熱脈衝流速儀試驗及其應用於地層透水性分布之研究

Abstract

地層透水性為水文地質的重要參數，然地層的非均質性往往造成地質材料隨著深度而有垂直方向的變化，傳統上應用於井孔中量測地層垂向水力傳導係數分布的方法大多精度欠佳或效益不彰。熱脈衝流速儀則為近期發展中針對井孔中垂向流速量測的一項技術；本研究在實驗室建立模擬系統，藉以分析驗證熱脈衝流速儀的量測結果以及造成誤差的物理機制，並嘗試發展可以擴展流速儀量測範圍的導流裝置。試測結果顯示熱脈衝流速儀之量測準確度因流速環境不同而改變，誤差隨井管內流速降低而增加，於本實驗系統的流量環境下，誤差可由4.6%上升至94.4%。反覆試驗的精確度在低流量環境下則表現較佳，以變異係數表示約為0.4%至5.8%。我們發現誤差主要來自於熱的自然對流以及摩擦損失效應。熱的自然對流在低流速環境下為主要誤差來源，而摩擦損失效應則因流速儀擺設位置的不同以及流速儀幾何構造的差異而有不同的影響。藉由分析實驗室模擬系統的量測結果，可推導出相關之校正公式。繼而選取裝設於非均質沉積物含水層及裂隙岩體的觀測井進行現地試驗，並輔以井站岩芯資料、地球物理井測以及現地水力試驗結果，嘗試估算地層各區段之水力傳導係數和透水性分布。試驗結果顯示熱脈衝流速儀在非均質沉積物含水層中可達到25公分量測間距的解析能力；另可測得井孔內的垂向自然水流，並偵測到大部分井孔內的地下水流僅由單一裂隙流出；對於裂隙岩體當中相對透水的裂隙位置，岩芯或井孔聲波造影的記錄並無法提供直接的判斷依據，但配合移動式導流器以及校正實驗式的使用，即使在低流速的環境下，熱脈衝流速儀對於地層當中相對透水的區段位置亦具有相當良好的辨識能力。若整合其他現地水力試驗的結果，將可提供更有效的地層透水性量測方式。

Heat-pulse flowmeter can be used to measure low flow velocities in a borehole; however, bias in the results due to measurement error is often encountered. A carefully designed water circulation system was established in the laboratory to evaluate the accuracy and precision of flow velocity measured by heat-pulse flowmeter in various conditions. A movable diverter was also developed to extend the operation flow range assembled on the flowmeter. Test results indicated that the coefficient of variation for repeated measurements, ranging from 0.4% to 5.8%, tends to increase with flow velocity. The measurement error increases from 4.6% to 94.4% as the average flow velocity decreases from 1.37 cm/sec to 0.18 cm/sec. We found that the error resulted primarily from free convection and frictional loss. Free convection plays an important role in heat transport at low flow velocities. Frictional effect varies with the position of measurement and geometric shape of the inlet and flow-through cell of the flowmeter. Based on the laboratory test data, a calibration equation for the measured flow velocity was derived by the least-squares regression analysis. Our laboratory experimental results suggested that, to avoid a large error, the heat-pulse flowmeter measurement is better conducted in laminar flow and the effect of free convection should be eliminated at any flow velocities. Field measurement of the vertical flow velocity using the heat-pulse flowmeter was then tested in a 23-m deep screened well in an alluvial aquifer to characterize the distribution of hydraulic conductivity along the screen. Measurement results indicate that groundwater flow is concentrated in two highly permeable sections. Their horizontal hydraulic conductivities are 3.7 to 6.4 times greater than the equivalent hydraulic conductivity of the whole aquifer, suggesting that contaminant migration rate could be underestimated in a heterogeneous aquifer. Two more field tests were conducted in open-holes in the fractured rock formation to characterize the preferential flow path. The field test results indicated that, with a proper calibration, the heat-pulse flowmeter measurement is capable of characterizing the vertical distribution of hydraulic conductivity or preferential flow. The position of the highly permeable fracture zone can be identified within the range of 25 cm and a single opened fracture was identified when most flow discharged in a specific location. However, the large aperture and high density of fractures were not certainly correlate well to the permeable zone. Comparing our test results with those obtained from other techniques, we found that heat-pulse flowmeter measurement is more efficient for locating permeable fractures.