An experimental ground-magnetic and VLF-EM traverse over a buried paleochannel near Salisbury, Maryland

Abstract

Since, 1963, the Maryland Geological Survey and the U.S. Geological Survey have been engaged in a study of the Pleistocene Series beneath a 230 square kilometer area around Salisbury, Maryland. (Hansen, 1966; Weigle, 1972; Zohdy and others, 1974, p.56) North of Salisbury a deep Pleistocene paleochannel was discovered; this channel was carved into an erosional plain at the top of the Miocene deposit, was filled and blanketed subsequently with deposits (mostly sand and gravel) of Pleistocene age. The channel is a prolific source of ground water, and has been outlined for a least 15 kilometers by means of power-augering and gamma-logging. A geophysical survey using ground magnetics and VLF-EM was made over the channel in an attempt to see if a more efficient method than augering could be developed to map the paleochannel at depth; the survey traverse was made over one of the better defined parts of the paleochannel (see figure 1.) where airborne magnetics and VLF studies could easily be done as a potential follow-up. We hoped to be able to map resistivity changes between aquifer and aquiclude facies with the VLF-EM, and perhaps identify other facies changes in black-sand concentrations with ground magnetics. The data is presented in Table 1 and plotted in figure 2. The traverse extended from north to south, with station spacing of 100 meters, for an over all length of 4.5 kilometers.