Bechtel, E.

Bechtel, T. D.

Borgioli, Giovanni

Bulletti, Andrea

Capineri, Lorenzo

Falorni, Pierluigi

Inagaki, M.

Ivashov, S. I.

Windsor, C. G.

Zhuravlev, A. V.

English

Florence Research

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 757Depth Information from Holographic Radar ScansC. G. Windsor1, A. Bulletti2, L. Capineri2, P. Falorni2, S. Valenini2, M. Inagaki3,T. Bechtel4, E. Bechtel4, A. Zhuravlev5, and S. Ivashov51116, New Road, East Hagbourne, OX11 9LD, UK2Dipartimento Elettronica e Telecomunicazioni, Universita` di Firenze, Firenze, Italy3Walnut Ltd, 4-2-54 Sakaecho Tachikawa 190, Japan4Enviroscan Inc, 1051 Columbia Avenue, Lancaster, PA 17603, USA5Remote Sensing Laboratory, Bauman Moscow State Technical University, RussiaAbstract— Holographic radar has several potentially important advantages over conventionalpulsed radar for buried object location and identification. In particular the sensor can belightweight, of low cost and provide images of the object with good resolution. This work reportsthat it can give some depth information also. Two simple experiments using the Rascan systemoperating at 5 discrete frequencies between 1.5 and 2.0GHz are described. The first “calibration”experiment used an aluminium plate buried in sand and inclined at a known angle to give a rangeof depths between 0 and 85 ± 5mm over a length of 300mm. The sensor was manually movedon a thin plate of glass placed over the sand above the plate. The scan down the plate showedcharacteristic bands in the backgound-corrected reflected intensity — the “zebra effect”. Thefive reflected amplitudes at each frequency are seen to vary differently with depth. These are in-terpreted qualitatively in terms of the simple theory involving interference between the reflectedwave and the incident wave at the sensor position. This theory does demonstrate the zebraeffect and suggests that the amplitude variation with frequency is characteristic of the depth.The cyclic nature of the zebra stripes means that the more frequencies available, the better thedepth discrimination. Quantitatively the true theory is complex, especially in the presence ofthe glass. However we suggest that the measurements can at least be used as calibration signalsfor a metal reflector at a given depth. The second “test” experiment involved nine US penniesburied in sand at known depths between 0 and 56mm with a lateral separation of about 50mm.A background-corrected total reflected intensity (the summed modulus of the reflected amplitudeover all frequencies) revealed the outline of each penny quite distinctly. For each penny the setof reflected signal amplitudes at each frequency was determined at the position over the pennywhere the signal was maximized. This “best” response for each penny gave a response over thefive frequencies which was distinctive and is indeed characteristic of the depth. A precise simu-lation theory is not available but the best frequency response of each penny could be comparedwith the frequency response curves from the aluminium plate as a function of the known depth.A least squares fit was made between the best amplitudes of as a function of frequency for eachpenny, compared to the amplitude variation of each frequency as a function of known depthin the inclined aluminium plate experiment. With arbitrary amplitude scaling a least squaresresidual minimum as a function of depth was always obtained for any one penny. There weresome problems. The amplitude scaling varied for each penny, presumably as its surface qualityand orientation altered the intensity reflected into the sensor. Also the signal quality deterio-rated with depth, and the frequency variation became less distinct at depths greater than 4 cm.However for the six pennies covering the depth range up to 3.4 cm the fitted depth agreed withthe actual measured depth to within a standard deviation of 3mm.1. INTRODUCTIONGround penetrating radar has many advantages for the detection of buried objects [1]. Howeverthe cost of conventional pulsed systems of order $200,000 has limited its use particularly in theimportant area of mine detection. The invention of holographic radar [2] offers similar or betterperformance for around $5000 [3]. Holographic radar’s simplicity arises from its use of continuoussignals at several different frequencies. The signal is extracted through interference between thewave reflected from the object with the emitted wave. The image of the buried object is built upby moving a scanning head and observing both amplitude and phase changes in the signal. In theRascan system there are five discrete frequencies between 1.5 and 2.0GHz and both parallel andperpendicular polarisations [4]. The antenna is moved by hand across the surface to be scannedas shown in Figure 1. The small attached wheel measures the distance along the scan and theoperator uses a ruler to determine the scan positions in the perpendicular direction.The working of the holographic radar system can be expressed most simply by assuming aplanar antenna and receiver and a planar reflector at a depth d into the ground. The equation for758 PIERS Proceedings, Moscow, Russia, August 18–21, 2009Figure 1: The Rascan system in use on the sand bed at Enviroscan, USA.-1-0.500.51-1 1 3 5Depth into ground (cm)AmplitudeReflector-1.5-1.0-0.50.00.51.01 2 3 4 5 6 7 8 9 10 11 12 13Depth (cm)Amplitude3.63.73.83.94Frequency (GHz)Velocity  12Attenuation F=0.3 (a)        (b)V= 5 10.Incident waveReflected waveFigure 2: (a) The incident and reflected wave amplitude as a function of depth into the ground with areflector present at 3.5 cm depth. The phase and amplitude of the two signals are identical at the reflector.(b) The interference signal at the antenna position as a function of the reflector depth as a function offrequency. A velocity of 20 cm, and some frequency dependent exponential attenuation has been assumed.an incident plane wave travelling along a one-dimensional depth variable x, at time t is yinc(x) =cos(2piνt + 2pix/V + φ0) where ν is the frequency, V is the velocity in the material, and φ0 is aphase factor. The dark continuous line in Figure 2 shows an incident wave of this sort travellinginto the ground with some small attenuation factor exp(−F (ν)x. The reflected wave from theobject at unknown depth d, shown by the dashed line, will travel back in the opposite directionand will have the same phase (or the opposite phase depending on the material) at that spatialposition at all times, a different amplitude A, but the same frequency and velocity. Its equation isyref (x) = A cos(2piνt + 2pi(d − x)/V + φ0). When the reflected wave arrives back at the antennainterference holography is performed by effective multiplication of the incident and reflected signalsat the position of the antenna. The resulting holographic signal yhol(0) is given by the integrationover all phase anglesyhol(0) =∫0,2picos(2piνt+ φ0)A cos(2piνt+ 4pid/V + φ0) exp(−F (ν)2d.dφ= 1/2A cos(4piνd/V ) exp(−F (ν)2d (1)and so is sinusoidal with a period depending on frequency, depth and velocity, with an amplitudedepending on the position of the reflector as shown in Figure 2(b).The amplitude is immediately seen be periodic in depth, with a period 2νd/V dependent onthe frequency. This is the origin of the intensity bands or “zebra effect” seen from sloping objects.It is seen that, at any one depth, the order of the signal as a function of frequency, varies andis to some extent characteristic of the depth. This is particularly true at depths within the firstoscillation period. At greater depths, the dependances of amplitude on frequency with depth arebecome similar although remain distinct. It is this distinct variation that offers the opportunity todetermine an object’s depth from its variation of reflected amplitude with frequency.However the simple Equation (1) is not valid in practical situations. In fact the incident waveis created by an antenna of finite shape and distinct geometry. The reflecting object in unlikelyto be an infinite plane. There are likely to be other reflecting surfaces, particularly the groundreflections, which cannot be neglected. The ground itself, soil or sand, will introduce its ownbackground scattering. All these effects can in principle be included and a mathematical model ofthe synthesized microwave hologram for point objects has been presented [4]. At present we haveProgress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 759no such comprehensive model. The method of this paper is to use experimental observations as themodel for understanding the reflections from unknown objects.2. THE INCLINED ALUMINIUM PLATE EXPERIMENT — THE CALIBRATIONA simple sheet of aluminium was used in this experiment to calibrate the holographic signals froman extended metallic reflector as a function of depth. The sheet was about 2mm thick and 350mmlong and was buried in the sand pit so that it had an inclination of 85±5mm in 300mm horizontaldistance (15.8◦). The top edge of the plate just touched the surface as shown in Figure 3. TheRascan head was scanned along the inclined direction at intervals of 10mm with the scans separatedin the direction perpendicular to the inclination of 10mm. There were 60 measurements along thedirection of incliation and 30 measurements in the perpendicular direction. Measurements weretaken in both parallel and perpendicular directions but the results were very similar and only theparallel direction is considered here.The scan is seen to contain a considerable area of “background”, an “edge” area of complexbehaviour which we shall neglect, and an area of “uniform” response from which we can extract asessentially one-dimensional response from the extended plate as a function of depth. A backgroundanalysis was first made. This is essential if quantitative amplitudes are to be extracted. A histogramof amplitudes was made over 6 of the scans within the background area and the mean taken foreach frequency, and subtracted from the raw amplitude data. 4 scans within the within the uniformarea were averaged and a mean amplidude level evaluated for each frequency as a function of thecalculated depth.(a) (b)Figure 3: (a) The layout of the inclined aluminium sheet experiment. The Rascan head was manuallyscanned parallel to the direction of inclination. (b) The grey scale presentation of the holographic data ateach frequency presented in the plane of the sheet.0100200300400500400 1400 2400 3400 AmplitudeFrequency3.6 GHz3.7 GHz3.8 GHz3.9 GHz4.0 GHzAluminium Plate (Parallel)Figure 4: Background determination From six scans in the upper half of the image. The amplitude of eachpixel in the image is plotted as a histogram. The centres of the amplitude histogram peak for each frequencyis quite distinct.Aluminium plate-2000-1000010002000300040000 10 20 30 40 5 60AmplitudeF3.6: AverageF3.7: AverageF3.8: AverageF3.9: AverageF4.0: Average5 1000Distance across plate (cm)Approximate depth (cm)Figure 5: The background-corrected averaged signal from the inclined aluminium plate as a function ofdistance along the plate for the five distinct frequencies (F) shown.Having subtracted the mean background level at each frequency, four scans in the lower regionof the image showing a strong signal were ageraged together for each frequency. The results shown760 PIERS Proceedings, Moscow, Russia, August 18–21, 2009in Figure 5 show a generally decaying set of amplitudes with the characteristic “zebra” bands ofpositive and negative amplitude with respect to the background at that frequency. The centres ofthe bands move characteristically with frequency and this is the key to the depth information.Figure 5 shows generally a strong similarity to the theoretical description in Figure 2(b). Themain difference is around the zero nominal depth position where the experimental curves extendaround a centimetre below the nominal zero position. This is not because of the plastic sheetoverlaying the sand. In holographic radar the signal from this would be expected to be independentof position and change only the background signal. The most likely causes were mentioned above.The antenna has finite extent and the wavelength of the radar is of order x cm in sand. Both ofthese cause resolution effects making the measured signal extend beyond the edge of the plate.Of course a similar effect occurs at the deep edge of the plate and the reflected signal is seen todisappear to background levels at around 40 cm lateral displacement.3. THE BURIED PENNIES EXPERIMENT — THE TEST SITUATIONNine US pennies were buried in sand at increasing depths from on the surface to 56mm deep in thesame area of sand used in the aluminium plate experiment. Figure 6 shows them as buried, beforecovering with more sand. The same 60 cm RASCAN was made over the pennies in the direction ofincreasing depth. Figure 7 shows the resulting images at each frequency.The scans across the pennies were analysed to find that scan which gave the largest amplitudeover each penny. Most of these lay on a single scan, but a few pennies lay off this line and gave amaximum response on other scans. These maxium amplidudes are shown as a function of pennynumber and frequency in Figure 8. It is a curious graph. The most shallow penny #1 gives only amodest singal with relatively little change of amplitude with frequency. The nominally 10mm deeppenny #2 shows the largest amplitude with a large frequency variation. Pennies #3 to #7 showsimilar amplitudes but a distinctive frequency variation. Pennies #8 and #9 are distinctly weaker.These data were analyzed for each penny by least squares fitting the 5 amplitudes from eachfrequency with the set of amplitudes as a function of depth derived from the aluminium sheetexperiment. It was necessary to introduce a arbitrary scaling factor for each penny. Possibly thethickness, reflectivity and specular reflection of the angle of each penny were different leading to adifferent amplitude factor. However the factor is assumed independent of frequency. Figure 9 showsthe actual and fitted amplitudes for the most shallow five pennies. Finally we show in Figure 10by closed circles the best fitted depths for each penny plotted against the actual measured depthswith crosses and error bars. The fit is always within the error bars for the pennies where a goodfit was obtained and the standard deviation of the fitted and actual depths is only about 3mm.Figure 6: The buried pennies experiment. shows a photograph of the pennies in the sand before being buried.After covering the pennies with more sand their depths were measured by poking with a wooden stick. Theradar scans also show a line of metal washers which proved too close together. For accurate analysis, andsome foreign Coins of random depths.Figure 7: The manual scan of the pennies experiment as a function of increasing penny depth. The variablenature of the phase of the signal is clear. Even the amplitude of the signal appears almost random. Whilethe penny signals appear reasonably separated, this cannot be said of the washers, which have not thereforebeen analysed.Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 761Pennies experiment-600-400-200020040060080010000 1 2 3 4 5 6 7 8 9 10Penny number (: depth)Amplitude3.63.73.83.94Frequency Depthd(mm)= 10 17 25 30 34 40 42 56Figure 8: The maximum set of amplitudes seen in any one scan from each penny as a function of frequency.-500-300-1001003005007009003.6 3.7 3.8 3.9 4Frequency (GHz)Amplitude1:  0 mm2: 10 mm3: 17 mm4: 25 mm5: 30 mm1:  5 mm2: 11 mm3: 19 mm4: 22 mm5: 30 mmAluminium plate fitsActual penny amplitudes and depths Figure 9: The measured set of amplitudes for each penny (crossesand full lines) fitted to those measured from the aluminium plateexperiment as a function of depth (closed circles and dashed lines).The measured depths and the fitted depths are shown. Pennies atgreater depths failed to give a good signal.01020304050601 2 3 4 5 6 7 8 9Penny numberDepth (mm)Measured depth (mm)Predicted depth (mm)Figure 10: The fitted depths of eachpenny (closed circles) compared tothe a measured depths (crosses withlikely errors).4. CONCLUSIONSThe sloping aluminium plate experiment and the simple theory of beating the incident wave witha reflected wave with phase shift 4piν/d (d = depth) clearly showed the origins and explanation ofthe stiped “zebra” effect from inclined reflectors.A good fit to theory is not possible because of the complications of the full probe emitter andobject geometry.The US pennies experiment was analyzed to determine the depths of isolated buried objects.The distinctive frequency response of each penny enabled the depth to be estimated by looking forthe same amplitude pattern within the aluminium plate experiment.The fit to experiment for the first 6 pennies was good to ±3mm. More frequencies would leadto a more robust depth algorithm.REFERENCES1. GPR Equipment and Service Providers, US Department of Transport pamphlet:http://www.fhwa.dot.gov/infrastructure/asstmgmt/gprbroc.pdf.2. Daniels, D. J., “Surface penetrating radar for industrial and security applications,” MicrowaveJournal, 68–82, December 1994.3. Chapursky, V. V., S. I. Ivashov, V. V. Razevig, A. P. Sheyko, and I. A. Vasilye, “Microwavehologram reconstruction for the RASCAN type subsurface radar,” Proceedings of the NinthInternational Conference on Ground Penetrating Radar, GPR 2002, 520–526, Santa Barbara,California, USA, April 29, 2002.4. Capineri, L., S. Ivashov, T. Bechtel, A. Zhuravlev, P. Falorni, C. Windsor, G. Borgioli,I. Vasiliev, and A. Sheyko, 12th International Conference on Ground Penetrating Radar, GPR2008, Birmingham, UK, 2008.5. Ivashov, S. I., et al., “Holographic subsurface radar technique and it applications,” 12th Inter-national Conference on Ground Penetrating Radar, GPR 2008, Birmingham, UK, 2008.

Depth Information from Holographic Radar Scans

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Depth Information from Holographic Radar Scans

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