Abstract: Apparatus is described for eliminating monochromatic interfering frequency components of seismic signals detected during the course of seismic exploration comprising an electrical circuit for producing a monochromatic nulling signal that has the same frequency as the interfering signal. The monochromatic nulling signal is produced by generating a truncated triangular wave having the same period as the interfering signal and filtering the truncated triangular wave to remove all components thereof except those having the period of the interfering signal. Circuit means are provided for controlling the amount of truncation of the truncated triangular wave, and for controlling the time phase thereof. The filtered signal is combined with the geophone signal in such a manner that the filtered signal at least partially cancels out the interfering component of the seismic signal.