Borehole time domain pulse electromagnetic method for detecting electrical resistivity of stratum out of metal bushing

The invention discloses a borehole time domain pulse electromagnetic method for detecting the electrical resistivity of the stratum out of a metal bushing, comprising the following steps of: 1) building a large-power pulse emitting source underground; receiving the time variable data of induced electromotive force e produced by a magnetic field vertical component Bz recorded in the recording system; and simultaneously recording emitting wave type and receiving signal; 2) emitting and receiving several times, and overlapping signals to improve the signal to noise ratio; 3) calculating bushing response according to the bushing parameter and the recorded emitting source current wave type to obtain the induced electromotive force delta epsilon f relative to the stratum out of metal bushing; 4) correcting the relative induced electromotive force; 5) one-dimensionally reversing and transforming the time variable observed signal into the radial variation information of the electrical resistivity of the stratum aroun