Analysis of Signal Transmission for Use of Logging While Drilling

Effective downhole-to-surface transmission of the logging signals is one of the critical techniques for logging while drilling (LWD). This letter aims to evaluate an advanced scheme of signal transmission for the LWD proposed by Novatek, with an emphasis on the electromagnetic (EM) coupling between two adjacent pipes and the impedance matching between the coupler and the coaxial cables imbedded in drilling pipes. The numerical modeling of the EM coupling structure has been made by first using the full-wave simulation tool, and then the influence of the parameters of the coupler on the coupling coefficient has been discussed based on circuit simulation software. The optimization of the critical parameters has been made based on the given analysis. It has been found from the analysis that the air gap between the upper and the lower ferrite cores in the coupler is a key factor with remarkable influences on the impedance matching and the coupling coefficient.