Quantitative Appraisal of Noise in Time Domain Induced Polarization Data

The possibility to acquire rapidly time domain IP data with multi channel systems is spreading the application of this technique. The quality of such IP data measured in the field is an important, yet not well documented matter. We present a systematic study of noise levels and sources in time domain IP data, carried out with repeated measurements, and different instrumental set ups, in two distinct geological settings. The results show that relative (random) noise to signal values are usually in excess of 10 % at early times, higher at late times. Measuring too early after placing the (stainless steel) electrodes in the ground seems to affect the early times of the decay. Periodic sub harmonics of the 50 Hz can be found in some occasions superimposed to the IP data. It is very important to obtain good electrical contact between electrodes and ground in order to lower both random and coherent noise, in particular capacitive coupling. Inductive coupling is reduced to the minimum using separated cables and electrodes for sending the current and measuring the potential. Full analysis of the whole IP decay curve is crucial for assessing data quality.