Abstract
In spite of the prevalence of high common mode rejection ratio (CMRR) input amplifiers, notch filters are routinely used in seismic recording operations to reduce common mode induced interference. An electrical model of the recording environment which predicts the degradation in system CMRR caused by cable imperfections such as imbalance and leakage is described in this paper. System CMRRs as low as 20 dB can be caused by poor quality cables. A new method of controlling common mode interference has been developed which has none of the disadvantages of notch filters. The method minimizes the correlation between the common mode and differential mode signals at the recording instrument. This process has no effect on the desired seismic signals, has minimal effect on the system noise, and typically results in a system CMRR in excess of 100 dB.
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