1. A method for removing noise from data measured in a controlled source electromagnetic survey of a subterranean region, comprising: a. obtaining a temporal frequency component from the data, said temporal frequency being selected from said controlled source's frequency spectrum; b. selecting a cutoff frequency, f Cutoff , said cutoff frequency being selected to differentiate sharply varying response, inconsistent with the source's signal and earth resistivity, from gradually varying response; c. applying a low-pass filter to said temporal frequency component of said data, said low-pass filter being designed to suppress frequencies above said cutoff frequency; and d. generating thereby a sequence of filtered data values.