EH4电磁成像系统在杭州湾地区晚第四纪地层中的应用

Application of Stratagem EH4 Electromagnetic Imaging System in the Late Quaternary Strata in the Hangzhou Bay Area

超音频电磁—大地电磁测深法(EH4)具有不受高阻层屏蔽影响、性能稳定、轻巧便携、操作方便、施工简单和成本低廉等优点,非常适合浅层地质勘探。本文以杭州湾南岸新湾地区的EH4电磁勘探测量结果为例,论述了它在晚第四纪地层基底识别、岩性和沉积相划分、储层和天然气层判别等方面的应用结果。研究表明,EH4电磁成像系统获得的新湾地区晚第四纪地层电阻率—深度剖面,能够详细地反映晚第四纪地层基底连续变化情况,新湾地区晚第四纪沉积层基底为一凹凸不平的不整合面,变化极其复杂。不同沉积环境下形成的沉积物类型和粒度均不同,随着沉积物粒度的减小电阻率亦减小,因而借助电阻率的变化,可以大致划分沉积物及沉积相类型。通过单点电阻率曲线和电阻率—深度剖面图,可以清楚地反映地下介质电阻率变化情况,气层厚度、气层与水层电阻率差值越大,气层在电阻率曲线上的显示越强,反之则越弱。电阻率曲线可以确定砂质透镜体的位置和分布范围,并判断出气层深度和厚度。

The Stratagem EH4 Electromagnetic Imaging System is characterized by unshielding of high resistivity layers, stable capability, portability, facility operation, simple utilization and lower cost etc. It can be used in geological prospecting of shallow layer. This paper base on the surveying results obtained with Stratagem EH4 Electromagnetic Imaging System in Xinwan, Hangzhou Bay area, elaborated its applications in recongnizing the late Quaternary strata basement, compartmentalizing lithologies and lithofacies, distinguishing reservoirs and natural gas layers. The research indicates that the resistivitydepth section with Stratagem EH4 Electromagnetic Imaging System in Xinwan area can perfectly reveal the continous change of the late Quaternary strata basement which is a concave and convex unconformity. The type and granularity of sediments deposited in different sedimentary environments is diverse. The resistivity decreases with the granularity of sediments, so we can compartmentalize the type of sediments and sedimentary facies. The resistivity curve and resistivity-depth section can clearly show the variation of subsurface medium＇s resistivity. The bigger thickness of gas layer and the more risistivity dispersion of gas layer and water layer, the stronger displaying of resistivity curve about gas layer, whereas the weaker displaying of it. The resistivity curve can be used to determine the location and distribution of sandy lenses and to deduce the depth and thickness of gas layers.