Research On 2D Transient Electromagnetics Forward Modeling And Quasi-2D Inversion And Its Application On Seafloor Massive Sulfide Detection

The tonnage of the SMS(seafloor massive sulfide)deposits associated with global hydrothermal activities is estimated to be about 6×10~8 t,which makes it a highly potential mineral resource in the future.SMS deposits are rich in elements such as Fe,Cu,Ag,Au,etc.Therefore,the electrical conductivity of the deposits and their surrounding host rock often exhibits huge contrast,which can be well detected by transient electromagnetic instruments.Although the transient electromagnetic method has been widely used on land explorations,it is still an innovation in the detection of SMS deposits.In this paper,a one-dimensional and two-dimensional forward modeling study and comparative analysis of marine transient electromagnetic methods are carried out to provide a basis for accurate detection of the SMS deposits in regard of their depths,positions and sizes.Then,a research on quasi-two-dimensional inversion scheme is carried out,which is then applied to process and interpret the collected transient electromagnetic data to create a complete set of marine transient electromagnetic data collection,processing,and interpretation routine.It is shown that this method can not only locate the position of SMS deposits,but also obtain the buried depth and thickness of the e SMS(extinct seafloor massive sulfide)deposits.The main research contents of this paper include:Firstly,a one-dimensional layered medium forward modeling of the in-loop source and the coincident-loop source was deducted,and equivalent models of SMS deposits under different parameters such as conductivity,thickness,and altitude were established to study the transient electromagnetic response pattern.Secondly,a two-dimensional SMS deposit model was created and applied with unstructured mesh segmentation to achieve a two-dimensional transient electromagnetic forward modeling,and its result was compared with the one-dimensional forward results.Thirdly,the one-dimensional Occam inversion of transient electromagnetic responses was completed,the conductivity distribution in depth of the layered model was obtained,and the two-dimensional conductivity model was constructed to apply lateral constraints to the quasi two-dimensional inversion,so that the inversion results are more accurate and reliable.Then,a series of processing procedures such as data screening,preprocessing and pseudo two-dimensional inversion was applied to actual marine TEM data collected from the TAG hydrothermal field in the Atlantic Ocean.Finally,the conductivity information of TAG mount and MIR zone in the hydrothermal field were obtained from inversion,the distribution of the deposits were delineated,and the interpretation of the result was combined with geological data from drilled and grabbed samples.