Several key issues in 2.5D forward modeling of semi-airborne electromagnetic sounding in the frequency domain
The semi-aerial frequency domain electromagnetic sounding method based on pseudo-random transmission has the advantages of strong signal,fast acquisition speed and low cost.It has been developed rapidly in recent years,and its practical application has achieved good exploration results.This paper addresses some core issues in 2.5D numerical simulation in this field.Using the finite element numerical method,the loading mode of the galvanic couple source,grid division,boundary treatment,observation area,and distribution characteristics of the induced magnetic field with the transmitting and receiving distance and frequency are analyzed and discussed.Model calculations show that the simulation accuracy of finite element method can be improved by adopting uniform mesh division and adding absorbing boundary conditions.It is found that the field values of the three components Hx,Hy,and Hz of the induced magnetic field generated by the electric dipole field source and the long wire field source have the following characteristics in the air at a distance of 20 m from the ground:(1)When the field source is an electric dipole source,Hy has the following characteristics:The abnormality is not sensitive,but Hx and Hz and their converted apparent resistivity have obvious anomalous characteristics.The abnormality on both sides of the vertical line in the AB dipole source will appear"symmetrical characteristics":the low-resistance body on the left is characterized by"lower first and then higher",the lower resistance body on the right is"higher and then lower",and the left The high-resistance body is characterized as"low first,then high",and the high-resistance body on the right is"high first,then low".(2)When the field source is a long power supply wire,the three components of the induced magnetic field all reflect the abnormal body,the Hx abnormal feature is complex,the Hy and Hz features are obvious,the low-resistance body is characterized by low field value,and the high-resistance body is characterized by High field value features,the abnormal features of Hz are stronger than those of H.The dipole length of the grounded electromagnetic field source affects the characteristics of the electromagnetic field,especially the three components of the induced magnetic field,and the grid and boundary of the finite element method also affect the characteristics of the electromagnetic field.When the field source is a dipole power supply,the Hx characteristic of the induced magnetic field is obvious,but the Hy characteristic is insensitive,and the Hz anomaly is obvious but the characteristic is more complicated.This study enriches some electromagnetic field characteristics of non-ideal electric dipole sources(approximate point sources),and provides a scientific basis for the development of semi-airborne electromagnetic methods.
Electric dipole sourceGround long lead sourceSemi-airborne frequency domain soundingMulti-component induced magnetic fieldFinite element method
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维普
