Suppressing the very low-frequency noise by B-spline gating of transient electromagnetic data

Abstract

Abstract Very low-frequency (VLF) communication signals of 15–25 kHz contaminate the transient electromagnetic (TEM) data. The notch filter or Butterworth filter is commonly used to suppress VLF noise in addition to synchronous detection of TEM data such as gating and stacking, while also introducing the TEM signal distortion around the VLF band. We propose a B-spline gating, which suppresses the VLF noise while integrating TEM data in the gate interval without any extra filter. B-spline gating is designed to ensure that the zeros of its amplitude frequency response (AFR) are located around VLF frequencies by optimizing the knot vector of the B-spline. It is available for various VLF frequencies. We apply B-spline gating to synthetic TEM data compared with rectangular gating and full-tapered gating, the AFRs of B-spline and full-tapered gating are 25 dB lower at high frequencies than rectangular gating, and the amplitude of B-spline is more reduced at VLF frequency than full-tapered gating; the standard error of the B-spline gates is reduced by >15% compared to the full-tapered gates. The field example from a TEM survey in the Inner Mongolia Autonomous Region of China shows that the amplitude of B-spline gating frequency response in late gates is reduced by about 20 dB at 24.1 kHz VLF than that of full-tapered gating, and the standard error is lower. It is concluded that B-spline gating can suppress VLF noise while following the normal decay of TEM signal without distortion.