The self-potential method and its application in karst collapses

The groundwater that flowing through soil cave or karst collapse pit destroys the balance of positive 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 symmetrical in relation to the symmetry axis of zero value line in the plane. The main interference factors of natural 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 syncline 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 Carboniferous (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. Vertical fissures on the ground are quite developed. And the fissures in the abnormal regions detested by self-potential method are consistent with the trend of the conjectural fault fracture zones and the direction of the underground 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.