Standard LWD Data Processing


Science operator: Texas A&M University

Hole: U1518A

Expedition: 372

Location: Hikurangi Subduction Margin (SW Pacific Ocean)

Latitude: 38° 51.5368' S

Longitude: 178° 53.7606' E

Logging date: December 20, 2017

Sea floor depth (driller's): 2647.3 m DRF

Sea floor depth (logger's): 2642.4 m LRF

Total penetration: 123 m LSF


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-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 and temperature, gamma ray, velocity, porosity, bulk density, and nuclear magnetic resonance. The logging of Hole U1518A reached a total depth of 123 m LSF.




Depth shift: The original logs have been depth-shifted to the sea floor (-2642.4 m). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface. It differed 4.9 m from the driller's depth.


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).



Quality Control


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 93% of the logged interval, indicating high quality of the processed NMR data. The processed sonic slowness data (DTCO) is of good quality as well. Note that the GR log from the RAB tool shows anomalously high values of 150-700 gAPI at the depth intervals that are largely associated with missing RAB images.


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 further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition.

After the moratorium period you may direct your questions to:



Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-8777

E-mail: Tanzhuo Liu


Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-8777

E-mail: Cristina Broglia