Exploration of groundwater-bearing fracture zones integrating VLF and resistivity methods


Because of urgent needs of groundwater resources in the Tochal region of north Tehran, an integration of electric and electromagnetic methods was applied to investigate the location and the direction of water bearing fractures. First, the VLF electromagnetic profiling was performed on suitable places and two locations were detected for supplementary resistivity surveys. Then, a number of vertical electrical soundings using Schlumberger array were carried out on suitable locations. With the presumption of a layered earth, three soundings in every two locations were merged and the 2D electrical resistivity cross-sections were made and the best location for groundwater- bearing fracture zone was detected.
The VLF method has been applied successfully to map the resistivity contrast at boundaries of fractured zones having a high degree of connectivity (Parasnis, 1973). Further, the VLF method yields a higher depth of penetration in hard rock areas because of their high resistivity. Therefore, a combined study of VLF and DC resistivity has the potential to be successful (Bernard and valla, 1991; Benson et al., 1997).
For tilt angle measurements, magnetic field coupling with the fracture zone is important. Therefore, the VLF-transmitter should be located along the strike of the target. The depth of investigation is dependent on the frequency used and the resistivity of the host medium. Sharma and Baranwal (2005) suggested a method using real component curve of VLF measurements to locate water bearing fractures. In this method, every significant leap in the data curve indicates a conductive anomaly referring to a fracture in a hard rock medium.
Integrated geophysical studies were performed on the Tochal telecabin region, north of Tehran, Iran. Tochal telecabin is a cultural-sporting complex that begins from the end of Velenjak Street and continues to the vertex of Tochal Mountain. This study was accomplished by a VLF survey followed by a resistivity survey using Schlumberger array to find a suitable location for drilling a well in the Tochal telecabin region.
In order to find the locations that contain water-bearing fractures, nine VLF profiles were performed to study along the main road of the Tochal telecabin. First a suitable transmitter with sufficient power and admissible direction was found and then the measurements with 5m station spacing were done. It is important to note that almost all of the measurements were taken before sunrise when the city wireless transmitters had not begun to work. Because city wireless transmitters generate noise, the instrument cannot get a suitable signal. High noise level is a serious problem on a cold and snowy day. Moreover, the active installations and high voltage transporter cables affect the signal and increase the noise level.
VLF data were collected using an ABEM-WADI instrument. With regard to the pseudo current-density section of first VLF profile, we focused on two major zones along the road. For zone 1, since the probable fault direction in the formation was approximately in the E–W direction, the HWU transmitter in this direction with a frequency of 18.3 kHz located in Le Blance, France was used. For zone 2, since the probable fault direction in the formation was approximately in the N–S direction, the RCV transmitter in this direction with a frequency of 27.1 kHz located in Russia, was used. Next, the other profiles were surveyed along hydrogeologically suitable locations with probable fractures and fault.
Six Schlumberger resistivity soundings were performed in two zones using a DC resistivity meter. Sounding locations were selected by detailed study of the area with a VLF survey as well as by their hydrogeological suitability.
After gathering the measured data, for further detailed information of the subsurface, the measured real anomaly was filtered using the approach of Karous and Hjelt (1983). This process yields pseudo-section of relative current density variation