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

A method for detecting formation resistivity outside of metal casing using borehole time-domain electromagnetic pulse including steps of a) providing a borehole large power pulse transmitting source, recording an induced electromotive force µ, and full time digital recording a transmitted waveform and a received signal; b) conducting transmission-reception and superimposing received signals to improve signal to noise ratio; c) calculating corresponding casing response according to known casing parameters and recorded current waveform of the transmitting source to obtain a relative induced electromotive force ”µ f ; d) correcting relative induced electromotive force value; e) carrying out one-dimensional inversion and converting the observation signal into radial variation information of the formation resistivity; f) obtaining a two dimensional image of the longitudinal and radial resistivity distribution of outer cased formation resistivity for measured well sections; and g) determining residual oil distribution in a reservoir from outer cased reservoir resistivity distribution.