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Developing a Cave Potential Map for South Dakota's Wind Cave Using GIS
By Rodney D. Horrocks, Cave Resource Management Specialist, Wind Cave National Park, Hot Springs, South Dakota
Since the discovery of Wind Cave in South Dakota in 1881, numerous cavers have been drawn to explore its complex mazes. Since 1902, 166 km have been surveyed in the cave, and the boundaries have expanded to fill a 1.6-1.9 km rectangle. A famous diary quote by an early Wind Cave explorer, Alvin McDonald, says, "Have given up the idea of finding the end of Wind Cave." This is as true today as it was in 1891 when it was written. It is common to hear cavers participating in the current survey effort remark about the "endless" potential of the cave. As the boundaries of the cave have enlarged, many hypotheses on its potential extent have been proposed, even going so far as speculating on a connection with Jewel Cave, 29.44 km to the northwest.
For years, cavers based the potential size of Wind Cave on barometric wind studies done by Herb Connecticut in 1966. Those studies suggest that the current volume of the surveyed cave represents only 2.5 percent of the total volume of 5.5 x 1,010 m3. The current volume of the surveyed portions of Wind Cave is 1.4 x 109 m3. As the boundaries of the cave have enlarged, many hypotheses on passage extent have been proposed. This study, however, speculated that a GIS-based cave potential model might be a more accurate method to predict the amount of potential surveyable passage in Wind Cave than barometric wind studies could provide.
Historically, it had been difficult to get park managers to recognize that Wind Cave may extend beyond its current boundaries, especially when additional park infrastructure was being planned outside of known cave boundaries. Unless the cave survey had been extended below a particular area, it was assumed that no cave existed there. Although this concept of a cave potential map was first developed after hearing about these attitudes, actual development of the model identified its limitations as a management tool. However, this project was partially instigated because managers cannot wait until a cave of this magnitude has been completely surveyed to develop their management policies. It was realized that an estimation of the potential extent and surveyable length of Wind Cave and a means to address the Wind/Jewel connection theory constitute the main benefit of this model.
As the concept of a cave potential map developed, it was decided that this model could address four issues: (1) the maximum likely extent of Wind Cave, (2) the surveyable length of the cave, (3) the Wind/Jewel Cave connection hypothesis, and (4) surface land management shortcomings that existed near the known cave.
Although previous researchers have analyzed Wind Cave's potential extent based on individual disciplines, no one had attempted to use geology, hydrology, airflow, and cave survey data together to quantify the potential extent of Wind Cave. GIS provided the perfect tool to accomplish this task.
We initially analyzed the region surrounding Wind Cave and identified some preliminary large-scale factors that could limit cave passage development. We theorized that the current erosional surface and the water table could provide those limiting boundaries. Blowholes and the cave survey data also provided additional clues on the potential extent of the cave.
With these preliminary limiting boundaries in hand, we hypothesized that other geologic factors would also have had a significant effect on the development of Wind Cave including erosional surfaces, structural geologic factors, mode of speleogenesis, and paleo-injection points. We also hypothesized that such data as the profile view of the Wind Cave survey, cave radio location depths, surface outcrops, cave levels, airflow, and passage density would all offer additional clues on the likely extent of Wind Cave. Once we identified these additional limiting factors and data sources, individual GIS layers provided buffers and overlays for further refining potential boundaries of the cave. In the future, as we learn more about these limiting factors, the cave potential boundaries will need further modification.
GIS was used to accomplish several tasks including the development of a spatial model that was used to verify our preliminary cave potential map, the visualization of those results, and the development of maps to demonstrate cave potential and support management requirements and decisions. The GIS tools used were ArcView; ArcView Spatial Analyst; ArcView 3D Analyst; and CaveTools, a third party extension used to incorporate cave survey data into the GIS.
GIS data layers were collected and derived from a variety of sources. Digital line graph (DLG) files were used to create hydrography, hypsography, and transportation layers. Contour lines from the DLG hypsography layer were used to generate a triangular irregular network (TIN) elevation model from which slope, aspect, and other layers used for visualization were derived. Blowholes and cave entrance locations were imported from field GPS readings. Cave survey data was converted from COMPASS plot files to Esri shapefiles format and georeferenced based on GPS locations of surface survey stations. Digitized park boundaries, digital orthophoto quads (DOQs), and geologic maps were also incorporated into the GIS. One of the first maps produced using the GIS showed the relationship of the outcrop of the Madison Limestone to the current surveyed cave extent.
Using the ModelBuilder functionality in ArcView Spatial Analyst, an interactive model diagram was constructed incorporating various spatial processes such as buffering, proximity, and weighted overlays. A cave potential surface was generated by weighting proximity to certain features, such as known entrances and blowholes, and combining these derived cave potential surfaces using weighted overlays with other factors such as geology, potentiometric surfaces, and current cave extents. In addition, the volumetric constraints for cave potential were further defined by generating 3D surfaces that represented the limiting bounds of the intersection of the Madison Limestone along its strike and dip with the upper and lower limits of the historic and present water table elevations. Area calculations using the GIS were made of the known extent of the cave and the potential area for cave development, and these were used to calculate the potential cave length based on current parameters.
Analysis of the Wind/Jewel Cave connection hypothesis resulted in the conclusion that a connection between the two caves is an unlikely scenario. Although such a connection could not be totally eliminated as a possibility, it is a remote possibility.
Once the analysis had been completed, we used the buffers and overlays to draw an outline around the area that represents the likely maximum extent of Wind Cave, creating the Wind Cave Potential Map. Approximately 97 percent of those boundaries fall inside of the current boundaries of Wind Cave National Park. The current boundaries of the cave were found to be one-tenth of the area of the total potential of the cave, as identified by this exercise.
By calculating passage density for the current cave boundaries and then for the maximum potential boundaries, a minimum and maximum potential surveyable length was calculated for Wind Cave. We predicted that the cave could have approximately 400 km of surveyable passage, if the cave is not extended beyond the current boundaries. If the cave is extended to all edges of the potential boundary, Wind Cave could have almost 1,800 km of surveyable passage, assuming similar passage density throughout the potential area. Since the current 163 km surveyed represent no more than 40 percent of the minimum predicted length of the cave or as little as 9 percent of the maximum predicted length of the cave, it is obvious that a tremendous amount of surveyable passage remains in the system. However, based on airflow and cave development patterns, it is unlikely that the cave will continue in all directions to the edge of the identified cave potential boundaries; even if it did, it is unlikely that cavers would be able to physically push the cave to those boundaries.
For more information, contact Rod Horrocks (tel.: 605-745-1158, e-mail: Rod_Horrocks@nps.gov). This article is derived from an article that first appeared in the Journal of Cave and Karst Studies (April 2002, Vol. 64, No. 1). The complete article can be seen at www.caves.org/pub/journal/PDF/V64/v64n1-Horrocks.pdf.