On the feasibility of electric field dual-frequency sounding in the near field of an electric dipole source
Sounding in the non-plane wave region is an important driving force for the theoretical development of frequency domain electromagnetic exploration methods.The wide field electromagnetic method does not simplify the theoretical expression of the electromagnetic field in the non-plane wave region,and obtains the apparent resistivity of the full region through iterative solution,which greatly expands the observation space range and detec-tion depth of the controlled source frequency domain electromagnetic method.E-Ex model is the most commonly used wide field electromagnetic observation model,but its near field sounding ability is limited.In this paper,the feasibility of two near field dual-frequency sounding schemes of E-Ex mode for the wide field electromagnetic method is discussed.Firstly,the basic principles of two schemes,the so-called high order and low-order approximation,for calculating the apparent resistivity of near field dual-frequency sounding are described briefly.Then an one-dimensional model is used for numerical simulation,and the relationship between the intensity of dif-ferent field components which vary with the induction number is compared in the half-space model.The apparent resistivity of typical geoelectric model is calculated,and the influence of frequency interval and observation error on the calculation results is analyzed.The results show that the higher order approximation scheme is not feasible.Under ideal conditions,the low-order approximate dual-frequency sounding scheme can obtain the variation of the geoelectric information in the near field,which is more sensitive to the underground low resistivity layer,and is less affected by the observation angle,the length of the grounding wire dipole and the observation frequency interval.However,it is very sensitive to the error of observation data,and the small error in the total observation field will also lead to the failure of extracting the near field apparent resistivity information.Therefore,controlling the observation accuracy is the key to the practical application of the near field electric field dual-frequency sound-ing methods.
electromagnetic methodelectromagnetic soundingfrequency domainnear fieldwide field