Downhole Logging Summary
Leg 197 Shipboard Scientific Party
The bend in the Hawaiian-Emperor chain is the most cited example of a change in plate motion recorded in a fixed-hotspot frame of reference. Alternatively, the bend might primarily record a change in motion of the Hawaiian hotspot relative to the Pacific plate. The main objective of Leg 197 was to test the hypothesis of the fixity of the Hawaiian hotspot respect to the Earth's spin axis. Another important objective of Leg 197 was to trace the geochemical evolution of the Hawaiian plume through time. The main scientific results can be found in the Leg 197 Preliminary Report.
Figure 1: Bathymetric map showing the locations of the Leg 197 dill sites on the Emperor seamount chain and previous DSDP/ODP holes.
Leg 197 sought to test the hypothesis of a fixed Hawaiian hotspot by recovering cores of volcanic basement rock from several of the volcanic edifices of the Emperor Seamounts that would be suitable for paleomagnetic, paleolatitude, and radiometric age determinations. For this purpose, 4 sites were drilled in several seamounts on the Emperor chain (Figure 1), (Sites 1203 and 1204 at Detroit, 81 Ma, Site 1205 at Nintoku, 56 Ma and 1206 at Koko, 48 Ma). Unfortunately, time constraints prevented us from logging at Site 1204 and poor borehole conditions at Sites 1205 and 1206 did not allow downhole measurements to be recorded. Downhole measurements were performed in Hole 1203A, after it had been drilled to a total depth of 915 mbsf.
At Site 1203, three logging runs were successfully deployed:
- First run: Triple-Combo. This first tool string included the Accelerator Porosity Sonde (APS), the Dual Laterolog (DLL), the Hostile Environment Natural Gamma Sonde (HNGS), the High Temperature Lithodensity Tool (HLDT), and the Temperature/Acceleration/Pressure Tool (TAP).
- Second run: the FMS/Sonic tool string was deployed including the FMS (Formation Micro-Scanner), the GPIT (General Purpose Inclinometry Tool), the DSI (Dipole Sonic Imager) and the NGT (Natural Gamma Ray Tool). The first pass recorded FMS and Sonic data only in the basement section, from 915 to 420 mbsf. The second pass was done in the basement and the entire sedimentary section.
- Third run: Göttingen Borehole Magnetometer (GBM), which recorded data while traversing up and down the hole.
Logging data recorded in Hole 1203A range from poor to high quality. In the upper 450 mbsf, the calipers of the HLDS and FMS were not able to make contact with the walls of an enlarged borehole (Figure 2). As a consequence, FMS data in the sequence of pelagic and hemipelagic deposits covering the summit of the seamount are of low quality.
In the basement section, where the major objectives of Leg 197 were addressed, logging data are of good to excellent quality for most of the measured parameters. The HLDS and FMS calipers show that the borehole is enlarged in a few intervals that correspond to sedimentary or volcaniclastic interbeds (Figure 2). In these intervals, FMS data quality is highly variable, ranging from poor to good.
High contrast in petrophysical properties between basalt and sediment/volcaniclastic units favor their identification (Figure 2 and 3). Strong changes or shifts within the log data are observed for all measured geophysical parameters.
Figure 2: Log data summary for Hole 1203A (natural gamma, electrical resistivity, density, porosity, and acoustic measurements).
Variations in the downhole measurements reflect interbedded volcaniclastic sediment and basalt. In general, the basaltic rocks are characterized by high electrical resistivity (up to 10 Ohm.m), low porosity (< 0.5%), high density (up to 2.5 g/cm3), low natural gamma ray (< 20 gAPI) and high P and S-wave velocities (respectively as much as 4 km/s and 2 km/s). In contrast, sediment and volcaniclastic interbed are characterized by low resistivity (about 1 Ohm.m), low density (< 2 g/cm3), high porosity (up to 40%) and low P and S-wave velocities (respectively about 2 km/s and 1 km/s). Within the basement, 31 lithologic units identified in the recovered cores can also be distinguished by the downhole measurements. The FMS images allow good core-log integration, which enables us to revise flow thickness calculated from the recorded or curated drilling depth (Figure 3). From FMS images and log data, four main lithologies can be distinguished.
1. Upper Sedimentary Section (from the base of the pipe in the hole to 467 mbsf)
The sedimentary section presents fairly constant and low gamma values (< 15 gAPI). In the upper 450 m of the section, these low values are linked to the low abundance of clay minerals included in the nannofossil ooze. The gradual increase in density and decrease in porosity recorded in the sedimentary section reflects consolidation and compaction effects. From standard downhole measurements and FMS images, we can distinguish the boundary between sediment and basement at 467 mbsf.
2. Volcaniclastic sediment in basement rock sequence
Nine intervals are identified as volcaniclastic units (Figures 2 and 3).
Figure 3: Comparison of FMS image, wireline measurements (electrical resistivity, natural gamma ray, porosity, density) with the core derived lithology and logging lithology in basement.
Volcaniclastic units cause large differences within the statically normalized FMS images. Usually, pad contact is better than in the upper sediment section and consequently, data quality is higher. This allows us to identify the sedimentary fabric of the volcaniclastic layers. Layered bedding is evident in some intervals (Figure 4). In other intervals, a patchy texture is observed. These differences are clearly related to the wide range of particle size and sorting (lapilli tuff, lapillistone, basaltic tuff, volcaniclastic siltstones, sandstones)
Figure 4: Detailed FMS image displaying transition between basement Unit 7 (layered volcaniclastic beds) and Unit 8 (pillow lava).
3. Pillow basalts
Pillow lobes can be recognized on the FMS images. Pillow basalts appear as circular to elliptical images of varying sizes (diameter = 10-150 cm). Their rims are regions of enhanced conductivity compared with that of the central part of the pillows (Figures 4 and 5). Pillows commonly exhibit fractures and vesicle concentration along cooling rims as well as in the center part of the pillows.
Figure 5: Detailed FMS image displaying typical pillow lobes in basement (part of Unit 23 from 798 to 800 mbsf).
4. Massive basalts
Massive basalt mainly occur as single lava flows or as massive parts of pahoehoe flows, usually occurring near the base. The typical internal structure of a massive flow inferred from the FMS images, is recorded as (Figure 6): a highly porous zone at the top of the flow caused by horizontal fracturing as well as vesicle layers, and a massive part in the flow center. This massive part is accompanied by large vertical fractures several centimeters in width and lengths up to 1.5 m.
Figure 6: Detailed FMS image displaying typical massive intervals within thick pahoehoe flows units (Unit 11).
Magnetic results (GBM and GPIT).
Between the end of pipe and 460 mbsf only minor variations of the magnetic field with depth are observed. This is due to weakly magnetized pelagic and hemipelagic deposits of the sedimentary section. In the basement, strongly magnetized layers were detected that correlate with basaltic rocks. These stronger magnetized units are interrupted by intervals of volcaniclastic sediment. The GPIT and GBM record similar signals (Figure 7). When the present geomagnetic field of the Earth as recorded at the site is subtracted from these magnetometer data, the resulting values suggest that the Site 1203 basement rocks record normal polarity. In addition to the fluxgate sensors the angular rate about the vertical spin axis of the logging tool was measured using a fiber optic gyro. The rotation history of the tool is determined by the accumulation of the angular rate during a log run. It is the first time such a sensor has been used to monitor instrument rotation in a borehole. Between the rig floor and bottom hole the tool rotated almost 60 times about its vertical body axis.
Figure 7: Comparison of the magnetic field components measured with GBM and GPIT.
Florence Einaudi: Logging Staff Scientist, Laboratoire de Mesures en Forage, Université de Montpellier II, ISTEEM, cc 56, 34000 Montpellier, France
Arno Buysch: Logging Trainee, Angewandte Geophysik, RWTH Aachen, Lochnerstrasse 4-20, Aachen 52056, Germany
Johannes Stoll: JOIDES Logging Scientist, Institut für Geophysik, Universität Göttingen, Herzberger Landstr. 180, Göttingen 37075, Germany