Open AccessA strong topographic valley effect in AMT and VLF‐R measurements
Author(s) -
Fischer Gaston
Publication year - 1989
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.1989.tb06008.x
Subject(s) - electrical resistivity and conductivity , geology , electric field , resistive touchscreen , induced polarization , geophysics , polarization (electrochemistry) , magnetic field , mineralogy , geometry , geomorphology , physics , mathematics , chemistry , quantum mechanics , electrical engineering , engineering
SUMMARY The topographic effects of sharply embanked, deep and narrow valleys cut in resistive rock are very strong on MT, AMT and VLF‐R soundings when the widths of the valley floors are of the same order as the skin‐depths. The electromagnetic responses of such 2‐D valley models have been calculated by means of a finite element numerical method for both E‐ and H‐polarization field configurations. The most striking effects occur in the H‐polarization geometry, where the entire current flow must round the bottom of the valley in concentrated bundles, thereby creating large electric fields and high apparent resistivities P H . With E‐polarization there is a sharp reduction of both electric and magnetic fields at the centre of the valley, but the reduction is stronger for the electric field, so the apparent resistivity P E decreases below the matrix resistivity. When the bottom of the valley contains a fill of conducting material, like sediments or alluviums, it is found that both P H and P E are dramatically reduced, to values below even the fill resistivity. These effects are studied for a variety of ratios between skin‐depth and width of valley floor.