Mountain Lake is the only natural lake of significance in the unglaciated southern Appalachian Highlands. It is located near the summit of Salt Pond Mountain, Giles County Virginia, at an elevation of 1177 m. It is underlain by Ordovician and Silurian non-calcareous shale and sandstone of the Martinsburg, Juniata and Clinch formations. Historical (250 years) and sediment (6000 years) records suggest that the size of the lake has varied periodically. In the 1930s lake origin was proposed as due to valley damming by a lateral landslide (Hutchinson and Pickford, 1932) or damming by scree (Sharp, 1933). A later theory modified the landslide hypothesis to the primarily vertical collapse of a canyon feature in the Clinch (Parker et al., 1975). Fracture trace analysis now reveals a regional lineation feature associated with the lake. This feature is present surficially both downgradient from the lake to the northwest, and upgradient to the southeast. Sonar bathymetry and diver reconnaissance show it expressed as a (relatively sediment-free) narrow open crevice in the deepest (33 m) portion of the lake, probably a fault. Hydrologic observation and resistivity suggest preferential water movement along this fracture, as well as leakage directly from the lake. The present study suggests conduit erosion within this feature and periodic vertical downsettling of overlying Clinch material as the primary mechanism of lake origin and water-level fluctuations through time.
The response of an electromagnetic gradiometer (EMG) system to a subsurface wire is analyzed in terms of experimental and analytical modeling results. Our objective is to explore characteristics of the response and assess the fidelity of our model. The EMG system consists of a static transmitter and a man-portable sensor, which uses a pair of receivers that yield a gradiometric measurement. Experimental results were collected over a range of wire depths from 3.4 to 8.5 m. A number of different transmitter positions were explored, and the tests studied were conducted at 200 kHz. Modeling results were consistent with the experimental results and supported a number of key findings. Results are presented showing that, in order to maximize the strength of the wire response, the transmitter should be positioned approximately 5 m off the wire axis. Furthermore, in order to avoid unwanted transmitter influence on the response, the EMG should be at least 30 m from the transmitter. Using the experimental and modeling results, we found a linear relationship between the width of the magnitude response peak and the wire depth. Based on our experimental results, the EMG is able to yield a discernible target response at a depth of at least 7 m. Lastly, an example of how the model can be used to optimize survey planning is presented. This paper illustrates how an EMG can be used to locate underground wires with applications ranging from underground utility mapping to the detection of shallow subsurface tunnels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.