ATTENUATION PREDICTIONS AT EXTREMELY LOW-FREQUENCIES FOR MEASUREMENT-WHILE-DRILLING ELECTROMAGNETIC TELEMETRY SYSTEM

Extremely low frequencies (ELF) are employed to transmit data from underground to the ground surface for Measurement-While-Drilling Electromagnetic (MWD-EM) telemetry system. Based on electromagnetic field theory, the work is aimed at predicting the receivability of the signals on the ground surface. A unified analytic method that is suitable for vertical well, directional well, and horizontal well is presented. Attenuation properties are examined for various parameters, including the earth's conductivity, operating frequency and the length of the drill string. Frequency dependence of the receivability in reference to a noise level is illustrated for different depths of well and different earth cases. It is also demonstrated that the electric field distributions on the ground surface have the same features for the three types of well, and the measurements should be carried out near the well heads for any type of well. A scale model experiment is made to test our theoretical results. The measured data and the computed results are comparable.