A Broadband Electromagnetic Propagation Logging With Linear Frequency Modulation Signal

In this letter, a novel broadband electromagnetic (EM) propagation logging method with linear frequency modulation (LFM) continuous waves (LFMCWs) as well-logging signals has been proposed for logging while drilling (LWD). The logging response of the layered media to the LFMCWs under the noise environment has been numerically modeled using numerical mode matching (NMM) and inverse fast Fourier transform (IFFT). Based on the frequency spectrum of LFM signals, the NMM method is used first to calculate both the amplitude and the phase of the signals in receiving coils for every frequency point, and then, IFFT is applied to convert them to the logging response in the time domain. After that, the matched filter method has been imposed to realize pulse compression of the received signals and to improve the peak SNR (PSNR) significantly. As the logging responses of the EM propagation logging tool, the amplitude ratio and the phase difference between the compressed signals from the two separated coils at the peak point are obtained, respectively. Then, the apparent conductivity based on the amplitude ratio and the phase difference is obtained to have the logging curves. It has been shown from the simulation results that using LFMCWs as wideband logging signals can improve the SNR and the logging resolution remarkably, particularly under the condition with correlated noise that restrained the SNR and logging resolution strongly in traditional logging with a single-frequency continuous wave.