Ground Penetrating Radar (GPR) direct wave study = Μελέτη των απευθείας κυμάτων στη μέθοδο του ραντάρ υπεδάφους.

Eleni Aggelos Tokmaktsi


In this study, the major aim is to examine the spatial distribution of the electromagnetic (EM) signals around the ground penetrating radar (GPR) transmitting antenna (Tx). Since the recorded signals are mostly dictated by the strong direct wave signal, we have focused on the study of this response, attempting to locate areas around the Tx where the direct wave becomes minimum, while the signal strength of the reflected wave remains ideally unchanged. The location of those local minima in the direct signal may give rise to advantageous Tx-Rx configurations, where a clear reflection can be obtained with the least possible involvement of the direct wave, showcasing the reflections of targets lying at shallow depths. To perform such a study, field testing was performed at a site where a mostly horizontal subsurface reflector was located. This was confirmed by carrying out GPR grid measurements and validated using the electrical resistivity tomography (ERT) method. Wide Angle Reflection and Refraction (WARR) measurements were also performed to estimate the average EM wave velocity in the uppermost layer and define more precisely the depth of the subsurface features in the survey area. Having found an almost horizontal layer in the field, both the direct and reflected waves were studied by collecting static GPR data around a circle. This way both the direct and reflected waves were studied in terms of (a) changing the Tx-to-Rx antenna orientation (i.e., broadside and endfire), (b) changing the Tx-to-Rx antenna separation (by varying the circle’s radius) and (c) changing the Tx-to-Rx antenna angular position (i.e., one antenna was placed at a fixed location in the circle’s center while the other was moved radially in fixed angle steps around the first one). In addition to field testing, numerical modelling was performed to study these three (3) aforementioned factors that are known to impact the EM signal distribution around the GPR transducers. For the numerical modelling work, the GPR antennas were represented either by ideal Hertzian dipoles (point dipoles) or by resistively loaded bare dipoles.

Κύριος στόχος της διατριβής είναι η μελέτη της χωρικής κατανομής των ηλεκτρομαγνητικών (Η/Μ) κυμάτων γύρω από την κεραία του πομπού (Tx) στη μέθοδο του ραντάρ υπεδάφους / γεωραντάρ (GPR). Οι αφίξεις που καταφθάνουν και καταγράφονται από την κεραία του δέκτη (Rx) του GPR χαρακτηρίζονται κυρίως από την ισχυρή άφιξη του απευθείας κύματος. Το σήμα αυτό πολλές φορές «καλύπτει» ανακλάσεις από στόχους ενδιαφέροντος, ιδιαίτερα όταν αυτοί βρίσκονται κοντά στην επιφάνεια του εδάφους. Μελετάται λοιπόν το απευθείας κύμα με σκοπό την εύρεση χωρικών συντεταγμένων γύρω από τον πομπό του GPR όπου το κύμα αυτό ελαχιστοποιείται σε πλάτος ενώ παράλληλα, το ανακλώμενο κύμα (προερχόμενο από υπεδάφιους στόχους) παραμένει -ιδανικά- αμετάβλητο. Η εύρεση τέτοιων περιοχών μπορεί να οδηγήσει σε κατάλληλες διατάξεις των κεραιών πομπού και δέκτη (Tx-Rx) όπου η καταγραφή του ανακλώμενου κύματος δεν θα επηρεάζεται/μολύνεται από το απευθείας κύμα, με αποτέλεσμα να αναδεικνύονται ανακλάσεις που προέρχονται από επιφανειακούς στόχους. Πραγματοποιηθήκαν μετρήσεις πεδίου με τη μέθοδο του γεωραντάρ σε περιοχή όπου εντοπίστηκε ένας σχεδόν οριζόντιος ανακλαστήρας. Η οριζοντιότητα του ανακλαστήρα επιβεβαιώθηκε από μετρήσεις GPR σε κάνναβο καθώς και με την εφαρμογή γεωηλεκτρικών τομογραφιών (ERT). Η GPR μέθοδος μέτρησης της ευρυγωνίας ανάκλασης και διάθλασης (WARR) χρησιμοποιήθηκε για τον υπολογισμό της μέσης ταχύτητας του Η/Μ παλμού στο ανώτερο στρώμα έτσι ώστε να υπολογιστεί με μεγαλύτερη ακρίβεια το βάθος των υπεδάφιων στόχων στην περιοχή μελέτης. Τα απευθείας και ανακλώμενα κύματα μελετήθηκαν στην παραπάνω περιοχή μέσα από τη συλλογή στατικών δεδομένων GPR γύρω από μια κυκλική διάταξη. Έτσι, απευθείας και ανακλώμενα κύματα μελετήθηκαν: α) αλλάζοντας τη διάταξη πόλωσης των κεραιών Tx και Rx (broadside και endfire), β) αλλάζοντας την απόσταση Tx-Rx (αλλάζοντας δηλαδή την ακτίνα του κύκλου) και γ) αλλάζοντας τη σχετική γωνία των κεραιών (η μία κεραία παραμένει σταθερή στο κέντρο του κύκλου ενώ η άλλη κινείται κυκλικά γύρω από την άλλη σε προκαθορισμένα βήματα δειγματοληψίας). Επιπλέον, παρήχθησαν συνθετικά δεδομένα για να μελετηθούν οι προαναφερθέντες τρείς (3) παράγοντες, οι οποίοι είναι γνωστό ότι επηρεάζουν τη χωρική κατανομή του Η/Μ σήματος γύρω από μία κεραία GPR. Για τα συνθετικά δεδομένα, οι κεραίες γεωραντάρ (Tx και Rx) μοντελοποιήθηκαν είτε ως σημειακά, θεωρητικά δίπολα, είτε ως πραγματικές κεραίες διπόλων.

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