Processing Notes

IODP-MSP drilling and logging contractor: ESO

Hole: M0079A

Expedition: 381

Location: Gulf of Corinth (Ionian Sea)

Latitude: 38° 9' 30.243" N

Longitude: 22° 41' 43.316" E

Logging date:  November 29, 2017 –December 1,  2017

Sea floor depth (driller's): 862.96 mbsl

Sea floor depth (logger's): 857.1 mbsl

Total penetration: 704.9 mbsf (drillers)

Core recovery:  86.65%

Oldest sediment recovered: n/a at time of writing

Lithologies: mud, clay, sand

Data

During Expedition 381, slim-hole downhole logging services managed by the European Petrophysics Consortium (EPC) were contracted by the University of Montpellier (France). Wireline logging data were acquired in .tdf format and processed by the EPC with the software package WellCad (v5.2). Raw data are provided online in DLIS format. Processed standard data are available in ASCII format.

The wireline logs were recorded with stand-alone logging tools in stages. Stages were defined depending on drilling information, core descriptions and physical properties measured on cores. Bentonite (32 viscosity, 9 lbs/gal) was present in the borehole during all logging. No data were acquired during Stage 1 due to technical problems with the winch, and only a partial EM51 log was acquired during Stage 2 because the borehole was closed at  about 296.5 m WSF.

Through pipe (November 29-30, 2017) 

Run 1: ASGR512.

Stage 2 (November 30, 2017), Upper Section, Interval 2 

Run 1: EM51.

Stage 3 (November 30 –December 1, 2017), Upper Section, Interval 1

Run 1: EM51.

Run 2: 2PSA

Run 3: DLL3.

The logging data presented in the following table were processed by the European Petrophysics Consortium.

Processed Logging Data

* 2 versions of processed ASGR data are available. See Spectral gamma ray data.

The depths in the table above are for the processed logs, after applying a depth shift to the sea floor. Discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length. Typical reasons for depth discrepancies are ship heave and tides. Peak-to-peak heave motion was always inferior to 1 m during logging. The heave compensator was activated, thus reducing the vertical movement to a maximum amplitude of 10 cm.  Other reasons for depth discrepancies are wireline and pipe stretch, and the difficulty of getting an accurate sea floor depth from the 'bottom felt' depth in soft sediments.

Processing

Depth shift and depth match. For each run, depths corrections have been applied so the data are all referenced to the seafloor (m WMSF). Depth corrections include:

• Zero logger (pipe entrance in the roster box) to zero driller (rig floor);
• Rig floor to sea floor;
• Difference between the initial and final zero of the tool;
• Manual shift using distinctive features, wherever necessary: gamma ray logs crossing the sea floor or tools transitioning from the open hole to the pipe, both giving a clear signal of features that are consistent between different runs of a same stage.
•  

Environmental correction. Environmental corrections are designed to remove any effect from the borehole (size, roughness, temperature, and tool standoff) or the drilling fluid (bentonite) that may partially mask or disrupt the log response from the formation. No post-acquisition corrections were applied at this hole.

Sonic data. The 2PSA data were acquired at a frequency of 15 kHz. The data were processed in WellCad to calculate the compressional velocity. Due to the limited quality of the waveform, first arrivals picking was done manually. Time picks were saved and the acoustic velocities were calculated using the R1 and R2 receivers (spacing = 1ft).

Spectral gamma ray data. The gamma ray logs recorded through drill pipe should be used qualitatively only due to attenuation of the incoming signal. Further attenuation is noted at the joint between two pipes (drop in values every ~9-10 m) because the joints have a thicker metallic wall than the pipes. Two versions of the through pipe gamma ray logs are provided: one without any cleaning of the data, and one where data recorded though joints have been removed (values replaced by -999.25). The bottom ~95 m of the drill string have a thicker wall that further attenuate the signal received from the formation.

Resistivity and conductivity data.The DLL3 performs well in high resistivity formations and consolidated sediments. Due to the borehole width, the presence of bentonite mud in the borehole, the low resistivity in the formation and the nature of the sediments, care must be taken when using resistivity values, especially the shallow readings. The induction conductivity measured by the EM51 tool (magnetic susceptibility and conductivity), which is more effective in high-conductivity formations, should be preferred.

Quality Control

Borehole conditions can greatly influence log data quality. The lack of downhole images or caliper data in Hole M0079A does not allow for an assessment of the quality of the borehole and how it could affect the collected data. The frequent problems during the descent and the inability to progress further down the hole to total depth, point at possible swellings of the formation (recovered cores tended to expand too).

The sonic tools are ideally run with centralizers. Use of centralizers implies bow-springs able to compress to pass through the drill bit (inner diameter = 95 mm) and then to open more than double that diameter in open hole (outer diameter of the drill bit = 216 mm) to keep the tool centralized. No such bow-springs were available for the tool 2PSA, this the tool string was operated without centralizers. This most likely affected  the quality of the sonic data.

The spectral and total gamma ray acquired through pipe should be used with care as the signal emitted by the formation was attenuated by the drill pipe wall and the drill collars before reaching the tool.

For further questions about the logging data, please contact:

Erwan Le Ber

School of Geography, Geology and the Environment

University of Leicester 
University Road 
Leicester 
LE1 7RH

United Kingdom

Phone: 0116 252 3327

E-mail: elb51@le.ac.uk