DOWNHOLE TIME-DOMAIN PULSED ELECTROMAGNETIC METHOD FOR DETECTING RESISTIVITY OF STRATUM OUTSIDE METAL CASED PIPE

Disclosed is a downhole time-domain pulsed electromagnetic method for detecting the resistivity of a stratum outside a metal cased pipe and having steps of: 1) constructing a high-power pulse emission source in the well; receiving time-varying data of an induced electromotive force e generated by a magnetic-field vertical component Bz and recorded by a recording system; recording simultaneously a transmission waveform and a reception signal; 2) performing a plurality of transmission/reception, superpositioning a signal, increasing a signal-to-noise ratio; 3) calculating a cased pipe response on the basis of a cased pipe parameter and a recorded emission source current waveform; acquiring a relatively induced electromotive force ?e of the stratum outside the cased pipe; 4) calibrating the relatively induced electromotive force; 5) performing an one-dimensional inversion, converting a time-varying observation signal into radial change information of resistivity of the stratum around the well; 6) on the basis of