gpr antenna resolution - reinaldo alvarez cabrera

8/9/2019 GPR Antenna Resolution - Reinaldo Alvarez Cabrera

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concept, it is mar#ed with +1r for horiontal resolution.

Fig. 1 ertical and Hori!ontal resolution diagram" the golden dots are the targets.

How do we calculate them2ow that we #now what the vertical and horiontal resolution are, we should !e

a!le to get them out from the antenna datasheet the manufacturer supplies, right-*orry, !ut no. The antenna manufacturers will !e more than happy to give you a lots ofcommercials, !rochures, and almost +out of the geophysics te3t !oo#s data samples,!ut nothing else. There is one thing though that we can get from almost every antennamanufacturer on the mar#et, the center frequency of the antenna. %e aware thoughthat different manufacturers specify this parameter differently. & much !etter approachwould !e to as#, what is the transmitted pulse width- That would allow you to ma#eyour own conclusions on the center frequency in the way you are familiar with.

The reciprocity theorem states that a receiving antenna is as good at receiving as it is attransmitting if all physical and fa!rication parameters are #ept identical. *o, anythingthat gets transmitted can theoretically get received !ac# !y a similar device. Ta#ing intoaccount that the earth wor#s li#e a low pass filter, then we can surely state that whatwas not transmitted has a!solutely no chance of !eing received if it has higherfrequency than the e3isting ones in the frequency spectrum of the original signal. Theonly e3ception of that rule is if you are using an unshielded antenna and the source ofthe signal is an e3ternal transmitter. This concept is important !ecause " have personallyread a!out antennas marvelously receiving microwaves frequencies with pulse width of 4to 5 ns. 1ow that happened was unfortunately never e3plained to me, !ut "$ll pu!lish itout as soon as " #now all the facts.

Copyright 2007-2011 Geoscanners AB AN00211210!Nrev2"0


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Why is the transmitted pulse or the center frequency so important- "t is !ecause thevertical resolution is nothing else than half of the duration of the transmitted pulse in

time. "f we translate that into depth information then we can say that the verticalresolution is half the width of the transmitted pulse times the velocity in the media

Vr=Tpulsec

2RDP(/)

where

ris the vertical resolution

#$ulseis the transmitted pulse duration, this can !e calculated !y ta#ing theinversion of the fundamental or center frequency.

cis the speed of light in vacuum

%&is the relative dielectric permittivity of the media.

Please #eep in mind that this is an appro3imate formula that wor#s most of the time,!ut not always. The reason for that is that the transmitted pulse will suffer from lowpass filtering in the media it is traveling through. *preading losses of the signal alsoaffect the a!ove e3plained formula. This means that o!ects that are far away from thesurface will most li#ely have a different vertical resolution than those closer to thesource of the sounding pulse.

&nother important factor to consider when trying to estimate the vertical resolution isthe type of materials of the two close targets. 6aterials that produce strong reflections

are more li#ely to mas# the o!ects that are close to them, while materials that producewea# reflections will !e easier to detect due to a more local signature. This lastconclusion is, of course, true only if the wea# reflections are strong enough to !edetected relia!ly otherwise you might loose them entirely.

The !ottom line a!out vertical resolution is that promising too high a resolution is not agood practice. & more conservative approach of ta#ing twice as much as the calculatedvalue will put the survey in a !etter perspective. The level of e3pertise of the personinterpreting the survey data will also, without any dou!t, play an important roll in thismatter. GPR surveys, more often that not, don$t fail. They ust don$t live up to thepromises made and the e3pectations from the customer ordering them. 7sing all the

#nowledge the theory provides us and e3ercising common sense is a #ey to success.2ow, the second type of resolution, the horiontal resolution. This is a topic of

much de!ate and many people have their own opinion on what should !e the right wayof calculating the horiontal resolution. %asically, one could summarie that thehoriontal resolution depends on the following parameters

/. The amount of traces per unit of distance. "f you have /8 traces per meter thereis no chance of discriminating o!ects that are 98 mm apart from one another.&ctually "$d !e very surprise if you can actually discriminate o!ects that are/88mm apart from one another.

:. The !eam width of your antenna. This parameter is almost never specified in the

antennas datasheet and many people use appro3imations for the real value. "t is



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compared to the previous e3ample.

The setup for the pipes is almost identical to the setup for the layers e3cept now " used

/8mm steel pipes instead of the plates. & diagram presenting the setup is shown inigure 5.

Fig. etu$ for the e*$eriment using vertically s$aced $i$es

The results are much more difficult to interpret and at 98mm separation it is almostimpossi!le to identify the second target if you do not have previous #nowledge of what$s+!uried under. igure 9. shows that even at /88mm distance the strong reflectioncaused !y the first pipe ma#es it very difficult to detect the second one. 6anye3perienced GPR practitioners may argue with me that they would spot the second pipeat 98mm without any trou!le at all. That$s !ecause the two strong positive, !lac# in thiscase, hyper!olas would nicely collapse into dots when migration is applied to them.That might or might not !e the case, " personally would prefer to use a higher frequencyantenna and have them separated nicely.

Fig. ,*$eriment results for the two $i$es vertical resolution.



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or the horiontal resolution e3periments " used some :8mm pipes, !ut " placed them inthe same plane, parallel to the surface. The distance from the antenna to the pipes was

reduced to @9mm and the horiontal distance !etween the pipes was changed from:98mm to /88mm and 98mm for the closest separation.

Fig. / etu$ for the hori!ontal resolution e*$eriment.

The simple formula e3plained a!ove for the horiontal resolution gives an estimate forthe horiontal resolution of //Amm. This estimate seems to !e a little too high if youconsider igure A. where the results of the e3periment are shown.

Fig. 0 ,*$eriments results for the hori!ontal resolution se$aration

"t appears that the resolution is quite higher, in fact, one could easily say that /88mmseparation is quite >B. That is appro3imately /9C !etter than the estimate, !ut onemust consider all the factors affecting the outcome of the e3periment.. "n real surveyconditions the relative dielectric permittivity won$t !e even close to !eing a constant,neither is the radiated frequency a single spectral line. 1ence the fact that theestimated horiontal resolution shown in formula num!er two is only a very goodstarting point to estimate the via!ility of the survey.



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Conclusions

;oming !ac# to the question that started this discussion how many antennas should I

have? The answer would !e very simple, as many as the amount of radically differentsurvey o2s you need to underta3e.

"t is not my intention to further confuse anyone with the presented material. Thisconclusions are !ased on what " have read in the literature listed in the referencesection and on my personal e3periences. " !elieve it is a good practice to analye all thefactors affecting the outcome of a ground penetrating radar survey !efore rising too highhopes in the people that have placed their trust in this technology. *ometimes it can !etoo much trust and not enough facts that will ruin a survey result.

" leave you at the starting point for you to draw your own conclusions and do a little !itof e3perimentation to find out what wor#s !est for your particular line of surveys.

&ll the data used in this article was collected using a *"R?4888 control unit from G**" 7*&and our own antenna G;%?/988. The data was processed with our own software GPR*oftPR> and the document was prepared using the >pen>ffice writer.

"f you have any questions or suggestions please send an email to mailDgeoscanners.comand the words +GPR antenna resolution in the su!ect. "$ll answer to all emailsaccording to time availa!ility.

References

1. .4.5illard" 6.haari" 7.H.8ungey" 99" %esolution of 4% 2owtie antennas.

roceedings of the 19

th

Intenational :onference on 4round enetrating %adar"4% 99.

. ;awrence 8. :onyers" 4round-enetrating %adar for 6rchaeology" 6ltamira ressI8< 9-0=1-900-/

+. 7ohn 5. %eynolds" 6n introduction to 6$$lied and ,nvironmental 4eo$hysics"7ohn Wiley and ons" I8< 9-01-=/>9-1

. Harry 5. 7ol" 4round enetrating %adar #heory and 6$$lications" ,lsevier"I8< =0>-9--++>-0

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gpr antenna resolution - reinaldo alvarez cabrera

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