Identification of possible settlements using ground penetrating radar; case study: the intersection of Kargar St. above the Engelab square in Tehran

The Ground Penetrating Radar (GPR) method is high-resolution surveys with an electromagnetic pulse reflection method for shallow earth layers. This method is similar to reflection seismography, which operates on the basis of wave propagation and reflection. In a simple case, a GPR device consists of a signal generator. The signal generated by this generator is sent into the earth by the transmitter antenna. Waves travel through the earth at a high speed and when these waves hit an object or reflection surface, due to the change of electromagnetic impedance in these places, part of the waves will be reflected from the surface. The receiving antenna located in the device shows these reflections as a high peak and records the movement time and amplitudes of the reflections. The electromagnetic wave inside the earth moves downwards in the form of a three-dimensional cone, and at the same time, factors affect the speed and loss of the amplitude of these waves. In fact, the electromagnetic characteristics of the materials, which depend on the constituent materials and the amount of water in them, will have a great effect on the speed and loss of the range of the radar waves penetrating the ground. Some materials, such as polar ice, are transparent to ground-penetrating radar waves, and these waves can pass through it without much loss. Some other materials such as clays saturated with water and also sea water is cloudy to these waves and reflect or absorb these waves. Therefore, radar waves penetrating the ground are attenuated in short distances and cannot propagate in such environments.
    Considering the high-resolution power of the ground penetrating radar method, as well as the high speed and ease of acquisition, it can be said that this method is useful in investigating and detecting shallow subsurface targets similar to subsurface holes, canals and other targets that have a different dielectric constant from the surrounding environment. It is an acceptable method. In addition to the effect of the frequency of the waves sent from the GPR antenna, other factors such as soil moisture, the amount of fine-grained clay materials, or the fineness of the sediments in general, reduce the depth of investigation or penetration of GPR waves. This problem is mainly caused by the higher electrical conductivity due to the presence of moisture or fine-grained sediment particles compared to coarse-grained sediments. In this study, a ground penetrating radar device with 80 MHz antenna was used and seven profiles with a total length of 662 meters were taken to investigate the intersection of Kargar Street in Tehran. The radargrams have been processed and the anomalies related to the meetings have been identified on them. These anomalies are discussed in the next step on the positioning satellite images and on the obtained results. According to the estimated results, there is a good agreement between the anomalies detected in the parallel profiles, which can be proof of the acceptable identification of possible anomalies in the study area.