عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Determination of hydrodynamic coefficients of groundwater aquifers including hydraulic conductivity and transmissivity has great importance in hydrogeological studies because of extensive application in analysis, especially in groundwater modelling. These parameters are usually estimated from pumping test, carried out on drilled boreholes. To estimate aquifer's hydraulic parameters, pumping tests have been used widely as they provide a high degree of reliability. However, various assumptions (including: aquifer homogeneity, isotropy, thickness, well storage, continuity, and nature of the fluid flow) are required for the application of the methods' formulas to be valid under field conditions. In addition, these methods are costly, time-consuming and require a large data set to estimate aquifer's hydraulic parameters. Collecting of soil sample, drilling, determining of hydrogeological parameters and well geophysical logging are common methods for monitoring and characterizing of subsurface structures on small scales. In large scale or complex aquifers, using this data for investigation of groundwater hydrodynamic parameters is not reasonable. Therefore, combination of geophysics results with hydrogeological downhole values (for example pumping test) that has been named “hydrogeophysics” has great help for high-resolution subsurface monitoring. Geophysical exploration methods, such as geoelectrics, have attracted interest of hydrogeologists for exploring and delimiting aquifers with success, for several decades. Basic equations for geoelectrical exploration are developed assuming: the medium is porous; the matrix is generally an insulator and electrical currents flows through the water presents in the pore spaces. Aquifer's electrical resistivity is mainly influenced by porosity and fluid resistivity in the pores. The geoelectrical data recorded on the surface contain useful information about the aquifer which can be interpreted by experienced geophysicists for hydrogeological studies. The advantage of using such Vertical Electrical Sounding (VES) technique is no need for ground perforation; also, the dense VES soundings help with making high-resolution maps and faster gaining information regarding the hydraulic conductivity distribution. So far, numerous studies have been carried out on determining the hydrodynamic coefficients from resistivity geophysical data and empirical relationships. This method consumes lesser time and expenses in comparison to pumping test methods and provides more points for estimation of aquifer's hydrodynamic coefficients.
In this research, the mentioned method was introduced and used on aquifer resistivity data for west part of Pyran-shahr Plain, located in West Azarbayjan province, Iran. The hydraulic conductivity and transmissivity of aquifer have been estimated using this approach. At first, the electrical resistivity of the aquifer was measured in 15 sites. The VES measurements were performed with a maximum half current electrode separation of 500 m (the Schlumberger electrodes array was used) and the potential electrode spacing (MN) ranged from 2 m to 80 m. The computer interpretation software “IX1D” was used for data inversion. To reduce the ambiguity problems, geologic constrains and some borehole lithologic data were used to constrain data of the vertical electrical sounding. A good correlation was observed between the electrical resistivity derived 1D models and the borehole lithologs. Using interpreted geophysical parameters (electrical resistivity and thickness of the aquifer) and Archie equation, porosity values have been determined in each vertical electrical sounding. Then, values of intrinsic permeability and hydraulic conductivity have been calculated using soil grain size and Kozeny and Hubert relationships, respectively. Finally, by calculating of longitudinal conductance and experimental relationships, values of hydraulic transmissivity were determined and also over the past years, pumping tests have been performed in the west part of Pyran-shahr aquifer. Hydraulic conductivity and transmissivity results of the pumping tests were shown using Theis–Jacob method and were compared with results of hydrogeophysics method.
In this study, using resistivity data, hydraulic conductivity and transmissivity values of aquifer were estimated 23 (m/day) and 3200 (m2/day) while by pumping test data analysis 37.8 (m/day) and 2980 (m2/day) were obtained, respectively. The acceptable agreement of the values obtained by two methods indicates that the electrical resistivity methods can be useful in assessing the hydrodynamic coefficients of aquifers and suitable for estimation of hydraulic parameters; because they are relatively quick and inexpensive, compared to pumping and additionally the geophysical survey can be scaled by careful design. Necessity of accurate knowledge from aquifer grain size, cementation and tortuosity factors of sediments, and also heterogeneity of the aquifer can be mentioned as the shortcomings of the method. Finally, to upgrade the thematic maps after performing VES measurements in areas, lacking pumping test data, the relationships of resistivity with hydraulic parameters can be used.