0 引言
相较于传统的水文地质、地面物探调查,航空物探在水资源调查中具有覆盖面积大、效率高、不受地面障碍物限制等优势。频率域航空电磁法是根据地下物质的导电性、导磁性(有时也包括介电性或电化学性)的差异,通过航空仪器测量不同频率变化下的电磁场的空间分布和频率特性,进而推断地下不同物质的分布,从而解决各类地质问题的方法[8]。淡水电阻率一般大于25 Ω·m,在淡水区电阻率值主要取决于地层岩性的颗粒度,颗粒度由粗到细电阻率值逐渐降低;而咸水区,咸水呈相对低阻,矿化度越高电阻率越低,且当矿化度大于10 g/L时,电阻率值与岩性变化关系不大,主要由矿化度决定[9]。在国内外,航空电磁法已被广泛应用于水资源调查:20世纪80年代末,满延龙[10]就在河北南宫、曲周地区利用频率域航空电磁法进行水质填图,在咸水区划分淡水资源范围;孟庆敏[11]在西北敦煌地区使用三频航电划分了咸淡水界线,为当地淡水资源调查提供了物探资料;王卫平等[12]利用频率域航空电磁法在黄河口海侵地区寻找浅层淡水,取得了不错的效果;Lipinski等[13]使用直升机电磁法在怀俄明州波德河流域煤层气开采地区探测地下含水层,并计算矿化度,以评价开采过程对地下水的污染程度;Siemon等[14]在德国西北的波库姆岛使用频率域航空电磁法寻找浅层淡水资源,以缓解当地旅游业发展带来的淡水资源短缺问题;Chandra等[15]同样使用航空电磁法在印度普拉亚格拉寻找淡水资源;Jorgensen等、Gunnink等以及Gottschalk等[16⇓-18]将航空电磁法应用于海岸带海水入侵调查,在划分海水入侵界线上均有不错效果。从前人的研究结果可以看出,航空电磁法在地下水资源调查中应用广泛,且咸水和淡水的电性差异明显,利用频率域航空电磁法进行咸淡水调查具备物性基础。
雄安新区地处华北平原,地下水利用量达到总用水量的近70%。其中,河北平原浅层淡水开采潜力一般,已存在超采情况,局部已无可持续利用的潜力,需适度控制;区内微咸水、咸水资源开采潜力分别为较大、大,在技术条件允许的情况下,可以扩大开采[19],以缓解地下水用水压力。华北平原地层在新构造运动以来,持续的沉降造成了3次以上不同程度的海水入侵[20],造成雄安新区东测浅层存在(微)咸水(图1)。1998年的雄县和1999年的安新县2次水文地质调查,均证实研究区内存在浅层(微)咸水;安新王岳村附近测得矿化度达5.5 g/L,已达到咸水级别[22-23]。20世纪雄安新区两次水文地质调查仅在局部地区划分了咸淡水分界,且缺乏分界线的变化趋势探讨。因此,探明雄安新区浅层(微)咸淡水分界现状对于指导当地水资源开发利用具有重要意义。
图1
1 研究区概况
图2
2 数据获取及处理
本次研究使用中国地质调查局航空物探遥感中心于2018年在雄安新区开展的频率域航空电磁测量数据。使用设备为基于固定翼飞机(Y-12)的AGS-863型自研国产频率域航空电磁系统(图3)。AGS-863系统设计发射频率分别为520、2 020、8 020 Hz;3个频率磁矩分别为:323、102、122 Am2;电磁装置为垂直共面;收发距18.9 m。
图3
图3
测量系统及搭载飞机示意
Fig.3
Schematic diagram of measuring system and carrying aircraft
研究区设计测网覆盖雄安新区全境,共计9 500 km,测量比例尺1:5万,测线方向140°~320°。使用中国地质调查局航空物探遥感中心的GeoProbe Mager及加拿大Geosoft公司的Oasis Montaj两个软件完成采集数据的处理。主要的处理步骤包括:归一化校正和零点漂移校正。归一化校正是将电磁相应的电位值转换为×10-6,不同频率按照不同系数进行转换。测量系统在测量过程中随着环境温度等外界因素的影响,零值会产生漂移;而零点校正是利用测量前和测量后高空(500 m以上地面电磁信号为零)基线飞行测得的响应值之差来消除这类影响。处理完的原始数据使用丹麦奥胡斯大学的Aarhus Workbench进行反演,反演方法采用横向约束反演(laterally constrained inversion,简称LCI)[29]。LCI反演是一种拟二维反演方法,它是通过将剖面数据集成反演,并施加横向约束,同时反演出同一条测线上多测点的地电参数[30]。对反演得到每个点不同深度的电阻率数据进行可信深度评价(depth of investigation,简称DOI),低于DOI的数据会被删除[31]。
3 结果
3.1 浅层(微)咸淡水分界
图4
图4
雄安新区不同测线的电阻率剖面
Fig.4
Resistivity profile of different lines in Xiong'an New Area
为了更好探究研究区电阻率平面分布规律,笔者提取地表以下5~35 m 5个不同深度范围的电阻率形成等值线平面(图5)。从图中可以看出,在不同深度上测区均大致以NE—SW为界,电性特征存在明显差异:西北侧电性特征主要以高阻为主,而另外一侧则主要以低阻为主。在分界线以东大部分地区电阻率值在36 Ω·m以下,雄县东、安新县西南局部地区电阻率值在24 Ω·m以下;而分界线西侧地区几乎无低电阻率值区域,绝大部分地区电阻率值在45 Ω·m以上。
图5
图5
雄安新区不同深度电阻率等值线平面
Fig.5
Contour map of resistivity at different depths in Xiong'an New Area
统计资料显示,含矿化度2 g/L咸水的细粉砂电阻率值在15~24 Ω·m;而矿化度达到5 g/L时,电阻率一般在3~4.8 Ω·m[32]。因此,推断研究区东南侧电阻率值在24 Ω·m以下的低阻体主要由含(微)咸水的含水地层引起的,EN—WS方向上的电阻率特征分界线即为(微)咸淡水分界线。
3.2 分界线变化趋势分析
图6
图6
雄安新区-15 ~ -20 m电阻率平面等值线综合
Fig.6
Comprehensive map of resistivity contour at -15 ~ -20 m depth in Xiong'an New Area
由于水漏斗的存在,导致昝岗镇附近浅层地下水向漏斗中心流动,一定程度上阻止了浅层(微)咸水继续向米家务乡西北方向侵入。因此,1998年雄县水文地质调查所测的咸淡水分界线(以2 g/L为界)在昝岗—米家务—双堂一线没有明显变化。但昝岗水漏斗同时也增加了张岗—龙湾—史各庄—刘家铺区域(微)咸水侵入的可能性。
2014年研究区禁(限)采地下水以来,浅层地下水水位下降减缓,加之受降水量增加、南水北调补水、引黄济淀和种植结构调整等因素影响,白洋淀水位以及容城县周边地区浅层地下水水位上升明显,容城县浅层水漏斗面积也呈明显减少趋势[27]。由此导致大王—安新—端村一侧,对比1999年安新县水文地质调查分界线(以2 g/L为界)明显向东侧推移。但在局部地区依旧有(微)咸水区域存在,例如大王—三台之间的民用水井在2019年水文地质调查中测到微咸水。因此,如果浅层水位保持稳定,安新县东侧分界线将趋于稳定。
在安州—关城一侧,相较于1999年所划界线有较为明显的(微)咸水侵入趋势。1999年安新县水文地质调查时,在寨里镇辖区内未发现咸水。但2019年水文地质调查在寨里镇附近测得一口矿化度大于2 g/L的民用水井,这也从侧面印证了安州—关城一带是有侵入迹象。
2019年,安新县西南部(芦庄—高阳一带)浅层地下水降落漏斗面积较上年增加了16.5%[27],也是新区内增加最多的区域。这加剧了(微)咸水向芦庄—高阳方向的侵入。未来该地区浅层地下水水位下降趋势如果得不到缓解,芦庄—高阳方向(微)咸水范围会继续扩大。
4 结论
笔者通过对雄安新区的频率域航空电磁数据反演,得到了研究区地下5~35 m的电性分布特征,以此推断研究区内(微)咸水与淡水的分界线。得到以下结论:①频率域航空电磁法结合地面验证可以有效应用于浅层(微)咸水与淡水的分界线识别及变化趋势评估。②浅层地下水降落漏斗会影响咸水侵入的趋势:雄县东北处的昝岗—米家务—双堂一带(微)咸水侵入由于昝岗水漏斗的存在而趋于稳定,但水漏斗也同时加剧了昝岗镇地区南部(微)咸水入侵;安新县城西侧由于容城水漏斗水位上升,相较1999年(微)咸水范围有所减少;高阳水漏斗地区水位持续下降,导致芦庄—高阳方向(微)咸水范围将继续扩大。
新区建成后,常住人口的增长会导致用水需求增加。因此,地下水资源的合理开发利用变得尤为重要。容城县大部及雄县西北部地区的浅层地下水位近几年持续回升,且水质较佳,可作为饮用水供水水源。在容城县东南、雄县东部以及安新县南部等(微)咸水区,浅层地下水不可用作饮用水,但可以作为农业灌溉、工业用水的水源。农业灌溉应避免漫灌,尽量采取滴灌或渠灌。对于昝岗、高阳两个浅层地下水降落漏斗区,应做好用水规划,减少两个地区浅层地下水开采量,以缓解浅层地下水位下降带来的(微)咸水侵入。(微)咸水区浅层地下水过度开采且得不到及时补充,还会引起当地土壤盐碱化、地面沉降等问题。此外,应对(微)咸淡水边界地区进行定期监测,为当地浅层地下水的开采利用规划提供依据。
致谢
感谢中国地质调查局环境监测院李海涛教授、赵凯博士在文章编写过程中给予的帮助,提供的水文监测井数据对于本文的研究具有重要的验证作用。同时,感谢评审专家、期刊编辑提出的宝贵修改意见。
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DOI:10.11820/dlkxjz.2015.03.013
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受到水库拦蓄、抽水灌溉等人类活动的影响,白洋淀流域平原区河流断流,非承压含水层逐渐被疏干。下游的白洋淀无法得到河流和地下水的补给,时刻面临干涸。鉴于白洋淀对区域生态环境和气候有重要的影响,多次实施调水补淀工程。然而,白洋淀渗漏严重,调水补淀的效果不理想。为了保护白洋淀,亟需恢复地下水的可持续性,这首先需要查明地下水的更新能力。2009年,对白洋淀流域平原区地下水进行实地调查,并采集采集地表水和地下水水样共36个,分析其氚含量。利用线性插值法、吴秉钧法和连炎清法恢复了该地区降水氚含量的历史数据,结合数学物理模型估算了非承压地下水的更新能力。结果表明,氚含量较高的白洋淀水渗漏使得周边地下水氚含量增高。总体上,沿山区到淀区的方向非承压地下水更新速率逐渐由15.0%/a降低至4.0%/a。山前平原冲洪积扇区域是非承压地下水的主要补给区,平均更新速率达9.8%/a。冲积平原区域非承压地下水的平均更新速率仅为4.4%/a。为此,需要限制在冲积平原区域开采非承压地下水,以逐步恢复其更新速率,使其重新补给白洋淀。地下水的恢复过程可能较慢,但可从根本上解决调水补淀无法解决的干淀问题。
Renewal rate of groundwater in the Baiyangdian Lake Basin
[J].
DOI:10.11820/dlkxjz.2015.03.013
[本文引用: 1]
Human activities, such as water storage in reservoirs and pumping for irrigation, reduce stream flows and deplete unconfined aquifer in the plain area of the Baiyangdian Lake Basin. Without recharges from rivers and groundwater, the Baiyangdian Lake faces the risk of drying up. Considering the impacts of the Baiyangdian Lake on the local environment, biodiversity, and climate, many projects of water transfer for the sustainability of the lake were implemented. However, the result are unsatisfactory due to considerable leakages in the lake. In order to preserve the lake, sustainable groundwater use should be achieved and therefore, it is necessary to study the renewal rate of unconfined groundwater. In this study groundwater was surveyed on the plain area of the Baiyangdian Lake Basin in 2009. thirty six samples from groundwater and surface water were collected for measuring the content of tritium. Tritium content in precipitation since the mid-1950s was rebuilt by comparing the results from linear interpolation, Wu's method and Lian's method. Renewal rate of groundwater in the plain area was estimated based on the tritium data and the well-mixed model. Results show that leakage of the lake slightly increased tritium content in the ambient groundwater. Generally, the renewal rate of unconfined groundwater decreased from 15.0 %/a to 4.0 %/a between the mountain area and the lake. Alluvial fans are the main recharge area with an average renewal rate of 9.8 %/a. The renewal rate dropped to 4.4 %/a in the alluvial plain. We recommend that groundwater development should be restricted in the alluvial plain to recover the renewal rate of unconfined groundwater. Although such recovery may take a long time to occur, it is believed that recharging the lake by shallow groundwater is the only way to maintain a sustainable lake.
Layered and laterally constrained 2D inversion of resistivity data
[J].
DOI:10.1190/1.1759461
URL
[本文引用: 1]
In a sedimentary environment, quasi‐layered models often can represent the actual geology more accurately than smooth minimum‐structure models. We present a 2D inversion scheme with lateral constraints and sharp boundaries (LCI) for continuous resistivity data. All data and models are inverted as one system, producing layered solutions with laterally smooth transitions. The models are regularized through lateral constraints that tie interface depths or thicknesses and resistivities of adjacent layers. A priori information, used to resolve ambiguities and to add, for example, geological information, can be added at any point of the profile and migrates through the lateral constraints to parameters at adjacent sites. Similarly, information from areas with well‐resolved parameters migrates through the constraints to help resolve areas with poorly constrained parameters. The estimated model is complemented by a full sensitivity analysis of the model parameters supporting quantitative evaluation of the inversion result.
频率域航空电磁数据的加权横向约束反演
[J].
Weighted laterally-constrained inversion of frequency-domain airborne EM data
[J].
A global measure for depth of investigation
[J].
DOI:10.1190/geo2011-0393.1
URL
[本文引用: 1]
We tested a new robust concept for the calculation of depth of investigation (DOI) that is valid for any 1D electromagnetic (EM) geophysical model. A good estimate of DOI is crucial when building geologic and hydrological models from EM data sets because the validity of the models varies strongly with data noise and the resistivity of the layers themselves. For diffusive methods, such as ground-based and airborne electromagnetic, it is not possible to define an unambiguous depth below which there is no information on the resistivity structure and a measure of DOI is therefore to what depth the model can be considered reliable. The method we presented is based on the actual model output from the inversion process and we used the actual system response, contrary to assuming, e.g., planar waves over a homogeneous half-space, the widely used skin depth calculation. Equally important, the data noise and the number of data points are integrated into the calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and thus it can be used on any model type for which a sensitivity matrix can be calculated. Unlike other sensitivity matrix methods, we defined a global and absolute threshold value contrary to defining a relative (such as 5%), sensitivity limit. The threshold value will apply to all 1D inverted data and will thus produce comparable numbers of DOI.
