Inspection of Internal Structure of Walls by Subsurface Radar

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ABSTRACT
This paper deals with the application of ultra-wide band time-domain subsurface radars, equipped with special signal processing techniques, to realize non-invasive image testing of the building walls' internal structure that are made with brick, stone, concrete, reinforced concrete and other construction materials. There are two normally associated problems, qualitative and quantitative, considered. Limitations and shortcomings of radar imaging, due to inherent physical features as well as signal processing improving the quality of radar images, are discussed. Actual field data are used to illustrate applications of subsurface radar for non-destructive testing of walls' internal regions.

Key words: Subsurface Radar. Ground Penetrating Radar. Nondestructive Testing, Radar Imaging

Introduction
Subsurface radar or ground penetrating radar (GPR) is worldwide effective non-invasive technique to study and image internal regions of opaque matters such as soils, rocks, brick, concrete, wood, fresh water, snow, ice, vegetation etc. Common GPR system includes three principle components: active sensor (transmitter and receiver with their antennas) spaced on surface that is border between air and media under investigation, and control electronics, and computer (Figure la).

The radar surveillance of testing areas is implemented by successive placing of the antenna sensor along a straight transect line in continues or stepped mode to implement the synthetic-aperture technique. The last enables to obtain preferable spatial resolution in radar where the used UWB antennas, especially in time-domain GPR. have inherently pure directive features (Finkelstein, 1994).

Return signals (A-scan) in each point, where the radar sensor is placed, are used to form a 2-D image of electromagnetic transect or the B-scan images. Various forms of image presentation due to computer's graphic opportunities can be used to visualize radar return like demonstrated in Figures lb and lc. Moreover, practical radar data is strongly corrupted due to numerous unavoidable reasons. Therefore, computer processing and image enhancement must be implemented.

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