综合电法在探测海水入侵界面中的研究与应用——以莱州湾地区为例
Research and application of comprehensive electrical method in detecting saltwater intrusion interface: A case study of Laizhou Bay area
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收稿日期: 2020-01-19 修回日期: 2020-03-15 网络出版日期: 2020-06-20
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Received: 2020-01-19 Revised: 2020-03-15 Online: 2020-06-20
作者简介 About authors
苏永军(1981-),男,高级工程师,主要从事地球物理勘查及研究工作。Email:syj95123@163.com 。
应用EH-4电磁成像系统和二维、三维高密度电法在莱州湾地区进行海水入侵界面的探测,利用EH-4电磁成像系统和高密度电法剖面成果查明了研究区海水入侵界线,绘制了该区海水入侵界线成果平面图,并对电性特征进行了梳理。探测结果表明通过这些方法所获得的反演断面图含有丰富的地质信息,能为研究该区地质环境问题提供可靠的地质信息,使用这些物探方法探测莱州湾地区海水入侵界线是有效可行的。该结果为今后在该区开展地质调查研究奠定了基础。
关键词:
In this paper, the authors analyzed cases of applying EH-4 electromagnetic imaging system and 2D, 3D high density resistivity method to detecting seawater intrusion interfaces in Laizhou Bay area. The results show that the inversion profile of these methods provides rich and reliable geological information, which can help solve geological environment problem of this region, and these geophysical methods are effective and feasible in detecting seawater intrusion interfaces in Laizhou Bay area. Based on many profiles of EH-4 electromagnetic imaging system and high density resistivity method, the authors successfully figured out the distribution of the seawater intrusion interface, compiled the map of the seawater intrusion interface, and analyzed and sorted out electrical features of the region. These results has laid a solid foundation for the future geological survey in Laizhou Bay area.
Keywords:
本文引用格式
苏永军, 范翠松, 赵更新, 张国利, 刘宏伟, 孙大鹏.
SU Yong-Jun, FAN Cui-Song, ZHAO Geng-Xin, ZHANG Guo-Li, LIU Hong-Wei, SUN Da-Peng.
0 引言
1 研究区概况
研究区位于莱州湾南岸,属暖温带半湿润季风型气候,兼有海洋性气候。研究区地处莱州湾凹陷,主要地层为第四系和新近系、古近系,以黏土、粉砂、细砂为主,地势平坦,区内有多条河流通过,水系较发育,地表径流主要来自大气降水。
根据该区以往物性资料,含水地层岩性具有明显的电阻率差异(表1),具备开展电法探测海水入侵界线的物性前提和电阻率反演断面图确定咸淡水体分界线的条件。
表1 研究区含水地层电阻率
Table 1
地层岩性 | 电阻率/(Ω·m) | 备注 |
---|---|---|
高浓度卤水 | <1 | |
卤水 | 1~2 | |
咸水 | 2~10 | 微咸水电阻率值范围 12~25 Ω·m |
淡水 | 10~30 | |
不含水地层或基岩 | >30 |
2 研究思路
在收集和研究莱州湾地区海水入侵资料的基础上,在莱州湾地区垂直海岸带或近似垂直海岸带布置大点距(500 、300、200、100 m)EH-4测量剖面,从宏观上调查海水入侵界线大概位置,然后进一步在入侵界线附近做小点距(50、20、10、5 m)的EH-4测量剖面或高密度电法剖面,根据反演断面图上电阻率值的变化, 从微观局部确定每条探测剖面上海水入侵界线和咸淡水界面的分布,垂向的厚度变化、横向上延伸情况,建立各层位的相互关系,同时结合该区水化学分析等水文地质资料对反演剖面进行地电断面的推断解释,进而更精确地勾画莱州湾地区海水入侵界线和咸淡水界线。按照“点线面结合,重点控制”的原则,在重点海水入侵区布设“U”字形三维高密度电法探测,对莱州湾地区海水入侵速度、范围等进行动态监测,对该地区海水入侵现状进行评估,为莱州湾地区海水入侵趋势预测及防治措施等提供科学依据。
3 技术方法简介
3.1 EH-4电磁成像系统
EH-4电磁成像系统[10]是由美国Geometrics公司和EMI公司联合研制电磁成像系统,是通过对地面电磁场观测研究地下岩性电阻率分布特征的一种物探方法。其观测两个正交的电场分量(Ex,Ey)和磁场分量(Hx,Hy),就可以确定介质的电阻率值,计算公式为
式中:ρ为电阻率,f为频率,由于地下介质不均匀,算出的电阻率为视电阻率。根据电磁波的趋肤深度公式,进而推断研究区地电特征和地下构造。
3.2 高密度电阻率法
4 应用实例分析
莱州湾地区布设了大量EH-4电磁成像系统和高密度电法剖面,在重点海水入侵区布设三维高密度电法探测与动态监测,均取得较好的应用效果,为说明其应用效果,对每种方法进行实例效果分析。
4.1 EH-4电磁成像系统成果分析
侯镇南剖面Ⅱ位于寿光市侯镇南, 全长4.0 km,剖面点距为500 m,测线方位为77°。从该剖面视电阻率反演综合断面(图1)可以看出,剖面电阻率整体呈现南西高北东低的趋势。剖面0~2 500 m浅部电阻率值在10~30 Ω·m之间,推断为淡水和微咸水分布区;剖面2 500~4 000 m之间浅部电阻率值在5~10 Ω·m之间,推断为咸水分布;在剖面里层2 500 m附近,电阻率值在7~14 Ω·m之间,初步推断为该剖面咸淡水分界线。
图1
图1
侯镇南剖面Ⅱ 500 m点距反演断面(a)和推断解释成果(b)
Fig.1
The inversion(a) and interpretation profile(b) of Houzhen south Ⅱ with 500 m measuring point distance
图2
图2
侯镇南剖面Ⅱ 20 m点距反演断面(a)和推断解释成果(b)
Fig.2
The inversion(a) and interpretation profile(b) of Houzhen south Ⅱ with 20 m measuring point distance
4.2 二维高密度电法成果分析
二维高密度电法剖面位于寿光市侯镇南马家庄附近,剖面全长1.35 km,点距10 m,测线方位角77°,采用Dipole-Dipole装置进行数据采集,对获得的数据进行处理和反演,从反演综合断面(图3)可以看出,剖面1 800~2 440 m范围,电阻率在10~30 Ω·m之间,推断为淡水和微咸水分布区;剖面2 440~3 150 m范围内,地表至-10 m含水地层呈水平状,电阻率在15~30 Ω·m,推断为浅层地表淡水,-10~-60 m电阻率为1~10 Ω·m,推断为咸水体分布;-60 m以下电阻率为十几至近百 Ω·m,推断为深部淡水区;在剖面2 440 m处电阻率等值线变化大,结合水文地质资料分析,推断该处为咸淡水分界线,该推断结果与水化学法确定的海水入侵分界线基本吻合。
图3
图3
马家庄反演断面(a)和推断解释成果(b)
Fig.3
The inversion(a) and interpretation profile(b) of Majiazhuang
4.3 三维高密度电法成果分析
三维高密度电法剖面布设在已知咸淡水过渡带,位于寿光市侯镇南马家庄附近,方位角77°,极距10 m,线距10 m,采用Dipole-Dipole装置,滚动测量方式,对采集到的三维数据体进行反演处理,并据此进行反演推断解释。由图4可见,反演电阻率平稳连续,分为上下两层,地表至-70 m电阻率大于10 Ω·m,推断为淡水分布区,地表局部有高阻出现,是地表已知干扰物所致;-70~-94 m电阻率小于10 Ω·m,推断为海水入侵区;-60~-94 m电阻率在10 Ω·m左右,推断为咸淡水分界线。利用三维高密度电法所推断咸淡水界线结果与水化学法咸淡水分界线基本吻合。利用研究区布设的多条高密度电法和EH-4电磁成像系统剖面成果资料查明了该区海水入侵界线,绘制了研究区海水入侵界线成果平面图(图5)。
图4
图4
三维高密度电法反演断面和推断结果
Fig.4
Inversion result and interpretation of 3D high density resistivity method
图5
图5
莱州湾地区海水入侵界线平面
Fig.5
The map of saltwater intrusion interface in Laizhou bay
三维高密度电法反演成果图直观,便于成果的解释和表达,同时又精细,地质信息丰富,能进行更精准的地质解释,可在重点海水入侵区进行不同时段四维动态监测。
5 结语
1) 利用高密度电法和EH-4电磁成像系统剖面成果资料划出研究区咸淡水界线与水化学法确定的分界线基本吻合,表明采用大小点距结合、局部加密探测海水入侵界线的研究思路是可行的,选择的探测方法是有效的,可为解决此类问题提供借鉴。
2) 通过三维高密度电法在研究区已知咸淡水过渡带的成功尝试,说明三维高密度电法探测及监测海水入侵界线是有效可行的,其成果图件直观、地质信息丰富、便于地质成果的解释和表达,有很好的应用前景。
3) 利用大量高密度电法和EH-4电磁成像系统测量剖面,初步查明研究区电阻率值南高北低,与研究区地质特征吻合;同时对研究区地层电性特征进行梳理,所取得成果对研究莱州湾地区地质环境问题提供了基础地球物理依据。
4)EH-4电磁成像系统和高密度电法在探测海水入侵界线是有效可行的,具有无破坏性、成本低、快速、准确度高等优点,在探测海水入侵界线具有很好的应用前景。
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