Research on forward simulation of multi-electrode spacing combined profile curves of typical karst water-rich structures

Based on differential equation of a point source and the boundary conditions of electric field, and combined the popular high density electrode layout for exploring groundwater at present, we conduct 2D forward simulation of multi-electrode spacing combined profile curve for two common and water-rich structures with low resistance in karst area such as buried fault zone and karst cave. The simulation results show that the combined profile curves are sensitive to underground hidden low resistivity structrues , and the curves are well differentiated.For the water-rich cave, the orthogonal point is the most prominent when the depth of water-rich cave is about 1/2-2/3 times the electrode space; as the distance of the electrode continues to increase, the synchronous low trend gradually appears.The larger the space range of the underground low-resistance cave is, the better the curve differentiation of the combined profile is, and the more obvious abnormal of the orthogonal point presents. For the water-rich fault zone, when the electrode space increases, the orthogonal point slightly moves to the dip direction of the fault, but is not obvious. The position of the orthogonal point is mainly affected by the low resistivity in the shallow part. With the increase of the electrode space, the downward low resistivity zone play a considerable role, for which more and more measuring points present as low synchronization in the dip direction on the left side of the orthogonal point with smaller slope curve while larger slope curve from the right side of the orthogonal point, the apparent resistivity reaches the minimum value at the top of the fault. The apparent resistivity decreases gradually with the increase of the electrode space when the orthogonal points of combined profile curve are obvious, whereas the apparent resistivity tends to be stable with the increase of the electrode space when the abnormal shape of combined profile curve shows as low synchronization. Branches of combined profile curve on both sides are symmetrical to the horizontal symmetry model while asymmetric to the tilt model, and the inclination of low resistance body is consistent with the inclination of the steep side of the orthogonal point nearby. The simulation results can be applied to the interpretation of multi-electrode spacing combined profile curve in karst area.