Evaluation of DC, FDEM and IP resistivity methods for imaging perched saltwater and a shallow channel within coastal tidal flat sediments
Evaluation of DC, FDEM and IP resistivity methods for imaging perched saltwater and a shallow channel within coastal tidal flat sediments
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DOI：
10.1016/j.jappgeo.2011.09.002
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发表时间：
2011-12-01
影响因子：
2
通讯作者：
Binot, Franz
中科院分区：
文献类型：
--
作者：
Attwa, Mohamed;Guenther, Thomas;Binot, Franz
目前，电阻率法广泛应用于各种地质和环境现场调查。在地质学上，沿海潮滩沉积物在德国北部形成了浅的河道状特征。采用三种地电方法对北海海岸潮汐沉积物中的近地表沉积物进行了成像，其中包括浅层导电带。对直流电阻率（DC）、频域电磁（FDEM）和频谱诱导极化（SIP）三种方法进行了评估，以表明哪一种方法可以在研究区域条件下提供所需的空间分辨率。该评估还包括一个综合建模来评估直流电阻率成像技术。这些结果是利用这些地球物理方法表征沿海含水层的一个令人鼓舞的例子。反演结果表明，潮汐沉积物的地下电阻率分布在短距离内变化较快。在泥炭层和粘土层之上，观察到一层薄的高导电性层，反射出栖息的盐水。二维IP剖面显示，栖息的盐水仅限于不透水粘土层上方的斑块形式。由于粘土具有较高的膜极化特性，在激电成像中可以较好地识别出粘土层的边界。电磁剖面和直流剖面在潮汐沉积中表现出浅通道的特征。本文重点介绍了最佳FDEM场参数和EM在岩性研究中的作用。直流电合成模拟可以观察到两个主要的局限性：(a)在栖息咸水的下边界存在涂抹；(b)高导电层横向效应的放大。这些限制降低了直流成像的分辨率，以准确地定义我们的目标。由于激电响应取决于微观几何、流体化学和饱和度，因此二维激电结果证明了该方法在表征沿海潮汐沉积方面的适用性，并具有良好的分辨率。在本研究中，SIP方法的成功为进一步研究该地区潮汐沉积的水力参数提供了支持。研究结果提供了沿海含水层的概况，可作为完善沿海潮滩形态要素和沉积序列概念模型的基础。(C) 2011 Elsevier B.V.版权所有
Presently, electrical resistivity methods are applied in a wide variety of geological and environmental site investigations. Geologically, the coastal tidal flat sediments formed shallow channel-like features at the northern part of Germany. Three geoelectrical methods are applied to image the near surface sediments including a shallow conductive zone within the tidal deposits at the North Sea coast. These methods, direct current (DC) resistivity, frequency domain electromagnetic (FDEM) and spectral induced polarization (SIP), are evaluated to show which one can provide the required spatial resolution under study area conditions. This evaluation also includes a synthetic modeling to assess the DC resistivity imaging technique.The results constitute an encouraging example using these geophysical methods in characterizing the coastal aquifers. The inversion results show that the subsurface resistivity distribution of tidal sediments can change rapidly within a short distance. A thin high conductive layer is observed above the peat and clay layers reflecting a perched saltwater. The 2D IP section shows that the perched saltwater is restricted to patched forms above an impermeable layer of clay. According to the IP images the boundaries of the clay layer are recognized with a good resolution due to the high membrane polarization of the clays. The EM and DC profiles show a shallow channel-like feature within tidal deposits. In this paper, the best FDEM field parameters and the role of EM in lithologic studies are emphasized. Two main limitations can be observed from DC synthetic modeling: (a) A smearing in the lower boundary of the perched saltwater; (b) an amplification of the lateral effect of the highly conductive layer. These limitations decrease the resolution of DC imaging for accurate defining our targets. Because the IP response depends on microgeometry, fluid chemistry and saturation, the 2D IP results demonstrate the suitability of this method to characterize the tidal deposits in the coastal area with a good resolution. In this study, the success of SIP method supports further investigations into studying the hydraulic parameters of tidal deposits in this area. The obtained results during this investigation provide an overview of the coastal aquifer and they can serve as a basis for refining the conceptual model of morphological elements and sedimentary sequences of the coastal tidal flat. (C) 2011 Elsevier B.V. All rights reserved.