Integral Equation Method Forward Of Marine Controlled Source Electromagnetic Field

China's research on the marine controlled source electromagnetic method began in the 1970 s,but the real attention of the marine controlled source electromagnetic method has really started from the 1990 s.Development,it has gradually become the main method of geophysical exploration at sea.At present,the marine controlled source electromagnetic method is widely used in offshore oil and gas exploration and auxiliary evaluation of natural gas hydrate resources before drilling.In this paper,the boundary integral method forward modeling of the marine controlled source electromagnetic field is studied.Starting from the basic principles of Maxwell's equations and boundary element methods,combined with the boundary conditions of the electromagnetic method,according to the superposition principle of the field,the integral equations satisfying the primary and secondary fields are deduced.Firstly,the electric field and magnetic field expressions of the horizontal dipole source in uniform full space and layered medium are deduced respectively,and then forward simulation is programmed.Then,based on Anderson's proposed Hankel filter coefficients,the adaptive linear filter algorithm is applied to the forward modeling.Through multiple model examples,the normalized amplitude and phase changes of the electric and magnetic fields are obtained.Taking the Beibu Gulf marine geoelectric model as an example,the effects of different seawater depths,different reservoir thicknesses,and different transmission and receiving distances on the detection effect were analyzed.Finally,since one-dimensional forward modeling can only simulate layered geoelectric models,in order to further realize three-dimensional complex model forward modeling,the volume fractions are converted into area fractions using the Stratton-Chu integral formula,and then the surface of the anomalous body is meshed.Grid division.The integral equation method forward modeling of a three-dimensional complex model is implemented.Firstly,two three-dimensional ocean geoelectric models with anomalous bodies are designed.The results of filtering algorithm and boundary element method are used to verify the reliability of the boundary element method.Then,the boundary element method is used to calculate whether the anomalous body contains anomalous terrain under the seabed,and the influence of the undulating terrain on the detection of anomalous body is analyzed.The results of calculation examples show that in the detection of electrical sources of marine layered media,the deeper the seawater is,the smaller the amplitude and phase changes of the electric and magnetic fields are,the better the detection effect is;and the thickness of the oil layer is the effect of detection.There is no obvious influence;in the shallow sea area,select the appropriate frequency,the smaller the transmission distance,the smaller the amplitude change,and the detection effect is relatively better.When carrying out a three-dimensional complex model forward simulation,the results of comparative analysis show that the calculation results of the boundary element method are consistent with the calculation results of the filtering algorithm;in addition,when detecting anomalous bodies,the influence of subsurface topography on anomalous bodies cannot be ignored.