# Development of a Metalens for Radio Wave Ground- Penetrating Radars
## 1 Introduction  
Currently, there is great interest in subsurface sounding systems with high-frequency electromagnetic radiation. Such systems enable non-destructive underground exploration for industrial and archaeological purposes; searching for underground utilities; the detection of electrical wiring and pipes laid along buildings; the detection of defects in building structures; and diagnostics of road and railway tracks. Deformation of the road surface and the soil under it significantly affects the service life and safety of roads. The cause of the premature wear and destruction of the road surface is design errors, the violation of construction and repair technologies, the quality of the materials used, water logging of the roadway embankment with ground and surface waters, and, as a result, shedding of the soil. To predict the timing of repairs and the high-quality reconstruction of roads, it is necessary to have a complete understanding of the structure of the pavement, including the geological structure of the underlying soil. Therefore, it seems extremely important to develop technologies for operational diagnostics of the state of the railway embankment, capable of examining long sections of the road in a short time. Standard inspection methods call for taking samples of the roadbed under test, either by drilling and coring or by removing some of the top layers, to visually determine the internal condition of the structure. The disadvantages of these methods are their invasiveness and pinpoint determination of the thickness of the roadway. Due to the fact that the laying of the roadbed can be uneven, this method does not allow determining the thickness of the roadbed over the entire laid section. Compared with other methods of non-destructive testing, the use of GPR provides high accuracy and speed of determining inhomogeneities in the underground environment, as well as low cost. Currently, non-destructive testing methods for studying various soils are based on a variety of physical phenomena, such as acoustic methods (sonic and ultrasonic frequencies) [1]. With ultrasound, it is impossible to answer the question about the actual dimensions of the object. The ultrasonic method imposes the need to maintain direct contact with the surface being examined. The investigated medium must be elastic; otherwise, ultrasound examination is impossible due to the rapid attenuation of the waves. Another is infrared thermography [2]. The main disadvantages of infrared thermographyinclude the fact that the difference in temperature between the desired object in the embankment and the environment is often too small to fix it. The bottom line is, similar to the ultrasonic method, the thermal method is used for radio sounding of rails and sleepers, but it is ineffective for the embankment. Scattered magnetic flux method [3] is effective for searching for defects in steel objects, such as rails. However, the shallow scanning depth of the embankment and the high risk of signal distortion by the rails make this method unsuitable for our purposes. Optical methods [4; 5] and methods based on eddy currents [6] are very widely used due to the wide variety of possibilities for obtaining information. The possibility of their use for external control does not depend on the material of the object. This method, in fact, is a study of the surface of the embankment; however, it is not suitable for subsurface sounding. Methods based on electromagnetic radars [7] have been carefully studied. Furthermore, machine learning has been proposed for various methods, such as: support vector machine (SVM), neural network (NN), K-nearest neighbors (KNN), hidden Markov model (HMM), naive Bayes method (NB), decision tree (DT), etc. [8; 9]. However, the accuracy of traditional machine learning algorithms depends on manually selected features, so the above algorithms work correctly only on certain georadargrams.