A method for mapping sub-surface resistivity contrasts comprising making multi-channel transient electromagnetic (MTEM) measurements using at least one electric source with grounded input current electrodes or one magnetic source with current in a wire loop or multi loop and at least one electric field or magnetic field receiver, measuring the resultant earth response simultaneously at each receiver using a known recording system, measuring the system response by measuring directly the current in the wire at the source using effectively the same recording system, using the measured system response to recover the impulse response of the earth from each measured earth response, and creating from such impulse responses a sub-surface representation of resistivity contrasts.