マイクロ波のエネルギー吸収特性を利用した淡塩水境界面の計測  [in Japanese] MEASUREMENT OF INTERFACE BETWEEN FRESH WATER AND SALT WATER USING ENERGY ABSORPTION CHARACTERISTICS IN MICROWAVE  [in Japanese]

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Abstract

淡塩水境界面の位置計測に対する時間領域反射法(TDR)の有効性を評価するために,48cm長プローブを用いて,その先端から境界面までの距離<i>h</i><sub>i</sub>の計測の可否と精度を調べた.<i>h</i><sub>i</sub>を評価するために,本法では,プローブに与えたマイクロ波ステップパルスのTDR波形が,塩水によるエネルギー吸収によって急変する特徴を利用した.水質変化を想定し,淡水の電気伝導度σ<sub>fw</sub>が異なる条件で<i>h</i><sub>i</sub>を評価した結果,淡水中を伝播するパルスのエネルギー損失が大きい場合,すなわちσ<sub>fw</sub>が高く,淡水層が厚い場合に<i>h</i><sub>i</sub>計測が困難になった.しかし,高σ<sub>fw</sub>条件でも,淡水層が薄くなりエネルギー損失が抑制されると,<i>h</i><sub>i</sub>は計測可能となり,測定誤差は概ね1cm未満となった.また,たとえ境界面が変動しても,静的な水理条件では同等の精度で<i>h</i><sub>i</sub>を評価できることを確認した.

  To examine the effectiveness of time domain reflectometry (TDR) for finding the interface between fresh and salt water layers, the possibility and accuracy in measuring the interface level were investigated with a two-rod TDR probe of 48cm. In the TDR waveform, reflection coefficient values indicate the magnitude of electrical energy of a microwave step-pulse given by TDR cable tester. The level was estimated by analyzing the waveform feature that the coefficient value will abruptly reduce at the interface due to energy absorption by salt water. In the experiment, to reproduce various groundwater qualities in a coastal aquifer, the electrical conductivity of fresh water was arranged from 0.0005 to 0.4 S m<sup>-1</sup>. The fresh water was set to the upper layer than salt water of 5.2 S m<sup>-1</sup> in the experimental column. In the situations where the conductivity or layer thickness of fresh water was large, the energy loss into the water prevented us from measuring the level. However, when the thickness is thin for the high conductivity condition, the interface could be more detectable due to reduction of the loss. The root mean square errors for the measured levels were less than 1cm under static hydraulic conditions even if the measured levels fluctuated. These results may suggest that the proposed method for detecting the interface is effective.

Journal

  • Doboku Gakkai Ronbunshuu B

    Doboku Gakkai Ronbunshuu B 66(2), 189-195, 2010

    Japan Society of Civil Engineers

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