STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

Electrical Earth Resistivity Surveying in Illinois

Merlyn B. Buhle John E. Brueckmann

ILLINOIS STATE GEOLOGICAL SURVEY John C. Frye, Chief URBANA

CIRCULAR 376

1964

13051 00004 1958

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS

Merlyn B. Buhle and John E. Brueckmann

ABSTRACT

The electrical earth resistivity survey has supplemented geological data in the solution of water-supply and other prac- tical problems in Illinois for 28 years. The distinctive features of 1, 137 resistivity surveys conducted since 1935 are tabulated. Municipalities, schools, housing subdivisions, water-flood pro- jects, farms, and industrial, mining, and public interests have been served.

Extensive field experience has guided the development of instruments and eguipment that are versatile and reliable. The effective use of the equipmentrests upon an understanding of the local geologic setting. Non-geologic factors that can distort the apparent resistivity must be recognized and then minimized or avoided.

Resistivity surveying is based upon theoretical consider- ations that have been verified by the application of this geophys- ical tool in a variety of geologic settings. Many published methods of interpretation have been applied successfully, but because unique and valid solutions have not been found consist- ently with these methods, a less complicated butmore practical approach has been devised for handling field data.

INTRODUCTION

Electrical earth resistivity surveys, mainly in search of water-yielding sand and gravel deposits, have been conducted by the Illinois State Geological Survey since 1935. A total of 1, 137 surveys have been made in Illinois through 1963 for municipalities, industries, farms, and other interests (fig. 1). Although most of the surveys have been undertaken for ground -water exploration, some have been made in connection with mining investigations and studies of geologic structures.

Electrical earth resistivity surveying has been used mainly as a service to the citizens of the state, to the extent that equipment and manpower are available.

1

2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

Figure 1 - Distribution of resistivity surveys, 1932 through 1963.

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS 3

It has been applied primarily to the solution of practical exploration problems and not as a research tool.

This report gives the history of electrical earth resistivity investigations in Illinois, shows the extent of the surveying, describes the equipment and methods used, and summarizes the practical results.

HISTORY

Resistivity surveying was first conducted in Illinois in 1931 by Hubbert who investigated (1) the fluorspar area of southern Illinois, (2) oil structures in the south central part of the state, (3) the lead and zinc-bearing area of northwestern Illinois, and (4) the water supply and gravel deposits of the glacial drift (Hubbert, 1934). In 1932, further study was made of the location of faults in the fluorspar area by the earth resistivity method (Hubbert and Weller, 1934).

Beginning in 1935, electrical earth resistivity surveys were conducted as a service activity of the Survey. As shown in table 1, municipalities have used the service most consistently in connection with ground-water investigations. Priority usually has been given to municipalities because electrical earth resistivity pros- pecting for public water supply sources might benefit more persons than equal time spent on individual water supply projects. Also, the larger amount of water needed for municipalities requires a relatively thick water-bearing deposit, which is likely to be detected by resistivity surveying. Small water-bearing deposits, which might be satisfactory sources of water for a farm supply, are commonly difficult or im- possible to locate.

About 10 to 20 resistivity surveys have been conducted each year for munici- pal ground-water investigations. The increase in number of municipal surveys from 1952 to 1954 was occasioned by a severe drought, in the early 1950's, which jeop- ardized many public water supplies and brought about extensive exploration for alternative sources.

The tabulation of resistivity surveys for public schools reflects the flurry of construction of rural consolidated district schools in the middle 1950's. Similar flurries are reflected for other types of water supplies, for example, the increased need for locomotive boiler water for railroads during the Second World War and the increased need for water supplies for rest stops along interstate highways, beginning in 1963. During the middle 1940's, water-flood operations for secondary recovery of oil required exploration for large and dependable ground-water supplies. Most of these operations were well under way by the middle 1950's.

Beginning in 1950, requests for resistivity surveys for farms were accepted. The growth of this work, reaching a total of 74 surveys in 19 63, partly reflects the increasing modernization of the farm home. A few resistivity surveys have been made in connection with mining operations, sand and gravel deposits, and deter- mination of thickness of overburden.

Table 2 is a compilation of the basic information for all resistivity surveys made in Illinois through 1963. They are listed alphabetically within each county.

THEORY AND APPLICATION

The electrical earth resistivity survey is based upon LaPlace's theory of the distribution of equipotential surfaces within a homogeneous and isotropic medium.

4 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

The flow of an electrical current within naturally occurring materials below the surface of the earth is thought to be analogous to a special case of this theory in which an electrical current flows between two points on the bounding plane of a medium. Discussions of the mathematical aspects are given in standard textbooks (Heiland, 1940, p. 707-723; Jakosky, 1940, p. 277-305).

Following the approach of Wenner (1916), four electrodes are spaced egually along a straight line (fig. 2). By systematically enlarging the distance between electrodes, the electrical field is expanded to include a greater volume of earth materials. The value of the apparent resistivity obtained at each electrode sepa- ration approximates the average of the true resistivity of all the materials within the impressed field.

Unconsolidated materials of the glacial drift have a wide range of resistances to the flow of an electrical current. Experience has shown that variations in appar- ent resistivity, when recognized, may be used for locating and, to some degree, identifying subsurface materials.

Ideal conditions, including homogeneity and istropy of uniformly thick layers, rarely occur naturally. The primary variables that govern the detectability of buried aquifers within the glacial drift are depth, thickness, and contrast in actual resistivities of the aquifer and the enclosing material. This contrast in actual resistivities is determined largely by differences in the chemistry of the water in the two materials. A more detailed list of variables that control values of apparent resistivity could be compiled. Those mentioned above have the most obvious effect upon data collected under field conditions in Illinois.

Potentiometer

Figure 2 - Schematic resistivity instrumentation, Wenner electrode configuration, and idealized earth conditions.

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS 5

Thick beds of water-bearing sand and gravel near the surface have been repeatedly mapped by this method, although several exceptions to this generali- zation have been observed. A layer of water-laid silt apparently imposes a masking effect that obscures the presence of an underlying sand and gravel (Buhle, 1957). It is thought that a highly conductive or a highly resistive upper layer requires an extreme modification of the rule of thumb, which states that the electrode separation approximates the depth of current penetration. The rule can be adjusted to a par- ticular locality. Another exception occurs where the water within a sand and gravel is salty (e.g., near an old oil field) or acidic (e.g., near some mine dumps). Such waters reduce an otherwise high apparent resistivity to a very low value. The de- tectability of buried layers also is influenced by such geologic factors as the effective (interconnected) porosity, the "formation factor" of electric-log inter- pretation, and the presence of conductive materials within a sand and gravel (Hackett, 1956). In exploration from the surface, the percent and type of clay that will distort an otherwise high resistivity is not known. Experience has shown that fairly low resistivities have been obtained over thick deposits of coarse ma- terials that contained small amounts of clay.

Theoretical considerations support the observation that an anomaly decreases as the depth to a layer increases and as its thickness decreases. Small anomalies at depth in thick drift are more likely, therefore, to indicate desirable aquifers than are small anomalies in thin drift. It follows also that an aquifer may be undetec- table by resistivity methods. The range in aquifer dimensions required for different uses thereby becomes a limitation to the application of resistivity surveying in the detection of sources of ground water. When the water supply need can be met by an aquifer too thin to be detected by electrical means at its depth of occurrence, the method is inapplicable. For this reason, the method is more successful when ap- plied to those uses requiring large aquifers such as large municipal and industrial supplies than when applied to the location of small municipal or individual supplies that can be served by relatively thin aquifers.

Resistivity surveying has located near- surface faults in rocks that have different conductive properties. It is especially helpful in areas where unconsoli- dated materials cover the fault trace. However, application of resistivity methods has been limited to several areas of commercially valuable mineralization in Illi- nois. In most cases, resistivity surveying has not been employed because the faults have been adequately mapped on geologic criteria alone.

Resistivity methods have been used regularly to determine the thickness of low resistivity clay or shale that overlies a detectable layer of limestone or sand- stone. Such information is useful in the search for limestone resources and in estimating depths to bedrock under highways, foundations, and dam sites. Re- sistivity methods have been much less useful in determining the depth to the base of highly resistant layers.

The application of resistivity methods to exploration for commercial deposits of sand and gravel is largely confined to outlining their areal extent. The thickness of a deposit can be roughly estimated. The wide and overlapping ranges of resis- tivity values for mixtures of sand and gravel preclude the possibility of estimating the proportions of size grades within a deposit on the basis of resistivity data.

Clean sand and gravel cannot be differentiated from limestone or dolomite because both have high resistivities; clay or till filled channels or valleys cut in shale or till cannot be differentiated because both have low resistivities.

The direct detection of coal by resistivity prospecting has not been suc- cessful because coal lacks a contrast in its electrical properties with the shale

6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

or glacial till that encloses or covers it. It follows also that clay-filled channels in coal seams, known as "cut-outs", are not detectable.

Instrumentation and Equipment

At least nine different resistivity instruments have been used by the Illinois State Geological Survey during the past 25 years. Six of these instruments and the additional components that are necessary for resistivity surveying are shown in figure 3 .

The Megger (A) has been used successfully, but only where very shallow materials were investigated.

Most of the instruments have direct-current commutated circuits which re- semble closely those developed by Gish and Rooney (1925). The hand-driven commutator of the early design (B, E, and three instruments not shown) has been replaced in several subsequent instruments (C and D) by a synchronous vibrator. This alteration has provided somewhat smoother operation but no noticeable increase in accuracy. Two instruments (B and C) are now considered too cumbersome for efficient field work, although they provide reliable data.

Three instruments are now in regular use by the Illinois Geological Survey:

(1) A large instrument (D), placed in the trunk of a car and drawing power from a 12 -volt storage battery, is used for electrode separations to a maximum of 400 feet.

(2) A small, 16-|- pound, portable instrument (E), mounted on a tripod and drawing power from dry cell batteries, is used for electrode separations to a maximum of 140 feet.

(3) A small, 8i-pound, portable potential-drop ratio instrument (F), completely transistorized, mounted on a tripod, and drawing power from dry cell bat- teries, is used for electrode separations to a maximum of 100 or 120 feet. This instrument was developed commercially and is being used with success in Illinois.

The equipment used with these instruments has been developed to meet the following requirements: (1) simplicity of operation; (2) ease of maintenance; (3) reduction of time loss; and (4) portability. Many sophisticated and complex components built in the laboratory have been discarded after trial in the field when their need for delicate handling hindered the practical conduct of the work. The program of resistivity surveying conducted by the Survey demands that useful con- clusions be drawn with a minimal expenditure of time. The present equipment is the most serviceable yet devised for our purposes. Detailed information on the construction or source of this equipment is available at the Survey.

Four reels for the lines are used. Since longer lines run to the current electrodes than to the potential electrodes, two sizes of reels are used.

The lines are standard "test-lead" wire, which is flexible and non-kinking. Current lines are black; potential lines are red. While making a resistivity station, the lines are held in place by loops around a center stake. A break in one of the lines is usually near the center stake and is located usually by stretching sections of wire and noticing the place of excessive stretch in the insulation. A continuity meter with probes is used to trace breaks and is especially useful in determining which line contains the break.

The current lines are distance -coded with tape to eliminate measuring the required distance for each stake setting. Convenient plugs connect the lines to the instrument.

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS 7

Figure 3 - Resistivity instruments and accessories. (A) Megger type. (B) Hand- driven commutator type. (C and D) Synchronous vibrator types. (E) Small hand- driven commutator type. (F) Small transistorized potential-drop ratio type. (G) Current and potential electrodes. (H) Current and potential line reels.

8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

Jumper cables connecting the stakes to the reels are subject to severe use. An open circuit is frequently due to a broken jumper. Clip breakage is remedied usually by replacement. An insulated cable with battery clips on both ends has been found to be the most reliable for steady field use.

A 3/8" steel rod about 2 feet long with a pointed end and a "T" handle is used for current stakes. Heavily copper-clad steel stakes are used with the po- tential lines. The steel core precludes bending the stake while putting it into the ground and the copper minimizes the development of an electromotive force between the stake and the ground. When the copper coating is worn off the end of the stake, the exposed steel tip is cut off and the stake resharpened. The center stake is also copper-clad since it sometimes serves as the fifth electrode for a Lee par- titioning configuration.

Field Operation

Resistivity surveying in Illinois has demonstrated the need for integrating resistivity with geology. The customary first step in preparing for a resistivity survey is assembling the available information on the local geology. Geophysical interpretations are made within this framework, which also guides the procedure of the resistivity survey.

The Pleistocene deposits of Illinois are complex and extensive and rest upon bedrock whose character and topography are highly varied. Over large areas, however, a rather elementary type of resistivity investigation yields significant results. The basic problem is finding a layer of highly resistant sand and gravel within a section of less resistive material.

Large areas of Illinois are covered by glacial drift, which ranges from a few feet to more than 400 feet in thickness and which commonly contains layers of sand and gravel. The greatest thicknesses of drift commonly occur over buried bedrock valleys. In a large central area of the state, the drift is chiefly underlain by shales of low resistivity. In a geologic setting where the drift lies on shale, resistivity surveying has been very effective. Layers of sand and gravel in the drift contrast sharply in electrical properties with the clayey materials of the drift and of the bedrock.

In areas such as northern Illinois where the drift lies on fresh-water-bearing sandstones, dolomites, and limestones, the resistivity method also has been ef- fective although the development of water supplies from the bedrock without con- sideration of the drift generally has made resistivity surveying unnecessary. Extreme caution is necessary in these areas since high resistivity values caused by bedrock can be misinterpreted as indications of sand and gravel.

Resistivity surveying has not been used in areas where the bedrock crops out at the ground surface. Extensive areas of bedrock exposure are found particular- ly in northwestern and southern Illinois.

Throughout the state, sand and gravel deposits occur as river terraces and outwash plains and in the beds of rivers and streams. In such situations, resistivity surveys have outlined, often in detail, the permeable materials present.

Requests for resistivity surveys usually come to the Illinois State Geologi- cal Survey after initial attempts at well construction have been unsuccessful. In such circumstances, this geophysical tool has made significant contributions. However, surveys do not result invariably in clearly defined recommendations. If a survey is run without the detection of an anomaly, additional test holes

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS 9

usually are recommended to search for any electrically undetectable aquifers that might be present. When a resistivity survey is requested where previous drilling has not been conducted, it generally is recommended that the entire thickness of the glacial drift be explored by test drilling before resistivity is used. The loca- tion for this initial test is determined purely upon the basis of convenience. The test hole provides data for later resistivity prospecting.

The effectiveness of a resistivity survey depends largely upon the areal distribution of resistivity stations. The thickness of the drift, the size of the aquifer sought, and the nature of the geologic setting determine the pattern that is selected. The Survey has found that usually an interval between stations of .25 mile is suited to municipal surveys over thick drift. It is considered probable that an aquifer that cannot be detected by a grid of stations at such an interval would not be extensive enough or thick enough to provide the necessary production. Exceptions to this rule, of course, have been found. A shorter interval between stations such as 50, 100, or 200 feet is used for thin drift and stream and valley fill. Thick and widespread water-bearing formations are not commonly found in thin drift, and detailed information is needed to pick the most favorable locations for test drilling.

The electrode separations used at a resistivity station are also determined by the local geology. The initial operating assumption used is that the electrode separation approximates the depth of current penetration. The maximum electrode separation that is selected equals or slightly exceeds the estimated depth to bed- rock. The increment, by which this maximum value is reached, is determined by the\amountof information needed to define an anomaly. The increment most commonly used in Illinois is 20 feet with a maximum electrode separation of 100 or 120 feet. This contrasts with an increment of 5 feet and a maximum of 25 feet for work on a small creek flat and an increment of 40 feet and a maximum of 400 feet in areas of thick drift.

Two basic methods of taking data are the step traverse and the depth pro- file. A step traverse involves taking one reading at the same electrode separation at each Station along a traverse. This approach is seldom used at the Survey, unless the geologic information restricts a water-bearing formation to a particular depth. Depth profiles require a series of readings at different electrode separations at each station. The additional information provided usually is valuable for final interpretation. Reliable data for a depth profile will plot as a smooth curve (Cart- wright and Buhle, 1964). Rough and irregular curves generally are considered cause for repeating the station.

A large number of factors, operating singly or together, can distort the true value of the apparent resistivity. These factors include:

(1) Buried conductors: Pipelines, phone cables, oil and water tanks.

(2) Fences. Those made of wire strung between metal posts; damp wooden posts occasionally have been suspected of carrying some current.

(3) Overhead high voltage transmission lines.

(4) High voltage transformers on poles.

(5) High contact resistance at one or more electrodes, caused by very dry ground, a layer of cinders, a potential electrode in contact with a limestone cobble, etc.

(6) Water moving down- slope, or percolating into the ground immediately after a rain.

(7) Wet vegetation touching jumper clips, electrodes, or a current- carrying metal part of a reel.

(8) Frozen ground, not fully penetrated by potential electrodes at every reading.

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

Factors such as 1 and 2, which are constant at any one place, permit readings to be duplicated with accuracy, but these readings are too low. The location of a buried conductor usually is marked at the ground surface or it is inferred from the shape of a negative anomaly. For example, a series of stations along a highway, giving low resistivities in a region of high resistivity, suggest the presence of a buried telephone cable, water, gas, or oil pipe.

Distortions due to 1, 2, and 3 are minimized by relocating the lines per- pendicularly to the fence, pipeline, or overhead transmission lines . Experience has shown that factors 4 through 7 usually make the duplication of the data for a profile impossible. Factors 4 and 5 often require the abandonment of a station. The influence of factor 5 is minimized or eliminated by watering the ground to de- crease contact resistance or by removing the cinders to provide reliable contact. Factors 6 and 7 require waiting until the moisture has disappeared into the ground. It has been found best to avoid factor 8 entirely, except during the early autumn when only a thin crust of ice has had time to form. All of these factors usually lower the apparent resistivity, except for 5 and 8, which can cause high and erratic readings .

Interpretation

Geologic and hydrologic data from nearby wells or inferences based on pre- vailing geologic and hydrologic knowledge form the bases for interpreting resistivity data in ground-water exploration in Illinois. Theoretical and empirical methods of interpretation (Mooney and Wetzel, 1956; Muskat, 1945; Roman, 1934; Tagg, 1934; Wetzel and McMurry, 1937; Moore, 1945) have been applied successfully in some instances, but more commonly have failed to provide unique and valid solutions. Usually the choice between alternative interpretations resulting from different theoretical methods cannot be made until test drilling has been done, at which time direct interpretation of the resistivity data is possible.

In areas of glacial drift in Illinois, resistivity highs usually are associated with the presence of clean sand and gravel, and resistivity lows usually are associ- ated with the absence of such materials. Resistivity surveys in Illinois have been used accordingly to locate sites where clean sand and gravel is apparently present and where test drilling for permeable aquifers is warranted.

Exceptions to this prospecting guide have been noted. In some parts of Illinois low values of apparent resistivity and featureless depth profiles have been found over deposits of sand and gravel. Rarely, however, have high resistivities been found without sand and gravel, unless bedrock of high resistivity was present. In summary, low resistivities usually have suggested the absence of sand and gravel but have not proven it. High resistivities in the drift almost always have been in- dicative of clean granular material.

The importance of integrating resistivity data into the framework of the local geology cannot be overemphasized (Hackett, 1956). Resistivity data without geologic control can be useless or misleading.

The following method of processing and interpreting field data is regularly used at the Illinois State Geological Survey (for case histories, see Buhle, 1953; 1958). A sketch map is made of the area surveyed, including the location of each resistivity station. On one copy of the map, the values of apparent resistivity, which were obtained at a selected electrode separation, are plotted. The electrode separation selected for plotting usually is determined by the magnitude of the

ELECTRICAL EARTH RESISTIVITY SURVEYING IN ILLINOIS 11

resistivity value; the higher readings usually are sought. Geologic control, where available, guides the selection. If, for example, a sand formation is logged in a nearby well from 35-45 feet, data at an electrode separation of 40 or 50 feet are selected in order to detect variations associated with such a layer. Two or more maps often are used to present adequately the data of one survey. Each map then is contoured with lines of equal apparent resistivity.

As a final step, the depth profiles within the areas of higher resistivity are inspected for any hints that they may give of the nature and extent of the underlying material. In general, the electrode separation with the highest apparent resistivity approximates the depth to a horizon of highest actual resistivity. One or more theoretical or empirical methods may be applied experimentally to the data. When test holes are drilled, the significance of the shape of a depth profile some- times is established. After a distinctive depth profile is correlated with a known stratigraphic section, extrapolations are made into adjoining areas on the basis of resistivity data.

12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 376

REFERENCES

Buhle, M. B., 1953, Earth resistivity in ground-water studies in Illinois: AIME Trans., v. 196, p. 395-399.

Buhle, M. B., 1957, Uses and limitations of electrical prospecting for water sup- plies: Illinois Acad. Sci. Trans., v. 50, p. 167-171.

Buhle, M. B., 1958, Six case histories of resistivity prospecting in Illinois in Geophysical surveys in mining, hydrological and engineering projects: Leiden, E.J. Brill (European Association of Exploration Geophysicists), p. 205-213.

Cartwright, K., and Buhle, M. B., 1964, Discussion of paper by Carpenter and Bassarob: Groundwater, v. 2, no. 2, p. 54-55.

Gish, O. H., and Rooney, W. J., 1925, Measurement of resistivity of large masses of undisturbed earth: Terr. Mag. and Atmos. Elec. v. 30, no. 4, p. 161-188.

Hackett, J. E., 1956, Relation between earth resistivity and glacial deposits near Shelbyville, Illinois: 111. Geol. Survey. Circ. 223, 19 p.

Heiland, C.A., 1940, Geophysical Exploration: New York, Prentice -Hall, 1013 p.

Hubbert, M. K., 1934, Results of earth -resistivity survey on various geologic structures in Illinois: AIME Trans., v. 110, p. 9-29.

Hubbert, M. K., and Weller, J. M., 1934, Location of faults in Hardin County,

Illinois, by the earth-resistivity method. AIME Trans., v. 110, p. 40-47.

Jakosky, J. J., 1940, Exploration Geophysics: Los Angeles, Times -Mirror Press, 786 p.

Mooney, H. M., and Wetzel, W. W., 1956, The potentials about a point electrode and apparent resistivity curves for a two -three -four layered earth: Minne- apolis, Univ. of Minnesota Press, 146 p.

Moore, R. W. , 1945, An emperical method of interpretation of earth-resistivity measurement: AIME Trans., v. 164, p. 197-214.

Muskat, M., 1945, Interpretation of earth resistivity measurements: AIME Trans . , v. 164, p. 224-231.

Roman, I., 1934, Some interpretations of earth resistivity data: AIME Trans., v. 110, p. 183-201.

Tagg, G. F., 1934, Interpretation of earth-resistivity measurements: AIME Trans., v. 110, p. 135-145.

Wenner, F., 1916, A method of measuring resistivity in the earth: U. S. Bureau of Standards Bull., v. 12, p. 469-478.

Wetzel, W. W., and McMurry, H. V., 1937, A set of curves to assist in the interpre- tation of the three -layer resistivity problem: Geophysics, v. 2, no. 4, p. 329-341.

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Illinois State Geological Survey Circular 37 6 51 p., 3 figs., 2 tables, 1964

Printed by Authority of State of Illinois, Ch . 127, IRS, Par. 58.25.

CIRCULAR 376

ILLINOIS STATE GEOLOGICAL SURVEY

URBANA