Standard Wireline Data Processing
DSDP
operator and logging contractor:
Scripps Institution of Oceanography
Hole: 53
Leg: 6
Location: Iwo Jima Ridge (tropical NW Pacific)
Latitude: 18° 2.00' N
Longitude: 141° 11.50' E
Logging
date: July 1969
Sea
floor depth (from DSDP
IR): 4640 mbrf
Sea
floor depth (step in GR
log): 4630 mbrf.
Total
penetration: 200.6 mbsf
Total
core recovered: 12.1 m
(34.6 % of cored section)
Oldest
sediment cored: Miocene
Lithologies: ashes of andesitic and basaltic
composition (sediments), basaltic
flows and intercalated limestones.
The
logging data were recorded by Schlumberger and Dresser Atlas in analog form
only. The data was digitized by Centerline Data in 2004 and processed at the
Borehole Research Group at the Lamont-Doherty Earth Observatory in September
2004.
Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
1. AL/CL (Dresser-Atlas) | main |
94 |
171 |
87 |
AL of very bad quality; not digitized. |
repeat |
94 |
176 |
87 |
AL of very bad quality; not digitized | |
2. EL/SP (Sclumberger) | main | 88 | 181 | 87 | |
repeat | 88 | 181 | 87 | ||
3. GNT (Schlumberger) | main | 0 | 188 | 87 |
Very
little information was found about
logging operations in the Initial Reports of the DSDP, Leg 6. Hole 53 was
drilled and then cased down to a depth of 87 mbsf. Three logging runs were
completed, but no information could be found on the actual logging sequence. The acoustic log (AL) was very erratic,
due to low velocity formations and signal attenuation; for this reason it was
not digitized.
The
depths in the table are for the processed logs (after depth matching and 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.
Depth
match and depth shift to sea floor:
Depth matching is typically done in the following way. One log is chosen as
reference (base) log (usually the total gamma ray log from the run with the
greatest vertical extent and no sudden changes in cable speed), and then the
features in the equivalent logs from the other runs are matched to it in turn.
This matching is performed manually. The depth adjustments that were required
to bring the match log in line with the base log are then applied to all the
other logs from the same tool string.
In
the case of Hole 53 no depth match could be performed between passes, as there
are no common curves. The log header from the acoustic log, however, indicated
that the caliper had been recorded 4 meters too deep. The calipers from the main and repeat from the acoustic log
show a good match.
All
the logs were then shifted to the sea floor by subtracting 4640 m. This amount
corresponds to the value given in the DSDP Initial reports and on the log
headers; a close look at the GR log, however, shows a clear step in gamma ray
values at 4630 mbrf, 10 m higher than the drillers' water depth. Because we
assume that the cores sub-bottom depths were obtained using the value of 4640
m, for consistency sake we shifted the logs by the same amount (also see figure
on page 1325).
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 sonic velocity
log).
Gamma
ray logs recorded through bottom hole assembly (BHA) and drill pipe should be
used only qualitatively, because of the attenuation on the incoming signal. The
thick-walled BHA attenuates the signal more than the thinner-walled drill pipe.
(The GNT porosity can sometimes be used qualitatively through the BHA and pipe,
but most of the other logs will not give usable data.)
Hole
diameter was recorded by the by the 3-arm CL tool.
A
null value of -999.25 may replace invalid log values.
Additional
information about the drilling and logging operation can be found in the
Operations section of the Site Chapter in DSDP Initial Reports volume 6. For
further questions about the logs, please contact:
Trevor
Williams
Phone:
845-365-8626
Fax:
845-365-3182
E-mail:
trevor@ldeo.columbia.edu
Cristina
Broglia
Phone:
845-365-8343
Fax:
845-365-3182
E-mail:
chris@ldeo.columbia.edu