Signal processing methods for crosswell electromagnetic imaging system

Abstract

The principle of crosswell electromagnetic (EM) imaging is similar to that of induction logging. The difference between them is that conventional induction logging can only measure the formation conductivity within a few meters around the well. However, crosswell EM imaging can be used in two wells with a separation distance of up to 1000 m. Several novel signal processing methods for crosswell EM imaging systems are developed for calibration, casing correction, and hybrid Jacobian matrix calculation. To conveniently perform calibration on the ground, the EM field formula in a vertical layered media is derived based on Maxwell equations, and the relationships between the received signal and frequency, formation conductivity, and transceiver spacing are subsequently derived. Furthermore, a new calibration method is developed. The EM field formula for a radial layered media is derived, and the analytical expressions describing the relationship between the receiving signal and borehole parameters, metal casing parameters, formation parameters, and receiver location are obtained. The effects of mud resistivity, metal casing position, metal casing resistivity, permeability, cement ring resistivity, transmitting frequency, and receiver location on the crosswell EM receiving signal are simulated. It is determined that only the casing required a correction. The influence of the metal casing is corrected by establishing a database. The difference between the corrected data and data obtained without a metal casing is less than 1%. There is a large error in the calculation of the Jacobian matrix in some regions when Green’s function was used. A hybrid Jacobian matrix calculation method combining local perturbation and a global Green’s function is developed, which significantly reduced the error and improved the inversion accuracy.