Audio-frequency spectra of passive source electromagnetic fields under a cultural background: A case study from the middle and lower reaches of the Yangtze River, China

In order to study the spectra characteristics of audio-frequency passive electromagnetic (EM) fields under a cultural background, long-term audio magnetotelluric observation stations were deployed in two different typical ore districts in the middle and lower reaches of the Yangtze River (MLYR), east China, where it is widely regarded as in a typical natural and cultural mixed source environment. Data of orthogonal horizontal electric and magnetic fields were recorded continuously for months. Power spectra of the EM fields, signal coherences and apparent resistivities are estimated by employing the standard magnetotelluric robust data processing method. The observed data set is statistically analyzed by daily data and their medians for each field component. There are distinct band characteristics for the overall spectra. The spectra display very low power and are unstable in the high and low frequency “dead bands”. There are several distinguishable extreme values, including the maxima of the fundamental power-line frequency and its harmonics, the maxima of the Schumann resonances, and a minimum at 780 Hz. In the time scale, the spectra have variations both daily and annually. The spectral amplitude is greater in the nighttime than daytime only in the high frequency “dead band”, while it is greater in summer than autumn and winter in the whole observation frequency band, including the industrial power frequencies. In the spatial scale, the differences of magnetic spectrum amplitude are mainly reflected in the band of 0.35–10 Hz and the power-line frequencies between the two selected regions, which indicate the background noise level of the north is stronger than that of the south of the Yangtze River in MLYR. The results have given us a better understanding of the passive source EM fields in east China, which provide a guiding and practical value for both the natural and controlled source EM exploration methods in data collection, processing, and interpretation. •Long-term observation revealed distinct EM spectra characteristics.•“Dead bands” have a dynamic frequency range without exact boundaries.•Sources of the MLYR area have common characteristics and different local features.