Experience in Applying Resistivity Logging in Hydrogeological Studies in Kuzbass

V. V. Ponomarev

Increasing attention is being paid each year to the study of the hydrogeological conditions of the Kuznetsk Basin. The water supply of the basin’s major industrial centers, the study of conditions for mining coal beneath river valleys, and the conditions for opening up water-bearing deposits require complex, time-consuming, and costly hydrogeological and engineering-geological investigations.

Obtaining the initial data needed for designing a catchment structure, for solving problems of water supply or of a drainage system, and for developing a project for dewatering deposits is associated with the drilling of a series of special experimental and observation wells, the construction of which is often very complex and costly.

By applying the method of experimental pumping, pouring water into a well, or injecting it under increased pressure, it is possible to obtain numerical data on the filtration coefficient, specific discharge, or water absorption; to estimate approximately the radius of influence of the catchment; and to establish the nature of the interaction between wells and individual aquifers. Analysis and mathematical processing of these data make it possible to answer questions concerning the possible capacity of a water intake or the magnitude of water inflow into mine workings.

However, even after carrying out such complex investigations, in the course of processing the materials we are often compelled, consciously or unconsciously, to make a whole series of assumptions and to introduce into the mathematical analysis an element of conditionality that reduces the value of our calculations and recommendations.

Modern geophysics has many techniques and methods that make it possible to check the geological section and to answer a number of questions of interest to hydrogeologists. With the aid of geophysical methods it is possible to trace the areal distribution of a given complex of rocks, to establish with sufficient accuracy the lithological section in a well, to identify in it water-resistant and water-absorbing horizons, and so forth. Of the entire complex of geophysical methods, in the practice of hydrogeological investigations in Kuzbass in recent years only one method has begun to be applied: resistivity logging.

The positive results of investigations using this method, obtained in a number of areas of Central Asian coal deposits, compelled hydrogeologists and geophysicists to turn their attention to this type of logging. Unfortunately, we have only the first stage of application of this type of investigation in Kuzbass. But the data obtained have proved to be na—

so encouraging that we considered it necessary to share our considerations on the possibility of solving a number of problems that often arise in hydrogeological investigations.

Since resistivity logging is far from a new method, there is no need to dwell in detail on its theoretical basis. In brief, the principle of this method is as follows.

As early as 1916, the well-known Russian hydrologist D. I. Kochertin established a functional relationship between the change in the concentration of a solution of some salt introduced into a single working with permeable walls and the filtration velocity of groundwater.

This method received further theoretical and experimental development in the works of S. A. Kolya. To determine the filtration velocity, he proposed the following calculation formula:

\[ V=\frac{l\cdot 8r}{T}\lg\frac{C_1-C_0}{C_2-C_0}, \]

where \(r\) is the radius of the working,

\(C_0\) is the natural concentration of salts in natural water.

As can be seen, in order to determine the filtration velocity of groundwater it is sufficient to know the intensity of freshening, with time, of an electrolyte solution artificially created in the borehole. This is greatly facilitated and accelerated by the possibility of electrical recording of the change in the concentration of the solution, in which the specific resistance of the solution is recorded and which, as is known, is related to concentration by an inverse dependence.

As experiments have shown, neither the diffusion of salts from the borehole into the water-bearing rock nor the so-called hydrostatic convection, consisting in the removal of salt from the borehole owing to the difference in the specific gravity of the water and the solution, has a significant influence on the final results of the experiment.

I shall briefly recall the procedure for carrying out resistivity logging, which is as follows:

a) to estimate the natural concentration of salts in the groundwater and to select the resistance scale, a resistivity curve is recorded before the salinization of the water in the borehole;

b) the water in the borehole is uniformly, but as rapidly as possible, salinized, on the basis of creating a concentration of not more than \(2\ \text{g/l}\), by drawing through the borehole a porous bag filled with salt;

c) immediately after salinization, a second resistivity curve is recorded, characterizing the degree and uniformity of the salt concentration in the borehole;

d) the subsequent resistivity curves are recorded, depending on the rate of the freshening process, at increasing time intervals from 1 to 30 min and more, until the electrolyte has completely disappeared. Recording is carried out while raising the resistivity meter at a speed not exceeding \(10\ \text{m}\) per minute.

The resistivity curves obtained by the indicated method make it possible to establish very interesting phenomena in the section of a water-bearing rock mass and, in a number of cases, even to calculate the filtration velocity of groundwater and the filtration coefficient of the rocks.

Figure 1 presents the lithological column of borehole No. 147, drilled in deposits of Jurassic age in Central Kuzbass and tested in the interval from 82.3 to 125–133 m. The upper horizons of the borehole are isolated by casing pipes with a clay seal. The same drawing gives three logging diagrams, two of which—

are represented by resistivity curves recorded at different rates of water self-discharge.

Relying only on the lithological method of identifying aquifer horizons, we are entitled to characterize this borehole as imperfect, with the roof of the aquifer occurring at a depth of 85 m and with the base located somewhere below the bottom of the borehole. Neither a careful study of the rock core, nor the data of special hydrogeological observations could provide such materials as would make it possible to differentiate the entire water-saturated rock thickness according to the degree of permeability.

Fig. 1.

It is quite natural that, even anticipating the possible heterogeneity

of the filtration capacities of individual sandstone horizons, we nevertheless, because of the absence of objective data, are forced to abandon such differentiation and conditionally classify the entire sandstone unit as uniform in its water permeability.

The application of geophysical methods refutes this conception of the structure of the water-saturated rock mass. Even the recording of the first diagram, \(\rho = f(h)\), shows that the structure of the sandstone horizon is far from homogeneous. Along with firm and more or less clean sandstones, characterized by clearly expressed peaks on the diagram, clayey varieties of sandstones also take part in the structure of the aquifer; in their composition and in the content of clay particles they approach siltstones. Naturally, with such heterogeneity of composition it is difficult to expect uniformity of water-transmitting properties. This is fully confirmed by the character of the resistivity curves.

On the first and especially on the second diagram, the lower boundary of the zone of water-permeable sandstones is distinguished. This boundary lies at a depth of 116–117 m, below which the sandstones, and together with them the coal seam, are practically non-water-bearing, since their water yield is extremely small, and in this respect they may be regarded as a relative aquiclude.

The upper boundary coincides with the base of the clayey rocks penetrated by the borehole in the interval 72.8–85.0 m, although it is not sufficiently clearly reflected by the resistivity curves. Thus, only with the aid of resistivity logging does it appear possible to establish the thickness of the water-permeable part of the sandstones and thereby to change our conception of the hydrogeological adequacy of the experimental borehole.

On all three diagrams presented in Fig. 1, peaks corresponding to the horizon 91–92.5 m are clearly distinguished. The peak of the first diagram indicates the presence in this interval of rather firm, well-cemented, and sufficiently clean sandstones. The peaks on the second and third diagrams indicate an intensive inflow of water from this horizon, considerably greater than from other varieties of sandstone in the penetrated unit. Consequently, the sandstone unit penetrated by borehole No. 147 is not homogeneous in its water permeability, and the calculated values of the filtration coefficients obtained by computation are averaged for the entire water-permeable interval.

Interesting information may be obtained by comparing resistivity curves recorded at different intensities of self-discharge, which is a consequence of changes in the magnitude of the drawdown of the groundwater level. With a large rate of self-discharge (curve 3), the logging curves not only indicate more clearly the interval of water inflow into the borehole, but also testify to a substantial change in the paths of groundwater movement.

Let us turn our attention to diagram 2. At a dynamic level located at an elevation of \(+2.4\) m above the mouth of the borehole, water does not enter the latter over the entire exposed interval of the aquifer. Entering the borehole intensively at the 91–92.5 m horizon, the water flows along the borehole. Part of it rises to the mouth and discharges onto the surface, while part descends downward and is absorbed at a depth of 115–117 m by a horizon located at the base of the water-permeable sandstone unit.

But it was sufficient to increase the drawdown of the level by only 1 m (diagram 3) for this horizon also to begin yielding water, thereby increasing the self-discharge rate. It is quite natural that the character of the functio-

…dependence of discharge on drawdown could not fail to change in the presence of such features of groundwater movement.

The unevenness of the inflow of groundwater to the borehole along the section of the watered strata is apparently the most general case, one that is well correlated with the concept of facies variability of rocks and rhythmicity of sediment accumulation. And since the permeability of rocks in the section of the watered strata varies, the inflow of water per unit filtration area will be uneven; moreover, this unevenness should increase as the intensity of water withdrawal increases. This suppo-

Fig. 2.

Labels visible in the figure:

1. Borehole 201; \(\rho = f(h.\tau)\); \(h_{\mathrm{в}} + 0.5\ \text{m}\)
2. \(\rho = f(h.\tau)\); \(h_{\mathrm{в}} - 1.20\ \text{m}\)
3. \(\rho = f(h.\tau)\); \(h_{\mathrm{в}} - 2.14\ \text{m}\)

sition can be illustrated by the diagrams shown in Figure 2, which reflect the course of freshening of the solution at different magni-

the amount of drawdown and, consequently, with different quantities of water being pumped out.

Diagram 1 corresponds to the static position of the groundwater level and the natural filtration velocity of the underground flow. As can be seen, water enters the borehole fairly uniformly over the interval from 40–45 to 95 m. To a considerably lesser degree water enters in the lower part of the borehole and from its bottom, as well as from the sandstone lying in the soil of the conglomerate bed. Contrary to expectation, the conglomerate bed proved to be almost non-water-bearing, as is indicated by the almost complete absence or, in any case, very weak freshening of the solution in this interval.

Analyzing diagram 1, we would be justified in concluding that water enters the borehole uniformly throughout the entire exposed part of the water-bearing horizon. However, the artificial increase in the velocity of water movement by pumping it from the test borehole, and analysis of the resistivity curves obtained in this process, refutes our initial assumption about the structure of the water-bearing horizon.

In fact, diagram 2 indicates an uneven inflow of water into the borehole. The most intensive inflow of water occurs in the interval from 65 to 96 m, as is indicated by the rapid freshening of it in this part. Water also enters from other horizons, but at a lower velocity and, naturally, in smaller quantity. Consequently, the ratio of the quantity of water flowing through the borehole changed toward a marked predominance of inflow from the 66–96 m horizon. The relative share of inflow from this horizon increases still more with an increase in the velocity of groundwater movement. When the drawdown of the water level in the borehole is increased to 2.64 m, the inflow from the 66–96 horizon becomes predominant, while the relative share of water entering from the 40–66 m and below-96 m horizons decreases still further.

The nature of this phenomenon should be sought in the different degrees of water permeability, resulting from changes in the lithological composition of the rocks of the water-bearing horizon and in the degree of their fracturing.

Geophysical exploration methods, already at the first stage of their application in hydrogeological investigations, made it possible to illuminate objectively very important aspects of the structure of the water-saturated rock mass. As can be established from what has been set forth above, the permeability of rocks in a vertical section varies within fairly wide limits; moreover, it turns out that not all sandy and even gravelly varieties of rocks are equally water-abundant.

In a number of cases the magnitude of water yield and the degree of their permeability are so small that they cannot be regarded as a component part of the water-bearing horizon and must be excluded when determining its thickness. Consequently, the lithological method of distinguishing water-bearing horizons in a whole series of cases is fraught with large errors.

In deposits of Jurassic age, characterized by sharp facies variability of the sediments, there is no less sharp a change in the filtration properties of the rocks in the vertical section and along the strike of one and the same water-bearing horizon.

It is not without interest to note the fluctuation of the upper boundary of permeable rocks (Fig. 3). With the beds lying almost horizontally and with very similar elevations of the borehole collars, in borehole No. 201 the upper boundary of the permeable rocks corresponds to a depth of 42 m, while in borehole No. 200 it descends to 57–60 m, although stratigraphically higher occur not only coals but also conglomerate-like sandstones. In boreholes Nos. 203 and 204 the upper boundary, on the contrary, rises to a depth of 25–27 m. With a considerable similarity of the sections, such a fluctuation of the level can-

Borehole 201
\[ \rho = f(h, T) \]
Scale along the top: \(0,\ 6.8,\ 13.6,\ 20.4,\ 27.2\ \Omega\cdot\mathrm{m}/\mathrm{cm}\).
Depth scale: to \(96.2\) m.

Borehole 200
\[ \rho = f(h, T) \]
Scale along the top: \(0,\ 8.4,\ 16.8,\ 25.2,\ 33.6\ \Omega\cdot\mathrm{m}/\mathrm{cm}\).
Depth scale: to \(81.10\) m.

Borehole 197

Borehole 203
Marked depth: \(28.1\) m.

\[ \rho = f(h, T) \]
Scale along the top: \(0,\ 7.5,\ 15,\ 22.5,\ 30\ \Omega\cdot\mathrm{m}/\mathrm{cm}\).
Depth scale: to \(98.10\) m.

Borehole 204
\[ \rho = f(h, T) \]
Scale along the top: \(0,\ 4.5,\ 9.0,\ 13.5,\ 18\ \Omega\cdot\mathrm{m}/\mathrm{cm}\).
Marked depth: \(23.7\) m.
Depth scale: to \(85\) m.

Fig. 3.

can be explained only by changes in the material composition of the rocks and by the nature of the development of active fracturing, which is difficult to determine on the basis of visual observations alone.

Along with fluctuations in the upper boundary of permeable rocks, the location of the zones of maximum water inflow into the borehole also changes. As noted above, in borehole No. 201 this zone is clearly recorded in the interval from 65 to 95 m. In borehole No. 200 it descends below 80 m, while in borehole No. 203 a small interval is outlined, corresponding to a depth of 55 m, where the inflow is most intense, although on the whole it remains fairly uniform over the interval 45–80 m. A similar picture is also observed in borehole No. 204, which represents an example of a uniformly watered rock mass.

It cannot be said that the nonuniformity of the filtration properties of rocks was characteristic only of the deposits of the Jurassic system, which symbolize the concept of facies variability in sedimentation. It also occurs in a more homogeneous complex of deposits; an example of this is provided by the logging diagrams recorded in boreholes drilled through the deposits of the Kolchugino Series in the Belovo area (Fig. 4).

All the examples given above of the application of geophysical methods in hydrogeological investigations concern only the qualitative aspect of the question. Without resorting to complex hydrogeological investigations with interval-by-interval determination of the permeability of rocks, resistivity logging can be used to determine the depth of occurrence of watered rocks, the thickness of the aquifer, and the completion quality of boreholes, which makes it possible to approach objectively the choice of calculation formulas in determining the filtration coefficient of rocks.

Resistivity logging makes it possible to differentiate a rock mass according to its water permeability, which is a factor of no small importance in solving questions concerning the construction of cutoff curtains or in developing methods for driving mine workings.

But this, we repeat, is only the qualitative side of the question. Resistivity investigations also make it possible to obtain a quantitative assessment of the aquifer or of the watered rock mass. It is precisely for this reason that they are finding ever wider application, despite the fact that geophysics has at its disposal more precise methods for the qualitative evaluation of aquifers, such as, for example, the tracer-atom method.

Theoretically, and also practically, the possibility has been demonstrated of determining, with the aid of a resistivity meter, the filtration velocity of groundwater and, through it, the filtration coefficient of rocks. As is known, these quantities already reflect the quantitative side of the question and, along with others, constitute initial data for the most varied hydraulic-engineering calculations.

The question of studying the filtration velocity of groundwater in a single working by means of geophysical methods is at present very topical, although insufficiently developed. The existing method for determining the flow velocity of a groundwater stream by surveying the isolines of a “charged body” is not sufficiently refined.

By this method what is determined is not the filtration velocity of the entire groundwater stream or of its individual sections, but the value of the actual flow velocity of groundwater in the pores of a random part of the aquifer, which cannot be used for practical calculations.

One of the most essential advantages of the methodology employed is the possibility of obtaining a set of resistivity curves, which give a clear idea of the regime of dilution of the solution.

Well 314 \(\rho=f(h,T)\)

Procedure for recording the curves \(\rho=f(h,T)\)

Well 319

Dependence of the specific resistivity of solutions on concentration
\(\rho\), ohm·m/l
\(C\), g/l
(after V. N. Dakhnov)

Graph
\[ C-C_0=f(h,\tau\text{ fixed}) \]

\(C-C_0\), g/l

at \(v=21.5\) cm/m, \(C_0=0.36\) g/l, \(V_{\mathrm{GO}}=2.9\) m/day.

\[ K_f=4.0\ \text{m/day}. \]

\[ v\ \frac{\text{cm}}{\text{sec}} = \frac{1.82\,z\ \text{cm}}{T\ \text{sec}} \lg \frac{C_1-C_0\ \text{g/l}}{C_2-C_0\ \text{g/l}}. \]

Well 317 \(\rho=f(h,T)\)

Fig. 4.

over the entire interval of the borehole and thus make it possible to give a qualitative representation of the relative distribution of the filtration velocity in the borehole section.

However, this method gives the value of the filtration velocity at some point (within 1 m) and excludes the possibility of comparing its results with the filtration coefficient determined by the method of test pumping, since pumping gives the average value of the filtration coefficient for the entire thickness of the aquifer. This makes the results obtained incomparable. On detailed consideration of this question, this difficulty can be overcome.

In fact, if, in comparing these data, one confines oneself to one or several values of filtration velocities obtained by the resistivity method at different points of the borehole, this will indeed not lead to the desired result. But if one uses the entire set of determinations of the filtration velocity, uniformly covering the entire thickness of the bed \((\rho = f(h))\), then, using one of the methods for determining weighted-average velocities and knowing the magnitude of the hydraulic gradient, one can arrive at the determination of the average filtration coefficient for the borehole, already comparable with the filtration coefficient obtained by the method of test pumping.

However, experience with such comparison has shown that the values of the filtration coefficient obtained by these methods do not always fully agree, and in a number of cases there has been a substantial discrepancy between these values. This was explained by the characteristics of the filtering rocks and of the filtration regime, and required the introduction into the above formula for determining the filtration velocity of a correction coefficient characteristic of particular categories of rocks and hydrogeological conditions:

\[ n = \frac{k_1}{k_2}\, \frac{(\text{in resistivity determination})}{(\text{in determination by pumping})}. \]

The necessity of introducing a correction into the value of the filtration coefficient obtained by the resistivity method substantially limits the range of possible application of resistivity logging for obtaining a quantitative assessment of an aquifer. Although the reasons for the discrepancy in the values of the filtration coefficient obtained by methods of test pumping or injection and by resistivity logging have not yet been sufficiently studied, there are grounds to suppose that an essential role here belongs to the properties of the rocks themselves, which determine the principal features of the movement of groundwater in them. And if this is so, then under conditions of great variability in the composition and degree of fracturing of a large number of formations, the values of the correction coefficient will vary substantially, and the introduction of any one of these values, or even of an averaged value, cannot but impart a certain schematism to all the calculations. And if such a procedure may still be allowed in a preliminary appraisal of one area or another, then in detailed and responsible investigations the possibility of applying it, from our point of view, is doubtful.

At the same time, it must be noted that in a number of cases, in the course of hydrogeological investigations, very encouraging results were obtained in the sense of agreement between the values of the filtration coefficients obtained by pumping methods and by resistivity logging.

N. I. Grinbaum, who widely introduced geophysical methods into the practice of hydrogeological investigations, gives an example of complete agreement of the results of determining the filtration coefficients of pebble deposits, when the correction coefficient \(n = 1\). This indicates that

under certain hydrogeological conditions the results of resistivity-meter observations can provide a quite satisfactory solution to questions of the quantitative assessment of aquifers. But if in most cases resistivity logging by itself is not in a position to give a quantitative assessment of the water-bearing capacity of rocks, then, in combination with the usual methods of hydrogeological investigations, it proves possible to answer most of the questions posed and, in particular, to differentiate the watered rock sequence according to the degree of its permeability.

It is quite natural that, for refining the intervals of water inflow into a well, no substantiation is required of the accuracy of the filtration velocities determined by resistivity logging. The very nature of the variation of this quantity, even in relative units, and the totality of the resistivity-meter curves give a sufficiently clear idea of the relative changes in the filtration properties of the permeable part of the rocks in the section of the well.

Having obtained the average coefficient of filtration for the well as a whole by conducting pumping or injection, as the most accurate means of quantitative assessment of the water-bearing capacity of rocks, and using the graph of the distribution of filtration velocities (even if only relative ones) in the well section, one can, by simple calculations, determine the true value of the coefficient of filtration for individual beds or horizons that differ from one another in degree of permeability.

An example of the use of resistivity-logging data for checking and refining hydrogeological calculations was reproduced by us at an experimental hydrogeological site in the valley of the Bachat River, in the field of the Pioneer mine, Belovo district of Kuzbass.

In order to determine the water-bearing capacity and the coefficient of filtration of the rocks, water was pumped from the central well of the site while levels were measured simultaneously in all observation wells. Using the established dependence of discharge on drawdown and the data on the magnitude of the lowering of the level in the observation wells, the authors of the report, applying Dupuit’s formula for a perfect well (?), calculated an averaged coefficient of filtration for the entire rock thickness exposed in the well, which according to their calculations proved equal to 1.27 m/day.

Subsequent resistivity logging introduced substantial corrections—though not into the method for calculating the coefficient of filtration, but into its results. First of all, the logging established that practically not the entire exposed part of the section is water-bearing, but only a small interval within depths from 64.2 to 81.0 m. On the other hand, the character of the resistivity-meter curves indicates a far from uniform degree of freshening of the solution, which is a consequence of the variability of the permeable capacities of the rocks of this part of the section.

Analysis of the curves and repeated calculations made it possible to refine the value both of the averaged coefficient of filtration, which proved equal to 4 m/day, and of the filtration coefficients of individual intervals. The latter vary within the limits from 0.9 to 7.9 m/day. It is quite natural that the refinement and differentiation of the section that were carried out could not but affect the final conclusions and recommendations.

The resistivity-meter method of investigation rendered substantial assistance in the study of the hydrogeological conditions of the Jurassic artesian basin, located in the center of Kuzbass and of interest from the point of view of the presence there of enormous reserves of groundwater capable of solving the problem of water supply for many large industrial centers of the region.

Many years of investigations have shown an extremely high degree of water saturation in Jurassic rocks and enormous static and dynamic resources of these waters. Yet despite such prolonged investigations, many questions remained little studied or entirely unstudied, which affected the completeness of the conclusions and the representativeness of the recommendations.

One of the substantial shortcomings was the lack of information on the permeability of the various horizons and individual lithological varieties of rocks. And this information proved important not only from a theoretical but also from a practical point of view, since without knowledge of these features it proved impossible to make the proper choice of design for water-intake wells and to locate them within the area of the water intake. This is not accidental, since the calculation of filters in weak rocks is in a great many cases based on the concept of the “limiting critical filtration velocity,” which follows from the value of the filtration coefficient, while the distance between wells is regulated by the magnitude of the radius of influence, which, as is known, is also connected with it by a functional dependence.

Basing our calculations on an averaged filtration coefficient, we either reduce its true value, which leads to unjustified expenditures on construction of the filter, or, conversely, increase it, which leads to a large carry-out of particles and to sanding-up of wells because of disproportionately high velocities of water movement arising when the filtration area is reduced.

Having resistivity-logging data, we obtained the possibility of taking a more correct approach not only to evaluating the hydrogeological conditions of the area, but also to selecting the designs of production wells. The thickness of the Jurassic sediments no longer appears to us homogeneous in terms of water permeability. In some areas water is able to move through almost all the rocks composing the section, including clayey varieties, which here possess fracturing (well 204, Fig. 3).

In other areas the groundwater flow is limited in its thickness to only a small horizon; moreover, not infrequently even sandy varieties of rocks are practically impermeable. Such a change in the thickness of the filtering layer, occurring even within the limits of a single experimental hydrogeological unit, cannot fail to be taken into account when choosing the methodology of hydrogeological calculations.

Having before us a logging diagram, we can determine with sufficient accuracy the depth at which the filter should be installed, thereby avoiding excesses and without reducing the filtration area.

The data of resistivity logging and the above-described method of differentiating the section according to the degree of water permeability of the rocks make it possible to represent the section in isolines of filtration coefficients, which is of great importance in investigations for hydraulic-engineering structures.

The capabilities of geophysical methods should be widely used when conducting hydrogeological investigations. One cannot, however, refrain from wishing that these methods become more perfected with respect to both qualitative and quantitative assessment.

LITERATURE

1. Grinbaum N. I. Determination of groundwater filtration velocities by means of a resistivity meter, MUP of the Main Coal Geology Administration, trust “Geofizuglegeologiya,” Collection No. 4, Moscow, 1955.

Trust “Kuzbassuglegeologiya”