Processing Notes

IODP-MSP drilling and logging contractor: ESO

Hole: M0080A

Expedition: 381

Location: Gulf of Corinth (Ionian Sea)

Latitude: 38° 7' 12.1467" N

Longitude: 23° 5' 10.6138" E

Logging date: December 15 – 17, 2017

Sea floor depth (driller's): 350.22 mbsl

Sea floor depth (logger's): 349.2 mbsl

Total penetration: 534.10 mbsf (drillers)

Core recovery:  78%

Oldest sediment recovered: n/a at time of writing

Lithologies: mud, clay, sand, gravel, breccia

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, 8.8 lbs/gal) was present in the hole during logging through pipe and stages 1 and 2. Sea water was circulated during Stage 3.

Through pipe (December 15, 2017) 

Run 1: ASGR512

Stage 1 (December 15-16, 2017), Lower Section

Run 1: EM51

Run 2: 2PSA

Run 3: ASGR512

Stage 2 (December 16, 2017), Middle Section 

Run 1: EM51

Run 2: 2PSA

Run 3: DIL45

Run 4: ASGR512

Stage 3 (December 17, 2017), Upper Section

Run 1: EM51

Run 2: 2PSA

Run 3: ASGR512

Run 4: DIL45

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

Processed Logging Data

* 2 versions of processed through pipe ASGR512 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. Waves’ peak to peak was always inferior to 1 m during logging, and heave compensation was activated. Heave compensation reduced 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 10 and 15 kHz frequency in the upper and lower sections respectively. The sonic 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.

Conductivity data> After using the DLL3 in Hole M0079A, it was decided to use the DIL45 to collect deep and shallow conductivity data because the way the tool works (induction) and the measurement range were more adapted to the type of formation encountered. The DIL45 and EM51 are both induction tools and perform well in conductive formations. There is a good match in trends and range of values for the wo datasets.

Quality Control

Borehole conditions can greatly influence log data quality. The lack of downhole images or caliper data in Hole M0080A do not allow for an assessment of the quality of the borehole and how it could affect collected data. The frequent problems encountered when sending the tools down during the expedition, with difficulty to progress further down the hole, point at possible swellings (upper 200 m of cores recovered tended to expand too) and instability (sands, gravels) of the formation.

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