Geophysical Journal | 2001 volume 23 5

Thermobaric changes of electrical anisotropy of some rocks of the Kryvyi Rig ultradeep borehole (Ukraine)

© Lebedev T.S., Shepel S.I.

The nature and character of the changes of the electrical state and the corresponding features of mineral substance in the conditions of different depths of the lithosphere cannot be understood without detailed studies of the regularities and peculiarities of the distribution of the values of the coefficients of the anisotropy of specific electrical resistance (λO) and the relative dielectric constant (λε) for the main types of rocks in laboratory experiments at high temperature and pressure and the saturation with fluids of different degree of mineralization. These studies are of special interest for potentially highly electrically anisotropic mineral formations. We consider the results of an experimental study of the effect of the NaCl concentration (to 50 g/l), temperature (to 900°C) and quasi-hydrostatic pressure (to 2,0 GPa) on the change of the coefficients of the anisotropy of electrical resistance (dc and ac at 1 kHz frequency) and the relative dielectric constant (at 1 and 700 kHz frequency) of rocks of one of the regions of the Central Ukrainian Shield (USh). The electrical anisotropy was investigated on samples of a core of rocks of the Kryvyi Rig ultradeep borehole (KUDB) drilled here that were taken from depths to 5432 m. From the data of our laboratory studies we calculated the mean λO and λε values of plagiogranites, granodiorites, mylonites, different schist's, albite-carbonate rocks with graphite and irony, quartsites most electrically anisotropic were schists (λO=1,58÷2,18) albite - carbonate rocks with graphite (λO=1,15÷1,79) and irony quartzites (λO=89,9÷1300,0). Other types of the mineral formations from the KUDB sections show lower anisotropy coefficientsr (1,11÷1,85). This distribution is due to the clear layering of rocks, a notable anisotropy of their mineral components, cracking and cataclasis because the KUDB was drilled in a crustal fault zone. The absolute lr values are generally higher than the le ones and the lower-frequency range of their measurements is described by rather high electrical anicotrophy values. When proceeding from dry to water-saturated rocks we marked a notable lr decrease for all samples studied. The NaCl concentration increase to 50 g/l in the water saturating the rock decrease the λO by 3.9 times at direct curreat and by 2.5 times at the a.c. with 1 kHz frequency. For all fluid mineralization values observed here the specific electrical resistance anisotropy coefficient was higher at d.c. than at a.c. The electrical anisotropy of the water-saturated rocks is also clearly marked by the λε values. However, unlike the λO it notably increases with increasing humidity. An analysis of thermal and basic dependences of most anisotropic samples of irony quartzites and albitecarbonate rocks with graphite across and along the layering enabled us to determine the effect of the PT-parameters on the anisotropy value. It is seen that the λO of these rocks clearly decreases to 700°C and than practically does not change in the 700-900°C range. The initial temperature interval (to 250-300°C) is featured by more intensive λO decrease than that in the zone of higher T values. This is due to higher gradients of the ρ decrease perpendicular to the layering compared to practically unchangeable values along it layering. The total effect of T, P, degree of humid and mineralization in model thermobaric conditions generally decreases notably the electrical anisotropy of the said rocks with depth. But the latter, remains rather notable even at 15 km depth; the electrical resitance anisotropy coefficient will be hundreds of units.

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