Download - Geophysical determination of buried geological structures and their influence on aquifer characteristics

Geoexploration, 27 ( 1991 ) 1-24 Elsevier Science Publishers B.V., Amsterdam

Geophysical determination of buried geological structures and their influence on aquifer

characteristics

Rom,~n Alvarez lnstituto di Geografia, Universidad Nacional Aut6noma de M~xico, Ap. 20-850, 01000, M~xico, D.F.,

Mexico

(Received June I l, 1990; accepted after revision July 25, 1990)

ABSTRACT

Aivarez, R., 1991. Geophysical determination of buried geological structures and their influence on aquifer characteristics. Geoexploration, 27: 1-24.

Many coastal plains in the semi-arid regions of Mexico become fertile lands when properly irri~ gated. In the last thirty years extensive drilling in several places has disturbed the natural equilibrium of the aquifers; this is partly due to poor knowledge of their distribution and properties, as well as lack of adequate exploitation strategies. This study constitutes a case history of the valley of Guaymas in northwestern Mexico, in which three sets of data are considered: (a) a set of 262 wells, (b) four telluric lines of approximate total length of 90 km, and a set of 326 randomly distributed gravimetric stations. The valley dimensions are 20 km by 50 km; two aquifers have been located, one above 160 m and the other below 320 m. Models have been computed for the four telluric lines and four gravimetric sections. They suggest that sediments on the south-central portion of the valley have a thickness of 800 m. The basement becomes shallow toward the north and south portions of the valley, reaching depths ranging from 200 to 300 m. The valley is flanked by two buried depressions oriented in NNE-SSW direction; these regions reach depths of over 1000 m in some places and apparently constitute reservoirs in which the surface recharge waters are maintained relatively free of contami- nation from hydrothermal fluids. Such fluids are extracted from shallow wells (under 200 m ) in some areas in which the basement approaches the surface. It is concluded that performing geophysical studies on the aquifer's location, in order to determine its regional geological characteristics, is a cost- effective procedure, that allows the establishment of timely extraction strategies.

INTRODUCTION

Establishing the macroscopic characteristics of an aquifer, such as its extension, volume, depth, and continuity is often not done prior to its exploitation. The practice in many developing countries usually consists of performing a few, if any, geophysical studies in the area, and proceed to drilling. Such a practice has been useful, and still may be, in places in which the water demand is not intensive. Typically, exploitation is initiated in a given portion

2 R. ALVAREZ

of the aquifer and spreads ~ubsequently, as required by demand, to adjacent areas~ In some large valleys, drilling is carried out extensively at intervals which are considered convenient in order to avoid interference between neighboring wells. However, as the well density is increased it becomes apparent that the aquifer characteristics are not uniform and that, in some regions, production varies radically from one well to the next; often there are no surface indications (e.g., outcropping formations) for explaining such drastic variations. It is only after extensive drilling, and a number of unsuccessful attempts, that the aquifer characteristics can be confidently outlined; by the time such data are available saline intrusions may have developed and production problems start to become unmanageable. In this context, regional geophysical studies may prove extremely valuable in an area, in order to provide early indications of possible constraints on and limitations of the aquifer. The cost of such studies is comparable to that of drilling a few wefts and is rapidly compen- sated by the higher rate of successful attempts.

The northwestern portion of Mexico, between parallels 26 ° and 32°N is a semi-arid area in which water is a scarce resource. In the coastal plains aquifers have been exploited, and some times overexploited, in the fashion de- scribed above in order to supply the water demands of the population as well as those of agriculture. The land is usually quite productive when sufficient water supply is available. Unfortunately, the growing demands for this liquid have finally disturbed the equilibrium in aquifers that discharge into the ocean, inducing a series of changes, which in some cases have become irreversible.

AQUIFERS IN THE VALLEY OF GUAYMAS

The valley of Guaymas is located at geographical coordinates 27 ° 55'N and lI0°50'W (Fig. 1, inset); it is oriented N-S (50 kin) and has a width of approximately 20 km. On the eastern side it is flanked by Sierra del Bacatete and on the western side by Sierra de Santa Ursula; these two formations ar of granitic composition. The valley is limited on the south by the Sea of Cortez in the Gulf of California. The only topographic irregularity in an otherwise gently sloping valley is a series of basaltic outcrops in its southeastern portion.

Figure I shows the well distribution in the valley of Guaymas; most of these wells are less than 250 m in depth. Two aquifers have been identified: the upper and the lower aquifers. The upper aquifer has been in exploitation for over 30 years providing water for irrigation purposes as well as for human consumption in Guaymas and Empalme cities. Generally speaking, the upper aquifer has a thiclmess of around 160 m, consisting of unconsolidated, alter- nating 1 vers of gra ~,l, sandstone, and clay.

The. ~ ~ uifer alr~,.~y shows a transgression of saline water in the coastal region, including lands a few kilometers north of the railroad track, parallel

GEOPHYSICAL DETERMINATION OF BURIED GEOLOGICAL STRUCTURES 3

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to the coastline; the lands involved are already unsuitable for agricultural use owing to their high salinity. This is not surprising in view of the rate of extraction ( 180X 106 m3/yr) versus the estimated rate of recharge ( 100× 106 m3/yr) of the upper aquifer from surface waters. The hypothesis has been put

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forward that the lower aquifer may be helping recharge the upper one; however, there is no direct evidence of such a mechanism operating in the valley.

Underlying the upper aquifer there is a stratum with lenses of gravel, sandstone, and abundant macrofossils in its upper portion, known as the "blue clay" stratum. Its thickness is 160 m near the coastline, decreasing to 30 m near the location of "Guadalupe Victoria" (Fig. 1 ). Under the blue clay the lower aquifer is located, which consists of a conglomerate of intercalated gravel, sandstone, and clay. Its thickness is around 180 m and it overlies ig- neous and metamorphic rocks. Figure 2 shows the geological cross-section of the valley in its N-S direction. Making this type of interpretation is only possible when many wells are available; notice, however, that even J n this case there are large areas in which information must be inferred since most of the wells are shallow.

In order to complement the well formation, several geophysical studies were carried out in the valley. The results from a telluric and a gravimetric survey are presented hereir, together with computational interpretations of such data.

TELLURICS

Measuring of natural electromagnetic fields and electric currents (telluric currents) has been performed over three decades in order to determine the characteristics of geological formations. Different versions of the telluric and the magnetotelluric methods have been developed in this period. The latter method differs from the former in that, in addition to the determination of the field E, determination of the magnetic field H is necessary.

From the fields E and H, at a given frequency, the apparent resistivity of the geologic media can be determined through:

Pa

where to= 2~f. f is the frequency of the electromagnetic wave, and p is the magnetic permeability, which for most geologic formations is ~ ~ , the vac- uum permeability.

Equation ( l ) yields the apparent resistivity when both fields, E and H, are simultaneously determined at one location; this constitutes a magnetotelluric determination of the resistivity. In the telluric method, the assumption is made that field H does not vary significantly within a few kilometers; in particular, if field H is measured simultaneously at two neighboring locations, then H~ =/-/2. This assumption is usually found to be valid, and allows for the determination of relative changes in apparent resistivity between a base station and any other stations. Assume that the apparent resistivity is measured at two locations l and 2. Then:

6 R. ALVAREZ

Pa2 (E2/H2) 2 (E2~ 2 p, -(E,/H,)2=\-~n) (2)

since the angular frequency to is the same for both measurements, the magnetic permeability p - ~ at both locations and, by the above assumption, Hi = /~ .

In the telluric method it is only necessary to determine field E in order to obtain the apparent resistivity variations. With this method one gains in ex- ecution time of the survey (i.e., field H is not determined), at the cost of not obtaining the absolute values ofpa but only those relative to the base station, whose apparent resistivity is arbitrarily set to a value of 1.

The particular version of the telluric method used in this survey (Beyer, 1977) consists of the determination of field E in two contiguous intervals of 500 m each (Fig. 3) and at two frequency bands centered on 0.5 and 8 Hz. The measurement at the leading interval is denoted Ey while that of the lag- ging interval is denoted Ex. These signals are fed to an X-Y plotter, to the X, Yinputs, respectively, after being filtered and amplified. Thus, both fields are determined simultaneously cancelling out the irregular variations of the telluric currents. The ratio Ey/Ex shows in the X-Y plotter either as a straight line with a given inclination or, more often, as an ellipse whose major axis determines the value of the above ratio. At each station six to eight measurements are obtained at each frequency and the average inchnation of the ellipse is used to calculate the ratio E for such a station. As previously stated, the base station is assigned an arbitrary value of apparent resistivity of 1, and the other stations are referred to this value.

Since the skin depth of the electromagnetic wave is inversely proportional to the square root of frequency, one has two penetrations with this method. The shallow penetration corresponds to the 8 Hz signal while the deeper penetration corresponds to the band centered at 0.5 Hz. The results in Figs. 5, 7,

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9, and 11 show the telluric reponses in the valley of Guaymas; they will be further discussed.

TELLURIC MODELING

The faults (F) and structures (A, B,..., H) shown in Fig. 4 were qualita- tively inferred (Alvarez, 1984) as a preliminary interpretation of the field data. In the present study a teUuric modeling has been carded out for each one of' toe f o u r l ines. To c~lcu la te the m o d e l s a t h r e e - d i m e n s i o n a l . . n ~ , ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . £ J.~." ~ , ~ i L I ~,,o I I , I J "

te!!urif: algorithm was used (Ting and _.y_L . . . . I n8 I" u;~,,,~n-.ot.~, . . . . . . . n,,,,,,,~ .... , - ~ - - , .~,,--..~---,~. e t a l . ,

1984) to obtain the apparent resistivity values. The method requires the def- inition of right-rectangular prisms of given dimensions, depths of burial, and resistivities, from which secondary, or scattered, fields E and H are computed and the apparent resistivities at given locations are evaluated. In the present case stations were located along lines; thus 2D modeling would have been sufficient. However, at the crossings of lines L I-L2, L2-L3, and L2-L4 one can perform 3D modeling more adequately. The cross-sections of the models shown in Figs. 6, 8, 10, and 12 correspond to prisms whose dimensions perpendicular to the line were chosen symmetric with respect to the line, in order to avoid edge effects as much as possible. Determination of the apparent resistivities were obtained by means of geoelectric soundings of the Schlumber- ger type. The shallow layers show values of 30-40 l) .m, although in some places the resistivity reaches values of 5 u-m. The coastal area with the saline transgression corresponds to a low-resistivity area ranging from 5 to 15 fl-m. In the models the value of 800 fl-m was assigned to the electrical basement throughout the area, while a value of 40 l)-m was assigned to the aquifers. Values of 10 to 20 ft. m were required for some portions in order to reproduce the variations observed in the field survey. These low-resistivity values appear to be associated with the presence of the blue clay formation that divides the upper and lower aquifers in some places. However, it is difficult to make a definitive statement in this respect owing to the presence of thermal activity in the area, which may also contribute to the low-resistivity values through the injection of geothermal fluids. The model values are shown as apparent resistivities; the values shown were obtained through a trial and error calcu- lation of the depths of burial of the variour ~,v~sms in the models until satis- factory results were obtained, Several hours of computer time were required in a minicomputer in order to obtain the results shown. A more detailed dis- cussion of the model of each line is presented in the following sections.

TELLURIC LINES

Figure 4 shows the distribution of telluric stations in the valley of Guay- mas; nominal station separation is 500 m. Lines 1 and 4 are oriented approx-

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imately in the E-W direction; Line 3 runs NE-SW, and Line 2, the longest line, is oriented approximately NNE-SSW. A series of features had been qual- itatively inferred (Alvarez, 1984), such as faults (FI, F2) as well as several structural features (A, B, C), Numerical modeling of these results shall be discussed next.

Line I

Field data are shown in Fig. 5 for Line 1. Strong response variations are observed in the vicinity of station 12E, followed by a steady increase in the relative resistivity values. The lower frequency band of 0.5 Hz suggests that resistive bodies come close to the surface as the eastern flank of the valley is approached. The same tendency is observed for the 8 Hz frequency response between stations 13E and 32E; however, from station 32E to the end of the line the response variations suggest dipping of the basement blocks. Notice that variations in the 8 Hz data are exaggerated with respect to those at 0.5 Hz throughout the line; this characteristic is present in the field data of the

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10 R. ALVAREZ

four telluric survey lines and to a lesser extent in the computed model responses.

Figure 6 shows the model and its response at frequencies of 0.5 and 8 Hz. The maximum model depth is 2 kin, which is more than sufficient for the present purposes. The model suggests a horst and graben structure with its deepest portion between stations 12E and 24E. Between stations 4E and 12E an uplifted block is required to reproduce the observed variations. According to this model the upper and lower aquifers are characterized by a 40 ~2-m resistivity; the shallow regions of 10 g2.m are necessary in order to account for the local variations ofthe telluric response, and although in some portions they seem to correspond to the blue clay, a general statement in this respect cannot be made. In any event the purpose of the telluric survey was to deter-

125 MODEL RESPONSE FOR LINE 1

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mine the regional structural characteristics; the detailed distribution of layered formations cannot be adequately determined with this method.

14 R. ALVAREZ

Line 2

Figure 7 shows the field data along Line 2; the crossing points with Lines 1, 3, and 4 are shown. The response at 0.5 Hz shows that the lowest relative resistivities occur between stations 5N and 8S increasing toward the north and south portions of the line. The response at 8 Hz coincides fairly well with the above response in the northern section of the line; from station 20N to the south it shows wider variations but conforming to the same trends. Only on the southern region of the line a discrepancy is observed between the response of the two frequencies, similar to that observed in Line 1.

The model corresponding to Line 2 is shown in Fig. 8; the main features of the field data are reproduced. The horst and graben structure is also apparent in this model; the deepest sediment region is located around the center of the line at depths on the order of 1000 m. The electrical basement blocks rise toward the two ends of the line. Larger, less fractured blocks are required on the northern side of the line. Between stations 4N and 8S a 10 ~- m formation

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Fig. 11. Telluric results obtained along Line 4 at the two frequency bands shown. This is the shortest line surveyed.

GEOPHYSICAL D E T E R M I N A T I O N OF BURIED GEOLOGICAL STRUCTURES 15

is required to reach the lowest apparent resistivity values. It was previously mentioned that the surficial low-resistivity regions could be linked to the blue clay formation in some areas; this block, however, appears to be too deep to correspond to the blue clay. A more likely explanation is possibly associated with basement thermal activity, which is not uncommon in the area; geothermal waters may be rising through basement faults and invading the upper basement region. The viability of this explanation is based on production wells found in the northern part of the valley, which produce boron-rich waters at temperatures of around 55 o C. These waters are used for irrigation after cool' ing them off in channels and ponds.

Line 3

The field data for Line 3 are shown in Fig. 9. Notice that this line trends in the NE-SW direction. The lower frequency band indicates that the most conductive region on this line is located around station 0; however the relative resistivity variations are small compared to those observed in Lines l and 2.

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16 R.ALVAREZ

The model and its response appear in Fig. 10. The electrical basement is shallower and topographically more uniform, in agreement with the observa- tion made for the northern part of Line 2. Three shallow conductors are needed to simulate the field response.

Line 4

The field data for Line 4 appear in Fig. 11. This is the shortest line, inter- secting Line 2 at station 18N. A drastic dip in the response is observed between stations 4W and 0 in both frequency intervals. The modeled response is shown in Fig. 12. Around station 2W a conductive block 1 km in width is necessary at a depth of I km, as well as two shallow conductors of 10 f~-m, in order to account for such a dip. When this result is combined with that of Line 2 a small, graben-like structure is suggested as shown in Fig. 4.

GRAVIMETRY

A gravimetric survey was carried out in the valley of Guaymas consisting of 327 randomly distributed stations, with a station density of roughly one station per 2.5 km 2. Figure 13 is the result of processing the field data and contouring it according to a thin-plate algorithm (Gonz/dez-Casanova and Alvarez, 1985); the result shows the contoured residual anomaly map, in which the faults and structures obtained from the telluric survey have been superimposed, as well as four gravimetric lines GI, G2, G3, and G4. Lines G I and G2 roughly correspond to telluric Lines 1 and 2, while G3 and G4 have been chosen in order to better analyze the structural characteristics of the southern and northern parts of the valley.

The gravimetric map shows a large region of high values on the NE portion of the surveyed area. On the northern section the SE portion shows a series of maxima, which correspond fairly well with the outcropping formation known as Sierra de San Francisquito (Fig. 4). The SW portion shows a region of moderate minima. The area is flanked by low values on the eastern and western sides. Thus, the northern portion apparently consists of uplifted basement, and the southeastern section to a basaltic eruption. To the east and west of the surveyed area there are also outcropping basalts probably corresponding to fissure eruptions; these formations are elongated in the N-S direction, fairly parallel to Sierra de San Francisquito. Considering that the valley is emplaced in an area of tectonic activity, owing to its proximity to the spread- ing centers in the Guaymas basin of the Gulf of California, it is not surprising to find manifestations such as basaltic eruptions, and thermal activity in the area.

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GRAVIMETRIC MODELS

The method used to perform two-dimensional modeling along lines G 1, G2, G3, and G4 is based on the works of Talwani et al. (1959), Talwani and Heirtzler (1964), and Won and Bevis ( 1987 ). It considers a bidimensional flat earth; that is, all geological structures are considered of large extension in the direction perpendicular to that of the modeled profile. It is also considered that the earth has topography but no curvature, although, given the gentle slopes in the valley, this feature is of no major consequence to the present calculations. Finally, it is considered that the model extends _+ 30,000 km along the profile, in order to avoid edge effects. The density values used for the calculations are 2.67 g / c m 3 for the basement, and 2.0 g / c m 3 for the inter- leaved, unconsolidated sediments which constitute the aquifers.

Line G I

Figure 14 shows the gravimetric model of Line GI. This line corresponds directly to Line I of the telluric survey and should be compared to the telluric model shown in Fig. 6. Notice the general correlation between the topographic profile of the basement inferred from both models. The gravimetric model clearly shows that the valley is flanked by two depressions. Although to the east the response does not show the rising branch of the V-shaped

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Fig. 14. Gravimetric model along Line G 1. The field and calculated values are shown (in gravity units), as well as the basement topography producing the calculated response. Well PGO-15 provides a reference for the model. Compare to the corresponding telluric results (Fig. 6 ).

GEOPHYSICAL DETERMINATION OF BURIED GEOLOGICAL STRUCTURES | 9

depression its presence is unquestionable, since Sierra del Bacatete is outcropping a few kilometers to the east. The suggestion is made on the basis of this result, that the valley is flanked by two faults that are oriented approximately NNE-SSW. As will be seen, the results in lines G3 and G4 also support this suggestion, as well as the trends in the overall contoured response (Fig. 13). The uplifted block between stations 4E to 12E in the telluric survey correspond to an uplifted portion of the basement in the gravimetric response. The same is true around stations 26E to 30E. Well PGO-15 reached a depth of 406 m in sediments.

Line G2

The model corresponding to Line G2 is shown in Fig. 15. This model should be compared to the corresponding telluric model (Fig. 8). Again the general trends between the two models correspond to each other fairly well. Both show that between stations 4N and 8S there is the largest basement depression in the valley. The northern portion of the lines show uplifted basement, and the same holds true for the southern portion. The horst and graben structure of the valley suggested by the telluric results appears less defined in the case of the gravimetric results. The telluric and the gravimetric data correlate well with the geological cross-section inferred from the drillings (Fig. 2 ) except in the southernmost portion, where one deep well indicated the basement at a depth of 500 m. The problem originates in the orojection of the basement position in the region beneath the blue clay form=.~,on, where there is no well information and the basement appears to be much too shallow with respect to both telluric and gravimetric results.

Line G3

The model corresponding to this line which is closest to the coastline is shown in Fig. i 6. The outcropping basalts of Sierra de San Francisquito provide an excellent reference for the modeled profile. The model shows the two depressions at both ends of the line, supporting the statement previously made about the two faults flanking the valley. The region between stations ~: "" A,L~ Lv

12E corresponds to the reported saline intrusion in the area. This model shows that the basement is actually uplifted near the coast, but also shows that such an uplift occurs only toward the central portion of the valley.

Line G4

The model corresponding to Line G4 is shown in Fig. 17. The basement reaching near the surface is quite evident; this uplift can be visualized throughout the NE portion of the surveyed area and delimited by the + 15

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mgal contour in Fig. 13. The two depressions flanking the valley are also seen in this result. Although telluric Line 3 is at an angle with respect to Line G4 both show the basement blocks near the surface, which in turn is supported by the geological cross-section of Fig. 2.

CONCLUSIONS

The results of two independent geophysical surveys in the valley of Guay- mas, telludcs and gravity, have been modeled numerically. The comparison between the two models suggests that the valley contains a basement depression toward the south-central part, and that it is flanked by two depressions that run in the NNE-SSW direction and that may correspond to geological faults. These results help explain why the quality of the water extracted from the flanks of the valley is better than that of most of the central portions. The uplifted basement in the central portion contributes hydrothermal fluids that contaminate the upper aquifer and most likely the lower one, where it exists. Although similar phenomena may be occurring on the bottom of the flanks, the sediment thicknesses are considerably larger, probably blocking the fluids rising from the basement. Thus, recharge from the surface is apparently chan- nelled along the flanks of the valley, where it is extracted by means of shallow wells.

This study indicates that geophysical studies of a regional type may yield valuable criteria for exploitation of the aquifers in regions in which the demand for water is large, and where the dimensions of the aquifer are also large.

ACKNOWLEDGMENT

I acknowledge material help in the manuscript preparation from L. Navarro, A. S~inchez, J.C. del Olmo, and M. Garcia.

REFERENCES

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Beyer, J.H., 1977. Telluric and dc resistivity techniques applied to the geophysical investigation of basin and range geothermal systems. Part l: The E-field ratio telluric method. Ph.D. The- sis, Univ. of California, Berkeley.

Gonz~ilez-Casanova, P. and Alvarez, R., 1985. Splines in Geophysics. Geophysics, 50:2831- 2848.

Talwani, M. and Heirtzler, J.R., 1964. Computation of magnetic anomalies caused by two-dimensional bodies of arbitrary shape. Geological Sciences, 9. In: G.A. Parks (Editor), Com- puter in the Mineral Industries, Part I. Stanford Univ. Publ., pp. 464-480.

Talwani, M., Worzel, J.L. and Landisman, M., 1959. Rapid gravity computations for two-di°

24 R. ALVAREZ

mensional bodies with application to the Mendocino submarine fracture zone. J. Geophys. Res., 64: 45-59.

Ting, S.C. and Hohmann, G.W., 1981. Integral equation modeling of three-dimensional magnetotelluric response. Geophysics, 46:182-197.

Wannamaker, P.E., Hohmann, G.W. and Ward, S.H., 1984. Magnetotelluric responses ofthree- dimensional bodies in layered earths. Geophysics, 49:1517-1533.

Won, U.J. and Bevis, M., 1987. Computing the gravitational and magnetic anomalies due to a polygon, algorithms and Fortran subroutines. Geophysics, 52: 232-238.