Abstract

Since electromagnetic (EM) techniques are very sensitive to cultural noise, many cases exist where standard processing techniques do not yield satisfactory signal-to-noise ratios (S/N). For EM surveys in areas with coherent regional EM noise we have developed a local noise compensation (LNC) technique. This technique, for transient EM methods, uses several mobile receivers referenced to a nearby (local) base station. LNC is applied as a prestack processing technique, and it is intended for multichannel systems with close receiver spacings. The main premise for LNC is that the regional noise is highly correlated over a region around the base station and that any other, more localized, noise is small. These assumptions must be tested at the beginning of the survey.The base station remains fixed with measurements being recorded continuously. Meanwhile, measurements are made at various other mobile stations in the zone of coherent noise. During the subsequent signal processing and removal of the transient signal, the actual noise at the base station is calculated as a function of time. Then, this noise is subtracted from the time series measured at the mobile sites. The method results in improvements in data quality, in depth of investigation and in productivity.Field tests carried out in Germany demonstrate results in an environment where the initial test showed that the necessary assumptions were not satisfied, although improvements in the S/N of the single records were obtained. Field tests in the Peoples Republic of China show a successful application of the technique, whereby the length of the usable signal was increased by a factor of about ten.

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