# Passive Harmonic RFID System for Buried Assets Localization
## 1 Introduction  
Pipeline transportation is considered the safest, most progressive, and most economical mode of transportation. Every day, energy resources such as refined petroleum and natural gas, as well as water supplies, are all being transported through complex underground pipeline networks from production areas or ports of entry to consumers, airports, military bases, population centers, and industry. Most pipelines are typically designed to have a lifespan of more than 25 years. However, maintenance and expansion operations are always required to meet high demands for safety, reliability, and efficiency. During the excavation process, pipeline operators face a challenging and uncertain task in accurately determining the position of buried pipelines, especially in urban areas. This leads to accidental breakage of pipes that can be very costly and causes a significant delay in civil constructions [1]. In this context, there is an increasing need to use non-destructive techniques for detection and localization of buried pipes in order to avoid and minimize failures during working processes. There are two primary methods for detecting the locations of the underground infrastructures. The first method is based on electromagnetic induction [2, 3] and relies on materials being conductive or metallic, which excludes the possibility of detecting plastic pipes recently used by many pipe operators. The second method is GPR (ground penetrating radar), based on the propagation of a very short electromagnetic pulse (1-20 ns) in the frequency band of 10 MHz-2.5 GHz [4, 5]. Although GPR has proven its efficiency in locating different types of pipes (especially metal pipes) and is the mostwidespread method, the use of this technique is quite expensive and requires experienced operators to interpret the results of complex signal processing operations. In recent years, Radio Frequency Identification (RFID) technology has grown significantly and new applications have appeared, such as parking management based on RFID [6], tracking of objects in hospitals [7], and localization based on RFID tags [8]. However, there are also other challenges such as security aspects, interference issues [9], or undesired effects of the environment. In some of these applications, special tags for metal surfaces or embedded tags are required [5; 10; 11; 12]. Recently, RFID systems have experienced a significant interest as a potentially cost-effective and less complicated alternative technique for marking buried items [13; 14], as well as for quality monitoring of oil and gas pipelines [15]. The key challenges identified were the powering of these sensors and the communication of the data to the operator, as a result of the attenuation of the electromagnetic signals caused by the ground. To solve this problem, the use of active wireless systems [16] is a possible solution. However, as a long lifetime is required for this application, passive devices provide a more efficient solution compared with battery-powered devices. Also, passive green devices are preferred in order to avoid soil contamination due to dangerous chemical substances in battery cells. Therefore, an energy harvesting method to provide energy to the tag electronics is required. As the tags are buried, the easiest way is to obtain the power from the interrogating radio frequency (RF) signal. On the other hand, passive chipless RFID tags have been proposed for enhancement of the backscattered signal in the detection of non-conductive pipes using commercial GPR systems. Both resonant tags and chipless tags in time-domain have been proposed for this purpose [5; 17; 18; 19]. As the attenuation of the soil increases with the frequency, another critical decision is to select the frequency band of the tag. The patented system electronic marking system (EMS) by 3M [20] uses low-frequency tags that can yield great ground penetration depths. A similar solution is shown in the literature [21].