1989
DOI: 10.1111/j.1365-246x.1989.tb06008.x
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A strong topographic valley effect in AMT and VLF-R measurements

Abstract: 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 … Show more

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Cited by 11 publications
(5 citation statements)
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“…(1986), and, although frequency‐dependent, are generally calculated to be small except in some cases immediately adjacent to sharp breaks in slope. In the present case, a pertinent result (Fischer 1989) is that on the floor of a narrow valley the topographic effect can significantly enhance the electric field, and hence the observed apparent resistivity, perpendicular to the strike of the valley. No such enhancement is evident at either AF5 or AF4, where the apparent resistivities parallel to the Styx River valley are greater at all periods than those perpendicular to the orientation of the valley.…”
Section: Magnetotelluric Datamentioning
confidence: 62%
See 1 more Smart Citation
“…(1986), and, although frequency‐dependent, are generally calculated to be small except in some cases immediately adjacent to sharp breaks in slope. In the present case, a pertinent result (Fischer 1989) is that on the floor of a narrow valley the topographic effect can significantly enhance the electric field, and hence the observed apparent resistivity, perpendicular to the strike of the valley. No such enhancement is evident at either AF5 or AF4, where the apparent resistivities parallel to the Styx River valley are greater at all periods than those perpendicular to the orientation of the valley.…”
Section: Magnetotelluric Datamentioning
confidence: 62%
“…Despite the relatively rugged topography of the area, particularly in the vicinity of AF5 and AF4, the effect of topography on the observed MT data is considered to be insignificant. Topographic effects have been discussed by, among others, Fischer (1989) and Wannamaker et al . (1986), and, although frequency‐dependent, are generally calculated to be small except in some cases immediately adjacent to sharp breaks in slope.…”
Section: Magnetotelluric Datamentioning
confidence: 95%
“…exhibits topographical features, since continuity of the current flow along the free surface requires equation (11) to hold along the entire surface profile. This has recently been demonstrated in numerical calculations over a steeply embanked valley (Fischer 1989), where it can be seen that both E-and B-polarization phases are indeed continuous across the entire 2-D profile, although they d o seem to exhibit strong discontinuities in their slopes. This means that rule (11) applies equally well to the B-polarization M T phase @ B as it does to @ E , as was claimed in Section 2.1, and it is therefore continuous at the surface of any conducting 2-D structure.…”
Section: Continuity Of the Mt Phase At The Surface Of Conducting Strumentioning
confidence: 78%
“…In general, if the mean topographic gradient over a length scale equal to the skin depth is in the order of unity then there will be significant differences in the responses for flat earth models and for models with topography. The VLF (5-30 kHz) and RMT (10-240 kHz) methods use rather high frequencies therefore a topographic relief with amplitudes in the order of 10 m may affect the response depending on the resistivity of the region and used frequency (Arcone, 1978;Fischer, 1989;Karous, 1979;Liu and Becker, 1992). Initially, VLF was thought to work in the range of 3-30 kHz (Paal, 1968), in practice, however, it is limited to 15-30 kHz because only very sporadic transmitters are found in the range of 3-15 kHz.…”
Section: Introductionmentioning
confidence: 99%