the resistivity line surveyed directly over the caverns. Points of interest marking the extent of the explored caverns and the additional discovered voids are labeled on the Base Map and can be referenced on the resistivity profile. Resistive regions marked X to X’ indicate undiscovered voids to the west of the explored caverns. Robert Casavant, Arizona State Parks Science & Research Manager, has nicknamed the largest of these anomalies, the “West Wing.”