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Logging Summary

IODP Expedition 330:

Louisville Seamount Trail

Expedition 330 Scientific Party


    Figure 1. Bathymetric map showing Expedition 330 drill sites. The green circles denote the locations of the two logged sites (U1374 and U1376).

    The Louisville Seamount Trail is a 4300 km long volcanic chain that is inferred to have been built in the past 80 million years as the Pacific plate moved over a persistent melt anomaly or hotspot. It is the South Pacific counterpart of the much better studied Hawaiian-Emperor seamount chain. Drilling during ODP Leg 197 in the Emperor Seamounts documented a substantial ~15° southward motion of the Hawaiian hotspot prior to 47 Ma, calling into question whether the Pacific hotspots constitute a fixed frame of reference.

    The Expedition 330 drilling program was designed to examine: 1) the possible motion of the Louisville hotspot and its geodynamical implications; and 2) the eruptive cycle and geochemical evolution of this seamount trail. This expedition aimed to replicate the ODP Leg 197 drilling experiment by matching the ages of the proposed drill sites to the ages of Detroit, Suiko, Nintoku and Koko seamounts in the Emperor Chain. This would enable direct comparison of paleolatitude estimates between the two longest-lived hotspot systems in the Pacific. Both objectives will provide important data to investigate the possible link between the Louisville mantle plume and the formation of the Ontong Java Plateau. Finally, the thin cover of sediments on these seamounts may provide additional information on the subsidence history of the individual Louisville seamounts.

    The primary logging objectives of Expedition 330 were to collect high-resolution downhole physical property data and integrate them with core measurements. Wireline logging was planned for all sites (with one hole drilled per site). The purpose of this was to obtain a continuous in situ record of the formations encountered through the seamount.

    Five Louisville guyots were drilled and cored - volcanic basement was reached at four of these drilling targets. Downhole logging measurements were taken at two of the sites drilled, U1374 and U1376 respectively (Figure 1).

Logging Tools

    The logging program on Expedition 330 was designed to obtain continuous in situ physical property data needed to assist in lithologic identification and the recognition of structural characteristics of cored volcanic basement formations. Combinations of four wireline logging tool strings were deployed during the expedition: (1) the triple combination (triple combo) tool string, (2) the Formation Micro-Scanner (FMS)-sonic tool string, (3) the Ultrasonic Borehole Imager (UBI), and (4) the third-party Göttingen Borehole Magnetometer (GBM). The third-party GBM allows fully oriented component magnetic anomalies to be determined by measuring three orthogonal components of the magnetic field. The GBM includes three optical gyroscopes, which record the tool's overall rotation from the start of the measurement. This enables the cumulative rotation of the tool to be undone by retroactively unwinding the recorded rotation of the GBM using a Matlab software program (S. Ehmann, Technische Universität Braunschweig, Germany). Together, this allows for independent determination of the intensity, inclination, and declination of the magnetization in the borehole lava flow formations. For optimum data quality the GBM was deployed with a centralizer and a non-magnetic sinker bar directly above the GBM.

Logging Operations

    Figure 2. Summary of downhole and corresponding core physical properties measurements and log unit divisions, Hole U1374A.
    Figure 3. Comparison of the raw magnetic field data (not corrected for tool inclination) of GBM run 1 with the lithology.
    Figure 4. Composite showing several features imaged using the FMS, Hole U1374A.
    Figure 5. Summary of downhole and corresponding core physical properties measurements and log unit divisions, Hole U1376A.
    Figure 6. Composite of features imaged by FMS, Hole U1376A
    Figure 7. Comparison of the raw magnetic field data (not corrected for tool inclination) of the GBM run with the lithology

    Site U1374: Rigil Guyot

    Hole U1374A was successfully logged with four different tool strings: the triple combo, the FMS-sonic with Hostile Environment Natural Gamma Sonde (HNGS), UBI and GBM. The triple combo and FMS tool strings made two full passes, and the UBI was run in sections where the borehole diameter was optimal. The GBM made two full passes of the borehole. Measurement commenced on the rig floor and continued during deployment of the tool to TD and retrieval back up the hole to the rig floor. This ensured that the tool could be accurately oriented pre- and post-measurement.

Downhole logging data obtained from U1374A included natural, total and spectral gamma ray radiation, density, neutron porosity and electrical resistivity, electrical images, p-wave velocity (Figure 2), three-component magnetic field (Figure 3) and acoustic images (Figure 4). Following some initial issues with rock bridges in the borehole, and an extensive borehole preparation, an open hole section of 393.9 m was logged. The downhole log measurements (resisitivity, density, velocity and neutron porosity) were used to identify a total of nine log units in Hole U1374A with two in the section covered by the Bottom Hole Assembly (BHA)and seven in the volcanic sequences in the open hole interval. The third party GBM collected good quality magnetic data, which will be reoriented post-expedition. The raw data correlates well to the lithological changes observed in the core (Figure 3). Lithological and structural features are well imaged with the Formation MicroScanner (FMS) (Figure 4).

Site U1376: Burton Guyot

At Hole U1376A three logging tools stings were deployed: the triple combo (Figure 5) and FMS-sonic tool strings (Figure 6) made two full passes, and the GBM performed one full pass (starting and ending at the rig floor, as described above) (Figure 7). Measurement depths were adjusted to match across different logging runs, obtaining a wireline matched below seafloor (WMSF) depth scale.

The logged depth interval for Hole U1376A was 80.4 - 182.3 m WMSF. Resistivity, density, compressional velocity and neutron porosity derived from downhole logging measurements (triple combo) were used to identify a total of thirteen Log Units in Hole U1376A. Three of these Log Units were in the section covered by the BHA and ten in the volcanic sequences in the open hole interval. These defined Log Units correlated to changes from massive basalt flows to more brecciated units, and interlayered aphyric and olivine-phyric flow units.

The GBM was run once in Hole U1376A and collected good quality magnetic data, which will be reoriented post-expedition. The GBM data shows that a massive lava flow is not as homogeneous as it appears in the palaeomagnetic data obtained from the recovered cores at Hole U1376A (Figure 7). Additionally, in the unrecovered section of the hole, between ~130 and 140 mbsf, the GBM data shows strong variations. Post-expedition work, which will split the horizontal component into north and east components, should provide further insight into these observed variations.

Lithological and structural features are well imaged with the FMS (Figure 6), in particular fractures, clast size, shape and distribution and areas of solid, massive basalt versus brecciated material. Of particular importance is FMS coverage over the unrecovered section between ~130 and 140 mbsf because it will provide valuable information in reconstructing the lithology over this interval of the hole.

    Louise Anderson: Logging Staff Scientist, Borehole Research Group University of Leicester, University Road, Leicester, LE1 7RH, UK