The article describes the history of research and development of surface-penetrating radar in the USSR, starting from the 60s and ending today in Russia and neighboring states formed after the collapse of the USSR. It reports key applications ranging from Antarctic ice and soil exploration to subsequent use in humanitarian demining, non-destructive testing and cultural heritage site surveys. The paper is under line the outstanding role played in the initial period of this technology formation by Prof. M.I. Finkelstein and his colleagues. Their research had a significant impact on the subsequent development of ground penetrating radar in modern Russia and neighboring countries.

This article describes the history of research and development of surface-penetrating radar, starting from the 60s in the USSR, and ending today in Russia and neighboring states formed after the collapse of the USSR. The outstanding role played in the initial period of this technology development by Prof. M.I. Finkelstein and his colleagues is highlighted. Their research had a significant impact on subsequent developments in modern Russia and the neighboring countries.

It is not practical to obtain a large number of labeled data to train a supervised learning network in tunnel lining nondestructive testing with ground‐penetrating radar (GPR). To decrease the dependence of supervised learning on the number of labeled data, an improved self‐supervised learning algorithm—self‐attention dense contrastive learning (SA‐DenseCL)—is proposed and incorporated with a mask region‐convolution neural network (Mask R‐CNN), which is trained by unlabeled and labeled GPR data. The proposed SA‐DenseCL adds a self‐attention‐based relevant projection head to the DenseCL architecture of self‐supervised learning, capturing the spatially continuing information between adjacent GPR traces. In the workflow, some unlabeled GPR images are used to pre‐train the SA‐DenseCL network for feature extraction and obtaining the backbone weights, which is superior to the conventional pre‐training methods of supervised learning pre‐trained by ImageNet images. The weights of the pre‐trained backbone are then used to initialize the Mask R‐CNN through transfer learning. Subsequently, a limited number of labeled GPR images are used to fine‐tune the Mask R‐CNN for automatically identifying the locations of the reinforcement bars and voids and estimating the secondary lining thickness. The experimental results show that the average precision reaches 96.70%, 81.04%, and 94.67% in identifying reinforcement bar locations, detecting void defects, and estimating secondary lining thickness, respectively, which outperform the conventional methods that use ImageNet‐based supervised learning or GPR image‐based DenseCL for initializing the Mask R‐CNN backbone weights. It is observed that the improved self‐supervised learning‐based framework can improve the detection and estimation accuracy in GPR tunnel lining inspection.

Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. Usually in short-range microwave imaging systems, the back-scattered signal is used, when a combined transmit-receive antenna scans over a plane, forming a two-dimensional synthesized aperture, while the signal reflected from the object of observation is recorded, as a result of which a microwave hologram of the object is formed. The second option involves registering the forward-scattered signal, when the transmitting and receiving antennas are located on opposite sides of the object and scan synchronously. The purpose of this work is a theoretical and experimental comparison of these two sounding options, identifying the advantages and disadvantages of each option, taking into account the features that arise when solving various problems of microwave imaging. Keywords: microwave holography, mivrowave image, back-scattered signal, forward-scattered signal, range resolution.

Abstract The ancient pyramids keep a lot of mysteries and attract the attention of historians, archaeologists and just tourists from all over the world. Their assignment, construction methods and especially the discovery of previously unknown voids and structures inside them require further study using new technical solutions. The most famous of these structures is the Great (Khufu’s) Pyramid on the Giza plateau in Egypt. Since the middle of the last century, several non-destructive technical methods have been proposed for examining the internal structure of the pyramid. Electromagnetic translucence in the radio frequency range and the use of muon sensors are among them. Although the use of muon sensors has supposedly revealed two previously unknown voids within the Pyramid, independent confirmation of their presence is required. The article discusses the fundamental possibility of using gravimetry to examine the internal structure of the Great Pyramid, and analyzes combining it with other modern technical means including unmanned aerial vehicle (UAV) for the implementation of this project. The paper proposes the simplest model that allows evaluating the required accuracy of a microgravimeter capable detecting the supposed voids in the Pyramid. The advantage of this approach, in addition to its simplicity, is the ability easily checking the presented in paper results. The main purpose of writing the article is to draw the attention of the scientific community to another method of non-destructive testing for the study of the internal structure of the Great Pyramid and discussion further needed steps in this direction.

Glass fiber Reinforced Polyurethane Foam (RPUF) has found wide application primarily in the aerospace and construction industry due to its outstanding properties. Unlike pure non-reinforced Polyurethane Foam (PUF), it has increased strength characteristics and at the same time retains unique heat and sound insulation properties. The paper considers the features of reinforced polyurethane foam examination in the Microwave (MW) range. It is shown that there is a significant difference in the recorded MW images compared to pure polyurethane foam. This is expressed primarily in the fact that the reinforcing fibers have a dielectric constant that is different from enclosing polyurethane foam, which leads to scattering and reflection of the incident electromagnetic wave on them. Experimental studies have shown that for RPUF, in contrast to PUF, there is an optimal wavelength range in which the contrast of defects against the background of glass fiber reflections is of the greatest value. An experimental comparison of two methods of examination back-scattering and forward-scattering methods was also carried out for RPUF. It is shown that the forward-scattering technology of measurements, if it can be implemented, has certain advantages since allows reducing the contrast of background reflections from the reinforcing fibers.

Many constructions built by ancient civilization hold many mysteries and attract the attention of historians, archaeologists and tourists from all over the world. The most famous and enigmatical among them is the Great (Khufu's-Cheops') Pyramid on the Giza plateau in Egypt. The assignment, construction methods and especially the inside structure of the Pyramid have been the cause of heated debate among historians and researchers since ancient times. Unfortunately, not only researchers are interested in ancient structures and excavation sites of archaeological values, but also robbers and illegal seekers of archaeological values. This led to the partial destruction of the Pyramid by the order of the Arab caliph Al-Ma'mun in the 9th century AD. From the middle of the last century, the point of view has prevailed that only non-destructive testing methods are acceptable when examining ancient structures. In the 1960s, a technology was proposed for transilluminating the pyramids of Giza by muons, which are generated by cosmic rays in the Earth's atmosphere. This method gave promising results. Other means were also proposed, which include radar, as well as gravitational and vibration technologies. However, despite numerous attempts to use them, no significant success or discovery has been achieved. A discussion of these methods, their applications and partial successes is the subject of this article.

Abstract The ancient pyramids keep a lot of mysteries and attract the attention of historians, archaeologists and just tourists from all over the world. Their assignment, construction methods and especially the discovery of previously unknown voids and structures inside them require further study using new technical solutions. The most famous of these structures is the Great (Khufu’s) Pyramid on the Giza plateau in Egypt. Since the middle of the last century, several non-destructive technical methods have been proposed for examining the internal structure of the pyramid. Electromagnetic translucence in the radio frequency range and the use of muon sensors are among them. Although the use of muon sensors has supposedly revealed two previously unknown voids within the Pyramid, independent confirmation of their presence is required. The article discusses the fundamental possibility of using gravimetry to examine the internal structure of the Great Pyramid, and analyzes combining it with other modern technical means including unmanned aerial vehicle (UAV) for the implementation of this project. The paper proposes the simplest model that allows evaluating the required accuracy of a microgravimeter capable detecting the supposed voids in the Pyramid. The advantage of this approach, in addition to its simplicity, is the ability easily checking the presented in paper results. The main purpose of writing the article is to draw the attention of the scientific community to another method of non-destructive testing for the study of the internal structure of the Great Pyramid and discussion further needed steps in this direction.

The study, preservation and restoration of the cultural heritage objects of mankind are not only of great cultural importance but also have a significant economic component because cultural values of past centuries attract tourists from all over the world. The use of modern technical and scientific achievements in the field of non-destructive testing makes it possible to obtain new knowledge about cultural objects regarding their origin and dating, as well as to contribute to their better restoration and preservation. An important component of their use is additional opportunities to identify high quality fakes of original cultural objects that have historical significance. The capabilities of various non-destructive testing (NDT) methods used to examine cultural objects are characterized by their penetration depth, resolution, and sensitivity to material properties. Thus, in many cases, it is necessary to perform multi-sensor non-destructive testing and creating large data sets that require an efficient evaluation. This article considers an example of using microwave (MW) holographic sensors for the examining of an old Orthodox Russian Icon dated of the late 19th century. The paper describes the technology of microwave holography, which has recently been applied to the examination of art works. Unlike the well-studied X-ray method, MW holography makes it possible to examine objects with one-sided access. Its other advantages are the relative cheapness of the equipment and the safety of use due to the low level of radiation. The article describes a MW holograms reconstruction algorithm, as well as a method for improving the quality of obtained MW images. The data collected at MW research of the Icon are compared with the results of X-ray examination and confirmed by subsequent opening and visual examination performed by professional restorers.

Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. Usually in short-range microwave imaging systems, the back-scattered signal is used, when a combined transmit-receive antenna scans over a plane, forming a two-dimensional synthesized aperture, while the signal reflected from the object of observation is recorded, as a result of which a microwave hologram of the object is formed. The second option involves registering the forward-scattered signal, when the transmitting and receiving antennas are located on opposite sides of the object and scan synchronously. The purpose of this work is a theoretical and experimental comparison of these two sounding options, identifying the advantages and disadvantages of each option, taking into account the features that arise when solving various problems of microwave imaging.

The present paper is aimed at studying the possibility of applying the technologies of non-destructive testing of different materials by means of radar methods.The main parameter that predetermines these capabilities is the attenuation of the highfrequency signal in the specific type of material.In practically significant cases its determination is possible only by experimental methods.The attenuation coefficient of ultrahigh frequency oscillations of the K-band is studied for four types of test specimens used by Bulgarian organizations working in the field of aerospace industry.Tests are performed through a specially designed test bench and laboratory installations.

In this paper the possibility of radio frequency range using to survey the Great (Khufu’s) Pyramid in Egypt is considered. This task is especially urgent in light of the reports that have appeared on the alleged detection of previously unknown voids in the pyramid body by using muon sensors. Given that this method is indirect, in the absence of the possibility of drilling or making passes, independent confirmation is required based on other non-destructive testing methods. The results of mathematical modelling will help determine the further direction of research and expand the areas of applicability of radio holography when examining objects of the cultural heritage of mankind.

The study of ancient Egyptian monuments attracts the attention of experts from around the world. A recent event that confirms this is the discovery, using muon sensors, of previously unknown cavities in the Great Pyramid of Giza (or Khufu's Pyramid). Since it is unfeasible to directly confirm this discovery by drilling, another independent non-destructive method is necessary to confirm this discovery and provide accurate determination of the locations and shapes of the cavities. Following a literature review of the different methods used in evaluating cultural objects, this paper analyses a possible framework for simulation of a holographic radar for detecting openings or other unknown structures of interest to archaeologists/Egyptologists and the public.

Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. This technique has various applications such as: nondestructive testing, medical imaging, concealed weapon detection, through-the-wall imaging, etc. Obtaining radar images in these applications is based on processing phase and amplitude of the reflected signal recorded over an aperture (a microwave hologram). Recently, systems began to appear in which the radar part is supplemented by an RGB-D sensor, which allows to obtain new capabilities. For example, there is the a microwave screening system architecture in which inverse synthetic aperture is formed by the natural motion of the subject in the vicinity of a stationary linear antenna array. The microwave system is complemented with an synchronous RGB-D video sensor which captures the trajectory of the moving subject in 3D and allows coherent processing of the radar signal. Another system detects objects buried under irregular surface and uses RGB-D sensor for capturing the surface relief for suppressing reflection of the sounding signal from the surface. Calibration between a radar and an RGB-D sensor is an essential process for microwave and optical data fusion. This article presents a novel approach for calibration, using a planar calibration target which is made of radiotransparent material (such a foam plastic sheet) with square marker and six small metal balls embedded in target surface and representing point objects. The proposed method exploits 3D-3D correspondences between coordinates of point objects in two coordinate systems associated to the sensor and to the radar. One points set is extracted from optical data, using marked corners of the target as a base points. Second points set is obtained from microwave data as local maxima of 3D volume of data reconstructed from one-frequency microwave hologram. Computer modeling were performed using Autodesk 3ds Max software by which models of all components of the system were built and the optical image from the sensor was modeled. Test experiments were carried out using a measurement system composed of the following components: a compact vector network analyzer (VNA), two mechanical scanners with stepper motors, one transmitting and one receiving horn antennas, mounted on the VNA, an RGB-D sensor, a microcontroller board, and a computer. The high accuracy of the method is confirmed both by computer modeling and physical experiment. The accuracy of determination of relative position between the radar and the sensor is about one fifth of the signal wavelength used.

Holographic subsurface radar (HSR) is not currently in widespread usage. This is due to a historical perspective in the ground-penetrating radar (GPR) community that the high attenuation of electromagnetic waves in most media of interest and the inability to apply time-varying gain to the continuous-wave (CW) HSR signal preclude sufficient effective penetration depth. While it is true that the fundamental physics of HSR, with its use of a CW signal, does not allow amplification of later (i.e., deeper) arrivals in lossy media (as is possible with impulse subsurface radar (ISR)), HSR has distinct advantages. The most important of these is the ability to do shallow subsurface imaging with a resolution that is not possible with ISR. In addition, the design of an HSR system is simpler than for ISR due to the relatively low-tech transmitting and receiving antennae. This paper provides a review of the main principles of HSR through an optical analogy and describes possible algorithms for radar hologram reconstruction. We also present a review of the history of development of systems and applications of the RASCAN type, which is possibly the only commercially available holographic subsurface radar. Among the subsurface imaging and remote sensing applications considered are humanitarian demining, construction inspection, nondestructive testing of dielectric aerospace materials, surveys of historic architecture and artworks, paleontology, and security screening. Each application is illustrated with relevant data acquired in laboratory and/or field experiments.

RASCAN holographic radar has been used to observe a rostrum of an Upper Jurassic–Lower Cretaceous fossil crocodylomorph under the upper surface 21 mm thick limestone slab (Maiolica Formation). The specimen, recovered in the Altopiano di Asiago (Vicenza Province, Italy), is at present housed in the paleontological collections of the Rovereto Civic Museum (Trento Province, Italy). The holographic radar response correlates well with the actual fossil shape revealed on the reverse side of the slab. This study has been made using the RASCAN-4/7000 holographic radar which can penetrate through several centimeters of the limestone. It works by comparing the phase of a sinusoid electromagnetic wave reflected from the subsurface of the fossil with the internal reference of the transmitting antennae. The radar has receiving antennas with both cross and parallel polarizations relative to the transmitter. On a processed radar plan-view image, objects can often be identified directly by their shape and texture as was first demonstrated in a similar research field for dinosaurs' tracks investigation [1, 2]. This is a great simplification compared with the processing needed using impulse radar [3]. The RASCAN method is suitable for portable and on-site applications due to its small size, light weight and low power consumption. The images generated can be used for diagnostic purposes similar to X-ray methods, but without the complications that the use of ionizing radiation entails. Another advantage is the possibility to make measurements from just one side of the object under investigation. Comparison of microwave holographic radar and the X-Ray images are used here to evaluate the proposed method.

This paper explores the possibility of using a synthetic aperture radar to detect surface defects of rails and measure parameters of rail junctions. Experimental data were obtained with a setup consisting of a two-coordinate electromechanical scanner and a radar emitting continuous stepped-frequency signal in the range of 22.2 - 26.2 GHz. As an object of study, fragments of narrow-gauge rails were used, in which surface defects of various sizes and depths were created. A phase method for radar signal processing based on the backward propagation of its wavefront was developed, with which radar images of rails with defects were obtained. Experimental studies have shown that the developed rail surface imaging method allows detecting the presence and measuring characteristics of cracks on the tread, cleavages of the railhead, the width of the joint gap and the magnitude of the vertical step at the rails joint. High accuracy and sensitivity of the radar method, confirmed with the contact measurements matching, allow using it for fast noncontact diagnostics of the rails condition.

The evaluation of microwave (MW) imaging radar capabilities in non-destructive tests to detect internal structures or defects in the inner volume of thermal insulation coating has been carried out. The MW imaging with high spatial resolution has been performed with holographic subsurface radar operated in the frequency range from 22.2 to 26.2 GHz. The investigation of this non-destructive testing (NDT) technology is important to evaluate the feasibility for quality check of assembled polyurethane sandwich parts for thermal insulation. The samples of polyurethane foam thermal insulation have been fabricated in collaboration with a manufacturing industry of household appliances to have a representative set of real defects. The images obtained are of high quality and easy to be interpreted as they reveal the shapes and plane positions of the defect into samples with thickness up to 90 mm.