Lange, Arthur L. 1990. Natural-Potential Profiles at Jewel Cave National Monument. University of Wyoming National Park Service Research Center. 26 p + attachments.
Ever increasing pressures on our natural environment necessitate measures to detect and ameliorate problems of contamination, construction and overuse, before they become critical. This is particularly the case in our national parks and monuments where the tide of visitors is continually growing. The underground resource of natural caverns is especially vulnerable to damage, due to its fragile features, rare and delicate biota, open communication with groundwater, and its invisibility beneath the ground surface. It is important that we be able to recognize the existence of a cavern system prior to the planning of roads and facilities that may encroach upon or introduce waste and hazardous materials into the underground system.
A number of geophysical tools have been applied to the detection and mapping of caves and tunnels, including gravity, seismic and active electrical methods. In the case of voids at depths of 50 or more meters and of limited dimensions, particularly in areas of considerable relief, these methods lose resolving power; furthermore, they become costly to implement. As a practical alternative, the natural-potential method offers a rapid and economical technique whose response is based on the movement of groundwater not only at cave-depth but within the zone of infiltration between the cave and the surface. Although the natural-potential has been applied for many years in the exploration for minerals and for targeting dam leakage, its application to cave mapping has been undertaken only recently. Tests over caverns in Kentucky, Missouri, Texas, and Nevada have demonstrated its efficacy for detecting voids and underground streams at depths between the surface and forty or more meters (>130 feet).
Jewel Cave is a deep (30-140 m; 95-450 ft) and complex network of galleries and tubes in Madison limestone, lacking voids of a size readily targeted by other geophysical methods. The local relief exceeds 150 m (500 ft), and for the most part is covered by forest. All in all, it is an ideal area for testing the capability of the natural-potential method. Four trail lines were laid out, totaling 2.9 km (9500 ft) in length, to cross over portions of the mapped cavern representing both relatively simple as well as complex cave structure. The flagging and data acquisition on these four lines were completed by the two-person crew in three days.
The natural-potential profiles revealed a direct correlation of negative anomalies with mapped caverns at a variety of depths, as expected in a carbonate environment containing waters of pH>7, as is the case at Jewel Cave. Of the 825 m, or 2705 ft, of mapped cave crossed by the profiles only one 60 m- (200 ft-) segment of cavern, exceeding 50 m (154 ft) in depth, on Line C showed no evident potential low (possibly the effect of mis-location). Thus, in relative terms, 93% of the known cavern system crossed by the profiles produced a definitive response. No relationship between millivolt response and elevation or lithology was evident in the plots.
An additional 500 m (1640 ft) of relatively negative response appeared in the profiles, of which 200 m (656 ft) occurred over meadow and 150 m (492 ft) over fault zones, where in both cases, enhanced infiltration may be expected. These are also possible zones for cavern development beyond the mapped extent of the cave system.
An underground traverse commencing at the natural entrance extended to a survey point beneath Line A. With the exception of the outermost location, the cave ceiling registered more positive than the corresponding cave floor; in tow caves, the difference exceeded 300 mV. At a depth of 40 m (131 ft) beneath surface Line A, the cave ceiling measured 249 mV more positive than the surveyed point on the surface. These floor/ceiling relationships are concordant with those observed in the other cave systems tested; however, the milivolt differences were much high in Jewel Cave.
In conclusion, the natural potential method delineated approximately 93% of the mapped cavern crossed over by the survey lines, and indicated additional zones of likely uncharted cavern. It mapped clearly the cone of depression around a pumping well and the main east/west fault crossing the Monument. The method can be relied upon for detecting and mapping undiscovered cavern in the Jewel Cave environment, with possible follow-up confirmation over selected areas of concern by precise gravimetric observations.