مدل‌سازی دوبُعدی داده‌های بسامد بسیار کم VLF بر نیم‌رخی از گسل شمال تهران در منطقه شهران

Two dimensional single frequency scalar VLF data modeling has been studied in this research. VLF method is one of the electromagnetic investigation methods. Radio frequency waves are used for investigation of conductive deposits (Paal, 1968). Synthetic modeling and inversion of single frequency and multiple frequencies of VLF signals was done by Oskooi (2004) for two dimensional simple structures. VLF transmitter antennas are high vertical wires that have the alternative current.
Conductivity of subsurface structures can be measured by single very low frequency two dimensional inversion methods. Ground VLF data is fast suitable facility for study geological structures with maximum depth of 100 m. Temporal and spatial variations of VLF primary field on surveying should be noticed. There is a linear relation between horizontal and vertical component of magnetic field.
(1)
The complex tipper vector (A, B) is only dependent on a ground structure and is independent from a transmitter azimuth. The x axis is on the direction of VLF transmitter on the geological strike. Y axis is the profile direction. For each site there is a transfer function, called complex scalar tipper. This value is recorded by instrument and it is measured by:
(2)
Interpretation and modeling of the tipper scalar data have been performed using synthetic and field data at the frequency of 23300 Hz. Considering that the limitation of VLF transmitters, the waves are coming from NWC station in Australia. With noticing the relation (2), there are 59 real and 59 imaginary data numbers. There is 5 m between each station. Skin depth is the relation between the depth and frequency.
(3)
Electromagnetic waves in conductive zones propagate in low frequency. We can estimate the depth of anomaly considering the electrical resistivity variations.
Forward model of a fault have been considered to apply a two dimensional modeling. In the forward model of fault initial electrical resistivity is drawn logarithmic and the values are 100 ohmm and 700 ohmm. In a level of 2% noise was added to the data in order to compare the results with field results. In inversion model there is a resistive layer in a distance of 150 meter. The depth of resistive layer in both inversion and forward model is 20 m. The conductive region in both models has the electrical resistivity of about 100 ohm-meter.
Field data are recorded by WADI instrument from Shahran region at the North West of Tehran. Field data acquisition along a 250 m SN-profile crossing the North Tehran Fault (NTF) consist of 50 stations with 5 m spacing. The signal is reported at a frequency of 23300 Hz that is the NWC signals. The profile is incidence with North Tehran Fault. Travers direction among profile is South-North. Geological structure under the Shahran area is east-west. Geological study of that region is done by Hafizi and Vali (1999) for estimating the underground water sources in cracks using resistivity and IP methods.
We can conclude in that region in the station of 20, there is a crack so conductive materials such as underground water are near the surface in the depth of 10 meter. Under the conductive zone there is tuff formation with electrical resistivity of about 50-100 ohm. At the distance of 100-250 meter, conductive layer is lying under the tuffs.
Final model could resolve geological features spatially and the size of the anomaly and the location of the estimated one form the model are consistent. The tipper data has been depicted in terms of distance along the profile to realize the changes laterally. Using tipper data, the location of the anomalies can be diagnosed. In this investigation field data were collected from Shahran area in North West of Tehran. The main features of the NTF are determined properly using the presented VLF data inversion and interpretation.