A comparison between the results of ground penetrating radar and magnetic surveys in near surface investigations: A case study

Today, it is indispensable to transmit and retain fuels, water and other energy resources by buried pipes, tanks and cables in urban and non-urban areas. Thus, the generation of huge and costly underground networks for this purpose is inevitable. Following the creation of such networks, their maintenance, and prevention from possible destructions are needed. Otherwise, considerable financial losses and irreparable environmental contaminations may occur. Often, a sufficient physical contrast between these installations and their surrounding media exists, and as a result, these installations are considered as suitable targets for detection by ground penetrating radar (GPR) method. In this paper, GPR data, acquired from a GPR survey on buried pipes including metallic pipes for gas transmission, have been processed and interpreted to detect the buried metallic pipes and other subsurface anomalies. To investigate the advantages or efficiency and drawbacks of the GPR method in this application, we have compared the results of the GPR method with the results of the magnetic method performed on the same targets. The results of this comparison are given in the following.
The GPR method provides a higher resolution in comparison to the magnetic method. However, GPR waves in subsurface highly conductive media are intensively attenuated. Hence, the depth of penetration in the latter method is limited. As can be seen from the GPR depth sections obtained from the GPR survey in the study area, the depth of penetration is even less than two meters. The magnetic method, despite its weak resolution, can more successfully detect the targets in highly conductive media provided that there is a good magnetic susceptibility contrast between the target and the surrounding medium. In urban areas, where high noise levels exist magnetic surveys unlike GPR methods via shielded antenna cannot be successfully performed. In this research, first, both the GPR and magnetic surveys have been carried out along survey lines passing the buried gas pipelines and other shallow subsurface targets in Qaleh-Showkat area that is approximately located 10 km to the west of Shahrood city to detect the targets. Then, the results of the GPR and magnetic surveys have been compared. As a result, we have found that the GPR method provided higher resolution in the detection of small anomalies located at shallow depths and near each other. However, in the locations having high electrical conductivities, the GPR method, unlike the magnetic method, could detect very shallow subsurface targets or anomalies even high attenuation of the GPR waves happen in these situations.