NW Buenos Aires Province. Argentine
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Geoelectrical Methods Applications to freshwater-saline water relations. Case study: NW Buenos Aires Province. Argentine. Jerónimo Ainchil & Eduardo Kruse Facultad de Ciencias Astronómicas y Geofísicas Universidad Nacional de La Plata Observatorio Astronómico - Paseo del Bosque s/n - La Plata - Argentina E-mail: [email protected] ABSTRACT In different regions of the flatlands in the Province of Buenos Aires there is a predominance of high salinity groundwater. The only freshwater aquifer is not very thick and not very extensive. Geolectrical prospecting achieved an evaluation of such freshwater resources. The geoelectrical data processing includes mainly two steps. The first one is a mathematical inversion in one dimension for each field curve. The second step is an interactive fitting in accordance with the geological information. The electrical methods attempts to describe the resistivity distribution in depth. The method begins with the measuring of a field curve. These field measurements add ambiguity to the problem. In this paper we had looked for those parameters with a greater certainty for the field curves measured in the study area. The Vertical Electrical Sounding field curves are Q type. The chosen parameters were the depth of the conductivity layer and the unitary transverse resistance (T). The distribution of these parameters over the study area surface permits the construction of the maps of parameters. INTRODUCTION In different regions of the flatlands in the Province of Buenos Aires there is a predominance of high salinity groundwater. Over this water lens of scarce thickness of fresh water can be located. These bodies of water of limited extension are the only supply to some populations. Often they are environments where aeolian accumulations are present. In this accumulations the morphology, the thickness and the permeability of the silts facilitate the recharge of the excesses of water that exist in the hydrological balance. Geoelectrical prospecting is able to carry out an expeditious identification of such sources of drinkable water. This work proposes an interpretation of results to facilitate a better representation of the areal delimitation and estimate of the thickness of these bodies of fresh water. The study area is located in the northwest of the Pcia. of Buenos Aires, in Trenque Lauquen's vicinities (Figure1). Trenque Lauquen República Argentina Figure 1 GEOLOGIC SETTING The studied areas correspond to the extensive flatlands of Pcia of Buenos Aires, where the morphological and hydrogeological particularities have a strong influence in the hydrodynamic and hydrochemical characteristics of the shallow groundwater. The area is a great plain with a small slope of about 0.1 m/km. Minor topographic forms that are important hydrogeologically can be differentiated within the general morphology. There are slightly noticeable elevations alterning with depressions and ravine that give a relief of soft, somewhat aligned crinkles. They represent typical forms of paleodune environments. The region shows elongated ridges with a prevailingly southward direction. they are ten kilometers long and two kilometers wide. From an hydrogeological point of view is covered by recent permeable sediments, fine sand and silts forming the dunes where several overlying aeolian cycles have been recognized. In the interdune low finer sediment with lower relative permeability have been found. It is an arreic environment and floods occur in the low lands during humid periods (Kruse et al, 1985). At the surface, sandy aeolian sediments with thickness that range from a few centimeters up to four meters were found. They lie over silt with variable proportions of sand and clay, and usually calcareous material, known as "sedimentos pampeanos" (Fidalgo, 1975) MATERIALS AND METHODS The geophysical method used was the Vertical Electrical Sounding (VES) with the Schlumberger array. This method begins with the measuring of a field curve. Vertical Electric Sounding is based on the fact that the current penetrates continuously deeper with increasing separation of the current electrodes and, thereby, reflects information about the resistivity variation with depth. Two well-grounded current electrodes feed a commuted DC current into the earth with the current of I. The potential differences V are measured between two neighbouring and non-polarizable electrodes in the center of the array. The current electrodes are systematically moved outwards in steps. These field measurements add ambiguity to the problem. Based on the description above, each value of apparent resistivity of the curve is obtained (in ohm.m), according to the expression: V =K ap I where K is the geometric constant of the device that takes the disposition of the electrodes in the land into account. The type of the curves of apparent resistivity in environments such as our study case are the Q type. Field procedure The field soundings were grouped in two sectors. These both sectors had been defined a priori, from the geologic and morphologic point of view . Eighteen VES were measured in each sector, of about 1.5 km2. The maximum longitude of current electrode spacing (AB/2) was 500m. Two types of characteristic curves are shown (Figures 2 and 3), the first one with a layer of an intermediate resistivity ranging, the other one without this kind of layer. The Figure 2 presence of that layer is the objective of the prospecting. Figure 3 DATA PROCESSING AND INTERPRETATION The electrical methods attempt to describe the resistivity distribution in depth. The geoelectrical data processing mainly includes two steps. The first one is a mathematical inversion in one dimension for each field curve and the second step is an interactive fitting in accordance with the geological information. The field data (curves of apparent resistivity) was processed to establish a resistivity distribution in depth that mathematically fits the curve of observed data (to less than a band of experimental error). To obtain this distribution that is the objective of the method, varied mathematical techniques were used. The processing of the observed curves of apparent resistivity, was made using the PRINTERSEV 2.0 program (Giusso and Soto,1995) for horizontal stratified media (one dimension) According to an available sequence in the mentioned software, the steps are the following: C obtaining an initial model according to Zohdy (1989), C reduction of the number of parameters of the model (Orellana, 1982), C interactive manual adjustment (Johansen, 1975), C and evaluation of the ranges of equivalence of the pattern and inversion with information a priori (Pous et al, 1987). In all cases the answer of the model was obtained by convolution, using the lineal operator of Johansen (op.cit.) for it. This filter works in a sampling of the transformation of resistivity of 10 points for logarithmic decade. It has a longitude of 141 coefficients, which was considered sufficient to evaluate curves of resistivity contrasts like those that are present in the area. By way of a paid comment it should be pointed out that special attention should be lent to the selection of the filter. A frequent error is to interpret spurious oscillations taken place by the operators included in the disclosed softwares, particularly when processing curved with the resistivity contrasts like those measures in this region. RESULTS For the interpretation it was necessary to work with some parameter that reflects in a surface map the distribution of water of good quality in depth. The unitary traverse resistance T, that is one of the parameters of Dar Zarrouk, facilitates a good Map of T values (total) Town: Mari Lauquen County: Trenque Lauquen C C F F 9 Color Scale 10 T values [ohm.m.m] 17 11 8 36000 12 13 1 33000 14 19 30000 15 4 18 2 27000 5 16 3 24000 6 21000 Est. Mari Lauquen 18000 7 20 25 15000 12000 26 33 32 9000 34 6000 27 37 3000 35 0 21 28 31 5 22 a ut 29 R 23 24 30 36 Reference VES Figure 4 representation of that distribution. Examples of the maps are shown. They were obtained from the calculation of T in Mari Lauquen, Partido of Trenque Lauquen. In the first one (Figure 4) the value of total T is shown above the low resistivity layer, in fact the high conductivity layer, that constitutes the hydrogeological basement. This value of total T is usually distorted by the values of the ground surface. The non-saturated surface silts, present very high resistivity values. For this reason it is advisable to work with T of the thickness with resistivity values of saturated sediments ( Map of T values of the intermediate resistivity layer Town: Mari Lauquen County: Trenque Lauquen Color Scale T values [ohm.m.m] 9 10 17000 17 11 8 12 15000 13 1 14 19 13000 15 4 18 2 5 16 3 11000 6 9000 7 20 25 7000 26 32 33 5000 34 27 37 3000 35 21 28 31 1000 22 29 23 24 30 36 Reference VES Figure 5 Figure 5), that constitutes the objective of the evaluation. On the other hand, the used methodology allowed us to determine the depth of the conductive substratum accurately, considered in this work as hydrogeological basement . The Figure 6 shows the map of upper limit of the conductive substratum. Depth to high conductivity layer Town: Mari Lauquen County: Trenque Lauquen C C F F 9 Color Scale 10 Depth 17 11 8 12 13 100m. 1 14 19 90m. 15 4 18 2 5 80m. 16 3 6 70m. Es tac ion Mari Lauquen 60m. 7 20 25 50m. 26 40m. 32 33 30m. 34 27 20m. 37 35 10m. 21 28 31 22 0m. 5 29 ta u 23 R 24 30 36 Reference VES Figure 6 CONCLUSIONS The study and analysis of the T values map of the saturated sediments' resistivity and the map of high conductivity layer depth would allow to recognize the variations of the salinity of the groundwater and even to preliminarily estimate the available reserves of fresh water.