Estimation of Elastic Wave Velocity From Logging-While-Drilling Measurement in Unconsolidated Formation of Accretionary Wedge in Nankai Trough

The logging-while-drilling (LWD) acoustic technology has been developed to measure formation velocities/slownesses for rig time saving and real-time applications such as seismic-tie, pore pressure determination, and/or deep-sea drilling exploration. The formation compression-wave velocity is often hard to determine because of the drilling environment and wave dispersion effects. We develop a new model-based dispersive processing (MDP) method to accurately determine wave slowness from the LWD acoustic data. Two specific steps are included during the MDP procedure. In the first, the effective collar parameters are determined using the collar wave from the measurements. The second step is the inversion for the wave slowness by minimizing the difference between the theoretical curve and dispersion data. We have validated the result of the MDP inversion with synthetic data testing and applied it to the acoustic LWD data of the Ocean Drilling Program (ODP) 808I hole. The applications of the corrected slowness results to synthetic seismograms and porosity prediction analyses demonstrate the effectiveness of the correction method.