COMBINED GEOELECTRICAL AND DRILL-HOLE INVESTIGATIONS...

GEOPHYSICS, VOL. 39, h-0. 3 (JUSE 19741, P. 340-352, 8 FIGS., 1 T.hBLE

COMBINED GEOELECTRICAL AND DRILL-HOLE INVESTIGATIONS

FOR DETECTING FRESH-WATER AQUIFERS IN NORTHWESTERN

MISSOURI

REISHARD I(. FROHLICH*

.4 geoelectrical investigation was conducted in an area of northwestern Missouri which was pre- viously explored by drill holes. The purpose of the survey was to evaluate the effectiveness of the method in locating potable groundwater. It was found that resistivity depth soundings using the Schlumberger arrangement can partially localize and determine the thickness and depth of both near-surface and basal fresh-water-bearing gravel bodies in glacial deposits. The basal gravel is usually confined to preglacial stream channels and has a particular importance as a fresh-water aquifer.

Only in a few cases does the resistivity increase at the depth of the bedrock. More often the depth

INTRODUCTION

In northwestern Missouri, sizeable amounts of groundwater for domestic and/or industrial use are contained in aquifers of limited thickness and lateral extent. Besides the geometric dimensions of the aquifer, such parameters as porosity, permeability, water salinity, and water recharge are important for evaluating potential sources of potable groundwater. Conventionally, data for these parameters are obtained from a net of close- ly spaced drill holes in and around potential aquifers. This type of exploration is expensive where aquifers are deep beneath the ground surface. Surface geophysical methods can reduce exploration costs substantially by reducing the number of drill holes required.

.4n intensive search for water resources has been conducted in the northern part of Missouri.

soundings show an appreciab1.e increase of resistivity at a depth well within the bedrock. High permeability and porosity at a lower water con ductivity in the gravel is compensated by lower permeability and porosity at a higher water conductivity- in the bedrock. This can explain why the formation resistivities found from depth soundings are basically the same. .4n increase of resistivity in the deeper bedrock occurs due to a tightening of fissures containing salt water.

In one particular well, which penetrates a basal gravel, it can be shown that the salt content of the well water originates from salt water encroachment, most likely from the bedrock.

Large amounts of potable water have been found in buried glacial stream channels. The State of Missouri Division of Geological Survey and Water Resources has conducted an extensive drilling program for the detection of these aquifers in Grundy County, northwestern Missouri. The records obtained during this program have made it possible to compare the dril!-hole data with recent geoelectrical depth soundings and to evaluate the applicability of geophysical surface methods for the detection of ground-water in northwestern Missouri.

GLACIAL DEPOSITS IN NORTHWESTERN MISSOURI

The hydrogeology of the glacial drift in northwestern Missouri is complicated for water pros- pection. Aquifers occur at three different levels,

Presented at the 42nd Annual International SEG Meeting, Wovember 28, 1972, ceived by the Editor, May 8, 1973; revised manuscript received November 2, 1973.

Anaheim, Calif. Manuscript re-

* University of Rhode Island, Kingston, RI 02881.

@ 1974 Society of Exploration Geophysicists. All rights reserved.

340

Fresh-Water Aquifers 341

and they all have a rather irregular distribution (Fuller and Knight, 1967).

Although near-surface layers generally consist of sand and clay, there are a limited number of small gravel pockets. Occasionally, they are large enough for their water to be used for local house- hold and farm needs.

Large amounts of potable water are contained in gravel deposits at the base of the glacial drift. In glacial stream channels the gravel can be as much as SO m thick (Fuller and Russell, 19.56). The stream channels are cut into the bedrock of dolomitic limestone with intermittent layers of shale, all of Pennsylvanian age.

The largest amounts of water occur in the fissures and joints of the upper part of the bedrock. However, at average concentrations of 2000 ppm dissolved ions, this water must be considered as nonpotable and can only be used in a limited way after treatment. When water is drawn from the basal gravel, it is possible to draw salt water from the bedrock into the fresh water zone, and some wells are contaminated in this manner.

METHOD OF INVESTIGATION

Schlumberger depth soundings were applied in Grundy County where drilling results were already available for comparison. Meidav (1960) conducted Wenner depth soundings in the same area. His investigations were restricted to a de- scription of relative changes in apparent resistivity and resulted in qualitative depth estimates and comparisons.

Deppermann (1954) has shown in a theoretical study that Wenner depth soundings are more influenced by near surface lateral inhomogeneities than Schlumberger soundings. This explains why so many Wenner field curves show scattered measuring points, whereas measurements obtained with the Schlumberger arrangement more often show regular curves.

The depth soundings were located adjacent to drill holes across three east-west profiles (Figure I). Because the measuring technique today can be considered as a standard procedure, reference is made to textbooks, such as Kunetz (1966), and Keller and Frischknecht (1970).

In case of horizontal layers at moderate changes of resistivity, one can with certain reservations apply a rule of thumb which says that the depth of exploration (commonly named “depth pene-

tration”) ranges between one-third and one- fourth of the current-electrode separation L. The equipment was designed so that accurate read- ings were obtained for values of L beyond 500 m. Thus, the rock material between the surface and an average depth of about 1.50 m was investigated; this depth is larger than the greatest known depth to the bedrock. The potential- electrode spacing JlN was usually expanded, once from 1 to 3 m, when L was expanded for greater depth penetration.

The interpretation of depth sounding curves is based on the assumption of horizontal layers. Presently, it is possible to calculate theoretical multilayer curves of any resistivity-thickness combination (Mooney et al, 1966) and to obtain an ideal match between field and master curves. Because of the large number of multilayer master curves which would be necessary for interpretation, a more practical method known as the auxiliary point method is used. There are several procedures of this method, reviewed by Zohdy (1965), which are based on an approximative piecewise interpretation of multilayer field curves with two- and (or) three-layer master curves. The interpretation procedure of Hummel (1929) and Ebert (1943) begins at the left-hand branch of the field curve so that the depth of the first layer and resistivities of the upper two layers are determined. When both layers are combined into a hypothetical single layer with a fictitious resistivity and thickness, the third layer can be interpreted by a second match with another master curve.

The above method was applied in this study to interpret four- and five-layer field curves with a set of three-layer master curves (Deppermann, 1970) as described by several authors (Homilius, 1961; Orellana and Mooney, 1966). The partial curve matching used in connection with the auxiliary point method will decrease in accuracy with an increasing number of layers and, also, if there is a large contrast of layer-resistivities. The more layers and the higher the resistivity contrast, the smaller will be the section of the field curve which has to be matched with three-layer master curves in the successive steps of interpretation. Though the auxiliary point method using three-layer master curves leads to a better approximation than a method based on two-layer master curves, many field curves of this study gave equal results

Frohlich

31’

TRENTON MO

10’ 93.2,’

GALT MO

FIG. I. Location of drill holes and geoelectrical depth soundings in the area of investigation, Grundy County, northwestern Missouri.

when interpreted both ways. This indicates that the field curves are simple, the interpretation in terms of resistivity and depth is reliable, and that for this particular survey, the geoelectrical depth sounding method has a high degree of diagnostic power.

FIELD RESULTS

The interpretation of a majority of the 28 depth soundings was compared with data from adjacent drill holes (Figure 1). Drill-hole observations are reliable; however, drilling was discontinued when the hard bedrock was reached. Two examples of characteristic field curves over basal gravel are shown in Figure 2. Also in the figure, the results of different interpretations are compared with the drill-hole observations. The horizontal axis is used to designate both L/2 for the field curve and the depth of the layers. The first impression

of the field curve D.S. 1 suggests that it represents a four-layer case if one ignores the first two points to the left. The results of interpretation 1 for D.S. 1 and D.S. 21 are in good agreement with the drill-hole observations. Both curves are prototypes in respect to a small but significant difference of resistivity between yellow and gray clay. The right-hand side of both curves shows an increase of resistivity at greater depth, which coincides with the top of the basal gravel. The boundary between gravel and bedrock, observed from drill holes at 8.5 m underneath D.S. 1 and 120 m underneath D.S. 21, does not show on the field curves. Apparently the basal gravel has the same resistivity as the upper part of the limestone bedrock.

Another possibility of a solution must be in- investigated; it suggests a stepwise increase of resistivity from clay to basal gravel and finally to


FIG. 2. Geoelectrical depth soundings over basal gravel. The thick line represents the field curve and the dashed line the auxiliary curves with relations ppIpl in boxes. Parallel to the abscissa are the results of the interpretation presented as solutions 1 to 4. These are compared with the drill-hole observation at the bottom.

the upper part of the bedrock. In this case, the ascending branch from about L/2= 80 m for D.S. 1 and L/2-100 m for D.S. 21 presents an A-type curve with pl<p2<p3. Here pi is the fictitious resistivity for layers from the surface to the top of the basal gravel, and p? and pa are the resistivities of the basal gravel and the bedrock, respectively. The thickness of the gravel would be too small to show significantly on the field curve. Its influence is suppressed and that branch appears as a two-layer curve though it represents three layers.

The dashed lines in Figure 2 show auxiliary curves, and points A, B, and C are the origins of the master curves obtained during the process of partial curve matching. The thin curves at the

right-hand side are A-type, three-layer master curves beginning at C for D.S. 1, and at B for D.S. 21. They match the ascending branch of the field curves and are compatible with interpretations 1, 2, and 3. Without further information, interpretations 1 to 4 are equally valid. In this case, only careful comparison with drill-hole observations can decide the proper solution. Inter- pretations 1, 2, and 3 for U.S. 1 show the boundaries between 7.5 ohm-m and 2.5 ohm-m at 52 m (solution 1); between 7.5 ohm-m and 30 ohm-m at 56 m (solution 2); and between 7.5 ohm-m and 40 ohm-m at 60 m (solution 3). In all three cases the second resistivity value represents the suppressed basal gravel. The boundary presented by its top approaches with increasing gravel-

344

DRILL HOLE 39 38

I RESISTIVITY D S 10

I I

I

800 1)-

ASL

Frohlich

37 16 ,

7

5

h’ y J. 1 LIMESTONE m] GRAVEL VI,” FISSURES

FIG. 3. Geologic profile A-B based on drill-hole observations. Vertical columns show the results of depth soundings with figures presenting resistivities in ohm-m. The depth range of geoelectrical boundaries shows the smallest and greatest boundary depths which are compatible with the depth soundmgs.

resistivitl- that of solution 1. In the successive steps of interpretation from 1 to 3, its depth and resistivity approach values which were obtained with solution 4. The same can be shown for D.S. 21. Preference therefore is given to solution 4 which also presents an asymptotic transition of an A-type, three-layer case with pl<p2<p3 into a two-layer case with pI<p2 and pz=p3. Also, the depth of the bedrock observed at 85 m beneath D.S. 1 does not agree with any of the interpreted boundaries at 105 m, 110 m, and 120 m for solutions 1, 2, and 3, respectively. A similar line of thought can be developed for D.S. 21 leading to the conclusion that in both cases all models have to be rejected except for solution 4.

The auxiliary point method has the advantage over multilayer calculations that it shows more easily the influence of a single layer on the depth sounding. This is shown for D.S. 1 and D.S. 21 and resolves the important question of whether the increase of pa to the right-hand side of both curves is caused by the bedrock only, or if the basal gravel has a significant contribution. This

also illustrates that the basal gravel is thick enough to be detected with geoelectrical depth soundings.

In Figure 2 the origin of a set of two-layer master curves with p~>pl is placed on the auxiliary curve, which starts at B for D.S. 1 and at A for D.S. 21. The abscissa of the origin of the master curves (PI= 1; ml= 1) equals the depth of the bedrock. The origin is designated with 0 at 8.5 m for D.S. 1 and 120 m for D.S. 21. The fictitious thickness of the layers above the bedrock for the H-auxiliary curve agrees with the true thickness. The dotted curve presents a master curve with pq=S.p,, which is shifted to the right-hand side parallel to the horizontal axis. If the basal gravel were negligible, the dotted curves would agree, in both cases, with the right- hand branch of the field curve. The offset between field and master curve is larger for D.S. 21 than for D.S. 1 since the basal gravel is thicker for the first (40 m) than underneath D.S. 1 (21 m). The offset is far larger than could be accounted for by measuring inaccuracy, which leads to the


5 I

6

E] CLAY

DEPTH RANGE OF K] CLAY WITH SAND GEOELECTRICAL BOUNDARY

conclusion that the basal gravel has a substantial contribution to the increase of resistivity.

Figures 3 and 4 show how depths obtained from geoelectrical interpretations fit into the geologic profiles based on drill-hole observations. A near- surface gravel deposit was found in the western part of profile A-B (Figure 3). Its high resistivity, which lies in the range of 100 ohm-m to 135 ohm-m, allows a good distinction from layers underneath it.

The depth soundings very seldom show an increase of resistivity at the overburden-bedrock interface such as in D.S. 7, Figure 3, and D.S. 18, Figure 4. In most cases this boundary does not reflect a change of resistivity at all. An increase to higher resistivities was observed at a greater depth within the bedrock such as in D.S. 10, Figure 3, and D.S. 20, Figure 4.

Bedrock resistivity at a negligibly low amount of saline water was measured in the southwestern part of the study area with D.S. 5 (see Figure 1). Underneath an overburden of only 9.5 m resistivities between 260 ohm-m and 270 ohm-m

were assigned to bedrock, which showed dolomitic limestone with interbedded shale in a nearb! quarry.

An example for the lack of geolectrical response of the overburden-bedrock interface is shown in D.S. 20, Figure .i. By intercpreting the ascending branch on the right-hand side, it can be shown that all models- including suppression of an A- type curve-result in an increase to a layer of resistivity between 40 ohm-m and 60 ohm-m within a depth range between 95 m and 135 m. This is far below the top of the basal gravel as well as the bedrock, which were oh~ezved_from the adjacent drill hole no. 35 at 56 m and 75 m, respectively. Neither the gravel nor the upper part of the bedrock can be distinguished from the over- lying gray clay. This can be explained b> large amounts of salt water in the upper part of the bedrock, which most likely infiltrated the gravel. A similar situation was found for D.S. 10 in Figure 3. Occasionally, there may exist a very small difference in resistivity between basal gravel and bedrock as can be shown in Figure 5.

Frohlich

31 30 36

19 21

DEPTH RANGE OF GEOELECTRICAL BOUNDnRY

FIG. 4. Geologic profile C-D (Figure 1) with results of geoelectrical depth soundings.

D.S. 16 suggests that the gravel and possibly the upper part of the bedrock have a slightly higher resistivity than the continuing part of the bedrock. The three-layer master curve, with a resistivity ratio of l/0.5/ 1.5, matches the field curve except for the right-hand side. The master curve in Figure 5 is presented by the dashed curve. The field curve lies above it and tends to approach the master curve to the far right. The insertion of a fourth layer, between the layers of

9.5 ohm-m and 30 ohm-m, of a resistivity larger than 30 ohm-m would give a better match with the field curve. Using the auxiliary point method, such four-layer curves of HK-type are difficult to interpret with respect to the third layer if p3 is only slightly larger than ~4. Large changes of this la>-er-thickness will not significantly alter the master curve.

Summarizing the geoelectrical results in respect to the overburden bedrock boundary, WC find

FIG. 5. Geoelectrical depth soundings show an increase of resistivity below the depth of the bedrock. Parallel to the abscissa the resistivity-depth function is compared with the drilling results. The dashed curve in D.S. 16 presents a four-layer case, while the field curve here indicates a fifth layer.


that they fall into three groups:

1. In only a few cases an increase of resistivity was observed at this interface2. Many depth soundings show an increase of resistivity far below this interface. In such cases the gravel at the base of the overburden also does not reflect a change in resistivity. The upper part of the carbonate bedrock can have resistivities as low as 10 ohm-m. 3. Depth soundings over stream channels with large gravel deposits do not show a change of resistivity at this interface, though electrode separations were large enough for the current to penetrate sufficiently deep into the bedrock. Here the upper part of the bedrock shows resistivities between 40 ohm-m and SO ohm-m.

The lack of resistivityr contrast between overburden and carbonate bedrock may have its origin in a strong electrical anisotropy of the overburden. This very interesting effect, tnentioned by a referee, is of great importance for glacial deposits. Frequent interbedding of clay and sand deposits may lead to macroanisotropy, and an overestimation of the depth to bedrock. This may partially contribute to the overestimation in case 2. For the basal gravel fill, as in case 3, however, this is less probable because of the absence of interbedding. There remains the possibility of a strong microanisotropy, which could lead to an overestimation of the depth to bedrock.

Another phenomenon seems to have a major effect and also accounts for the low resistivity of the upper part of the bedrock. The saline water strongly influences the resistivity of the upper part of the bedrock. Low resistivities between 10 ohm-m and 1-5 ohm-m, as described in case 2; are caused by large and numerous openings in the bedrock, which are filled with saline water. Be- cause of their abundance it also penetrates into the overburden, which explains the fact that neither the gravel nor the upper portion of the bedrock can be distinguished from the low resistivity clay such as found in D.S. 20, Figure 5. Intermediate resistivities between 40 ohm-m and 60 ohm-m for the bedrock are still too low for a carbonate rock even if shales are interbedded. These resistivities can be explained by the pres- ence of salt water filling fissures of intermediate size and number as observed in case 3. At greater depth it is reasonable to assume that the bulk

volume of the fissures and openings decreases. This results in an increase of resistivity to values of 260 ohm-m and higher as found in D.S. 5 and D.S. 10.

Electrode separations were not large enough to systematically investigate the change of resistivity with depth in the upper part of the bedrock. Since boreholes were discontinued at the depth of the bedrock, the depth soundings may offer use- ful information on the salt water content in the bedrock.

The results of the depth soundings with respect to layer resistivities are shown in Figure 6. The resistivities of the gray and yellow clays are con- centrated at the left end of the scale and do not exceed 25 ohm-m. The basal gravel and upper part of the limestone bedrock range between 40 and 55 ohm-m. Finally, resistivities of the near- surface gravel pockets are widely spread between 60 and 110 ohm-m. The high resistivities in the deeper part of the bedrock are not shown here. The three rock units do not overlap and con sequently can be distinguished from each other.

PARTIAL REPLACEMENT OF DRILL HOLES

WITH DEPTH SOUNDINGS

A series of depth soundings were measured along two small profiles which crossed known gravel beds. Depth soundings 7, 24, 2.5, 26, and 27 (Figure 1) traverse a projected stream channel. The same series of depth soundings also helped investigate the northern extension of a near- surface gravel pocket, which was observed in drill holes 16 and 37 and was also found by D.S. 7 (Figure 3). In this particular area the bedrock seems to have only small amounts of salt water because an increase of resistivity occurs at the depth of the bedrock rather than beneath it. Figure 7 shows the five depth sounding curves with a resistivity profile underneath. The near- surface gravel varies in resistivity between SO ohm-m and 110 ohm-m and apparently is of larger extent than the commonly known gravel pockets at the surface in the area The bedrock resistivities vary between 75 ohm-m and 8.5 ohm-m, and the maximum depth of the stream channel is located between D.S. 24 and D.S. 2.5.

The right-hand side of D.S. 7, D.S. 21, and D.S. 25 are curves of H-type with a minimum. The equivalence principle makes the interpretation ambiguous since thickness and resistivity of the

RESISTIVITY ohm-m

FIG. 6. Histogram of layer resistivities compiled from 28 depth soundings.

intermediate layer can be interchanged as long as the quotient mz/pz is constant for pI>p2<pa, with rn2 being the thickness of the intermediate layer. The equivalence principle, however, is not applicable without restrictions particularly if the minimum, as in this case, is rather weak.

Pylaev (1948) has shown that only within certain limits can pi be changed, with a correspond- ing adjustment of m2, to keep the quotient constant (Bhattacharya and Patra, 1968). If these limits are exceeded, master curves will disagree with the field curve.

Interpreting the field curves with this in mind, we see that the given resistivity of 40 ohm-m for the intermediate layer presents the minimum resistivity which is acceptable under the principle of equivalence and which produces a good match between master and field curves. Therefore, the resistivity of 40 ohm-m for the intermediate layer over the stream channel agrees with basal gravel and fits into the geologic concept. Xorth of the glacial stream channel, the intermediate layer obtains resistivities between 22 and 24 ohm-m, which can be explained by a thinning of the basal gravel and replacement of finer sand with a possible clay component.

A profile farther east with three depth soundings (D.S. 13a, b, c) shows very smooth curves (Figure 8). They indicate an increase in resistivity at depths varying between 70 m and 90 m. Com- paring D.S. 13a with the adjacent drill hole no. 8 (Figure 3), we see that the depth agrees with the top of the basal gravel. The increase of resistivity

can be explained by the combined influence of basal gravel and bedrock limestone.

SALT WATER ENCROACHMENT

r\t present, there is some uncertainty as to whether the water in the basal gravel is saline. Water wells frequently are drilled to the bottom of the basal gravel, and it is possible that salt water encroaching from the bedrock into the fresh water aquifer has contaminated the well water. Drill hole no. 1 next to D.S. 1 (Figures 1 and 2) is a water well which penetrates to the bottom of the basal gravel. The conductivity of the well water was measured with a conductivity bridge, and its resistivity was found to be R, = 4.0 ohm- m. If it is assumed that this well water is repre- sentative of all water contained in the pore volume of the basal gravel, the porosity @ of the gravel can be interpreted from the layer or formation resistivity R, of the depth sounding. The relation between R,, R,, and @ is given by Archie’s law:

R, = a. R,.Wm,

in which a and m are rock constants. For consoli- dated rocks the constants have the values: a= 1 and 2.25>m> 1.6 (Keller and Frischknecht, 1970, p. 21). For unconsolidated rocks, as in the case of glacial deposits, Wyllie and Gregory (1953) ex- tended the use of Archie’s law to find values for the constants: a = 1 and m= 1.3. With R,,= 4.0 ohm-m and R,=46.5 ohm-m, resulting from in-


P-

Q- EL 1 OC 2t u-u-

Q E2 1 EOI 1 ob[ EL u-u-

8

%

D.S. 13 a 20 30 -IL111

Frohlich

D.S.13b 20 30 sL+.n

D. S. I3 c 10 20 30 9.2 (Am)

l-

~ L/2(m) 100

FIG. 8. Resistivity profile across a stream channel (Figure 1) showing low resistivity clay deposits with higher resistivity gravel deposits underneath.

terpretation of D.S. 1, the only unknown in Archie’s law is the porosity @ which is determined to be 0.~ or i-5 percent. This unusuaily small value for the porosity cannot explain the high water yield of the well. Different porosities were assumed, and the water resistivity R, was calculated for formation resistivities R,=40 ohm-m and 50 ohm-m. The latter two values are

the extreme values for the formation resistivity of the basal gravel as found by interpreting the depth soundings_ The water resistivity is calculated as follows: R,= R,.@’ 3.

Table 1 shows that only at a porosity of less than 20 percent is a water resistivity of 4.0 ohm-m or less compatible with a formation resistivity between 40 and 50 ohm-m. According to the yield


Table 1. Water resistivities calculated from Arcbie’s law using formation resistivities for different porosities.

Porosity

% 0 Ro=40 ohm-m R,=50 ohm-m

10 0.1 2.00 2.51

20 0.2 4.94 6.17

30 0.3 8.36 10.45

40 0.4 12.15 15.19

50 0.5 16.25 20.30

60 0.6 20.59 25.74

The equality of basal gravel and bedrock resistivity can be explained by the compensating effect of porosity and pore water salinity on the resistivity. When both increase in size, the resistivity will decrease. While the basal gravel has a higher porosity than the bedrock, the salinit) of its pore water is much lower than that of the bedrock. Both effects seem to compensate each other in respect to the formation resistivit\-. Therefore, only small differences in resistivity can be expected if, for instance, the increase of water salinity in the bedrock is not full>, compensated by a decrease of its porosity or b!. an increase of the porosity in the gravel.

of the water well, however, one must admit a Further structural information can be gained higher porosity for the basal gravel. It is there- by using additional cost-saving geophysical fore resonable to explain the disagreement be- methods. Particularly, the depth of the bedrock tween measured and calculated water resistivit) could be determined by shallow seismic refraction by the encroachment of salt water at the bottom measurements as suggested by Meidav (1960).

of the basal gravel while the major part of the Detailed gravity surveys (McGinnis et al, 1063) water in the gravel must have a higher resistivit) can be expected to respond to bedrock topography and, consequently, can be considered to be pot- leading to a localization of the huricd stream able. channels.

SUMMARY

Geoelectrical depth soundings, when used in conjunction with drill-hole data, can partially replace the more expensive method of drilling to obtain groundwater information. Since the layer resistivities of clay, near-surface and basal gravels, and dry bedrock do not overlap, the boundaries of the rock bodies can be found easil) in terms of depth. It was found that resistivity interpretation becomes unreliable if all three aquifers are on top of each other. In this case, the highly resistive near-surface gravels conceal the effect of deeper layers with smaller values of resistivity. The depth of glacial deposits can only be found if the bedrock does not have saline water in the upper part.

Detailed interpretation suggests that saline water is mainly restricted to the bedrock, while basal gravel deposits carry fresh water of higher resistivity. This also explains the fact that the bedrock~ in most cases has approximate!y the same formation resistivity as the basal gravel and that, consequently, this interface does not show on geoeiectrical depth soundings. This example shows that formation or layer resistivity is a complicated function of rock material, pore volume, permeability, and salinity of pore water.

ACKNOWLEDGMENTS

The author is indebted to the Missouri Geologi- cal Survey and Water Resources Division for the geologic data. This work was supported by a grant of the Water Resources Research Center Columbia, MO., under contract number :4- 046MO.

REFERENCES

Bhattacharya, I’. K., and Patra, H. P., 1968, Direct current geoelectrical soundings, in Methods in geo- chemistry and geophysics: Elsevier Publishing Co.

Deppermann, K., 1954, Die Abhangigkeit des schein- baren Widerstandes vom Sondenabstand bei der Vierpunkt-Methode: Geophys. Prosp. v. 2, no. 4, p. 262-273.

--- 1970, Unpublished set of three-layer master curves.

Ebert, _4., 1943, Grundlagen zur i2uswertung geo- elecktrischer Tiefenmessungen: Beitrlge zur Ang. Geoph., v. 10, p. l-17.

Fuller, D. L., and Knight, R. I)., 1967, Ground water in mineral and water resources of Missouri: Missouri Geoi. Surv. and Water Resources, 2nd Ser., v. 43, p. 281-313.

Fuller, D. I,., and Russell, u’. B., 1956, Water possibili- ties from the glacial drift of Grundy County: Mis- souri Geol. Surv. and water Resources, Water Re- sources Rep. I.

Homilius, J. 1961, Uher die Auswertung geoelectrischer Sondierungen im Palle eines vielfach geschichteten Untergrundes: 2. Geophysik, v. 27, p. 2822300.

352 Frohlich

Hummel, J. N., 1929, Der scheinbare spezifische for layered earth models: Geophysics, v. 31, p. 192- Widerstand: 2. Geophysik, v. 3-4, p. 80. 203.

Keller, G. V:, and Frischknecht, E‘. C., 1970, Electrical Orellana, E., and Mooney, H. M., 1966, Master tables methods m geophysical prospecting: New York, Pergamon Press.

and curves for vertical electrical sounding over layered structures: Madrid, Interciencia.

Kunetz, G., 1966, Principles of direct current re- Pylaev,, A. M., 1948, Rykovodstvo po Interpretatsi sistivity prospecting: Geoexpl. Monogr., Ser. 1, no. Vertlkal’nykh Electricheskikh Zondirovanii: Moskva, 1, Gebrueder Borntraeger, Berlin-Nikolassee. Gosgeolizdat, 168 p.

McGinnis, Lyle D., Kempton, John P., and Heigold, Wyllie, M. R. J., and Gregory, A. R., 1953, Formation Paul C., 1963! Relationship of gravity anomalies to a factors of unconsolidated porous media: Influence of drift-liiled ‘bedrock valley system in Nbrthern nlinois: Ill. State Geol., Surv., Urbana, Circ. 354.

particle shape and effect of cementation: J. Petr. Tech., v. 198, p. 103.

Meidav, Tsvi, 1960, An electrical resistivity survey for Zohdy, A. A. R., 196.5, The auxiliary point method of ground water: Geophysics, v. 2.5, no. 5,~. 1077-1093. electrical sounding interpretation, and its relationship

Mooney, H. M., Orellana, E., Pickett, H., and Torn- LO the Dar Zarrouk parameters: Geophysics, v. 30,p. heim, I,., 1966, A resistivity computing procedure 644-660.

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