Previous   |   Next

doi:10.2204/iodp.proc.324.103.2010

Downhole logging

Downhole logging data obtained from Hole U13346A included natural and spectral gamma ray, density, photoelectric factor (PEF), and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 14 logging units in Hole U1346A with 3 in the section covered by the BHA, 4 in the sedimentary sequences in the open hole interval, and 7 in the basaltic basement.

Operations

A wiper trip was completed throughout the open hole before the start of the wireline logging operations. The drill pipe was set at a depth of 118.9 m wireline matched depth below seafloor (WMSF), which is ~16.4 m above the sediment/basement interface. The hole was circulated with 38.5 bbl of attapulgite mud. Downhole logging operations lasted 11.7 h beginning at 1955 h on 18 September 2009. The wireline logging operations consisted of one tool string deployment and testing of the wireline heave compensator (WHC). Logging operations in Hole U1346A took place in rough sea conditions where the ship's heave was generally 4–5 m, with occasional excursions up to 6 m.

Tool string deployment

The wireline tool string deployment consisted of a 23.2 m long modified triple combo that included a logging equipment cable head (LEH-QT), digital telemetry cartridge (DTC-H), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Natural Gamma Ray Cartridge (HNGC), Litho-Density Sonde Cartridge (LDSC), Hostile Environment Litho-Density Sonde (HLDS), Digital Telemetry Adapter (DTA-A), General Purpose Inclinometry Tool (GPIT), and the Digital Dual Induction Tool model E (DITE). Downhole logs were recorded in three passes: (1) a downlog from seafloor to 169.1 m WMSF, (2) an uplog (Pass 1) from 177.6 to 150.3 m WMSF, and (3) a second uplog (Pass 2) from 177.6 m WMSF to seafloor.

After the downlog was stopped and prior to starting Pass 1, 1.5 h was spent assessing downhole tool motion and optimizing the efficiency of the WHC for the water depth at Hole U1346A and heave conditions at the time of the logging operations. Once the best possible WHC parameters were chosen for the prevailing heave conditions, the tool string was lowered to 177.6 m WMSF to begin the first uplog. Pass 1 was completed with the HLDS caliper arm closed as the downhole tool motion was being assessed while moving the tool string at a constant speed and to prevent potential tool damage. After downhole tool motion conditions were considered acceptable, Pass 2 was completed with the HLDS caliper arms opened.

Data processing

Logging data were recorded onboard the JOIDES Resolution by Schlumberger and archived in digital log information standard (DLIS) format. Data were sent by satellite transfer to Lamont-Doherty Earth Observatory-Borehole Research Group, processed there, and transferred back to the ship for archival in the shipboard database. Processing and data quality notes are given below.

Depth shifting

Depth shifts that were applied to logging data were performed by selecting a 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 features in equivalent logs from other passes are aligned by eye. In the case of Hole U1346A, the base log was the gamma ray profile from Pass 2 of the HNGS-HLDS-GPIT-DITE tool string. The original logs were first shifted to the seafloor (–3630.9 m WMSF). The seafloor depth was determined by the step in gamma ray value. This differs by 0.9 m from the seafloor depth given by the drillers (Table T12). The depth adjustments that were required to bring the match log in line with the base log were applied to all the other data sets from the same tool string.

Proper depth shifting of wireline logging depths relative to pipe depths was essential to achieve scientific goals. The targets that offered potential wireline logging depth references were the seafloor and the sediment/basement interface. However, data acquired across the seafloor resulted from logging through the BHA, so data from this interval are of poor quality.

Data quality

The quality of wireline logging data were assessed by evaluating whether logged values are reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Gamma ray logs recorded through the BHA should be used only qualitatively because of the attenuation of the incoming signal. The thick-walled BHA attenuates the signal more than the thinner walled drill pipe.

A wide (>30.5 cm) and/or irregular borehole affects most recordings, particularly those like the HLDS that require eccentralization and good contact with the borehole wall. The density log roughly correlates with the resistivity logs, but it is largely affected by the hole conditions. The hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL), and it shows a very irregular borehole with intervals exceeding the maximum caliper aperture. Good repeatability was observed between Pass 1 and Pass 2, particularly for measurements of electrical resistivity, gamma ray, density, and PEF.

Bulk density (HLDS) data were recorded at a sampling rate of 2.54 cm in addition to the standard sampling rate of 15.24 cm. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard the JOIDES Resolution. In normal processing, short-spacing data are smoothed to match long-spacing data; in the 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.

Preliminary results

Electrical resistivity measurements

A total of three electrical resistivity curves were obtained with the DITE, the deep induction phasor (IDPH), medium induction phasor (IMPH), and the spherically focused log (SFL). The SFL, IMPH, and IDPH resistivity profiles represent different depths of investigation into the formation (64, 76, and 152 cm) and different vertical resolutions (76, 152, and 213 cm). Downhole open hole electrical resistivity measurements covered 16.4 m of the bottommost sedimentary sequences and the uppermost 42.2 m of the basement lithostratigraphic units. The DITE was the only tool that reached the bottom of the logged interval in Hole U1346A because it was the bottommost tool in the logging tool string (Fig. F19 in the "Methods" chapter).

In the bottommost sedimentary sequences the IMPH values range from 0.9 to 4.1 Ωm, the IDPH values range from 0.6 to 18.6 Ωm, and the SFLU values range from 1.1 to 3.6 Ωm. In the uppermost basement lithostratigraphic units the IMPH measurements range from 2.1 to 101.5 Ωm, the IDPH values range from 2.6 to 93.5 Ωm, and the SFLU values range from 3 to 164 Ωm (Fig. F53).

All electrical resistivity curves show a gradual increase in the sedimentary sequences from the bottom of the BHA at 118.9 m WMSF to the sediment/basement interface at 135.3 m WMSF (Fig. F53).

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1346A with the HNGS. The total gamma ray measurements through the BHA show several anomalies:

1. From seafloor to ~11.7 m WMSF,

2. From 49.4 to 52.7 m WMSF, and

3. From 109.2 m to just past the BHA at ~121.6 m WMSF.

The two deepest anomalies could represent oceanic anoxic Events (OAEs) 1a and 1b previously identified at Sites 1207 and 1213 (Robinson et al., 2004).

Downhole open hole gamma ray measurements covered 16.4 m of the bottommost sedimentary sequences and the uppermost 22.6 m of the basement lithostratigraphic units. The short overall coverage (39.2 m) was mainly due to the placement of the BHA, the topmost position of the HNGS within the tool string (Fig. F53), and the total logging depth achieved in Hole U1346A.

Total gamma ray measurements in the bottommost sediments of Hole U1346A are highly variable, from 13.5 to 105.1 gAPI with a mean of 39.3 gAPI. Potassium values are also relatively high with values between 0.09 and 2.81 wt%, with a mean of 0.78 wt% (Fig. F54). Uranium values are mostly between 0.81 and 8.73 ppm, with a mean of 2.84 ppm. In contrast, thorium values are relatively low, from 0.05 to 1.55 ppm, with a mean of 0.51 ppm. The relatively high uranium peaks found in the sediment basement contact could be indicative of vigorous and focused hydrothermal fluid flow.

Gamma ray measurements in basaltic oceanic crust are typically low (e.g., Bartetzko et al., 2001; Barr et al., 2002); however, the basaltic basement drilled in Hole U1346A exhibits relatively high gamma ray values. Total gamma ray values (HSGR) obtained with the HNGS in the basement range from 14.7 to 62.5 gAPI, with a mean of 40.5 gAPI. Potassium values are also relatively high with basement values between 0.67 and 2.83 wt%. High potassium values caused by alteration were also confirmed by geochemical analyses (see "Geochemistry"). Shipboard ICP-AES analyses show a similar range in K₂O values as those obtained with the downhole logs (Fig. F54). Differences between the laboratory and downhole potassium analyses are most likely due to variations in depth scales (WSMF versus core depth below seafloor, method A) and variations in scales of measurements between the two methods. Uranium values are mostly between 0.09 and 3.28 ppm, with a mean of 1.16 ppm. Thorium ranges from 0.02 to 1.17 ppm, with a mean of 0.48 ppm. Elevated potassium values in the basaltic basement illustrate the high degree of alteration that was recorded in the cores recovered from Hole U1346A.

Density and photoelectric factor

Density values range from 1.09 to 1.96 g/cm³ over the lowermost sediment section of the open hole (Fig. F55). In the uppermost basement section, density values are between 1.07 and 2.77 g/cm³. A comparison between discrete physical property samples and downhole density log shows good agreement (Fig. F55). PEF values in the bottommost sediments range from 1.21 to 4.10 b/e^–, whereas in the basement values range between 1.31 and 5.14 b/e^–. Low density and PEF values correspond to intervals with enlarged borehole dimensions (Fig. F55).

Magnetic field measurements

Measurements of total magnetic moment, magnetic inclination, and magnetic intensity were obtained with the GPIT (Fig. F56). The mean magnetic inclination and total magnetic moment from 130 to 179.9 m WMSF are 77° and 1.2 Oe, respectively. The magnetic intensity is 1.2 Oe on the z-axis and varies between –0.4 and 0.4 Oe on the x- and y-axes.

Lithostratigraphic correlations

Preliminary interpretation of the downhole log data divides Hole U1346A into several logging units within three main sections: the section covered by the BHA, the sedimentary sequences in open hole, and the basaltic basement (Figs. F53, F54, F55, F57). Logging units in the section covered or partially influenced by the BHA were interpreted on the basis of the gamma ray downhole logs, and only intervals that showed significant anomalies were characterized as logging units. Logging units within the open hole section that contained sedimentary sequences were also interpreted on the basis of the gamma ray fluctuations, whereas the basaltic basement was characterized using the resistivity logs.

Three logging units were qualitatively identified in the section covered by the BHA (Fig. F57):

1. Logging Unit Ip (seafloor to ~13.4 m WMSF) shows a significant increase in total gamma ray measurements.

2. Logging Unit IIp (48–52.9 m WMSF) shows a sharp increase in gamma ray values, with the spectral gamma data showing that the peak has a significant contribution from uranium.

3. Logging Unit IIIp (109.3 m WMSF to the bottom of the BHA) shows a significant uranium peak of ~1 ppm.

The sedimentary sequence in open hole below the BHA was divided into four logging units based on gamma ray downhole logs (Fig. F53):

1. Logging Unit Is (118.9–126 m WMSF) has low gamma ray values of ~20 gAPI.

2. Logging Unit IIs (126–131.7 m WMSF) shows a large peak in gamma ray values to as much as ~105 gAPI with a large contribution from uranium of as much as 9 ppm. This logging unit may correspond to the limestones and silty limestones recovered in Core 324-U1346A-5R.

3. Logging Unit IIIs (131.7–134.1 m WMSF) is characterized by low gamma ray values averaging 34 gAPI.

4. Logging Unit IVs (134.1–135.3 m WMSF) shows high gamma ray values directly above the sediment/basement interface. This may correlate with a banded, partially deformed green limestone at the base of the sediment sequence (see "Sedimentology").

The basement sequence below 135.3 m WMSF is divided into seven logging units based on the downhole resistivity logs (Fig. F53):

1. Logging Unit Ib (135.3–144.8 m WMSF) shows a steady increase in resistivity from the sediment/basement interface to 137.5 m WMSF and then levels out with values of ~18 Ωm. Caliper data show the borehole is larger than the bit size in the unit with a diameter of 56 cm. Density averages 2.5 g/cm³.

2. Logging Unit IIb (144.8–148 m WMSF) shows lower resistivity values of ~7.7 Ωm.

3. Logging Unit IIIb (148–151.8 m WMSF) has resistivity values of ~30 Ωm with a peak of ~100 Ωm at 151.5 m WMSF.

4. Logging Unit IVb (151.8–157.1 m WMSF) is characterized by low resistivity values between 2.2 and 13 Ωm.

5. Logging Unit Vb (157.1–165.3 m WMSF) has resistivity values between 16 and 110 Ωm.

6. Logging Unit VIb (165.3 and 168 m WMSF) has resistivity values averaging ~15 Ωm.

7. Logging Unit VIIb (168–177.6 m WMSF) has resistivity values between 10 and 100 Ωm.

A comparison of the downhole potassium logs from Hole U1346A with other shallow oceanic basement environments such as the OJP (Hole 807C), East Pacific Rise (Hole 1256D), and the eastern flank of the Juan de Fuca Ridge (Hole U1301B) shows higher potassium contents at the Shirshov Massif (Fig. F58). The sediment/basement interface also shows elevated uranium concentrations in Hole U1346A in comparison to the other localities.

A qualitative match of the gamma ray and electrical resistivity logs to seismic Line TN037 5A was made by tying seafloor and sediment-basement gamma ray anomalies to prominent reflectors in the regional seismic interpretation (Fig. F59). This correlation also shows potential ties of logging Unit IIp in the sedimentary sequence that could represent an OAE previously interpreted in this area (Robinson et al., 2001) and logging Unit IIIb to a discontinuous basement reflector that may represent the top of a lava flow.

Top of page   |   Previous   |   Next