Noise reduction in electromagnetic time series to improve detection of seismic-induced signals

A method is described for removing the ionospheric-induced electromagnetic signals from electromagnetic time series in order to improve the detection of signals caused by oceanic, industrial, environmental, or seismic activity. The method is based on transforming the ionospheric-induced signals at a remote site to the local site using smoothly varying transfer functions. This assumes the remote site is relatively free of the non-ionospheric-induced signals. The transformed ionospheric-induced signal is then subtracted from the local time series. The method is tested on data recorded at Hollister and Parkfield that have been provided by F. Morrison. The sites are both near the San Andreas fault in California. Parkfield is used here as the remote site because it will be less contaminated by man-made sources since it is in a remote part of central California. The electric dipoles are approximately 100 m long with electrodes buried about 3 m deep and the magnetic sensors are three buried coils.