Rapid Sinusoidal Fitting Method for Near-Bit Gamma Imaging While Drilling

Seeking to solve the problem of low transmission rate of existing LWD data and the demands on real-time gamma imaging, sector gamma-ray imaging characteristics were simulated and analyzed. This occurred while a near bit gamma-ray imaging tool crossing inclined interface moving from low-level radioactive sandstone to high-level radioactive mudstone, and thus a rapid sinusoidal fitting method based on the sector gamma imaging was proposed. Based on the studies, a rapid sinusoidal fitting firmware algorithm for near-bit gamma imaging logging was designed. This algorithm combined the least squares frequency estimation and 3-parameter sine fitting to obtain the 4 parameters of the sinusoid, such as amplitude, frequency, phase and DC component, hence obtaining the multi-sector gamma imaging through fitting inversion. By using the fast sinusoidal fitting method, the gamma data was acquired in the simulated wellbore fabricated with the standard rock samples and layered dipping strata, and the fitting of multi-sector gamma imaging measurement was realized with a small fitting error. The results showed that the sinusoidal features of the 8-sector gamma image obtained by fitting inversion were clear and they could accurately reflect the information of the inclined strata interface, which verified the correctness and feasibility of the fast sinusoidal fitting method.