THE AUTOBALANCER—AN AUTOMATIC SYSTEM FOR THE REDUCTION OF POWER‐LINE INTERFERENCE WITH SEISMIC SIGNALS*

Since the early years of seismic surveying, field engineers and observers have been faced with the appearance of interference at power‐line frequency (and harmonics) on seismic records: 60 Hz in the Americas, 50 Hz in Eurasia; and 16 2/3 Hz (the train supply frequency in Germany, Austria, and Switzerland) in part of Europe. Such interference is picked up from the ground surface through electric leakage between geophone leads and ground.

To minimize power‐line interference, the autobalancer automatically performs a balancing procedure by adjusting two potentiometers on each channel—essentially the same operation as manually performed by seismic operators for decades. The first 24‐channel system became operational in 1977.

The first‐generation design comprised two 256‐position electronic switching potentiometers. Over 4 years of experience with this system in all parts of the world has shown its value under a variety of field conditions. Improvements up to 30 dB were often found, sometimes even up to 40 dB. Balancing time was about 30 s.

A second‐generation design was field‐tested in 1981. Potentiometer stepping is now performed at three levels: coarse, medium, and fine. This leads to faster operation and better resolution: balancing time is about 10 s; improvements up to 60 dB are occasionally found.

A later modification gives a further reduction of the balancing time to 5 s in 60 Hz areas or 6 s in 50 Hz areas.