SEECOM-5: A cost effective method of reducing common mode derived 50 Hz signals

Abstract

Any signal in the seismic spectrum which, at the input to the seismic amplifier, has an amplitude and/or phase difference between one leg of the twisted pair and the other, will be treated as a difference signal and amplified. All signals generated by coil movements in the geophones fall into this category. On the other hand, all common mode signals would ideally be attenuated severely since they have equal amplitudes and phases in each leg of the pair. (This is because of the high common mode attenuation performance of modern amplifiers.) In practice, however, this is not the case, as in many instances the seismic pairs are in substantial imbalance due to poorly manufactured or repaired cables, or due to high leakage currents to one another or to earth.