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. Circuit means are provided for detecting the phase of the residual signal resulting from incomplete cancellation of the interfering signal to provide an indication of the amplitude of the nulling signal. Means are also provided for detecting the time phase of the nulling signal. The truncation of the truncated triangular wave, and the time phase of the truncated triangular wave are controlled so as to result in more complete cancellation.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Divisional of copending application Ser. No. 421,384 filed Dec. 3, 1973 now U.S. Pat. No. 3,889,229.
