Research On The Joint Inversion Of Magnetotelluric Sounding And Long Offset Transient Electromagnetic Methods

Each individual geophysical method can only reflect a certain aspect of geological formations.Using this limited geophysical information,the geophysical inversion can not detailed reflect the geological formations.While combining kinds of geophysical data,joint iversion can effectively improve the inversion result,which gets closer to the geological formation.And non-uniqueness is much less of a problem.The Magnetotelluric Sounding method(MT)uses the natural alternating electromagnetic field as its source,and its exploration depth ranges from tens of meters to hundreds of kilometers.Measuring the secondary induced field,Long offset transient electromagnetic(LOTEM)is a kind of active source method.LOTEM is sensitive to high conductors,and has higher resolution.In this paper,depending on the study of MT and LOTEM joint inversion,and the analysis of different ways to assign objective function weights,we can draw a conclusion that this joint inversion method can improve the resolution at shallow area meanwhile keep large exploration depth.Based on Maxwell's equations,that the response characteristics of electromagnetic fields was simulated by MT and LOTEM forward modeling has been realized.And the sensitive characteristics to geoelectric parameters was analyzed.It concluded that MT and LOTEM have better sensitivity at high frequency or to low resistance geological body.With the increasing of exploration depth,the sensitivity decays rapidly.At the shallow area,LOTEM is more sensitivity than MT,while MT has deeper exploration depth than LOTEM.Thus,based on inversion parameter selected and weighting distributed,the MT and LOTEM joint inversion can not only guarantee high resolution but also large exploration depth.Based on the OCCAM inversion algorithm,the MT and LOTEM joint inversion algorithm design was fulfilled.Also this joint inversion algorithm was implemented through C++ coding.At the same time,four different weight distribution methods of the objective function were deduced and realized(namely: fixed,SNR,fitting,sensitivity).The algorithm correctness and the superiority of joint inversion were verified by processing the synth data.It verified that the proposed joint inversion method can combine the advantages of MT(large exploration depth)and LOTEM(high resolution)into one.Therefore the joint inversion can achieve accurate identification and precise detection to geological structures.This paper also compared and analyzed the inversion results of the four different weight distribution methods.As for the differences of the inversion results,the corresponding reasons were analyzed.According to the actual geological stracture of a graphite mine in the southwest China,in this paper,a complex model was designed to verify that the joint inversion can adapt to the comprehensive enviroment.Comparing the results of joint inversion and MT or LOTEM alone inversion,the joint inversion result is more in line with the designed graphite ore.In the aspects of characterizing the shape of the graphite ore and delineating its spatial distribution,the joint inversion has greater advantages than the alone.The practice showes that the suggested joint inversion method can maintain the large exploration depth while effectively improve the shallow resolution.At the same time,the joint inversion is superior in speeding up the inversion iteration,limiting the non-uniqueness,and ensuring the inversion stability.Thus,the study of this paper will provide guidance for future geophysical methodological research and an additional choice for actual exploration.