Study On The 3-D Forward And Inversion Of Induced-polarization In Borehole

The induced polarization method is an important branch of electrical prospecting,which is widely used in the fields of hydrology,engineering,environment,prospecting and archaeology,the electrical structure of the actual geological target body is threedimensional,the one-dimension and the two-dimensional situation can not meet the needs of practical geological interpretation,especially the exploration target is gradually turned to the deep part,so it is necessary to developed effective threedimensional induced polarization method,but the results obtained by using the data collected from the traditional ground equipment are not ideal,which is reflected in the true value of the inversion of the abnormal weight of the inversion and the physical property of the inversion to the theoretical model.So it is necessary to take different methods of observation.Such as surface layout,that is surface excitation and reception;borehole-surface layout,that is borehole excitation and reception;borehole-borehole layout,that is borehole excitation and borehole reception;surface-borehole layout,that is surface excitation and borehole reception.Therefore,the combination of surface and borehole layouts can greatly improve the accuracy of inversion and facilitate geological interpretation.Basing on the finite element method to finished the three-dimensional forward modeling of apparent resistivity firstly,we based on the equivalent resistivity method to achieve apparent polarizability forward response through a high and a low resistance models with borehole-surface observation.The results show that the DC resistivity and induced polarization is sensitive to low resistivity and high polarization body but opposite to insensitive to high resistance and high polarization body.Through surfaceborehole and borehole-borehole observations in step model we can see the apparent polarizability of low resistance body is stretching to low value.Then,the resistivity inversion and the polarizability inversion are performed.The inversion method is based on regularization and Occam inversion which is constrainted with model smoothing.This thesis selects long column low resistance and high polarizability as an inversion theoretical model.According to the effect of various device combinations,it can be seen that the shape and physical properties of the model are different from the real model if there is only surface data in the inversion,and the morphological location and physical properties of the model are inversed better if the surface data and borehole-borehole data combined in the inversion,but there is still a gap between the physical properties and the real model,When using borehole-surface record layouts,the contour,position and physical properties of the model are well inversed,the resistivity of inversion is close to theoretical model,and the polarizability is close to 85% of the real value,and he result of the inversion is the best when combine surface,borehole-borehole,borehole-surface and surface-borehole record layouts,and the abnormal area is very close to the real model which also illustrates the importance of borehole data in inversion and the effect of multiple layout combinations in inversion.When considering the actual cost and the result of inversion,borehole-surface and borehole-borehole record layouts will lead to good inversion results without costing much money,which provides the basis for actual production and geological interpretation.