Detection of subsurface installations and analysis of GPR pulse characteristics

Detection of subsurface installations and knowing their characteristics and accurate burial places are some challenges that the engineers involved always face with. Urban development, and thus, the necessity for the development of new and convenient installation networks and also the need for maintenance of old installation networks indicate the significance of accurate placement of the subsurface installations. In this regard, non-destructive methods have been considered to be used by many engineers in this field in order to save time and costs and also to reduce the disturbances due to unnecessary drillings in urban areas. In general, geophysical methods are among the most important non-destructive methods. One of shallow subsurface geophysical methods which has obtained some degree of success in this regard is the ground penetrating radar (GPR) method. In this research work, the GPR method is applied in several case studies having different subsurface conditions, and thus, the capability of the GPR method is investigated for accurate placement of subsurface installations in these case studies. Furthermore, we use a different procedure in data interpretation and analysis to analyze the characteristics of amplitude, phase and frequencies of GPR pulses in order to derive maximum information from GPR sections.
    Three-dimensional (3D) data survey in one of case studies carried out in this research work had made it possible to process, interpret, and visualize subsurface structures in 3D form. Besides, resistivity surveys in the study area had provided us with the possibility of investigation and correlation of the GPR data results with the resistivity findings. Considering that a survey line had been carried out on the subsurface media containing high electrical conductivities, the unfavorable influence of the conductivities of survey media on the GPR measurements made along the survey line was indicated. Since the resistivities (or conductivities) of the subsurface media play an important role in the penetration of GPR waves, we can estimate the resistivities of the subsurface media by analyzing the frequency spectrum of the GPR pulses propagated in the media. Furthermore, the systematic noise affecting the GPR measurements is recognized by this frequency–spectrum analysis as this noise has frequencies different from those of signals. This study also considers GPR and resistivity investigations along a survey line crossing subsurface metallic and non-metallic installations, and thus, detailed investigations made on these subsurface targets. In this area, an estimation of the diameter of the non-metallic pipe installed for transferring of water in the subsurface has been made with a good accuracy from the acquired GPR time section along the survey line. Moreover, the analysis of amplitude and phase characteristics of the GPR pulses from some parts of the survey line has made it possible to distinguish or separate GPR responses of metallic installations from non-metallic ones. Finally, by considering GPR measurements along a survey line passing a subsurface non-metallic pipe, we investigated the effect of the fluid inside the pipe.