Influence of the Hydrogeological Features of the Baley Ore District on the Results of Electrical Prospecting Work

I. P. Novikov, V. A. Kortunov

(Presented by the scientific seminar of the Department of Geophysical Methods of Exploration)

As is known, the results of electrical prospecting work are greatly influenced by the hydrogeological features of the areas under investigation. Neglect of these features, or an inability to take them properly into account, may in many cases lead the geophysical researcher to an incorrect interpretation of the nature of the electrical-prospecting anomalies obtained; as a consequence, the effectiveness of this type of investigation will decline, and in some cases may even be reduced to nothing. This applies especially to regions with hummocky and mountainous relief and to regions where permafrost is developed. The Baley district described by us also belongs to such regions.

Of all the hydrogeological features of the Baley ore district, two factors exert a substantial influence on the results of electrical-prospecting investigations: the type of groundwater and the development of permafrost.

The type of groundwater, and the confinement of waters of a given type to particular geological formations or structures, together with mineralogical, structural-textural, and other factors, strongly affect the electrical conductivity of rocks. The specific electrical resistivity of waters, in turn, depends on the chemical composition and concentration of salts dissolved in these waters.

The influence of the chemical composition of waters on the magnitude of the specific resistivity of rocks is small. The greatest influence on the conductivity of aqueous solutions, and consequently of rocks, is exerted by the concentration of salts in them. Under natural conditions, the concentration of salts dissolved in waters varies within fairly wide limits—from fractions of a milligram per liter (surface fresh waters) to tens and even hundreds of grams per liter (formation waters). Consequently, the conductivity of waters and of rocks saturated with these waters also varies within wide limits.

In those cases when the concentration of salts in the waters is insignificant, which is characteristic of the Baley district, the determining influence on the spe-

The electrical resistance of rocks is affected by their water saturation, which, in turn, depends mainly on the porosity and fracturing of the rocks, determined by textural-structural features. Thus, the most porous and fractured, and consequently the most water-saturated, geological formations may be marked by anomalies as low-resistivity objects.

The development of permafrost also substantially affects the magnitude of the specific resistance of rocks. Usually permafrost increases the resistance of rocks if only free water is taken into account in the calculation. In light of modern concepts, a soil system contains three types of water: gravitational (otherwise free, bulk), boundary (otherwise firmly bound, adsorbed, film), and near-boundary (loosely bound) water (Tyutyunov, 1961). Gravitational water fills large pores and cavities in rocks. Boundary water is a layer of water, comparable with the thickness of a molecule, held on the surface of the solid phase by forces of molecular attraction. This water does not expand during freezing and does not release the latent heat of ice formation. Adsorbed water, which constitutes the boundary phase, differs from free water in its chemical and thermodynamic properties: its hydrogen bonds are destroyed or deformed, and it is at a higher energy level.

The near-boundary phase is transitional from boundary to free water. It lies within the sphere of weakened action of the force field of mineral particles and, to some extent, possesses the properties of both boundary and free water.

If, in studying the influence of permafrost on the specific resistance of rocks, only free water is taken into account, then on this basis it is easy to conclude that at temperatures below −0.2° the resistance of frozen rocks should increase sharply to the values of the resistance of ice, since at temperatures from 0° to minus 0.2° all free water passes into the solid state. However, from the practice of studying regions with developed permafrost it has been established that at temperatures below −0.2° only coarse-grained (sands, pebble beds, etc.), fractured, and crushed frozen rocks possess high resistance values. Fine-dispersed frozen rocks (clays, loams, rocks that have undergone intensive milonitization), even at comparatively low temperatures, have an inappropriately low resistance. The low resistance of fine-dispersed rocks is caused by the presence of unfrozen bound water of the boundary and near-boundary layers. Tyutyunov (1961), proceeding from the physicochemical processes occurring in dispersed media, considers that the movement of water in these media is determined by the gradient of the chemical potential of the interacting phases, i.e., mineral particles and water. With decreasing temperature, the magnitude of the chemical potential of the surface of mineral particles increases, and consequently the mobility of the water film increases. In addition, with decreasing temperature the influence of the electric field on adsorbed water molecules increases, leading to intensified displacement of ions and their transition from the solid phase into the liquid phase, within which ion motion occurs more freely.

At present, for some rocks, temperature limits have been established within which significant phase transformations of water occur. Outside the limits of the regions of significant phase transformations of water in fine-dispersed rocks, as temperature decreases, the amount of unfrozen water asymptotically decreases, and in the limit (according to Jung at −180°) it all passes into ice. In accordance with this, an asymptotic increase in the specific electrical resis—

resistance of frozen rocks. In coarse-grained rocks, however, as well as in fractured and crushed rocks, beyond the zone of significant phase transformations, a negligible amount of unfrozen water remains. It is practically considered that in the temperature interval from 0° to minus 0.2° all of it turns into ice. Therefore, beyond this zone the indicated rocks have high resistances, the magnitude of which changes little with decreasing temperature.

However, it should be noted that fractured and crushed rocks in areas of permafrost development do not always, as was noted above, have high specific resistances.

Studies at Baley, for example, have established that the indicated rocks are very often recorded with lowered and even low specific electrical resistances. This is explained by the insulating influence of deep waters circulating along fractures in the rocks. As a result of the thermal action of these waters, deep and through taliks arise in frozen rocks. In the formation of these taliks, infiltration of near-surface and surface (atmospheric) waters also plays a major role.

In the area of the Baley district under consideration, three groups of factors exert the determining influence on the development of permafrost and of various types of groundwaters: geomorphological, tectonic, and lithological. This, in turn, makes it possible to distinguish within the area two districts substantially different in hydrogeological features (Khnyrev, 1959). The first district includes the watersheds of the Bortsovochny and Petrovsky ranges; the second—the foothills of the ranges and the Undino-Dain depression.

For the first district the following types of waters are characteristic: waters of eluvial-deluvial deposits of watersheds and slopes, and of alluvial and proluvial deposits of valley bottoms—padei; waters of fractures in the weathering crust, and fracture waters of tectonic disturbances.

Within the second district, waters of eluvial-deluvial deposits on the slopes of uplands and of alluvial-proluvial deposits of floodplains and depressions of the ancient relief are distinguished; waters associated with tectonic disturbances in granites, volcanogenic-sedimentary, and terrigenous formations; waters confined to bedding fractures and waters of quartz veins.

Permafrost in the territory under consideration, as in many areas of Transbaikalia, is developed mainly within the second hydrogeological district. For the first district only seasonal freezing of the soil of the active layer is typical.

The foregoing ultimately allows us to note the following features of the influence of the district’s hydrogeological conditions on its geological-geophysical—more precisely, geoelectrical—characteristics. First, we shall note the influence of the conditions of development of various types of water:

1. The specific electrical resistance of the district’s geological formations is determined chiefly by their water saturation. This conclusion has been drawn from the following considerations: in the rocks of the district there are no appreciable accumulations of conducting minerals, while the chemical composition of the waters, as indicated above, does not exert a substantial influence on specific resistance, and the mineralization of almost all types of groundwaters of the district is insignificant.

2. Thus, owing to increased water saturation, the following types of geological formations of the area under consideration are marked by lowered specific resistances:

a) eluvial-deluvial thawed unconsolidated deposits within the Bortsovochny and Petrovsky ranges, containing groundwaters;

b) intensely crushed structural eluvium in ridge areas, containing fissure waters of the weathering crust;

c) alluvial deposits in talik areas within the Undino-Dainskaya depression and adjacent valleys;

d) areas of intensely crushed bedrock in zones of tectonic dislocations, and quartz ores in areas of their intense tectonic fragmentation, if the corresponding zones of increased fracturing are saturated with fissure waters. Thus, for example, the Taseevskoe deposit is marked by the lowest resistivity values in comparison with the adjacent areas. It should also be noted here that at present, owing to intensive development of the deposit, the hydrogeological conditions of the latter have changed greatly (the groundwater level has dropped considerably, the upper part of the section has become dewatered, the regime of water circulation has changed, etc.). In view of these circumstances, it should be expected that the resistivity of the rocks of the deposit has increased considerably (this applies mainly to the Cretaceous deposits of the Balei suite).

1. Areas of conductive water-saturated geological formations have the following characteristic forms:

a) water-saturated eluvial-deluvial deposits have the form of thin and shallow (5–10) lenses, having an irregular configuration in plan. Electrical-prospecting anomalies from these lenses are observed chiefly in relief lows, i.e., in areas of increased thickness and water saturation of loose deposits;

b) areas of water-saturated weathering crust (fissured structural eluvium) differ from lenses of eluvial-deluvial deposits by having greater vertical dimensions (up to 40–50 m), lower porosity and water saturation, and, consequently, higher specific resistivity. In areas of increased thickness of deluvial deposits, electrical-prospecting anomalies from pockets of the weathering crust are “obscured” and “masked” by these deposits. The detection of anomalies from pockets of the weathering crust should be expected only in areas with a uniform thickness and water saturation of the surficial deposits, chiefly within ridge slopes;

c) for talus water-saturated alluvium, the characteristic forms of occurrence are lenses that are more or less uniform in area and lie horizontally. The thickness of such lenses in talik areas may approach the total thickness of the alluvium. In areas of development of suprapermafrost, interpermafrost, and subpermafrost waters, the geoelectrical section of the alluvium may acquire a layered structure—an alternation of lenses of increased and decreased conductivity;

d) finally, water-saturated crushed rocks of tectonic zones apparently have the form of steeply dipping sheet-like bodies with a clearly expressed strike and considerable extension to depth. The thickness of such zones in most cases is considerably less than their dimensions along strike. Depending on the degree of fragmentation of the rocks of these zones—and, consequently, on their water saturation—as well as on the thickness of the overlying loose deposits and their moisture content, the indicated zones are marked either by narrow, elongated anomalous fields of reduced and low resistivity values (significant water saturation of the zones, small thickness of loose overlying deposits equal to or less than the thickness of the zones), or by an insignificant lowering of the resistivity field without any clearly expressed boundaries, or may not be marked at all in the case of large thicknesses of overlying loose deposits exceeding the thickness of the zones by 5 or more times. To the foregoing it should be added that the conductivity of such zones in comparison with

water-saturated loose formations will be lower, owing to the lesser porosity and fracturing, and consequently also the lesser water saturation, of tectonically fragmented rocks.

Noting the influence of permafrost on the electrical properties of rocks, we shall first point to the role of seasonal freezing. The zone of seasonal freezing forms the first layer from the surface, of comparatively small thickness (up to 4–5), but usually of high resistivity. Seasonal changes in the dimensions of this zone, changes over time in its electrical properties, and also the different conductivity of coarse-grained and fine-dispersed formations make the results of electrical-prospecting investigations obtained at different times of the year with small spacings of the current line difficult to compare. As a consequence, it does not seem possible to carry out a quantitative interpretation of the left branches of vertical-sounding curves, or to interpret data from electrical profiling methods that are sensitive to inhomogeneities in the upper part of the section.

It was indicated above, in general terms, what role permafrost plays in forming the geoelectrical character of various geological formations. Within the Baley district, as a result of the development of permafrost, the following geological formations possess elevated and high resistivities:

a) deluvial-proluvial deposits of the foothills of the Borshchovochny and Petrovsky ranges;

b) deposits of the weathering crust, having considerable thickness;

c) coarse-grained (pebble, sand, gravel) alluvial deposits of the Undino-Dainskaya depression and the valleys adjoining it.

In comparison with the formations listed, frozen fine-dispersed alluvial deposits of the district (loams, clays, milonites), in light of current concepts of the physicochemical processes occurring in permafrost zones, should be characterized by lower specific resistivities.

LITERATURE

I. A. Tyutyunov. Introduction to the Theory of the Formation of Frozen Rocks. Publishing House of the Academy of Sciences of the USSR, 1961.

I. A. Tyutyunov. Physicochemical Changes of Rocks in the Far North. Physicochemical processes in freezing and frozen rocks. Publishing House of the Academy of Sciences of the USSR, 1961.

G. I. Khnyrev, O. A. Loginova. Report on hydrogeological works of the Geological Prospecting Expedition of the “Baleyzoloto” Combine for 1954–59. Archives of the “Baleyzoloto” Combine.