Extracting shallow geophysical information from electromagnetic data with pseudorandom current using an equivalent sampling method

Abstract Using a pseudorandom emission source and extracting the Earth's impulse response from electromagnetic data based on a correlation identification method can obtain geophysical information at higher resolution and shallower detection depth than traditional low-frequency step emission sources. To obtain the identified Earth impulse response with earlier sampling time and higher accuracy, we propose a series of data-processing methods. First, based on the Bostick algorithm, we derived the order and transmission frequency of pseudorandom sources suitable for practical detection of horizontal and vertical resolutions. Second, we adopted the equivalent sampling method and designed fractional multiple sampling frequencies based on the actual required measurement sampling frequency. By reconstructing multi-period low sampling rate data into single-period high-resolution data, the earliest sampling time of the identified Earth impulse response advanced. Subsequently, we proposed a cyclic correlation identification method that suppresses the identified side lobe, suppresses random noise, and improves the accuracy of the identified Earth’s impulse response. Finally, we used the proposed method to identify and process measured data in the Alxa area of Inner Mongolia, confirming its reliability for extracting shallow measurement data.