Topographic effects on telluric field measurements

Harinarayana, T.; Sarma, S. V. S.

doi:10.1007/bf00876660

Cited by 10 publications

(9 citation statements)

References 6 publications

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“…A simple analytical solution for telluric problems can be achieved using the Schwartz-Christoffel transform (Thayer, 1975;Harinarayana and Sarma, 1982). In this case telluric currents flow perpendicular to the strike between the surface and a perfect insulating basement.…”

Section: A Comparison With An Analytical Approximationmentioning

confidence: 99%

“…Two-dimensional numerical approximations have been carried out using transmission system and network analogies (Ku et al, 1973;Mozley, 1982), finite elements (Wannamaker et al, 1986) and a modification of Rayleigh scattering theory (Jiracek, 1973) for modeling topographic features (Reddig and Jiracek, 1984). The simpler telluric case has been analysed with the aid of conformal transformation (Thayer, 1975;Harinarayana and Sarma, 1982 ). In addition it has been suggested that it may also be possible to normalize impedance tensor elements to compensate for the topographic distortions (Mozley, 1982 ) .…”

Section: Introductionmentioning

confidence: 99%

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Distortions of magnetotelluric sounding curves due to slope

1989

International Journal of Rock Mechanics and Mining Sciences &am

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This study considers a particular case of 2D modeling, namely the topographic effect on MT data. A simple way to remove topographic effects is to incorporate surface elevations, or depressions, into the forward model. In cases where a 2D approximation is valid, any 2D algorithm may be used for the calculations. Such a scheme allows model results and observations to be compared along a profile. This approach, however, does not necessarily provide useful answers since the topography may, and in fact nearly always does, couple with the geoelectric structure and/or varies on a more rapid scale than the observations. In order to understand the effect of topography on MT data the degree and characteristics of the distortion introduced by a simple slope are considered. The slope model constitutes a general result with distortion characteristics that can be extended to other features such as a depression or a hill. The introduction of additional features such as a superimposed geoelectric structure would greatly complicate the problem due to electromagnetic coupling effects which would necessarily be a function of frequency. The single slope, or half-graben, is probably the simplest class of general near-surface distortions that affect MT data. The period range used is from 0.02 s (50 Hz) to 100 s (0.01 Hz), i.e. both audio and magnetotelluric observations are considered. The elevation of the slope is fixed at a 'throw' of 1 km and slope angles range from 0" to 90". The spatial and frequency distortions due to the slope are considered in detail. At a particular location the distorting effect of the slope introduces a false gradient into an otherwise 1D sounding curve. The topographic distortion introduced is investigated by applying a 1D inverse procedure to the obtained results. The way in which false layers may be introduced into the geoelectric section is illustrated. An analytical solution to a purely DC topographic problem is also considered. The analytical results are compared with the corresponding low frequency results obtained with the numerical models of the present study.

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Section: A Comparison With An Analytical Approximationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Distortions of magnetotelluric sounding curves due to slope

1989

International Journal of Rock Mechanics and Mining Sciences &am

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“…The studies by Wescot and Hessler (1962) and Faradzhev et al (1972) are examples of analogue model studies. Analytic solutions based on conformal mapping techniques have been carried out by Kunetz and De Géry (1956), Thayer (1975) and Harinarayana and Sarma (1982). Numerical techniques that can be applied to various terrain geometries can also be used to remove topographic effects from data.…”

Section: Introductionmentioning

confidence: 99%

THE IMPORTANCE OF TOPOGRAPHIC CORRECTIONS ON MAGNETOTELLURIC RESPONSE DATA FROM RUGGED REGIONS OF ANATOLIA

Gürer¹,

Ilkişik²

1997

Geophysical Prospecting

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Topographic irregularities cause some distortions of magnetotelluric (MT) fields. In the vicinity of a topographic feature, the TM‐mode distortion increases with the height and inclination of the slope. It is well‐known that TM‐mode (E⊥) topographic effects are much greater than TE‐mode (Eı) distortions. We have made a study of MT anomalies in TM‐mode due to two‐dimensional topography. In order to reduce these effects, the distortion tensor stripping technique was used. After corrections, the resulting data can be interpreted as if they were obtained over a flat surface and depend only on the subsurface structure. However, this technique sometimes causes some geometrical distortions of the real subsurface structure. One of our aims is to overcome this failure. We have modified the correction coefficients by considering the actual one‐dimensional geology. Model studies showed that our approach is especially useful in removing the terrain effects on complex 2D subsurface structures. The other purpose of this study is to emphasize the importance of a proper terrain correction for data from sites having mountainous topography over complex geology, e.g. strike‐slip faults, suture zones and rift valleys. Some examples of MT data sets collected from the North Anatolian Fault Zone and from the thrust regions of the Western Taurides will be presented.

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“…Far less common are the studies of topographical effects, where the upper surface of the model is not level. Wannamaker, Stodt & Rijo (1986) and Harinarayana & Sarma (1982) have recently reviewed most of the theoretical work on the subject, especially a technique based on Rayleigh electromagnetic scattering by Jiracek (1973), Jiracek, Reddig & Kojima (1986) and Reddig & Jiracek (1984). Several other schemes only consider topography through its effects as deflector of a stationary current flow, corresponding to what is described as an H-polarization configuration in the limit of a vanishing frequency.…”

Section: Introductionmentioning

confidence: 99%

“…Several other schemes only consider topography through its effects as deflector of a stationary current flow, corresponding to what is described as an H-polarization configuration in the limit of a vanishing frequency. In this DC limit the problem can be solved by the method of conformal mapping, or Schwarz-Christoffel transformations (Kunetz & Chastenet De GCry 1956;Thayer, 1975;Harinarayana & Sarma, 1982;Harinarayana 1986).…”

Section: Introductionmentioning

confidence: 99%

A strong topographic valley effect in AMT and VLF-R measurements

Fischer

1989

Geophysical Journal International

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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 pH. 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 pE 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 pH and pE 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.

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Topographic effects on telluric field measurements

Cited by 10 publications

References 6 publications

Distortions of magnetotelluric sounding curves due to slope

Distortions of magnetotelluric sounding curves due to slope

THE IMPORTANCE OF TOPOGRAPHIC CORRECTIONS ON MAGNETOTELLURIC RESPONSE DATA FROM RUGGED REGIONS OF ANATOLIA

A strong topographic valley effect in AMT and VLF-R measurements

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