Use of Holographic Subsurface Radars at Examining Cultural Heritage Objects
Russian Science Foundation Project No. 21-19-00043

Traditional areas of radar using, as a rule, consider a relatively homogeneous space with a low level of attenuation as a propagation medium for example air or space. Subsurface radar is characterized by environments under investigation with a high degree of heterogeneity and absorption level: soil, building structures, sea and ocean ice, and others. The problems associated with the registration and restoration of MW holograms in such environments require the development of appropriate mathematical methods and algorithms.

One of the areas that can become the object of application of the developed methods is the study and preservation of monuments of the cultural heritage of mankind.

The preservation and competent restoration of the mankind cultural heritage objects requires a thorough preliminary examination of their physical condition and internal structure. In term of the presented project, the following main problems should be solved:

• Investigation of the structure of the cultural objects;
• Assessment of heterogeneity of structural materials;
• Detection of hidden undocumented items;
• Detection of cracks and depth of its propagation;
• Determination of areas with high humidity.

Usually, restorers and culture experts use for solving emerging problems various methods of non-destructive testing, which were proved in common practice: X-ray, infrared and acoustics devices and other methods.

In recent years, microwave imaging has become widespread. This is a method of introscopy that allows visually studying the internal objects in media that are opaque in the optical-wavelength range. The team of authors of the presented project developed a new class of devices working on this technology: holographic subsurface radars RASCAN. Radars are produced in lots and supplied to both Russian customers and abroad. The prize of the Russian Federation Government in the field of science and technology was awarded for the development of RASCAN radars.

These devices are used for non-destructive testing of composite materials, examination of building structures, detection of bugging devices and in other areas. The use of RASCAN radars is absolutely safe for staff because the emission power is only 3–5 mW, which is two orders lower than the output of cell phones. This is a significant advantage over X-ray technology, where ionizing radiation is dangerous at almost any exposure level and requires special precautionary measures. Another advantage of radar is the ability to implement analysis with one-side access to the object, while x-ray installations, with rare exceptions, require access to the object from both sides. This is the operational concept of medical devices or devices for baggage screening of passengers. Although X-ray machines have a potentially higher spatial resolution due to a significantly shorter wavelength than microwave, experiments which were performed in collaboration with Institute of Nuclear Physics of Moscow State University showed that in some cases holographic subsurface radars have a higher sensitivity to structural defects in comparison with X-ray installations. Such a phenomenon is related to the different principle of interaction of these types of radiation with matter.

With the help of RASCAN radars, several objects, which have cultural and historical value, have already been examined. In particular, the floors of the Senate building in St. Petersburg, which was converted into the Constitutional Court of the Russian Federation, were studied. Two other tested objects were located in Italy. These objects were: the Church of St. Biaggio (S. Biagio) in the province of Tuscany and St. Marcos Crux (Croce di San Marco) in Florence. The Padua University, Italy uses the RASCAN radar to diagnose ancient frescoes and mosaics in order to detect later layers that were performed during their restoration and renovation in the XVI–XIX centuries, when the concepts of preservation of the author's right did not exist. In the USA, these devices were used to diagnose termite damage of old buildings wooden structures.

The results of these studies and others were reported at the international conference "Heritage, science and technology", which was held on November 15–16, 2017 as a part of the VI St. Petersburg International Cultural Forum, and at the second International conference "Study of works of art and objects of cultural and historical heritage. New technologies and their application", which was also held in St. Petersburg on July 25-27, 2018 under supervision of the Ministry of Culture of the Russian Federation along with The British Institute of Non-Destructive Testing (BINDT) and the assistance of the Russian Museum, St. Petersburg as well as published in indexed international journals. It should be noted that currently microwave methods don’t have widespread application for the diagnosis of cultural heritage. The exception is the impulse subsurface radars, which are widely used in archaeology. Research of new methods of electromagnetic imaging in inhomogeneous media with losses and creation of devices for inspection of cultural heritage objects is the main task of the project. At the same time, it is necessary to solve a number of theoretical and experimental problems that related to the choice of optimal wave ranges depending on the task to be solved and the determination of the object inspection method (with monostatic, two-way bistatic or as general case the multiposition antennae). During the project, research will be carried out on methods for reconstructing microwave holograms on objects of complex shape and heterogeneous internal structure that are characteristic of objects of cultural heritage.

During the project performance it is planned to choose and examine the objects most suitable for their diagnosis by electromagnetic emission: old buildings, churches and works of art before, during and after restoration.

Particular interest is the development of methods for microwave diagnosis of the Khufu pyramid, Egypt, in connection with the alleged detection of a concealed chamber in it, which happened due to usage of muon sensors. Reports about it discovering have recently appeared in scientific papers. Methods of microwave imaging, as authors of the project suppose, allow to determine the location and size of the internal cavities of the pyramid more accurately and confirm or deny the presence of unknown voids in the pyramid. In this case, it is necessary to develop numerical methods for reconstructing microwave holograms for objects with complex form including the one that has typical form for Egypt pyramids. The results of the project and the developed equipment will be tested primarily in the process of restoration of national architectural monuments under supervision the Ministry of culture of the Russian Federation and the Russian Museum, St. Petersburg.

Publications of the project participants related to the survey of cultural heritage objects are presented here.