The feasibility of marine CSEM method for detecting offshore freshened groundwater reservoirs
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摘要:
海底淡水是一种天然淡水资源，主要位于近海大陆架区域，来自陆地渗透水或是由海平面升降所形成的古河道中。传统地震方法在储层流体盐度变化的识别中存在一定的局限性，而海洋可控源电磁法（CSEM）对高阻薄层敏感，有利于通过观测淡水储层和围岩电阻率差异引起的电磁异常来探测淡水资源。本文研究区域位于长江口嵊泗古河道，基于“嵊泗一井”水文地球物理测井数据建立长江口嵊泗古河道淡水储层地电模型，并利用数值模拟方法分析该区域淡水储层海洋可控源电磁响应特征。结果表明，利用海洋CSEM方法对高阻薄层强敏感度的特征探测海底淡水资源具有一定的优势，能够有效探测到高阻薄层引起的电磁场异常响应，具有较好的淡水储层识别能力。因此，该方法应用于长江口嵊泗古河道淡水储层探测是可行的。
Abstract:Offshore freshened groundwater (OFG) is a natural freshwater resource located mainly in the continental shelf region, from either onshore coastal aquifers or paleo-channels formed in sea-level lowstands. Conventional seismic methods have certain limitations in identifying salinity changes. Fortunately, the marine CSEM (controlled source electromagnetic method) is sensitive to high-resistivity thin layers, which is beneficial for detection of OFG by analyzing electromagnetic anomalies caused by the contrast of resistivity between the freshwater reservoirs and surrounding sediments. Paleo-channels in Shengsi in the Yangtze River estuary were studied. Based on the hydrogeological and logging data of "Shengsi No. 1 Well", a geoelectric model was established to analyze the marine CSEM responses. Results indicate that the marine CSEM could effectively detect electromagnetic anomalies caused by high-resistivity thin layers, and has good ability to locate underground freshwater reservoirs; its application for the detection of OFG in the Shengsi paleo-channel is feasible.
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Key words:
- marine CSEM /
- OFG /
- detectivity /
- Shengsi
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表 1 嵊泗一井水文测井解释成果表[30]
Table 1. Results of hydrological logging interpretation of Shengsi No. 1 well
岩性 深度/m 厚度/m $\bar \rho_s $ /(Ω·M)$\varPhi $ /%含水层性质 中细粗砂互层 95.6～139.1 43.5 1.6～4 5.7～48 咸水 细砂 140.6～142.8 2.2 1.0 5.7～30 咸水 粉细砂 145.6～151.4 5.8 2 30 咸水 亚砂土 155.4～158.0 2.6 3 46 咸水 粉细砂夹亚黏土 158.0～173.3 15.3 6.6～10.6 7～25 淡（微咸）水 中砂 173.3～179.6 6.3 14 20 淡（微咸）水 粉细砂 179.6～180.8 1.2 10.50 15 淡（微咸）水 中细砂 180.8～184.4 3.6 12 15～20 淡（微咸）水 粉细砂 184.4～185.0 0.6 6 15 淡（微咸）水 粉细砂 186.1～192.3 6.2 6 10～29 淡（微咸）水 亚砂土 192.3～198.4 6.1 3 28 咸水 中粗砂 198.4～219.0 20.6 1.2～2.8 20.9 咸水 粉砂（土） 229.0～231.4 2.4 2.40 0～32 咸水 细砂 231.4～233.1 1.7 8.80 0～53 咸水 粉砂（土） 233.1～234.5 1.4 4 23 咸水 砂砾石 234.5～245.0 10.5 76 13～32 淡水 含砾粗砂 246.2～248.0 1.8 20 27～45 淡水 中细砂 248.0～250.8 2.8 11 6～27 淡水 砂砾石 250.80～256.00 5.2 76 6～30 淡水 泥质粗砂 263.0～266.6 3.6 16 0～27 咸水 泥质中砂 266.6～270.8 4.2 10 13～40 咸水 含砾泥质中砂 270.8～274.2 3.4 15 13 咸水 
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