Sampling rate and necessary conditions for geoelectric structure reconstruction in transient electromagnetic exploration.

To improve the detection precision of the transient electromagnetic method, dense observation points and windows are necessary. However, to keep the number of samples in every acquisition window unchanged, the sampling rate of principal transient electromagnetic instruments decreases with transmitter frequency, and the increased windows reduce the original signal-to-noise ratio at the cost of decreasing the number of samples participating in the statistical average. Taking the V8 receiver from Phoenix Geophysics as an example, this study analyzes the main steps in the signal detection of a transient electromagnetic instrument. Following the Wiener-Khinchin law of large numbers, herein, a fixed, high-sampling-rate receiver scheme, which does not decrease the number of samples participating in the statistical average while increasing the windows, is proposed. Starting from the Nyquist sampling theorem, we provide the definition of detection precision with the dimension of length. Then, according to the necessary conditions for reconstructing a geoelectric structure, we determine the relationship between a survey grid space and a detection depth space corresponding to window interval as well as the minimum period in a geoelectric signal spectrum. The exploration of the multilayer, water-filled goaf of the Dazigou Coal Mine in Datong shows the aliasing caused by undersampling and the fine geoelectric section obtained using additional conditions under a geophysical premise. The necessary conditions for the reconstruction of the geoelectric structure indicate that satisfying the Nyquist sampling theorem may be insufficient in reconstructing the geoelectric structure, but the structure must not be reconstructed completely without not satisfying this theorem. [ABSTRACT FROM AUTHOR]