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Ground-penetrating radar (GPR) has been applied to determine the subsurface karstic features during the construction of the national highway in the south-eastern part of Slovenia. The highway construction is situated mostly in the dinaric karstic region with a high density of karstic features visible on the surface. Ground-penetrating radar prospecting was done in all areas where a slope was cut into the limestone bedrock. The main purpose of the survey was to map potentially hazardous zones in the highway subsurface and to detect and characterize the karst. The ground-penetrating radar method was used because of the heterogeneous nature of the karst. With its high degree of karsticifaction and geological diversity all conventional methods failed. One of GPR’s main advantages is that, while the penetration depth is limited to several meters, the obtained resolution can be on the scale of centimeters and the measured profile is continuous. Because of the ground-penetrating radar’s limitations with respect to depth, the range surveying was done simultaneously with the road construction using 200-MHz bistatic antenna on the level of the highway plane. All the 2D radargrams were constructed in 3D models where the measurements were made in raster with 2 meters between a single GPR profile. This two-meters spacing was determined as the optimal value in which only a minimal resolution-price tradeoff was made. The gathered results were tested and compared to experimental drillings and excavations so that any anomalies and reflections were calibrated. The drilling was conducted twice, first to calibrate the radargram reflections and secondly to check and confirm the calibration success. Altogether, over 30 boreholes were drilled at various previously selected locations. The data obtained from the drilling proved to be very helpful with the calibration since anomalies found during the drilling were almost exclusively (over 95%) a result of the propagation of radar waves from the limestone to an air void or from the limestone to a clay pocket. Drilling test boreholes proved to be a very useful tool for the calibration of the GPR anomalies recorded in 2D radargrams. Such a process showed a near 100 % accuracy with respect to interpreting the subsurface features, with 77% correctly interpreted as caves or clay pockets and 23% wrongly interpreted, where the interpretation was a void but it was indeed partly a clay-filled and partly an air-filled void. The completed survey also showed simultaneous surveying with GPR and road construction is a very efficient and economical way to predict various karstic features and the density of the karstic forms. Georadarske meritve so bile izvajane za detekcijo podpovršinskih kraških pojavov ob izgradnji nacionalne avtoceste v jugo-vzhodnem delu Slovenije. Predvidena trasa avtoceste je locirana večinoma v dinarskem krasu z veliko gostoto kraških pojavov, ki so vidni že na površju. Preiskave z georadarjem so bile izvedene v vseh območjih, kjer so bili projektirani globoki vkopi v kraški apnenec, z namenom detekcije ter karakterizacije kraških pojavov. Preiskave z georadarjem so bile izbrane zaradi ekstremno heterogene narave kraškega terena, kjer konvencionalne metode preiskovanja ne pridejo v poštev oziroma so finančno preveč zahtevne. Ena glavnih prednosti preiskav z georadarjem je, čeprav je globina penetracije omejena na nekaj metrov, ločljivost ter preiskanost terena, saj lahko v idealnih pogojih zagotovimo ločljivost nekaj centimetrov na celotnem kontinuiranem profilu. Zaradi dejavnikov, ki omejujejo globinski doseg preiskav z georadarjem, so bile preiskave izvajane istočasno z samo gradnjo avtoceste. Preiskave smo na podlagi preučitve korelacij med dosegom ter ločljivostjo izvajali z 2000 MHz oddajno – sprejemno zaščiteno anteno po cestnem telesu pred polaganjem asfaltnega sloja. Iz 2D radarski profilov (radargrame) smo skonstruirali 3D modele površja, kjer so bile označene močnejše EM anomalije. Različni razmaki med posameznimi 2D profili so bili testirani z namenom ugotovitve optimalne finančno – časovne kombinacije, ki je še vedno zagotavljala potrebno ločljivost. Pridobljeni rezultati so bili kalibrirani ter testirani s testnim vrtanjem. Vrtanje je bilo izvajano dvakrat, in sicer, prvič za kalibracijo georadarskih anomalij z radargramov z dejanskimi kraškimi pojavi na terenu ter drugič za preverjanje rezultatov. Skupno je bilo izvrtanih več kot 30 vrtin na predhodno določenih lokacijah. Podatki s testnega vrtanja so se izkazali kot zelo dobri, saj so bile ugotovljene anomalije v veliki večini (nad 95%) posledica uklonov med apnencem in prazninami oziroma apnencem ter glinenimi žepi. Vrtanje testnih vrtin se je izkazalo kot zelo primerno orodje za kalibracijo georadarskih anomalij zajetih na radargramih. Tak pristop se je izkazal, kot zelo natančen pri interpretiranju georadarskih anomalij kraških pojavov, 77% anomalij je bilo po kalibracijskem vrtanju pravilno interpretiranih kot jama oziroma glineni žep, 23% napačno interpretiranih pa so bili interpretirani kot glineni žep namesto praznine ali obratno. Istočasna preiskava z georadarjem ter gradnjo avtoceste se je izkazala kot zelo uspešna, saj je tako (po odstranitvi glinenih slojev) razmerje med globinskim dosegom ter ločljivostjo optimalno. |
