Standard Wireline Data Processing (Phase 3)

IODP -USIO logging contractor: LDEO-BRG

Hole: U1309D (Phase 3)

Expedition: 305

Location: Atlantis Massif - Mid Atlantic Ridge (central N Atlantic)

Latitude: 30° 10.120' N

Longitude: 42° 7.113’ W

Logging date: February 24-25, 2005

Sea floor depth (driller's): 1656 mbrf

Total penetration: 1415.5 mbsf

Total core recovered: 80.2 % of cored section

Oldest sediment cored: none

Lithologies: diabase, gabbro, oxide gabbro, dunitic troctolite

Data

The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.

Logging Runs-Phase 3

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Bit depth (mbsf)	Notes
1. DLL/APS/HLDS/GPITHNGS/TAP	Main	177	1410	191
	Repeat	1088	1278
2. FMS/GPIT/SGT	Pass 1	730	1413
	Pass 2	624	1411

Hole U1309D was drilled and logged during Expedition 304. During Expedition 305, Hole U1309D was deepened and logged in two phases. In Logging Phase 3, Hole U1309D was logged to total depth; the data are of excellent quality. The first two tool strings reached the bottom of the hole without problems. Temperatures at the bottom of the hole reached 119° C, according to the TAP tool on the first tool string. The FMS/GPIT/SGT tool string initially contained the DSI sonic tool, but difficulties were experienced downhole, and it was found to have a damaged joint between the transmitter and receiver section. The DSI was removed and the tool string was run back into the hole.

The third tool string, WST-3, experienced unstable voltage and loss of communication downhole, and was replaced with the WST tool. Difficulties were experienced running the WST into the hole, the caliper apparently opening while in the pipe. High heave (4 m) conditions were probably a factor. When the tool was being returned to the surface, the wireline was found to be twisted/knotted, and 580 m of wireline had to be recovered by T-bar and cut.

The depths in the table are for the processed logs (after depth matching between passes 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.

Processing

Depth match and depth shift to sea floor: The original logs were shifted to the sea floor (-1656 m), and then depth matched. Depth matching was a two-step process: first, FMS/GPIT/SGT pass 2 was matched to the reference run from the Phase 2 logging of Hole U1309D, and second, the other passes were matched to the depth matched logs from FMS/GPIT/SGT pass 2. Because the extremely low GR values (<10 API) made matching by GR alone difficult, several logs were used for matching, including resistivity, caliper, and FMS-average.

Because the gamma ray logs did not cross the sea floor, the seafloor depth given by the drillers was used (1656 m).

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.

High resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm, respectively. SGT gamma ray data were sampled every 5.08 cm. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low-density correction) the results are improved, because the short spacing has better vertical resolution.

Quality Control

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.

A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS/APS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the FMS tool (C1 and C2). The hole is generally in gauge (10 inches wide), but with some generally thin sections extending to about 18 inches wide (e.g. 416-421 mbsf). The lower down in the hole, the fewer the number of wide sections.

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 IODP Initial Reports Volume 305. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia