Detection of fractures in Ash Meadows southwestern Nevada by electromagnetic terrain-conductivity measurements

摘要

Electromagnetic surveys using a terrain-conductivity measuring device have been used to detect individual fractures along a fault zone in a ground-water discharge area in Ash Meadows, southwestern Nevada. A fracture identified on aerial photographs and confirmed by excavation was surveyed using the conductivity meter to develop techniques in recognizing fractures. Precise coil alignment was critical in detecting the anomalies associated with the fracture. Rough desert terrain required modification of the conductivity meter, the addition of a unipod connected to the transmitter and receiver coils to enable quick, precise coil alignment. Generally, a decrease in the apparent conductivity readings in the vertical dipole configuration were observed when the transmitter coil passed over the fracture. In the horizontal dipole configuration, readings along the survey line at the different exploration depths tended to converge at the fracture, with the degree of convergence dependent upon overburden thickness.