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Leg 191 Logging Summary

Shipboard Scientific Party

Leg 191 Objectives

Ocean Drilling Program Leg 191 had two main goals:

(1) to drill and case a borehole at a site in the northwest Pacific Ocean, between Japan and the Shatsky Rise and install therein a seismic observatory. The seismic observatory was successfully installed at Site 1179 and left ready for activation by a future ROV cruise. During Late 2000, the system was successfully started and data were obtained from the borehole seismometer.

(2) to test the drilling and casing emplacement capabilities of the Hard Rock Re-entry System (HRRS or hammer drill) on a basaltic outcrop at Shatsky Rise.

Figure 1: Location map of seismic station coverage in the northwest Pacific. Black circles indicate land seismic stations, whereas red circles are current and proposed seafloor borehole observatories. Leg 186 (Site 1150 and 1151), Leg 191 (Site 1179) and future Leg 195 (planned Site WP-1) locations are reported on this map.

Site 1179:

Site 1179 is located in the northwest Pacific Ocean, East of Japan (Figure 1), in lithosphere of anomaly M8 age (129 m.y.). Five holes were drilled at Site 1179 (Figure 2). Cores were recovered in Holes A-B-C and D at various depths. A sedimentary column of 377 m thick was cored (Holes A, B, and C) in addition to 98 m of basaltic basement (Hole D). Hole E was devoted to the seismometer installation. The sedimentary column has been divided into four lithologic units (Scientific Party).

Unit I consists of 223.5 m of clay and radiolarian-bearing diatom ooze of late Miocene to recent age. Ash beds are common in this unit, recording volcanic activity from the western Pacific island arcs.

Unit II is a clay-rich and diatom-bearing radiolarian ooze of late Miocene age with a thickness of 22.5 m.

Unit III contains barren, brown pelagic clay in a layer 37.5 m in thickness.

Unit IV yielded poor recovery with only chert and porcellanite fragments from an unknown sedimentary matrix within 93.7 m above basement.

Basement: The 98 m igneous section consists of Early Cretaceous aphyric ocean ridge basalts. The section consists of massive flows and pillows with small amounts of inter-unit sediments and volcanic breccias.

Figure 2: Lithologic summary column for Site 1179.

Wireline logging

Objectives

Geophysical measurements were important for the long-term seismic observatory installation. The logging program was initially designed to measure the physical properties, hole shape, porosity, in situ stress and fracture characteristics of the sediments and basement. To achieve these objectives the initial logging plan included Triple Combo, FMS-Sonic and UBI.

Logging Operations

The Triple Combo tool string was first deployed for physical measurements. It includes the Accelerator Porosity Sonde (APS), the Hostile Environment Natural Gamma Sonde (HNGS), the High Temperature Lithodensity Tool (HLDT), the Phasor Dual Induction Spherically Focused Resistivity Tool (DIT-E) and the Temperature / Acceleration / Pressure Tool (TAP). The new LDEO high resolution Multisensor Gamma-ray Tool (MGT) was attached to the top of the toolstring.

Downhole conditions prevented the complete drilled interval from being logged. The first triple combo pass was logged from 300 to 203 mbsf with the standard triple combo probes. After this pass, the tool string was again lowered but a bridge was encountered at about 260 mbsf. After several unsuccessful attempts to pass the tool string through the bridge, only the upper part of the borehole was logged and the remaining passes started from this point. The second and third passes were used for recording MGT data only and the fourth pass to run the standard Triple Combo.

Data quality

Poor borehole conditions ranging from washed out to constricted along the whole logged interval resulted in poor quality logs, especially for tools that require eccentralization and good contact between the tool and the borehole wall (APS and HLDT). Caliper data are very rough (Figure 3), ranging from 7 up to 16.5 inches. From 246 mbsf to the pipe depth, the caliper remained fully opened (16.5 in. which is the full range of the tool). Furthermore, the HLDT developed a functioning problem with the far spaced detector resulting in erroneous spikes in the density and photoelectric measurements. Despite the degraded borehole conditions, gamma ray (both HNGS and MGT) and resistivity (DITE) produced good quality data and recorded the lithologic changes well.

 

Figure 3: Natural radioactivity measurements (HNGS and MGT) in Hole 1179D are shown with caliper data and lithologic units determined from core description from Hole 1179C. The MGT natural radioactivity measurements are in CPS (counts per second) and the HNGS measurements are in API units.  

Results

Natural Radioactivity

Natural radioactivity was measured downhole by both the HNGS and the newly developed MGT. Both tools have scintillation detectors that measure the natural gamma radiation emitted by the formation due to radioactive decay. In general the total gamma-ray record shows an overall increase in natural radioactivity with depth (Figure 3). There is, however, a significant decrease in the log data at 246 mbsf, this correlates with similar tool responses at this depth for resistivity, and is probably linked to the tool response due to the poor hole conditions at this depth (caliper jumping from 7 to 16.5 in.). A number of distinctive peaks in the gamma ray measurements occur in the uppermost part of the logged section at depths of 182.9, 188.0, 195.7, and 242.2 mbsf and correspond to occurrences of ash layers described in the sediment cores.

The data from the MGT are well correlated with total counts from the HNGS and core data (Multi Sensor Track). The MGT measurements present much higher vertical resolution and they allow a better identification of the layers (Figure 4).  

Figure 4: Natural radioactivity measurements in Hole 1179D, comparison between the HNGS and the newly developed MGT. On this graph are also reported the Hole 1179C core natural radioactivity measurements.

Electrical Measurements

The resistivity data clearly delineate the main lithologic types observed in the sediment column at Hole 1179C (Figure 5). The electrical resistivity in lithologic Unit I (ooze) is low, with small increases in resistivity at 180, 188, 200 and 212 mbsf. These increases could be associated with ash layers or burrows noted in the core description and gamma ray measurements. The boundary between Units I and II is marked by a slight increase in resistivity (only 0.01 Ohm.m of difference between Unit I and II). The DIT data clearly recorded the lithologic change between Unit II and III, marked by an increase in the electrical resistivity at 243 mbsf. The boundary between lithologic Units III and IV is marked by a strong increase in resistivity (from 0.7 to 3.5 Ohm.m). The chert layer consists of alternating high resistivity layers (nodules) and interbedded low resistivity layers (clay rich layers).

Figure 5: Electrical measurements in Hole 1179D are shown with caliper data and lithologic units determined from core description from Hole 1179C. The diagram shows the good correlations between natural radioactivity and resistivity measurements. Both measurements allow the ash layer identification, marked by increase in resistivity and natural gamma ray.

Logging Scientists:

Florence Einaudi- Logging Staff Scientist - Laboratoire de Mesures en Forage, ODP NEB, ISTEEM, cc49 Universite de Montpellier II 34 095 Montpellier Cedex France

Alex Meltser - Logging Engineer - Lamont-Doherty Earth Observatory, Columbia university, Borehoel Research Group, Palisades, NY 10964, USA

Sarah Haggas - JOIDES Logger - Department of Geology, University of Leicester, Leicester, LE1 7RH, UK



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