Design and Applications of Multi-Frequency Holographic Subsurface Radar: Review and Case Histories

Abstract

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.

An Example of Microwave Holography Investigation of an Old Orthodox Russian Icon Dated to 19th Century

“…Another advantage of holographic subsurface radars is their complete safety for operating personnel, since the power of the emitted signal used in them is two orders of magnitude less than the power of a mobile phone and the devices themselves can be made compact and easy to use [7].…”

“…Various NDT methods are used to examine cultural objects, such as X-ray [1] and ultrasound [2] technologies, infrared thermography [3], terahertz spectroscopy [4], as well as subsurface radars of both impulse [5] and holographic [6] types. The last devices have been used in recent years to obtain images of the internal structure of objects and detect hidden defects, such as delamination, chipping, surface and subsurface cracks, reinforcement corrosion, the presence of small air voids, and moisture distribution [7,8]. All these listed methods are characterized by different spatial resolution and penetration depth, as well as different sensitivity to changes in the physical properties of materials.…”

“…Holographic subsurface radar recorded MW holograms is a special type of GPR. It emits multi-frequency continuous wave signal and is distinguished by its relative simplicity of design and the ability to adjust the operating frequency range in accordance with the requirements of the task being solved [7]. The paper considers an example of using MW holographic sensors for examining an old Orthodox Russian Icon dated to the end of 19th century.…”

Microwave time reversal for nondestructive testing of buried small damage in composite materials

“…Due to these performances, composite materials have been rapidly developed in recent decades and are widely used in various fields such as aerospace, electronics, and medical treatment. The case of composite materials backed by metal plates is an important application environment in many fields that have only single-side access, such as through wall target detection [1], subsurface radar [2], nondestructive testing (NDT) of thermal insulation coatings [3,4], and breast tumor detection [5,6]. Some damages, including voids and cracks, will destroy the strength and mechanical performance of materials, causing safety hazards [7].…”

Feasibility of Using a 300 GHz Radar to Detect Fractures and Lithological Changes in Rocks

“…Currently, automated machine learning and classification models have been applied to processing GPR data in order to classify geological structures, obtain dielectric permittivity variations and identify boundaries [6][7][8][9][10]. In addition, among the different types of GPR radars, it is worth mentioning multi-frequency holographic radars, which have better spatial resolutions than impulse subsurface radars [11]. However, one of the limitations of GPRs is the impossibility of measuring millimeter and sub-millimeter fractures due to the range of frequencies that are usually used.…”