Standard LWD Data Processing
Science operator: Texas A&M University
Hole: U1520B
Expedition: 372
Location: Hikurangi Subduction Margin (SW Pacific Ocean)
Latitude: 38° 58.1587' S
Longitude: 179° 7.9233' E
Logging date: December 29-30, 2017
Sea floor depth (driller's): 3538.3 m DRF
Sea floor depth (logger's): 3531.0 DSF
Total penetration: 757 m DSF
Oldest sediment recovered: Early Miocene (NB: LWD Hole - Site cored during IODP 375)
Lithology: Silty clay to clayey silt with interbeds of silt and sand; marl, calcareous mudstone, and chalk with debris flow deposits and volcanic ash/tuff
Data
The logging data were recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in December 2017.
The logging of Hole U1520B was performed on December 29-30 of 2017, reaching the total depth of 757 m DSF. The logging-while-drilling (LWD) tool string was assembled and contained the geoVISION, SonicScope, NeoScope, TeleScope, and proVISION tools. The LWD data include downhole measurements of borehole caliper, resistivity at bit, annular pressure, gamma ray, velocity, porosity, bulk density, and nuclear magnetic resonance.
Depth shift: Original logs have been depth-shifted to the sea floor (-3531.0 m). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface.
Gamma ray data processing: Processing of the data was performed in real-time onboard by Schlumberger personnel. Gamma ray data is measured as natural gamma ray (GR): the GR from the RAB tool has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Neutron porosity data processing: Processing of the data was performed onshore by Schlumberger personnel. The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature, and weight).
Sonic slowness data: Processing of the data was performed onshore by Schlumberger personnel. The sonic slowness data was sampled with a sampling rate of 0.1524 m (0.5 ft).
Resistivity data: The array resistivity compensated data was sampled with a sampling rate of 0.1524 m (0.5 ft). The RAB resistivity was sampled with a sampling rate of 0.03048 m (1.2 in).
During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. The best data is acquired in a circular borehole; this is particularly true for the density tool, which uses clamp-on stabilizers to eliminate mud standoff and to ensure proper contact with the borehole wall. A data quality indicator is given by the ultrasonic caliper (UCAV) measurement of hole diameter. For NMR logs, the data quality is generally high when the distance between the tool sensor and the wall of the borehole is less than 1 inch. The data quality may be degraded when this distance is greater than 1 inch. The caliper log shows less than 1 inch of the washout for about 75% of the logged interval, indicating moderate quality of the processed NMR data. However, there are a number of missing data intervals at depths of 215-225, 270-293, 302-312, 340-358, 410-436, and 502-511 m WMSF due probably to some technical problems. The processed sonic slowness data (DTCO) is of good quality.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the International Ocean Discovery Program, Expedition 372.
For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.