Effects of Probe Installation Method, Crack Existence and High Temperature on Apparent Dielectric Constant Determined by Time Domain Reflectometry in Rocks

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  • TDR法による岩石の比誘電率測定に及ぼすプローブの設置方法，亀裂の有無および高温の影響
  • TDRホウ ニ ヨル ガンセキ ノ ヒ ユウデンリツ ソクテイ ニ オヨボス プローブ ノ セッチ ホウホウ キレツ ノ ウム オヨビ コウオン ノ エイキョウ

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<p>The relations between apparent dielectric constant and volumetric water content of Akeyo sandstone, Kimachl sandstone and Shirakawa tuff were tested to measure the water content of rock by the time domain reflectometry (TDR) method. In addition, the values of apparent dielectric constant under three different conditions of the TDR probe installation were compared, as well as the effect on the existence of crack in a rock specimen. Moreover, to understand the effect of high temperature, the apparent dielectric constants of dry rock specimens were examined by changing the temperature from normal one to 80℃.</p><p>The apparent dielectric constant measured by the TDR method sensitively increased with the increase of the volumetric water content of rock specimen, showing that the TDR method is applicable to measure the water content of rock. However, under the same water-content condition, the apparent dielectric constant significantly changed when the probe installation method was different. In particular, the apparent dielectric constant was underestimated when air had remained in gaps between the probe rod and the guide hole in the rock specimen. The improved piston-type probe, which was developed in this study, could minimize the effect of the gaps. Comparison between the apparent dielectric constant of rock specimen with single crack and that of intact rock specimen showed that the apparent dielectric constant obtained by the TDR method was influenced by the existence of crack in the specimen. In particular, when air remained in the crack, the apparent dielectric constant was notably underestimated. The apparent dielectric constant of dry rock specimen slightly increased with the increase of temperature. This result suggests that the apparent dielectric constant of wet rock likely to change significantly with the increase of temperature, because the dielectric constant of water is sensitive to the change of temperature. Further investigation is necessary to reveal the effect of high temperature on the apparent dielectric constant of wet rock.</p>

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