Investigations of the Rahivere peat deposit by ground-penetrating radar

Rahivere turbamaardla uuringud georadariga Magistritöö käsitleb 2009. a veebruaris Vooremaa idaossa jäävas Rahivere turbamaardlas teostatud georadari uuringuid. Rahivere turbamaardla paikneb voortevahelises nõos ning tema pindala on 53,63 ha ja aktiivseks reservvaruks 97 000 tonni. Maardlale ja selle vahetusse ümbrusesse rajati keskmiselt 100 meetriste intervallide tagant 13 kirde-edelasuunalist radariprofiili ja üks loode-kagu suunaline profiil kogupikkusega 9 km. Mõõtmisteks kasutati Läti päritolu georadarit „Zond 12e“ sagedusel 300 MHz. Radargrammidel eristub turbakihi lamam valdavalt pideva ja selge peegeldusena, mis paikneb võrreldes servadega nõo keskosas oluliselt sügavamal (üle 4 m). Radargrammide alusel määratud turba paksuse ning elektromagnetlainete levikukiiruse kontrollimiseks puuriti maardla piiresse erinevatele profiilidele käsipuuriga kokku 10 auku. Maardla leiti koosnevast vähelagunenud (ülal) ja hästilagunenud (all) turbast. Turba lamamiks on liiv ja aleuroliit (nõo servaaladel) ning limnilise tekkega savi (nõo keskosas). Radargrammid interpreteeriti kasutades turba permitiivsusena väärtust 69,5 ning saadud andmepunktide alusel loodi arvutiprogrammiga ArcGIS turba levikut kirjeldavad pinnad ja arvutati turba mahud. Pinna genereerimiseks sobivaima meetodi leidmiseks katsetati erinevaid interpolatsioonitehnikaid. Lisaks erinevate meetodite võrdlemisele analüüsiti pinnale arvutatud mahu seost tema tegemiseks kasutatud informatsiooni hulgaga. Selleks tehti iga meetodiga 3 pinda: esiteks kõikide profiilide andmepunktidega (14 profiili), teiseks iga teise profiili andmepunktidega (6 profiili) ning kolmandaks iga kolmanda profiili andmetega (4 profiili). Sobivaimaks vahendiks pinna loomisel võib pidada andmepunktide triangulatsiooni (ArcGISi TIN). Leiti, et optimaalseks georadari profiilide vahekauguseks maardla turbamahu leidmiseks on 200 meetrit. SUMMARY Investigations of the Rahivere peat deposit by ground-penetrating radar Ivo Sibul Rahivere peat deposit lies in the eastern part of Vooremaa drumlin field. According to the Estonian records of mineral resources of the Environmental Registry it has an area of 53,63 hectares and reserves of 97 thousand tons. Calculation of these reserves is based solely on borehole data. Current study proposes an alternative method for peat volume calculation. Fourteen georadar profiles were created in Rahivere peat bog, covering the area of peat deposit and it’s nearest vicinity. Thirteen of the profiles were NE-SWoriented and juxtaposed approximately 100 meters from each other; one profile connects 4 profiles perpendicularly. Latvian „Zond 12e” ground-penetrating radar at the frequency of 300 MHz and common offset method was utilized during field work. Additionally, common mid-point technique (antennae 300 & 500 MHz) provided some information for estimating the dielectrical permittivity of peat (69,5). Radargrams reveal that thickness of the peat deposits extends four meters in the centre of the valley. In order to verify the radargrams’interpretation as well as the evaluated velocity of electromagnetic waves in peat, ten boreholes (depth from 1,75 to 5,5 m) were drilled manually upon the profiles. According to the borehole data, minerotrophic peat, where present, is covered by the ombrotrophic one. Mineral soil consists of silt and sand in the peripheral areas of the valley and limnoglacial clay in the central part. Peat thickness data were introduced as a point feature class into ArcGIS geodatabase. These spatially coordinated points were source data for some wellknown interpolation methods (spline, natural neigbor, IDW, kriging, TIN). The aim of generating raster and TIN surfaces was peat volume calculation. At first, all the georadar information was used. Then, data points were reduced by eliminating 8 of the profiles and finally, only 4 of them attended in the processing. Distance between the profiles was 100, 200 and 300 meters, respectively. Calculated peat volumes correspond well with official information. TIN method is given the preference after comparing the isolines of the surfaces. 200 m step is suggested as the optimum for georadar profiling in peat deposits