IODP-MSP
drilling and logging contractor: ESO
Hole: M0021B
Expedition: 310
Location:
Latitude: 17° 29.3427' S
Longitude:
149° 24.1692' W
Logging date: November 16, 2005
Sea
floor depth (driller's): 92.25 mbrf (81.70 mbsl)
Sea
floor depth (logger's): 92.25 mbrf
Total
penetration: 32.21 mbsf
Total
core recovered:
21.12 m (65.57 % of cored section)
Oldest
sediment recovered:
Pleistocene sequence
Lithologies: Reef
framework, algal crusts, and microbialite matrix
The logging data was
recorded by the
Tool string |
Pass |
Top depth (mbsf) |
Bottom depth (mbsf) |
Pipe depth (mbsf) |
Notes |
1. DIL45 |
|
3.27 |
17.07 |
2.55 |
|
2. OBI40 |
|
2.58 |
17.46 |
2.55 |
|
3. ABI40 |
|
2.54 |
17.46 |
2.55 |
|
4. ASGR |
|
0 |
17.12 |
2.55 |
|
5. IDRONAUT |
|
1.28 |
17.08 |
2.55 |
|
6. 2PCA |
|
1.82 |
17.01 |
2.55 |
|
7. 2PSA |
|
3.20 |
7.80 |
2.55 |
|
A complete
list of tool and log acronyms is available at http://brg.ldeo.columbia.edu/data/iodp-eso/exp310/exp_documents/iodp-eso-310-acronyms.html.
After completion of the coring, the drill string was pulled and the coring bit was changed for an open shoe casing to provide borehole stability in unstable sections and a smooth exit and entry of logging tools. In addition, a wiper trip was performed with fresh sea water (no drilling mud was used). Difficult borehole conditions often required the boreholes to be logged in key intervals where the HQ drill string was used as a temporary casing. All measurements were performed under open borehole conditions (no casing) with the exception of a few spectral gamma ray logs which were run through the steel pipes to obtain continuous geophysical information over the entire interval cored.
Hole
M0021B was drilled and logged during Expedition 310. Logging operations were
conducted from 17.46 mbsf upwards with data coverage
by all slimhole tools in the open borehole without
repositioning the open shoe casing (2.55 mbsf). Borehole
conditions were very hostile and the borehole was highly unstable, particularly
below 15 mbsf. The calipers show a large increase in
diameter below this depth and the logging tools could not pass below an
obstruction at around 17 mbsf despite repetitive cleaning
and hammering efforts. Optical images were badly affected by cloudy borehole
fluid, although improve in quality towards the top of the log. Total gamma radiation
was low in much of the logged section. Sonic velocities were only obtained over
a short section and were highly variable.
The depths
in the table are for the processed logs (after applying a depth shift to the
sea floor). Generally, 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, wireline and pipe
stretch, tides, and the difficulty of getting an accurate sea floor from the
'bottom felt' depth in soft sediment. However, for
Depth
shift: The original logs were first shifted to the sea floor
using the driller’s depth to seafloor (-92.25 m below rig floor). For
Environmental
corrections: None were applied.
Acoustic data: The 2PSA tool was generally run at a frequency of 10 kHz in Pass 1 and 1 kHz in Pass 2 in order to calculate compressional and Stoneley velocities respectively. However, in borehole M0021B only one run was carried out at 10 kHz. The data was filtered (frequency filter) in such a way that only the energy around the induced frequency (source) was analyzed. Waveform picking was done manually in the LogCrucher software package to ensure good quality data. Time picks were saved and the acoustic velocities were calculated (using the receiver spacing of 1 ft). All presented acoustic data is accurate. Where no clear first arrivals in the waveform were present in at least two receivers, a value of zero was entered in the database.
Spectral gamma ray: Gamma ray logs recorded through drill pipe
should be used only qualitatively due to attenuation of the incoming signal.
The quality
of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different passes of
the same tool, and by correspondence between logs affected by the same
formation property (e.g. the resistivity log should show similar features to
the acoustic log).
The quality of the ASGR Spectral Natural Gamma data is
directly related to lithology in combination with
logging speed. Despite logging speeds of 1.1 m/minute and a taking a sample
every 10 cm (collecting gamma ray emissions of the formation for approximately
6 seconds for every sample) the amount of total counts obtained are still very
low. This degrades the quality of the statistics that separates the raw counts
into activity values of naturally occurring radioactive elements such as
potassium (K), uranium (U) and thorium (Th). Negative
K values are indicative of incorrect statistics. Gamma ray logs recorded through drill pipe
should be used only qualitatively due to attenuation of the incoming signal.
Gamma ray logs recorded through drill pipe should be used only qualitatively
due to attenuation of the incoming signal.
Due to a short time period between the completion of coring
(including wiper trip) and logging, the IDRONAUT data should be treated with
great care. The hydrological properties of the borehole fluid measured with
this tool represent more of a mixture between fresh sea water (used for coring
and for the wiper trips) and true formation pore water.
A wide
and/or irregular borehole affects most recordings, particularly those that
require eccentralization and a good contact with the
borehole wall. Hole diameter was measured by the
caliper tool (2PCA) and can also be calculated from the acoustic imaging tool
(ABI40).
A null
value of -999.25 may replace invalid log values.
Additional
information about the drilling and logging operations can be found in the
Operations section of the Site Chapter in IODP Proceedings of Expedition 310.
For further questions about the data, please contact:
Jennifer Inwood
University
of Leicester
Phone:
011-44-116-252-3327
Fax: 011-44--116-252-3918
E-mail: iodp@le.ac.uk
For any web
site-related problem please contact:
E-mail: logdb@ldeo.columbia.edu