NHP 

pp 

809-A 

Recognition of Natural Brine by 

Electrical Soundings Near the 

Salt Fork of the Brazos River, 

Kent and Stonewall Counties, Texas 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 809-A 
NHP PP 809-A 

Zohdy, Adel A.R. and Jackson 

Recognition of Natural Brine 

by Electrical Soundings Near 
Recognition of Natural Brine by 

Electrical Soundings Near the 

Salt Fork of the Brazos River, 

Kent and Stonewall Counties, Texas 

By ADELA. R. ZOHDY and DALLAS B. JACKSON 

ORIGIN AND MANAGEMENT OF SALT SPRINGS AND SEEPS IN THE 

UPPER BRAZOS RIVER BASIN, TEXAS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 809-A 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1973 
UNITED STATES DEPARTMENT OF THE INT~RIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600095 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402- Price 40 cents domestic postpaid or 30 cents GPO Bookstore 

Stock Number 2401-02373 
CONTENTS 

Page 

Abstract ···········································------····-··-·------·-······--·-··-·--····-·· A1 

Introduction ----------------·····-------------------------····------------·-···-·-----······ 1 

Field procedures and methods of interpretation ··---·--·--------· 2 

Soundings 1 and 2, near Jayton··----------------·-···--·-··---··------------ 2 

Soundings 3-9, near Girard ---------·············································· 3 

Page 

Soundings 10-a and 10-b, near Swenson ................................ A5 

Soundings 11-15, near Aspermont and Old Glory ................ 5 

Synthetic VES curve .................................................................. 9 

Summary and conclusions ........................................................ 12 

References cited .......................................................................... 13 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing the location of vertical electrical sounding stations .......................................................................... A2 

2. Diagram of electrode configurations (plan view) .................................................................................................................... 3 

3. VES 1 curve and interpretation .................................................................................................................................................. 4 

4. VES 2 curve and interpretation ................................................................................................................................................... 5 

5. VES curves 3-6, 8, 9 and DES 7, near Girard.......................................................................................................................... 6 

6. Geoelectric section, near Girard ·····································-··········································································································· 7 

7. VES curves 10-a and 10-b, obtained over outcrops of gypsum beds, near Swenson........................................................ 7 

8. VES curves 11 and 15, obtained at about 7 miles west of Aspermont.............................................................................. 8 

9. VES curves 12, 13, and 14. obtained west, north, and east of Aspermont, respectively .................................................. 9 

10. Digitized laterolog data from Stonewall County ...................................................................................................................... 10 

11. Dar Zarrouk curves........................................................................................................................................................................ 11 

12. Synthetic VES curve calculated for layering obtained from numerically smoothed Dar Zarrouk curve ...................... 12 

13. Synthetic VES curve calculated for layering obtained from graphically smoothed Dar Zarrouk curve plus a 

20-foot layer of 75 ohm-meters inserted at a depth of 180 feet .................................................................................... 13 

III 

ORIGIN AND MANAGEMENT OF SALT SPRINGS AND SEEPS IN THE 

UPPER BRAZOS RIVER BASIN, TEXAS 

RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS 

NEAR THE SALT FORK OF THE BRAZOS RIVER, 

KENT AND STONEWALL COUNTIES, TEXAS 

By ADELA. R. ZoHDY and DALLAS B. JACKSON 

ABSTRACT 

Fifteen electrical soundings were made near the Salt Fork 

of the Brazos River using the Schlum.berger and equatorial 

electrode arrays to evaluate the usefulness of the direct-current 

resistivity method in determining the depth of a fresh-watersalt-water interface in a geologic setting containing gypsum and 

anhydrite layers. The sounding data were interpreted by curve 

matching and computer modeling, and indicate that the resistivity method is useful where the interface lies above the gypsum series and also where the effective relative conductance of 

the brine-saturated layers below the gysum layers is large. This 

type of successful application was obtained from soundings 

made near the town of Jayton and also in an area approximately 

7 miles west of Aspermont. 

Near the town of Girard, the fresh-water-salt-water interface 

was difficult to detect because of the presence of high-resistivity 

anhydrite beds that lie directly beneath the interface. In the 

immediate vicinity of Aspermont and north of Old Glory, preliminary interpretation of the sounding data indicates the 

presence of brackish- to fresh-water-saturated sedimentary 

rocks. No direct evidence is observed on the sounding curves 

of the presence of very low resistivity brine-saturated rocks 

nor of high-resistivity anhydrite beds. Geologic and laterolog 

data obtained from a deep well in the area north of Aspermont 

indicate that the section is composed primarily of alternating 

beds of shale and dolomite of very low and very high resistivities, respectively, which mask the presence of the brine. The 

calculation of synthetic electrical sounding curves from the 

laterolog data was made with the help of smoothed Dar Zarrouk 

curves. Analyses of the synthetic sounding curves indicate that 

the logarithmic average resistivity of the shale-dolomite 

sequence may attain values characteristic of sedimentary rocks 

saturated with fresh to brackish water rather than with brine. 

The analyses indicate also that the electric section in this area 

has an anisotropy of about 2.1-2.7. 

INTRODUCTION 

The Brazos River traverses the eastern part of the 

Permian Basin and is potentially one of the principal 

sources of water in Texas. However, it is highly contaminated with brine and carries an average daily load 

of about 1,650 tons of sodium chloride into the Possum 

Kingdom Reservoir, about 110 miles west of Dallas, 

Texas. More than 85 percent of this salt is contributed 

by the Salt Fork of the Brazos River, where it originates 

in brine springs and seeps. An understanding of the 

origin and subsurface hydrology of the natural brine is 

fundamental in applying engineering measures to control the flow of salt water into streams. The geologic 

and hydrologic framework is briefly outlined in a report 

by Stevens and Hardt (1965). Other chapters in this 

professional paper on the origin and management of 

salt springs and seeps in the upper Brazos River basin 

provide information on the analysis of geophysical logs 

of boreholes, the hydrogeology and geochemistry of 

fresh water and brine, and the problems of management 

and control of salt springs and seeps. 

In May 1966, the U.S. Geological Survey made 15 

VES (vertical electrical soundings) to determine the 

usefulness of the direct-current resistivity method in 

evaluating the depth to a fresh-water-salt-water interface near the Salt Fork of the Brazos River where the 

geologic setting is characterized by the presence of 

both clastic and evaporite rocks. Figure 1 shows the 

location of the VES stations in the vicinity of Aspermont, Girard, Jayton, Old Glory, and Swenson, Tex. 

Stevens and Hardt (1965) report that, in this area 

the total dissolved -solids content of deep saline ground 

water ranges from about 20,000 to 250,000 ppm (parts 

per million), whereas overlying ground water contains 

only about 2,000-5,000 ppm dissolved solids. This significant change in the saJinity of the formation fluids 

results in an excellent contrast in the electrical properties of the sedimentary rocks saturated with these 

fluids. Therefore the VES method should be highly 

effective in delineating the fresh-water-salt-water interface (Zohdy and others, 1969). However, the presence of 

numerous high-rasistivity gypsum and anhydrite beds 

Al 
A2 ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

T-26 

1 ~ .,5 

EXPLANATION 

.3 

Electrical sounding location and number 

0T-11 

U.S. Geological Survey test hole and number 

6 

Approximate location of deep oil well 

FIGURE I.-Location of vertical electrical sounding stations. 

in the geologic section can limit the effectiveness of the 

method in detecting the fresh-water-salt-water interface sought. The results of the electrical soundings 

reported here illustrate the geologic conditions required 

for the successful delineation of the fresh-watersalt-water interface in the presence of gypsum and 

anhydrite deposits. 

FIELD PROCEDURES AND METHODS 

OF INTERPRETATION 

Fourteen vertical electrical soundings were made 

using the quadrupole Schlumberger array, and one 

sounding was made using the bipole-dipole equatorial 

array (Bhattacharya and Patra, 1968; Berdichevskii 

and Petrovskii, 1956; Zohdy and Jackson, 1969; Zohdy, 

1969b). Figure 2 shows the Schlumberger and the 

bipole-dipole equatorial arrays. 

Interpretation of the sounding curves was made by 

curve matching techniques using theoretically calculated sets of master curves (Compagnie Generale de 

Geophysique, 1963; Orellana and Mooney, 1966) that 

were used in conjunction with the auxiliary point diagrams and other related graphical methods (Bhattacharya and Patra, 1968; Dakhnov, 1953; Kalenov, 1957; 

Keller and Frischknecht, 1966; Kunetz, 1966, Zohdy, 

1965; Zohdy, 1968). In addition, a few of the graphically obtained earth models were checked by computing 

the multilayer sounding curves on an IBM 360/65 digital computer using a modified version of Mooney's program (Mooney and others, 1966). 

SOUNDINGS 1 AND 2, NEAR JAYTON 

The first two electrical soundings, VES 1 and VES 2, 

were made near the town of Jayton where the depth to 

the fresh-water-salt-water interface is about 200 feet 

below land surface. Figure 3 shows the curve of VES 1 

which was made near test well T -12. The depth to the 

interface at the well was reported by the driller to be 

about 60 f~et below land surface. Qualitative examination of the VES 1 curve reveals a four-layer geoelectric 

section of the QH-type with p1 >p~ >p:l <p-t (Kalenov, 

1957) and shows the detection of a layer with the 
RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK A3 

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FIGURE 2.-Electrode configurations (plan view). Arrays: a, 

Schlumberger, b, bipole-dipole equatorial. A, B, current electrodes; M, N, potential electrodes; R, distance from center 

of source bipole AB to center of measuring dipole MN; R, 

distance from current electrode to center of measuring dipole 

MN or effective electrode spacing of equatorial array. 

exceptionally low resistivity of less than 2 ohm-meters 

(brine-saturated sedimentary rocks) at shallow depth, 

as evidenced by the steep descent of the left branch of 

the curve. Quantitative interpretation of the curve indicated that a depth of 30 feet to the interface is most 

reasonable and that the maximum depth to the freshwater-salt-water interface is about 35 feet instead of 

the reported value of 60 feet. The depth to the interface at T-12 was reevaluated hydrogeologically at a 

later date (P. R. Stevens, written commun., 1969). 

Both geophysical logs and "thief" samples of water 

from the well indicate that the interface is at 30 feet 

which is in excellent agreement with the interpretation 

of VES 1 curve. The calculation of a four-layer VES 

curve on the computer for the earth model shown in 

figure 3 matches the observed VES 1 curve satisfactorily. 

The curve of VES 2 is shown in figure 4. It represents 

at least a seven-layer geoelectric section of the 

KHKHK-type (Kalenov, 1957; Keller and Frischknecht, 1966) in which the fresh-water-salt-water 

interface is represented by the boundary between the 

fourth and fifth layers. Preliminary curve-matching 

interpretation indicated that the depth to the interface 

is in the range of 200-225 feet. This estimate correlates 

very favorably with the estimated depth of 205 feet 

according to geologic data (W. F. Hardt, oral commun., 

1966). A depth of 200 feet is obtained from the VES 

curve by assuming the resistivity of the fourth layer to 

be 100 ohm-meters. Therefore, in order to make the 

interpreted depth agree precisely with the depth 

obtained from the geologic information, it is sufficient 

to change the assumed resistivity of the fourth layer 

from 100 ohm-meters to 112 ohm-meters. This adjustment in the resistivity of the fourth layer is calculated 

by using the principle of equivalence (Bhattacharya 

and Patra, 1968; Kalenov, 1957). A seven-layer VES 

curve was calculated on the computer and was found 

to match the observed curve of VES 2 as shown in 

figure4. 

The results obtained from VES 1 and 2 are very 

encouraging in terms of the usefulness of the VES 

method for mapping the depth to the fresh-watersalt-water interface in the vicinity of Jayton. The success of the method here is attributed to the large value 

of the effective relative conductance (ratio of effective 

relative thickness (Flathe, 1963) to resistivity) of the 

brine-saturated layers. Furthermore, the resistivity 

values at VES 1 and 2 indicate that the interface must 

lie above any highly resistive gypsum or anhydrite beds. 

SOUNDINGS 3-9, NEAR GIRARD 

Seven vertical electrical soundings were made along 

an east-west profile 6 miles long across Duck Creek, 

northwest of the town of Girard. Five soundings were 

made west of Duck Creek and two east of it. The freshwater-salt-water interface along this profile is known to 

be about 500 feet below land surface (Stevens and 

Hardt, 1965). The interface occurs within a gypsum 

series, and it is underlain by several beds of high-resistivity anhydrite. 

The VES curves shown in figure 5, indicate that the 

interface may be detected west of Duck Creek but it 

is very difficult to detect east of the creek. The geoelectric section shown in figure 6 is based on the interpretation of curves 3 through 9. The true resistivity of the 

gypsum beds above the interface is interpreted to be 

about 560 ohm-meters. This resistivity value is based 

on the interpretation of VES 4 in accordance with the 

depth to the interface in test hole T-10. The brinesaturated material below the interface seems to have a 

resistivity of about 40 ohm-meters according to the 

interpretation of the equatorial sounding DES 7. For 

a brine-saturated material, a resistivity of 40 ohmmeters is very high indeed, but this is attributable to 

the low porosity of the rocks containing the brine and 

(or) to the presence of anhydrite beds of very high 

resistivity in association with brine-saturated layers of 

very low resistivity (the two forming an anisotropic 

medium of average resistivity of about 40 ohm-meters). 

In fact the anhydrite beds are probably more resistive 

east of Duck Creek where they mask the effect of the 

brine almost completely on VES curves 8 and 9. Similar 

highly resistive materials saturated with salt water 

were found on the island of Oahu, Hawaii, where basal-
A4 ORIGIN AND M/ NAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

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SCHLUMBERGER ELECTRODE SPACING ( A

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FIGURE 3.-VES 1 curve and interpretation. Theoretical curve calculated by computer for given earth model. Numbers in bar 

designate interpreted true resistivity, in ohm-meters. Numbers below bar designate depth, in feet, below land surface. 

tic lava saturated with sea water has a resistivity of 

about 30 ohm-meters (Zohdy and Jackson, 1969). 

The position of the fresh-water-salt-water interface 

in the Girard area is difficult to determine by the VES 

method because of the presence of gypsum beds of high 

resistivity above the interface and, more important, 

because of the presence of anhydrite beds of very high 

resistivity below it. However, the results of the VES 

interpretations shown in figure 6 indjcate that other 

valuable information is furnished by the electrical 

sounding data. The Salado Formation (ofLatePermian 

age) in test well T-10 is easily identified on the VES 

curves 3, 4, 5, 6, and 8 by the formation of a well-defined 

minimum on these curves. The true resistivity of the 

Salado Formation is interpreted from VES 4 to be 

about 8 ohm-meters in accordance with the principle of 

equivalence and the well data of T-10 about its thickness. The rising branch to the right of the minimum on 

VES curve 4 indicates the detection of high-resistivity 

gypsum beds near the top of the red beds of the Tansill 

Formation (of the Artesia Group) and the other underlying formations (Yates, Seven Rivers, and Queen). 

To the west of VES 4, the interpretation of VES 

curve 3 indicates that the geoelectric section is essentially the same as at VES 4 and at test well T-10. However, to the east of VES 4, the data of VES curves 5, 6, 

8, and 9 indicate thflt the depth to the top of high 

resistivity gypsum beds, within the red beds, increases 

appreciably. A moderately low resistivity of 30 to 40 

ohm-meters characterizes the geoelectric section to a 

depth of as much as 380 feet beneath VES 6 even 

though Permian red beds were penetrated at much shallower depths in some shallow wells, such as well 801, 

near Duck Creek. This means that if the stratigraphic 

sequence is the same as that in well T-10, the Artesia 

group beneath VES 6, 7, 8, and 9, has a much lower 

resistivity than beneath VES 3, 4, and 5 ( 30-40 ohmmeters as opposed to 560 ohm-meters). An explanation 
RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK A5 

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}'IGURE 4.-VES 2 curve and interpretation. Theoretical curve calculated by computer for given earth model. Numbers in bar 

designate interpreted true resistivity, in ohm-meters. Numbers below bar designate depth, in feet, below land surface. 

for this low resistivity is that solution openings in the 

gypsum beds beneath and around Duck Creek must 

have lowered their resistivity. If this is true, the resistivity method could be used effectively to map the zones 

around Duck Creek where solution openings exist in 

the gypsum beds and differentiate these zones from 

others where competent gypsum layers exist. 

SOUNDINGS 10-a and 10-b, NEAR SWENSON 

Two soundings, VES 10-a and 10-b, were made 

approximately 2 miles south of the town of Swenson. 

These soundings were made on gypsum outcrops to 

evaluate the true resistivity of gypsum beds in that 

area. The VES curves 10-a and 10-b are shown in 

figure 7. Interpretation of the three-layer K-type curve 

(Kalenov, 1957; Keller and Frischknecht, 1966) of 

VES 10-a, indicates that the true resistivity of the 

gypsum bed is at least 350 ohm-meters and might be 

1.000 ohm-meters or more. However, the curve of VES 

10-b, which was obtained on an adjacent outcrop having less than one-half foot of soil cover, indicates that 

the true resistivity of that gypsum bed might be as low 

as 120 ohm-meters. These values show that individual 

gypsum layers have a large range of resistivities, and 

that the resistivity depends on the amount of fracture 

and solution porosity and the resistivity of the material 

filling the fractures. 

SOUNDINGS 11-15, NEAR ASPERMONT 

AND OLD GLORY 

VES 11 and 15 were made near test hole T-26, about 

7 miles due west of Aspermont. Figure 8 shows the 

curves of these two soundings. The presence of a series 

of high-resistivity gypsum beds is clearly indicated by 

the prominent maximums on the curves. The sharp 

decrease in apparent resistivity following the maximums, however, is diagnostic of brine-saturated sedimentary rocks. Preliminary quantitative interpretation 
A6 ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

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FIGURE 5.-VES curves 3-6, 8, 9 and DES 7, near Girard. VES 3, 4, 5, 6, and DES 7, west 

of Duck Creek; VES 8 and 9, east of Duck Creek, DES 7, dipole equatorial sounding, 

expanded to the west. 
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RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK A7 

EXPLANATION 

'3 

Electrical sounding and number 

~0 

Test hole and number 

Boundary delineating rocks 

of similar resistivity 

Dashed where approximately located 

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DATUM IS MEAN SEA LEVEL 

VERTICAL EXAGGERATION X 10 

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FIGURE 6.--Geoelectric section, near Girard. Numbers in layers designate interpreted true resistivities, in ohm-meters. 

of VES 11 curve yields a depth of about 120 feet to the 

fresh-water-salt-water interface. This depth is in good 

agreement with the depth of 140 feet at T-26. Calcula-

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tions using the principle of equivalence indicate that if 

the resistivity of the gypsum beds is assumed to be 370 

ohm-meters, instead of 480 ohm-meters, which was 

assumed on the preliminary curve matching, then the 

depth to the interface from the VES 11 data would 

agree perfectly with the geologic data from T -26. Interpretation of VES 15 curve using the same resistivity 

value of 370 ohm-meters for the gypsum beds yields 

the same depth of 140 feet to the interface at VES 15. 

The brine-saturated sedimentary rocks beneath VES 

11 and 15 have a resistivity of about 1-3 ohm-meters,. 

which is similar to the values obtained near Jayton. 

The geoelectric section in the vicinity of VES 11 and 

15 is composed of alternating beds of high and low resistivities. The main gypsum series, however, as well as 

the underlying brine-saturated sedimentary rocks, can 

be easily identified on the VES curves. The ease with 

which the brine-saturated layers are identified on the 

curves is attributed to their favorable effective relative 

conductance. 

Ves 12, 13, and 14 were made west, north, and east 

of Aspermont. Figure 9 shows the curves of these 

10 100 AB 1000 soundings and their interpretation. The first electric 

SCHLUMBERGER ELECTRODE SPACING (-2-). IN FEET layer on all three curves has a thickness of about 20 

FIG""""' .- curves an , o over ou rops TTD,. 7 VES 10-a d 10-b btained tc 1· feet and a low resistivity ranging from about 4.4 ohmof gypsum beds, near Swenson. meters on VES 13, to about 9 ohm-meters on VES 14. 
-AS ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXASen 

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SCHLUMBERGER ELECTRODE SPACING (A2

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FIGURE 8.-VES curves 11 and 15, obtained at about 7 miles west of Aspermont. Dashed curve designates curve corrected for 

wrong jumps caused by a lateral heterogeneity near potential electrodes. 

The second layer on VES 12 and 13 is about 70 feet 

thick; it has a moderate to high resistivity of about 

60 ohm-meters on VES 12, and about 175 ohm-meters 

onVES 13. 

According to P. R. Stevens (written commun., 1967) 

the first two layers on VES 12 and 13 probably represent the Blaine Formation which is composed of interbedded gypsum, shale, and dolomite. The first 20 feet 

however, probably are composed entirely of shale, silt, 

and some clay of 4.4--6.2 ohm-meters, the rise in resistivity to values of 60 and 175 ohm-meters indicates 

intercalated gypsum and dolomite. 

Beneath VES 14, the top 20 feet probably are strata 

of the Flowerpot Shale which are quite similar in their 

electrical properties (9 ohm-meters) to the shaly part 

of the Blaine Formation (4.4-6.2 ohm-meters). The 

second layer beneath VES 14 is much thicker (300 

feet) and has lower resistivity (35 ohm-meters) than 

the second layers beneath VES 12 and 13. The moderate resistivity of the second layer beneath VES 14 indicates that gypsum and dolomite beds probably are 

absent and that the section is composed primarily of 

sand and silt. 

Test well T -30, approximately 2 miles southwest of 

VES 14 and about 2.5 miles northwest of the town of 

Old Glory, starts in the Flowerpot Shale, which overlies the San Angelo Sandstone gradationally at a depth 

of approximately 68 feet; at a depth of about 158 feet 

the top of the Clear Fork Group is penetrated. The 

Clear Fork Group is composed of the Choza, the Vale, 

and the Arroyo Formations, which are composed mainly 

of shale, sandstone, and dolomite. The second layer in 

VES 14, therefore, correlates with the San Angelo Sandstone, which probably is thicker at VES 14 than at 

T-30. 

Thus the interpretation of the first two layers of 

VES 12, 13 and 14 is in generally good agreement with 

the geologic information. However, on all three sounding curves there is basically no evidence that would 

indicate the presence of brine of very low resistivity. 

The third layer on VES 12 is interpreted to have a 

resistivity of 13 ohm-meters; on VES 13, 15 ohm-
RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK A9 

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10 100 1000 10,000 

SCHLUMBERGER ELECTRODE ~PACING ( 11!), IN FEET 

FIGURE 9.-VES curves 12, 13, and 14, obtained west, north, and east of Aspermont, respectively. Numbers in bars designate 

interpreted true resistivities, in ohm-meters. Numbers below bars designate depth, in feet, below land surface. 

meters; and on VES 14, 22 ohm-meters. These resistivities are not characteristic of brine-saturated sedimentary rocks; nevertheless, water-sample and drilling 

data indicate that brine exists in the Clear Fork Group. 

This apparent paradox (which also applies for the data 

obtained from DES 7 west of Girard), was clarified by 

analyzing laterolog resistivity data from a deep well 

north of Aspermont, and by calculating and interpreting synthetic VES curves. The analysis indicated that 

dolomites of high resistivity combine with shales and 

brine of low resistivity to form an anisotropic layer with 

an average logarithmic resistivity (square root of product of resistivities measured in the vertical and horizontal directions) that is not characteristic of brinesaturated sedimentary rocks. 

SYNTHETIC VES CURVES 

Geophysical-well-logging data are available from several wells in the Aspermont area. However, resistivity 

data (rather than thickness data only), that could be 

utilized in evaluating the interpretation of the VES 

curves 12, 13, and 14 are not available except from a 

focused electric log of a drilled well 4, 700 feet deep in 

section 183 of Stonewall County. The electric log of this 

well is a Schlumberger laterolog LL-7. The measured 

apparent resistivity was digitized at a depth interval of 

10 feet to form a geoelectric section of 74 layers of different resistivities. An electric basement having infinite 
A10 ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

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DEPTH, IN FEET BELOW LAND SURFACE 

FIGURE 10.-Digitized laterolog data from Stonewall County, presented on log-log scale. 

resistivity was assumed to exist at a depth of 4, 700 feet 

and was added to the section as the 75th layer. Figure 

10 shows a plot of the distribution of resistivity with 

depth on a log-log scale. 

The calculation of the sounding curve that would 

have been obtained for this geoelectric section could 

not be made on the computer because of the large resistivity contrasts and large number of layers that are 

involved in this electric section. A method was devised 

by the senior author to overcome this difficulty. The 

basic idea of the method is summarized as follows. The 

Dar Zarrouk curve (Maillet, 1947; Orellana, 1963; 

Zohdy, 1965) for any finite number of layers can be 

calculated easily, although the calculation of the corresponding sounding curve may not be feasible. Consequently, whenever this situation arises, one can 

calculate the Dar Zarrouk curve, smooth it (either 

graphically or by a numerical filter), then calculate the 

resistivities and thicknesses of a new set of layers from 

the smoothed Dar Zarrouk curve. This new set will consist of a fewer number of layers but will be electrically 

equivalent to the original set of layers. A sounding 

curve can be calculated for this new set of layers, unless 

further smoothing of the Dar Zarrouk curve is necessary. 

Figure 11 shows the Dar Zarrouk curve calculated 

for the 75 layers of the digitized laterolog and two 

smoothed Dar Zarrouk curves that were calculated by 

graphical smoothing and by a numerical filter. The 

number of layers above the assumed basement is 

reduced from 7 4 to 16. The layering calculated from 

the numerically smoothed Dar Zarrouk curve and the 

corresponding synthetic sounding curve are shown in 

figure 12. The total thickness of the layers is increased 

from the original4,700 feet to 7,300 feet, owing to the 

electrical anisotropy that results from combining certain layers into equivalent layers. 

The next step in this analysis is to interpret the syn-
RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK All 

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10 100 1000 10,000 100,000 

PSEUDODEPTH ( ;\Z), IN FEET BELOW LAND SURFACE 

FIGURE 11.-Dar Zarrouk curves. a, Dar Zarrouk curve for 75layers obtained from digitized laterolog. b, equivalent Dar Zarrouk 

curve for 17 layers obtained by graphical smoothing of curve a. c, equivalent Dar Zarrouk curve for 17 layers obtained by 

numerical filtering of curve a. 

thetic sounding curve as if it represents a field curve 

and as if the layering it represents is unknown. The 

results of the graphical interpretation of the synthetic 

sounding curve are shown in figure 12 and indicate that 

the synthetic curve can be interpreted readily in terms 

of six layers, of which the third layer has a resistivity of 

5 ohm-meters and the fourth layer a resistivity of 8.6 

ohm-meters. The 5-ohm-meter material (which is 

hardly detectable on the sounding curve) could be 

interpreted as containing brine but the 8.6-ohm-meter 

material (indicated on the sounding curve by the long 

horizontal segment between AB/2 == 1,500 feet and 

5,000 feet) would be interpreted generally as sand containing brackish water and (or) as clay but not necessarily as sedimentary rocks saturated with brine. 

Furthermore, the total depth to the assumed highresistivity basement would be about 12,800 feet instead 

of 4, 700 feet (which is the depth assumed in the original 

model) . Therefore the total anisotropy, A., of the section 

is equal to 

12,800 

A. 4,700 2.72, 

which is a very significant anisotropy. 

Several equivalent layering models were calculated 

by using graphical smoothing and by applying different 

numerical filters to the original Dar Zarrouk curve of 

the digitized laterolog. The calculation of the corresponding synthetic VES curves for these models and 

their subsequent graphical interpretation by master 

VES curves indicated that the interpreted third layer 

of low resistivity material of about 5 ohm-meters (interpretable as brine) could be obscured completed by the 

presence of a relatively thin layer (20 feet) of moderate resistivity (75 ohm-meters) at a depth between 

100 and 200 feet. 

Figure 13 shows the layering obtained from the 

graphical smoothing of the original Dar Zarrouk curve 

plus a hypothetical 20-foot layer of 75 ohm-meters 

inserted at a depth of 180 feet. The synthetic VES 

curve for this section is interpretable in terms of a 5-

layer section in which there are no layers having resistivities less than 8 ohm-meters below a depth of 20 feet. 

The foregoing analysis indicates that the detection 

of the fresh-water-salt-water interface on aVES curve 

obtained near this well in Stonewall County can be 

difficult. However, the analysis sheds a new light on the 

interpretation of VES 12, 13, and 14 where the thirdlayer resistivities of 13, 15, and 22 ohm-meters, respectively, probably represent the logarithmic-average 

resistivity of the anisotropic section in which brine is 

present but is masked by the effect of highly resistive 

dolomite layers. 
A12 ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

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SCHLUMBERGER ELECTRODE SPACING EAf). OR DEPTH BELOW LAND SURFACE, IN FEET 

FIGURE 12.-Synthetic VES curve calculated for layering obtained from numerically smoothed Dar Zarrouk curve, a, synthetic 

VES curve. b, layering calculated from numerically smoothed Dar Zarrouk curve, c. layering from graphical interpretation 

of synthetic VES curve. Numbers in bar designates interpreted true resistivity in ohm-meters. 

SUMMARY AND CONCLUSIONS 

Electrical soundings VES 1 and 2, made near the town 

of Jayton, and VES 11 and 15, made about 7 miles west 

of Aspermont, indicate that the VES method can be 

used successfully to evaluate the depth to the freshwater-salt-water interface. In fact, the resistivity data 

of VES 1 revealed that the initial geologic evaluation 

of the depth to the interface, in test hole T-12, was in 

error and later geologic findings confirmed the depth 

predicted by the electrical sounding. From geologic control the interpreted resistivity values at VES 2, 11, and 

15 were adjusted slightly so that the calculated depth 

to the interface agreed with known geologic data. 

In the area near Girard, the application of electrical 

soundings showed that the conventional VES method 

cannot be used to determine consistently the depth to , 

the interface. The difficulty is attributed to the presence 

of highly resistive anhydrite beds which mask the effect 

of low-resistivity, brine-saturated rocks, and the probable anisotropy of the 40 ohm-meter material at the 

bottom of the section from VES 3 to VES 9. The masking effect was particularly evident on soundings made 

east of Duck Creek. The methods of differential soundings (Zohdy, 1969a) in conjunction with electromagnetic soundings might yield more diagnostic results in 

the Girard area. 

In the immediate vicinity of Aspermont the VES 

data cannot be used to evaluate the depth to the freshwater-salt-water interface because of the presence of 

dolomite beds of high resistivity which combine with 

the low-resistivity shale beds and brine to form an anisotropic layer of average logarithmic resistivity ranging 

from about 8.6 ohm-meters (for a synthetic sounding) 

to about 22 ohm-meters (for VES 14). These resistivity 

values are not characteristic of brine-saturated sedimentary rocks. The study of synthetic sounding curves 
RECOGNITION OF NATURAL BRINE BY ELECTRICAL SOUNDINGS NEAR SALT FORK Al3 

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SCHLUMBERGER ELECTRODE SPACING (A2

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FIGURE 13.-Synthetic VES curve calculated for layering obtained from graphically smoothed Dar Zarrouk curve plus a 20-foot 

layer of 75 ohm-meters inserted at a depth of 180 feet. a, synthetic VES curve. b, geoelectric layering. c, layering from graphical interpretation of VES curve. Numbers in bar designate interpreted true resistivities in ohm-meters. 

based on electric logs helps to explain the factors which 

complicate the interpretation of YES data to determine 

the depth of the interface in the Aspermont area. 

REFERENCES CITED 

Berdichevskii, M. N., and Petrovskii, A. D., 1956, Methods of 

bilateral equatorial sounding: Prikladnaya Geofizika, v. 

14, p. 97-114 [English translation by Ivan Mitten, U.S. 

Geol. Survey Library, Denver, Colo., 37 p.]. 

Bhattacharya, P. K., and Patra, H. P., 1968, Direct current geoelectric sounding, princples and interpretations: New York, 

Elsevier, 135 p. 

Compagnie Glmerale de Geophysique, 1963, Master curves for 

electrical sounding: The Hague, European Assoc. of 

Exploration Geophysicists. 

Dakhnov, F. N., 1953, Electrical prospecting for petroleum and 

gas deposits (in Russian) : Moscow, Gostoptekhizdat, 497 p. 

Flathe, H., 1963, Five-layer master curves for hydrogeological 

interpretation of geoelectrical resistivity measurements 

above a two-story aquifer: Geophys. Prosp., v. 11, p. 

471-508. 

Kalenov, E. N., 1957, Interpretation of vertical electrical sounding curves (in Russian): Moscow, Gostoptekhizdat, 471 p. 

Keller, G. V., and Frischknecht, F. C., 1966, Electrical methods 

in geophysical prospecting: New York, Pergamon Press, 

517 p. 

Kunetz, Geza, 1966, Principles of direct current resistivity 

prospecting: Berlin,Gebriider Bortraeger, 101 p. 

Maillet, R., 1947, The fundamental equations of electrical prospecting: Geophysics, v. 3, p. 529-556. 

Mooney, H. M., Orellana, E., Pickett, H., and Tornheim, L., 

1966, A resistivity computation method for layered earth 

models: Geophysics, v. 31, p 192-203. 

Orellana, Ernesto, 1963, Properties and drawing of the so-called 

Dar Zarrouk curves: Geophysics, v. 28, no. 1, p. 99-110. 

Orellana, Ernesto, and Mooney, H. M., 1966, Master tables and· 

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

Stevens, P. R., and Hardt, W. F., 1965, Preliminary report on 

the investigation of salt springs and seeps in a portion of 

the Permian basin in Texas: U.S. Geol. Survey open-file 

report, 19 p. 
Al4 ORIGIN AND MANAGEMENT, SALT SPRING AND SEEPS, UPPER BRAZOS RIVER BASIN, TEXAS 

Zohdy, A. A. R., 1965, The auxiliary point method of electrical 

sounding interpretation, and its relationship to the Dar 

Zarrouk parameters: Geophysics, v. 30, no. 4, p. 644-660. 

---1968, A rapid graphical method for the interpretation of 

A- and H-type electrical soundings: Geophysics, v. 33, no. 

5, p. 822-833. 

---1969a, A new method for differential resistivity sounding: Geophysics, v. 34, no. 6, p. 924-943. 

---1969b, The use of Schlumberger and equatorial soundings in groundwater investigations near El Paso, Texas: 

Geophysics, v. 34, no. 5, p. 713-728. 

Zohdy, A. A. R., and Jackson, D. B., 1969, Application of deep 

electrical soundings for groundwater exploration in 

Hawaii: Geophysics, v. 34, no. 4, p. 584-600. 

Zohdy, A. A. R., Jackson, D. B., Mattick, R. E., and Peterson, 

D. L., 1969, Geophysical surveys for ground water at White 

Sands Missile Range, New Mexico: U.S. Geol. Survey 

open-file rept., 31 p., 44 p. graphs, 8 p. tables, 17 figs. 

*u.s. GOVERNMENT PRINTING OFFICE: 1973 o-!51!5-6!59/76