Mapping Caves

Mark Altaweel

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Caves are formed due to a variety of geological processes, including weathering, tectonic movement, and pressure.

Benefits of mapping and studying caves

Caves are often recreational or treated as tourists sites; they also serve as important scientific places of study, including being used to study climate change. Caves have also been studied and considered as potential storage areas for methane, one of the most potent greenhouse gases.

Submerged caves are important areas for water resources used to supply population centers. Additionally, caves have important cultural signifigance, as many caves serve as holy places or have connections to narratives important to ethnic/national identities.

A yellowed map of Mammoth Cave from 1835.
A map of Mammoth Cave, the largest cave system in the world. Map: 1835 via LOC.gov.

Challenges of mapping caves

While caves can be important to different stakeholders, mapping them has been challenging, particularly large, complex caves that can stretch for many kilometers underground. Mapping caves is important both for management, that is to monitor and protect caves, but also to understand their wider benefits to society.

New technologies and techniques have helped create more accurate maps of caves.

Mapping caves with a compass

One can create a reasonable map of a cave system using a compass, measuring tape or laser range finder, and an inclinometer to measure slope. With a compass, correction between magnetic and truth north is always needed and caves often change due to tectonic or erosional activity that may require periodic updating to maps.

Software to map caves from measurements

Current software that can automatically create maps from basic measuring tools include CaveWhere. This mapping software can take 2D data and project 3D maps; profiles of caves can be warped and fit to larger maps used for given areas. Overall, the software provides a simple to use interface where field-collected data can be entered to create a map quickly.[1] 

Similar open source GNU GPL licensed software, Therion, can be used for similar functions. The software provides texturing, stereo (anaglyph) rendering, and off screen rendering.[2] 

Other popular tools include COMPASS and Walls, which can also provide declination to correct for the difference between true and magnetic north.[3]

Mapping complex caves

While current software has become more sophisticated and easier to use for users, the challenge for speleologists, that is those who study caves, is mapping particularly complex, long caves, including those that have underground rivers or caverns that might be well hidden behind fallen debris or simply having narrow openings.

A picture of cave columns covered in "cave popcorn".
Complex cave systems can be more difficult to map. Photo: Cave columns in the Caverns of Sonora. Photo: Alex Demas, USGS, public domain.

Electrical resistivity tomography

Electrical Resistivity Tomography (ERT) has emerged as one geophysical method to help speleologists better map caves. The method uses electrical currents that are sent through the bedrock or surface, with the voids in the cave providing a high contrast to the solid rock overlying caves.[4] 

Furthermore, for ERT, one can map caves from the outside using pins and wiring set at various intervals, depending on the desired depth of the electric current penetration, where pins setup further apart enables a generally deeper ground signal. With ERT, 2D and 3D maps can be created, with potentially long distances covered by mapping profiles enabled by ERT.

Airborne electromagnetic mapping

Another related method, airborne electromagnetic mapping (AEM), has been used to map submerged cave systems. This technique involves transmitting an electromagnetic signal from a plane or helicopter.

An annotated photo of a white helicopter dangling an electromagnetic loop over a sandy area with blue skies in the background.
Airborne electromagnetic mapping can help to map complex cave systems by mapping the electromagnetic response of the subsurface to detect changes in underground features. Photo: USGS, public domain with annotations by Caitlin Dempsey.

Similar to ERT, conductivity contrasts in voids allows caves to emerge in the imaging and enables mapping to be performed even in difficult caves such as those that are submerged.[5] 

Using drones to map caves

While still not discussed in any great length in the scientific literature, the next steps for mapping caves might be the use of AEM or similar electromagnetic techniques using UAVs, as these offer a low-cost way to map using this method and might be more suitable for different elevations needed for UAV systems to fly so that the depth of signals can be adjusted as needed. Variations of this can include unmanned underwater vehicles (UUV) for underwater caves.[6]

Caves are some of our last terrestrial and even underwater unexplored places, as even caves that are well known to us are often not fully explored and long-lost tunnels are hidden. Mapping software has enabled the creation of much more accurate maps that we can be adjusted and placed within larger maps of the Earth.

The real challenge is exploring all the voids present in any cave system. Techniques such as ERT and AEM are some of the best geophysical methods used. Although even these techniques may be too coarse or not provide enough detail.

Furthermore, entire voids can be missed if signals do not reach given spaces. The challenge, therefore, will be to create other techniques that may improve overall accuracy and save time in capturing needed detail to map caves. 

References

[1]    For more on CaveWhere, see:  https://cavewhere.com/.

[2]    For more on Therion, see:  https://therion.speleo.sk/.

[3]    For more on COMPASS, see:  https://www.fountainware.com/compass/index.htm.  Walls can be downloaded here: https://www.texasspeleologicalsurvey.org/software/walls/tsswalls.php.

[4]    For more on using ERT to study caves, see:  Vargemezis, G., Fikos, I., Tsourlos, P., 2015. Application of Electrical Resistivity Tomography Method to the Mapping of Explored Caves and Detection of Possible New Chambers: Case Studies from Greece. Presented at the 8th Congress of the Balkan Geophysical Society, Chania, Greece. https://doi.org/10.3997/2214-4609.201414132.

[5]    For more on mapping using AEM techniques for caves, see:  Schiller, A., Supper, R., Schattauer, I., Motschka, K., Merediz Alonso, G., Lopéz Tamayo, A., 2017. Advanced Airborne Electromagnetics for Capturing Hydrogeological Parameters Over the Coastal Karst System of Tulum, Mexico, in: Renard, P., Bertrand, C. (Eds.), EuroKarst 2016, Neuchâtel, Advances in Karst Science. Springer International Publishing, Cham, pp. 35–43. https://doi.org/10.1007/978-3-319-45465-8_4.

[6]    For more on using UAVs with AEM, see:  Becken, M., Kotowski, P.O., Schmalzl, J., Symons, G., Braunch, K., 2022. Semi-Airborne Electromagnetic Exploration Using a Scalar Magnetometer Suspended below a Multicopter. First Break 40, 37–46. https://doi.org/10.3997/1365-2397.fb2022064

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.

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