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Fig. 3. Arrangement of objects
in the mockup wall
Ability of executing one-sided sounding of a wall, instead of double-sided sounding as X-ray apparatus does
Ability to find out not only metal objects, but also non-metallic ones
Safety of radar's using for operator.
Airport Luggage Scanning
Counterintelligence activities for detecting bugging devices
Inquiry activities of law-enforcement agencies
Inspection of building structures for determining the position of reinforcement, voids and other heterogeneities
Surveying of especially critical constructions (airport runways, bridges, etc.) for determining their latent flaws
Detection of cracks in underground parts of buildings and structures for prevention of the water infiltration.
A subsurface radar using a multifrequency signal has been developed. It is designated for surveying building structures and works. The characteristic feature of this device is the possibility of obtaining sounding plane microwave images featuring a high resolution attaining 1...2 cm. T he main applications of this device include the survey of building structures to reveal their heterogene-ities and defects and the investigation of premises to detect bugging devices.
Key words: building structure, defects, heterogeneities, microwave images, subsurface radar.
The existing methods of the nondestructive testing of building structures have a number of deficiencies. For instance, X-ray devices require two-side access to a building, which is difficult in some cases or most commonly impossible. However, X-ray equipment has found a wide application in medicine, for baggage checking at airports and in those technological processes where the two-side access to an object under investigation does not produce any problems. Ultrasonic equipment proves to be inefficient in media containing a great number of microcracks and heterogeneities. Its field of application includes the study of continuous and relatively continuous media involving a small number of defects and allogenic inclusions. From this viewpoint the use of the RF range appears to be most promising because reflection sounding is possible, i.e. the receipt and radiation of RF waves is performed from one side of a surface sounded. This allows for examining walls, ceilings, enrichments, etc. in finished buildings. Thus, the quality control of their construction and repair is possible. The proposed method using a specially made antenna permits the survey in corners between walls, which is hardly feasible by using other techniques. A further advantage of radar sounding is a relatively long wavelength lambda within the microwave band used because this wavelength does not produce reflections from insignificant natural heterogeneities such as cracks and the operational hollows of bricks and other building materials which are small as compared with lambda.
Recently short impulse time domain subsurface radar systems have already found applications for sounding various building structures and coverings both for detecting hidden objects in them (Botros et al., 1984) and for defect diagnostics (Carter et al., 1992; Maierhofer et al., 1996; Papaioannou et al., 1996). The sounding of building structures presents certain difficulties, because the required resolution is very low and ranges from 1 cm to 5...10 cm depending on a problem to be solved. These resolution requirements are related to the characteristic size of heterogeneities, which are found in different building structures.
Especially high requirements arise when detecting various bugging devices hidden in the walls or the enclosures and enrichments of rooms. The size of these devices is very small and their design is such that they are difficult to be detected against the background of reflections from the natural heterogeneities of building structures. These heterogeneities can include steel reinforcement bars of concrete, pipes and electric cables, fastening nails and clamps, etc.
The resolution of subsurface radar has conventionally been improved by reducing the pulse duration in pulse radar or by widening the frequency band in FWCW radar. However, these measures result in more complicated and more costly equipment used for this work.
The building structure sounding radar discussed in this paper operates on several fixed frequencies, and a signal is received by using two cross polarizations on each frequency. Microwave images obtained have a high resolution in the sounding plane of the medium and allow for restoring the shape of objects by which assumptions can be made as to their nature and purpose.
The general view of RASCAN radar developed in TsNIIRES is presented on Figure 1. This is subsurface radar using a signal whose frequency varies according to the step law.
The radar includes a transmitter radiating on 5 frequencies within the 3.6 GHz to 4.0 GHz band and two receivers operating in the same wavelength band. The transmitter power is below 10 mW. The antenna of one of these receivers has the same polarization as the transmitter antenna has. The antenna of the other receiver effects cross-polarization reception. All the HF elements of the radar are mounted on an antenna unit and accommodated in a common body. The HF part of the radar is shown in the foreground of the picture on Figure 1. It is connected by a LF cable to an interface unit, which in turn is connected to the parallel port of the computer. And the computer itself does not require any modification except for installing appropriate software. The interface unit and the computer are shown in the background on Figure 1.
Microwave images are obtained by the mechanical scanning of the antenna unit over the surface of an object under sounding. The rate of scanning of the surface investigated is that which allows for taking the image of 1 m2 of the surface over the time interval from 5 min to 20 min. depending on the required space resolution. The levels of received signals are measured every 1 cm or 2 cm both along the X - axis and the Y - axis. A simplified block diagram of the radar is shown on Figure 2. Data was entered into the computer through a specially developed interface, which was connected, to the parallel port of the computer.
When taking measurements, the oscillator was switched serially from one operating frequency to the other one (f = 3.6, 3.7, 3.8, 3.9, 4.0 GHz). The frequency choice was in agreement with the requirement of changing the contrast of any object in respect to the background level at the boundaries of the frequency band. This is especially important for performing measurements in heterogeneous media and makes it possible to provide a sufficiently contrast observation of an object if only for one of the frequencies and, as is shown below, allows for detecting objects placed in the same line of sight but at different depths.
The electromagnetic radiation is reflected from objects possessing the dielectric permittivity contrast in respect to the medium in which they are located. By virtue of this fact obtained images show not only metal objects but also dielectric heterogeneities, e.g. voids, which distinguishes this device from metal detectors which are of considerable current use. Water and the increased humidity parts of structures are also of high contrast.
To demonstrate the efficiency of RASCAN radar, the mockup wall was sounded. The mockup wall was presented by a packet consisting of seven 1 m by 1.2 m plaster boards 10.5 cm thick in the aggregate, and there were different objects placed between the wall layers. Objects to be detected included two metal wires and seven 25-mm coins. One of the coins was placed under the left-hand wire and the other one was under the right-hand wire. Besides, a 3cm x 3 cm square opening was made in the second plaster layer and the opening depth was identical to the board thickness, i.e. 1.5 cm. The arrangement of the objects within the wall mockup is given on Figure 3. The size of the shadowed surface on the diagram was 0.6 m by 0.6 m.
The figure placed at each of the objects states the ordinal number of a layer, as viewed from above, under which this object is located, i.e. the object with figure 2 is between the 2nd and 3rd layers of dry plaster. A recess was made in the 3rd and 4th layers where a pistol mockup was placed; its barrel length was 13.5 cm and its grip height was 9.7 cm. A grid was plotted on the diagram for convenience. The grid spacing is 3 cm. The experimental results for the sounding of the different parts of the mockup wall are given on Figure 4 and 5.
Figure 4 presents the microwave image of the part of the mockup enclosed by a dotted line on Figure 3. This figure shows five microwave images, which have been obtained by using the parallel polarizations of the radar received and transmitted signals. These images are arranged from top to bottom, as the frequency increases. Both of the wires, seven coins and the opening are observed on this picture.
A relatively high contrast of the opening can be explained by the difference between the dielectric permittivity of the air filling the void and the dielectric permittivity of the plaster board material. Let us note that the level of the contrast of objects and its sign in relation to the background varies depending on the depth of their position. This is related to the features of the design of a two-way channel wherein a signal reflected from an object is added to the signal of a transmit antenna. Note that the coins placed both before and behind the wire are seen very well. The possibility of the observation of an object located behind another object in its shadow is related to the differences in the phases of signals reflected from objects located at different depths. By changing the frequency of a sounding signal, we can reduce the contrast of a nearby object and enhance the contrast of an object positioned at a greater depth behind the former one.
Figure 5 presents similar microwave images of objects obtained by using the cross polarization of received and transmitted signals. This polarization results in increasing the contrast of lengthy objects (wires) and in reducing the contrast of lumped objects (coins and opening).
The low radiation power of the radar equal to 10 mW assures its environmental safety. For the same reason the radar while in operation does not produce interferences to other devices operating in the same wavelength band. The discussed type of subsurface radar can find applications in the following fields:
Counterintelligence activities for detecting bugging devices;
Operative inquiry activities of law enforcement bodies;
Sounding of building structures for determining the position of reinforcement, voids and other heterogeneities;
Sounding of especially critical building works (airport runways, bridges, crossings, etc.) for determining their latent flaws.
Botros A. Z., A. D. Olver, L. G. Cuthbert, and G. Farmer, Microwave detection of hidden objects in wall. Electron. Lett., 1984, no. 20, pp. 374-380.
Carter, C. R., T. Chung, T. Masliwec and D. G. Manning, Analysis of radar reflections from asphalt covered bridge deck structures. Geological Survey of Canada. Paper. - 1992, no. 90-4, pp. 33-40.
Maierhofer Ch., H. Wiggenhauser, M. Brandfass, A. Pitsch and K. J. Langenberg, Localization of Tendon Ducts Behind Reinforcement in Concrete Using Radar in Combination with Inverse Scattering. Proceedings of 6th International Conference on Ground Penetrating Radar. GPR'96 Tohoku University Faculty of Engineering, Sendai, Japan, 30 Sep. - 3 Oct., 1996, pp. 257-260.
Papaioannou M. G., S. P. Papamarinopoulos and P Stefanopoulos, Geophysical Studies in Hermitage Museum, the Czar's Former Winter Palace, in St. Petersburg. Proceedings of 6th International Conference on Ground Penetrating Radar. GPR'96 Tohoku University Faculty of Engineering, Sendai, Japan, 30 Sep. - 3 Oct., 1996, pp. 101-106.