This place is traditionally and for a long time used for student practice to show effects of rock anisotropy.
In place of anisotropic rock outcrop (S1) ARS with any array gives appreciably extended diagrams. In the last years our studies were concentrated on territory, where the anisotropic basement was covered with the sandstone layer. All attempts to receive here effect of anisotropy with Schlumberger array were unsuccessful. Only non-linear array (like dipole equatorial) has enabled to estimate confidently properties of the anisotropic basement. To understand the reason of these effects, in two sites (S1 and S2), where the anisotropic basement outcrops and is covered with a thin sandstone layer, we made azimuthal soundings with AMN (E) and dipole equatorial (D) array. AR soundings were measured on 12 azimuths with a step 30° and with spacing from 1.5 up to 50 m and from 5 up to 75 m in S1 and S2 accordingly. At each sounding MN line remained fixed in the center, and electrode A or dipole AB was moved around. The azimuthal diagrams for various spacings for two arrays are shown on fig.2. VES statistical processing has allowed to draw sounding results as probability distribution in coordinates apparent resistivity - distance, the examples of which for D-array are shown on fig.3. In both cases the distributions are bimodal and marked by two modal curves drown also on fig.3. The upper curve in site 1 corresponds to longitudinal array orientation and looks like two-layered one with resistivity ratio greater then 1. The lower curve corresponds to transversal array orientation and looks like three-layered H-type curve. Difference between these curves is about ten times in resistivity level. The curves for S1 show that the medium in this place is not uniform half-space and the reason of that is probably in effect of weathering. VES curves for the site 2 with overburden are also different for longitudinal and transversal directions. Longitudinal curve looks as descending two-layered one, whereas transversal curve looks as three-layered H-type curve. Theoretical accounts have confirmed, that the difference between both curves and the layer H on the last curve arises from the anisotropy influence. The further data processing was made with the help of spectral approach. Spectral approach allows to define a strike azimuth and an error of that estimation. On fig.4 azimuths' estimations for different spacings and their errors for E and D array are submitted. The error for E array is much more that for D-array. The error for D array is small and frequently is not seen clearly on fig.4.
Fig.5 gives an example of spectral transformation of AR soundings into sounding curves without influence of anisotropy. That can be done with 0-harmonics of AR soundings. As we saw on fig.2, ARS diagrams for E and D arrays were different, but 0-harmonics for both arrays on fig.5 are very similar, and can be interpreted as usual VES curves.
On fig. 6 azimuthal diagrams for nearly one hundred ARS sites on Patil are presented. To the South from line B the anisotropic basement is covered with the layer of sandstone. The north part of the area is situated on the outcrop of the basement. Some diagrams at the North were measured with D-array spacing from 20 to 40 m. All other ARS were measured with array spacing from 40 to 120 m. The rotation of D-array was executed around the spacing center. The choice of D-array electrode spacing depending on proposed thickness of sandstone cover, was justified with the help of theoretical modeling (Bolshakov et al. 1995). The difference in the shape of the AR diagrams S2, 8, 7, 6, 5, 10, 4 results from the gradual increase in the thickness of the upper layer. The difference in the shape of AR diagrams 2 and 25 is a result of a change in array spacing (40 and 120 m). Delineation of the strike in the basement rocks with AR diagrams helped to map a fold, situated in the right part of fig.6. The form of the fold is marked by lines with the letter F. The abrupt edge of Patil hill in the South and the East is marked by a line with letter E. Line A-A is VES profile (see its results on fig.1).
V.A. Shevnin, I.N. Modin
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