Research On Forward Modeling Of Out-of-quadrant Phase Data In Magnetotelluric Sounding

The term “out-of-quadrant phase” often refers to the phenomenon that the phase value of the off-diagonal element of the impedance tensor in the measured data of magnetotelluric sounding exceeds the normal phase range of the first or third quadrant.This phenomenon was first formally proposed in the late 1980 s.Since then,geophysicists have discovered phase anomalies in various parts of the world.Due to the limitations of the processing techniques at that time,scholars could only model with two-dimensional isotropic medium.However,the isotropic two-dimensional inversion technology cannot effectively fit these out-of-quadrant phase curves in the low frequency band.Therefore,in the two-dimensional inversion,data with such phases must be eliminated,which undoubtedly reduces the reliability of the inversion results.This indicates that the research methods based on one-dimensional or two-dimensional isotropic media may not be used to explain the phase anomaly phenomenon,so this phenomenon is often speculated to be caused by the current distortion effect or the complex three-dimensional structure of the underground in the early research.In recent years,with the popularization of magnetotelluric method technology,as well as the rapid development of forward and inversion techniques for 3D isotropic media,the simulattion on the phenomenon of out-of-quadrant phase by constructing a synthetic 3D isotropic model becomes possible.At the same time,more and more field data have been found to contain out-of-quadrant phases.However,so far there are still insufficient studies on the 3D modeling and interpretation of the observed out-of-quadrant phase data,as well as a systematic comparison on the cause of this phenomenon.The theoretical basis on the generation of out-of-quadrant phases are urgently in needs of improving.This study intends to summarize the results of previous studies on the out-of-quadrant phase phenomenon.We attempt to reproduce the various hypotheses previously proposed through the numerical modelling,to find the key structure that can cause the out-of-quadrant phenomenon.After analyzing the data with such phase anomalies from the Nelson Batholith area,the contact region between the northeastern margin of the Qinghai-Tibet Plateau and North China block,and the eastern section of the Eastern Qinling orogenic belt,3D isotropic models were obtained by inverting the data using the 3D Mod EM program.The models were then decomposed and reorganized,to obtain the key structures that the out-of-quadrant phases dependence on.Through the continuously simplified and forward modeling,we summerize the common feature of the structures that re-produce the out-of-quadrant phase,for the three survey areas.Then,the small area model that can produce the response of the phenomenon beyond the quadrant is continuously simplified by forward comparison.In the process of simplification,the common points of the structures that produce the phase out-of-quadrant phenomenon are summarized.Based on the specific structure of the three-dimensional electrical model obtained by the inversion,two types of theoretical three-dimensional models that may be in line with the reality are finally constructed,namely: embedded feature models and spirals Style feature model.Based on the actual geological data,the two types of models are reasonably deduced,and the two types of models are used to give a reasonable explanation for the phenomenon of phase-out-of-quadrant in the study area of this paper.