Application of comprehensive geophysical prospecting method in concealed karst area: A case study from Ning 2019H18 drilling platform in Changning shale gas area
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摘要: 岩溶塌陷具有隐蔽性和突发性,是碳酸盐岩地区工程建设过程中常见的不良地质问题。长宁地区岩溶发育广泛,其页岩气开发利用受困于岩溶塌陷危害,综合运用微动、高精度重力、瞬态面波和等值反磁通多项物探方法,调查长宁页岩气区块宁2019H18钻井平台下伏地质体特征,查明土洞和溶洞的发育状况,结合钻孔数据进行验证,主要成果包括: ①微重力探测呈现重力值南东高、北西低,重力场响应特征表明该场地南东放向基岩密度较大且完整,而场地NE向基岩密度低; ②微动剖面响应结果与微重力成果对应较好,场地速度结构南东高、北西低,岩石的波速与密度呈正相关; ③进一步查明了场地下方软弱岩层的分布范围,场地SN向基岩完整性好,场地NW向存在着一条裂隙带,裂隙带内充填较为破碎松软的全风化砂岩,其承载力较低。通过物探三维可视化成果,可以为下一步场地建设指明方向,为类似工程案例提供科学依据。Abstract: Karst collapse is a common geological problem in the process of engineering construction in carbonate rock areas because of its concealment and sudden occurrence. The karst collapse is widely developed in Changning area, therefore, the exploitation and utilization of shale gas is hampered by extensive karst collapse. In this paper, the authors used micromotion, high-precision gravity, transient surface wave and equivalent anti-flux geophysical prospecting methods to investigate the subsurface geological condition of Ning 2019H18 drilling platform, and ascertained the development of earth cave and karst cave, which were tested by the drilling data. The main achievements are as follous. ①The microgravity is high in southeast and low in northwest, and the gravity response indicates that the southeast bedrock has large density and complete structure, and that the northwest bedrock has low density. ②The micromotion response profile is well corresponded with the microgravity, and the court page is high in southeast and low in northwest. And the wave velocity is positively related to its density. ③The distribution of underground soft strata was also verified, and the southeast bedrocks were completely preserved. A fractured zone exists in the northwest of the area, filled with cracked and soft completely weathered sandstane, which has low bearing capacity. The 3D visualized physical prospecting achievements could provide directions for the next establishment of the area, and some scientific references for similar engineering cases.
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