Interpretation of VLF Resistivity Data for Ground Water Contamination Surveys

Abstract

Very low frequency (VLF) military communications systems provide a primary field that can be used for shallow geophysical surveys to locate ground water contamination and vertical geologic contacts. Useful properties that can be easily obtained from the interaction of the earth and the primary field are the magnitude of the vertical secondary magnetic field, the surface impedence, and the phase angle between the electrical and magnetic horizontal components. The variations in the secondary magnetic field can be related to vertical geologic contacts, such as the edges of landfill trenches. The surface impedence yields an apparent terrain conductivity, which can be used to locate low‐resistivity anomalies often associated with contaminated ground water. The phase angle gives information on vertical variations in resistivity, phase angles less than 45° indicating increasing resistivity with depth. The depth of penetration of the VLF field is about one skin depth. For a frequency of 20 kHz, the skin depth in meters is approximately equal to 3.67 where p is terrain resistivity in ohmmeters.

Extending Electromagnetic Methods to Map Coastal Pore Water Salinities

“…Both the instruments can be oriented in either the horizontal magnetic‐dipole (HMD) or the vertical magnetic‐dipole (VMD) mode. Coil spacing and height, alternating current frequency, and dipole mode determine instrument sensitivity and exploration depth (McNeill 1980; Stewart and Bretnall 1986).…”

Developments and applications of modern airborne electromagnetic surveys : proceedings of the U.S. Geological Survey Workshop on Developments and Applications of Modern Airborne Electromagnetic Surveys, October 7-9, 1987

“…For the vertical coil (horizontal dipole) configuration, the effective penetration depth is approximately 0.75 times the intercoil spacing. Materials above the effective penetration depth contribute 70 percent of the instrument response measured at the surface (Stewart and Bretnall, 1986). Another consequence of operation at low induction numbers is that there is very little coupling between induced current loops in the ground.…”

Διερεύνηση Της Θαλάσσιας Διείσδυσης Σε Παράκτιους Υδροφορείς Σε Δελταϊκό Περιβάλλον

“…Το βάθος έρευνας των κάθετων γεωηλεκτρικών μετρήσεων ανέρχεται περίπου στο 10 -20% του αναπτύγματος των ηλεκτροδίων του ρεύματος (Stewart and Bretnall, 1986). Η βαθιά διείσδυση προϋποθέτει μεγάλα διαστήματα ανάμεσα στα ηλεκτρόδια του ρεύματος, τα οποία πρέπει να εκτείνονται κατά μήκος μιας ευθείας γραμμής (Telford et al, 1990 (Zohdy, 1973και Zohdy 1975.…”