摘要:
流经土洞或岩溶塌陷坑中的地下水破坏了土层的固-液界面离子双电层扩散区中正、负电荷的平衡,引起地下介质(土层)自然极化,形成自上而下的自然电场。土洞或岩溶塌陷在平面上产生自然电位似同心圆负心异常、在剖面上引起极小值异常;自然电位梯度在剖面上出现过零点异常,在平面上呈以零值线为对称轴的正负对称的似“蝴蝶状”异常。岩溶区自然电场法干扰因素主要有工业游散电流等人文干扰电磁场、地下金属管道(线)自然极化电场、上升或下降泉自然电场、山地电场、炭质灰岩自然电场等。针对岩溶区自然电场法干扰因素特点,本文提出采用避让法、排除法、在线监测法进行消除,并将之应用于广西来宾市吉利村岩溶塌陷探测中。结果表明,自然电位剖面曲线在地面沉陷区中心附近出现极小值异常,自然电位梯度剖面曲线也在相同部位显示过零点异常。水文地质监测钻孔在相应的位置上分别揭露了塌陷堆积物、溶洞和断层破碎带,从而验证了上述自然电场法异常的属性。
关键词:
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岩溶
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土洞
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岩溶塌陷
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自然电场
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异常特征
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探测效果
Abstract:
The groundwater that flowing through soil cave or karst collapse pit destroys the balance of posi-tive and negative charges in the ion double electrode layer between the solid-liquid interface in soil,which induces spontaneous polarization of underground medium (soil layer)and forms downward natural electric field.The soil cave and karst collapse cause self-potential negative anomaly that looks like concentric ring in the plane view and cause minimum value anomaly in the profile.The self-potential gradient appears zero crossing point anomaly in the profile and appears "butterfly-shaped" anomaly that is negative-positive sym-metrical in relation to the symmetry axis of zero value line in the plane.The main interference factors of nat-ural electric field in karst region are industrial stray currents,spontaneous polarization electric fields coming from underground metal pipe/line,natural electric fields produced by ascending springs or descending spring,mountain potential fields and natural electric fields of Carbonaceous limestone,etc.According to the characteristics of the interference factors led by self-potential in karst area,avoiding method,eliminating method and online monitoring method are put forth in the paper in order to eliminate the interference factor. The self-potential method has been successfully applied in the karst collapse area in Jili Village,Laibin City, Guangxi.The collapse is located inside the Guilin-Laibin fault zone and on the sides of the Datoushan syn-cline axis of the secondary fold between the transition zone of the east limb of the Pingtang anticline and the west limb of the Laibin syncline.The landforms there are characterized of karst accumulation and residual peak pimple plain.The overlying layers are gravel silty clays of the Quaternary eluvial and slope-alluvia,the underling rocks are mainly thick bedded limestone with dolomitic limestone interlayer of the Middle Carbon-iferous (C2 ).A set of NNE trending fault develops parallel to the fold axis accompanied by a group of the NWW trending tension-shear fault in the nearly vertical fault direction which cuts and offsets the main NNE faults.The broken rocks and karst features as well as karst collapse are widely spread and quite developed in fault fracture zones.The actual measured self-potential curve has minimum value anomaly around the center of the subsidence area.At the same position the self-potential gradient profile has zero value anomaly.Verti-cal fissures on the ground are quite developed.And the fissures in the abnormal regions detested by self-po-tential method are consistent with the trend of the conjectural fault fracture zones and the direction of the un-derground river.Collapse deposits,caves and fault fracture zones are exposed at the same position by hydrogeologic monitoring borehole,which verifies the properties of self-potential anomalies have been discussed above.