Quality analysis of sonic logs and comparison to seismic checkshots during NanTroSEIZE Stage1 LWD

NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) Stage 1A, the first IODP (Integrated Ocean Drilling Program) expedition by the drilling vessel "gChikyu"h, was completed in off-shore Kii peninsula, Japan. Natural gamma ray, resistivity, resistivity image, density, porosity, sonic velocity, and seismic checkshot surveys were obtained with various LWD (Logging While Drilling) tools during the Expedition 314. Our target geology was faults and the accretionary prism, therefore the formation was very sticky and fractured. Moreover because the holes were drilled with a riserless system and without mud control, it was very difficult to keep the hole condition stable. The ultrasonic caliper indicates large washouts and the sonic data are noisy in most zones at all sites. The hole diameters are classified into three washout conditions: 9in, 9.5in and larger than 10in. The sonic quality is classified into six conditions: mud slowness, dispersive very slow compressional slowness, good quality, low S/N, no compressional signal and others. The good quality intervals of sonic logging are consistent with the intervals of 9in hole diameter. The low S/N and no compressional signal intervals of sonic logging are consistent with the intervals of larger than 9.5in hole diameter. The sonic quality has good correlation with hole conditions, especially hole diameter. The sonic quality log bring us not only velocity values but also hole conditions and formation information. This is first time in the scientific ocean drilling history to use the LWD seismic checkshot survey tool, which measures seismic signals transmitted by an air gun source at the surface with a downhole tool consisting of a hydrophone and a 3-component geophone. The new technology allows us to monitor partial waveforms and the first arrival time in real time. It assisted in evaluating the data quality and in navigating the drill bit location while drilling. Although the operation faced several tool failures under strong challenges (Kuroshio current and borehole conditions) and the memory data was noisy, the velocity profiles were very consistent with those of 3D surface seismic data and sonic logs. In the low S/N and no compressional signal intervals of sonic logging, the quality was low but the values of velocities are reliable judging from the seismic checkshot results.