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Logging Summary

IODP Expedition 341:

Southern Alaska Margin

Expedition 341 Scientific Party


    Figure 1. Bathymetric map of the Gulf of Alaska, showing locations of IODP Expedition 341 sites.

    The Gulf of Alaska borders the St. Elias orogen of Alaska and Canada, the highest coastal mountain range on Earth. The St. Elias range is <30 Ma in age and much of the mountain building in the region took place during the Neogene, a period of significant global climate change distinguished by the transition into a colder, more variable climate dominated by the onset and intensification of major northern hemisphere glaciations. A global increase in erosion rates and sediment delivery to ocean basins occurred along with the transition into the icehouse world. This situation makes the Gulf of Alaska a unique location to study the complex interplay between tectonics (deformation and exhumation), sedimentation, and climate. Tectonic processes influence regional climate by raising mountains that affect orographic precipitation patterns. Climate changes affect tectonics through changes in erosion rates and distribution of mass and stress within orogenic wedges. The aim of IODP Expedition 341 was to examine the response of an orogenic system to the climatically driven establishment of a highly erosive glacial system.

    A cross-margin transect of sites was planned to investigate the pre-glacial, glacigenic, and post-glacial sediments accumulated in the Gulf of Alaska during the critical period of climatic transition, between the middle Miocene and late Pleistocene/Holocene. Five sites were drilled during Expedition 341, extending from the continental shelf across the slope and offshore into the deep sea Surveyor Fan (Figure 1).

    A complete overview of the expedition operations and preliminary scientific results can be found in the Expedition 341 Preliminary Report.

Logging Tools and Operations

    Figure 2. Logging tool strings used during Expedition 341.

    Downhole logging operations were carried out at four of the five drilled sites. Logged Sites U1420 and U1421 are located on the continental shelf and slope, respectively, while Sites U1417 and U1418 are further offshore in the Surveyor Fan. At Sites U1417 and U1418, average core recovery was relatively good (70-86%) so logging data provide a complementary data set. At Sites U1420 and U1421, however, core recovery was poor (14-24%) and thus logging data provide the primary means of sediment characterization. Slope Site U1419 was not logged due to its shallow depth of penetration (<200 m below seafloor).

    The logging program for Expedition 341 was designed to measure in situ geophysical and lithological properties of thick deposits of glacial sediments, as well as the underlying pre-glacial and overlying post-glacial sediments. Five different tool strings were deployed during Expedition 341 (Figure 2). Two standard IODP wireline tool strings were run at Sites U1417 and U1418: the triple combo (measuring gamma radiation, density, resistivity, deep reading magnetic susceptibility, and borehole diameter) and the FMS-Sonic tool string (measuring gamma radiation and sonic velocity and recording resistivity images). Due to concerns about hole stability based on poor core recovery and challenging coring conditions in the shelf and slope environments, a Sonic-Induction tool string was the primary logging string deployed at Sites U1420 and U1421. This string was designed to provide the highest priority measurements in a single logging run, including gamma radiation, sonic velocity, resistivity, and borehole diameter. The VSI tool string (measuring acoustic travel-time) was run at Sites U1417, U1418, and U1421 to collect vertical seismic profiles but adequate travel-times were recorded only at Sites U1417 and U1421. One technical highlight of the expedition was the first at-sea deployment of the full Magnetic Susceptibility Sonde (MSS) tool string (comprising both deep reading and high-resolution magnetic susceptibility sensors), carried out as an additional run at Site U1417.

    Hole preparation prior to logging varied based on formation conditions. In Holes U1417E and U1418F where drilling progressed relatively smoothly, seawater was used as the logging fluid. Due to more difficult drilling conditions in Holes U1420A and U1421A, these holes were stabilized with heavy logging mud prior to logging (mud weight: 10.5 ppg for U1420A, 11.8 ppg for U1421A). All logging operations took place in favorable sea conditions.

    Logging operations are summarized in the following table:


Logging Data and Results


    We present here a summary of logging data collected during this expedition and show some highlights for each site. The drill pipe was raised to between 81 and 99 mbsf prior to logging because of hole instability in shallow sediments, so logs are typically recorded only deeper than ~100 mbsf. Logging data are initially referenced to depth below the rig floor; after logging is completed, all data are shifted to depth below seafloor and depth-matched to remove offsets between different logging runs and passes. The resulting depth scale, used for all log data presented below, is wireline matched depth below seafloor (WMSF).


Site U1417

Figure 3. Summary of logging data recorded with the Triple Combo in Hole U1417E.

Site U1417 is situated in the distal deep sea Surveyor Fan at ~4190 m water depth, ~60 km from the Surveyor Channel that delivers sediment to the site via overbank deposits. The site was chosen to provide a long record of Neogene glacial and tectonic processes, including the onset of tidewater glaciation, the formation of the Surveyor Channel and Fan, early uplift of the St. Elias orogen, and preglacial to glacial conditions. Five holes were drilled, penetrating 709.5 m below seafloor, with ages extending from Miocene to Holocene. Core recovery ranged from 42% to 99%, with lowest recovery rates deeper than ~230 mbsf. Four principle lithologies were drilled: clastic wedge sedimentation, bioturbated mud, interbedded biogenic and glacigenic sediment, and glacigenic sediment.

Downhole logs were recorded in Hole U1417E with four different tool strings (Triple Combo, FMS-Sonic, VSI, and MSS). The caliper log showed an irregular borehole with diameter ranging from <5 inches to >18 inches, significantly larger than the bit size shallower than 305 m WMSF. Deeper than 305 m WMSF, there is reasonable agreement between the gamma ray and density logs and the core gamma ray attenuation (GRA) and moisture-and-density (MAD) data, suggesting that these data are good quality (see Figure 3). A series of bridges were encountered during logging; the Triple Combo reached an impassable obstruction at ~80 m above total depth and each successive tool string was blocked at a shallower depth.

Figure 4. Summary of logging data recorded with the FMS-Sonic tool string in Hole U1417E.

Based on hole condition and characteristic trends and features in the data, two distinct logging units were identified for Hole U1417E (Figure 3, Figure 4). Logging Unit 1 is defined primarily by highly variable borehole diameter. Within this unit, gamma ray, magnetic susceptibility, and compressional wave velocity (Vp) logs are the most robust to borehole conditions. Logging Unit 2 is characterized by improved borehole conditions and the quality of the logging data is higher throughout this unit. Unit 2 was subdivided into two sub-units. Sub-Unit 2A shows elevated gamma ray, density, and velocity values relative to the unit above. Magnetic susceptibility shows higher amplitude variability and co-varies with gamma ray. Sub-Unit 2B is distinguished by an initial, rapid decrease in density, resistivity, and velocity with depth, followed by general increases. Magnetic susceptibility displays less dynamic range in this sub-unit. Together, these data indicate that downhole logs are likely responding to variations in lithology.

The VSI tool string was run in Hole U1417E to conduct a vertical seismic profile (VSP) experiment. The VSI caliper arm had difficulty achieving good clamping force due to the rugose borehole wall and soft sediments at the shallow sub-seafloor depth reached by the tool string (max depth 218 m WMSF). Despite these limitations, adequate travel times were recorded with two shots at one depth station during the VSP.

Results from the full MSS tool string run in Hole U1417E are described below (see “Magnetic susceptibility measurements”).

Site U1418

Figure 5. Summary of logging data recorded with the Triple Combo in Hole U1418F.

Site U1418 is located on a slightly elevated portion of the proximal Surveyor Fan at ~3670 m water depth, between a modern channel that feeds the Aleutian Trench to the west and the relict Bering Channel to the east. The site was chosen to develop a high temporal resolution proximal record of glacial-interglacial dynamics, fan development, and paleoceanography. Six holes were drilled, reaching as deep as 948.7 mbsf, with ages extending back to early Pleistocene. Core recovery ranged from 72% to 100%. Three primary lithologies were drilled: a mass transport deposit, glacigenic sediment, and interbedded biogenic and glacigenic sediment.

Downhole logs were recorded with two tool strings (Triple Combo, FMS-Sonic); the VSI tool string was also run but no good shots were recorded. Borehole conditions were variable, with numerous washed-out intervals where hole diameter exceeded the 18-inch limit of the caliper arm. The tool strings deployed encountered blockages at different depths between the first logging run (Triple Combo) and the last logging run (FMS-Sonic), indicating that hole conditions were deteriorating over the course of logging operations.

Figure 6. Summary of logging data recorded with the FMS-Sonic tool string in Hole U1418F.

The logged interval was assigned to a single logging unit because the character of the logs changes gradually downhole with no major steps in the base levels (Figure 5, Figure 6). At the scale of this unit, total gamma ray values range from ~35 to 55 gAPI units, with the exception of anomalously low values corresponding to washed-out intervals. The natural radiation signal is generally dominated by potassium and thorium content, with uranium contributing in minor way. For the most part, the three radioactive elements co-vary, suggesting that they are mainly responding to clay mineralogy or content. Density measurements do not show specific characteristics in the logged interval. Resistivity data show a slight decrease with depth over the logged interval, which is counter to the expected increasing trend with depth due to compaction; however, lower mean values below 462 m WMSF may simply be a response to the wider borehole diameter in this interval. Magnetic susceptibility values vary around a relatively consistent mean value downhole, and velocity measurements show a generally increasing trend with depth.

Site U1420

Figure 7
Figure 7. Summary of logging data recorded in Hole U1420A.

Site U1420 is situated at ~250 m water depth within the Bering Trough, a shelf-crossing trough thought to have formed by the Bering Glacier advancing across the shelf during glacial maxima. The goal of drilling Site U1420 was to test the hypothesis that the onset of ice streams resulted in correspondingly high rates of erosion that could affect orogenic processes and behavior, and to establish age control for the glacial advance. One hole was drilled at Site U1420, penetrating to 1020.8 mbsf. Due to limited core recovery (<14%), glacigenic sediment was the only primary lithology identified at this site and the age of drilled sediments was determined to be middle Pleistocene to Holocene.

Because of concerns about borehole stability based on poor core recovery and challenging coring conditions, a sonic-induction tool string (see “Logging tools and operations” above for description) was the only tool string deployed in Hole U1420A. Due to an obstruction or collapse of the borehole, the tool string was only able to record data between ~90 and 290 m WMSF. The caliper log indicates that borehole diameter exceeded 18 inches in the upper and lower sections of the logged interval. Borehole size was smaller (~15 inches) between ~140 and 200 m WMSF. Even in this large diameter hole, the data seem to be of good quality, as the measurements show relatively consistent variability through multiple passes of the logged interval.

The logged interval in Hole U1420A was assigned to a single logging unit based on the minimal measurements recorded and the limited depth interval of the logging data in the context of the entire drilled depth. However, on the basis of distinctive changes in resistivity and velocity measurements, logging Unit 1 was divided into five subunits (Figure 7). Logging Subunit 1A is characterized by a relatively constant mean trend in all data types, with little net downhole variation. Subunit 1B is distinguished by abrupt decreases in gamma ray and resistivity measurements. The deep resistivity curve likely measured formation, whereas the shallow and medium curves most likely reflect the resistivity of the borehole fluid, given the large borehole diameter. Gamma radiation, resistivity, and compressional velocity (Vp) all increase at the Subunit 1B/1C boundary, whereas gamma radiation decreases and resistivity and Vp increase across the Subunit 1C/1D boundary. All three of these logs then decrease and the caliper increases in Subunit 1E. Overall, resistivity in Hole U1420A is generally >3.0 ohm-m, with the deepest resistivity curve showing values >8 ohm-m in Subunit 1D. The relatively high Vp values (~1700 to >2500 m/s) measured within this shallow logged interval support the idea that high measured resistivity at Site U1420 may be due to a combination of reduced pore water salinity (observed in porewater geochemistry) and lithology (possibly a clast-rich diamict, consistent with glacigenic origin).

Site U1421

Figure 7
Figure 8. Summary of logging data recorded in Hole U1421A.

Site U1421 is located seaward of the glacial Bering Trough mouth at ~720 m water depth. The key drilling objective was to establish age control for a series of seismic sequences that correlate to shelf sequences drilled at Site U1420. Three holes were drilled at Site U1421, penetrating to a maximum depth of 702.7 mbsf. Average core recovery ranged from 20% to 100% but in the logged interval, between ~96 m WMSF and total depth, the average recovery was only 10%. Two primary lithologies were identified based on Site U1421 cores - glacigenic sediment and interbedded biogenic and glacigenic sediment - and sediments were determined to be between middle Pleistocene and Holocene age.

Two tool strings were deployed in Hole U1421A on the basis of potentially unstable borehole conditions and limited time at the end of the expedition: the sonic-induction tool string (see “Logging tools and operations” above) and the VSI tool string. With the exception of some thin washouts, borehole diameter varied smoothly and rarely exceeded 18 inches. The character of the borehole wall in Hole U1421A on the shelf (Figure 8) is a distinct change from the rugose character observed at the deeper water sites (U1417 and U1418; Figure 3, Figure 5). There is a distinct separation between the shallow resistivity curve and the medium and deep resistivity curves through much of the logged interval in Hole U1421A. Given that the borehole diameter is within the depth of investigation of all three curves, this could indicate that the shallow borehole wall was invaded by logging mud, which has relatively low resistivity.

Gamma ray was measured through the entire hole, through the drill pipe from the seawater-seafloor boundary to the base of the pipe, and in open hole down to total depth. The gamma ray signal is highly attenuated when the tool was inside the bottom-hole assembly (above ~96 m WMSF); however, there is still a reasonable agreement between gamma ray from downhole logs and core logs, and similar trends are shown in both data sets.

The logged interval in Hole U1421A was divided into four units based on distinct changes in character and trends in gamma ray, resistivity, and compressional wave velocity (Vp) logs (Figure 8). Logging Unit 1 is characterized by relatively high gamma ray values, with a slight decreasing trend with depth, while resistivity and Vp have no trend with depth. Logging Unit 2 is distinguished by a general decrease in gamma ray values and a bimodal pattern in resistivity and Vp, defined by intervals of high, relatively constant values separated by intervals of very low values. The Unit 2/3 boundary is marked by a dramatic decrease in resistivity and Vp. The greatest fluctuations in all logged parameters is observed in Unit 3 and variations may be correlated with changes in borehole diameter, which may in turn be related to lithology variations. Logging Unit 4 is characterized by relatively high values and distinctly lower variability in all log data.

The VSP in Hole U1421A provides data for establishing a link between core and log data (recorded in depth) and seismic surveys (recorded in two-way traveltime) at the location of the borehole. Six stations yielded traveltimes ranging from 1.278 s TWTT below sea level at 284.7 m WMSF to 1.641 s TWTT at the deepest station at 687 m WMSF. A linear trend is observed in the VSP traveltime data consistent with the increasing trend with depth in the velocity log (Figure 8).

Magnetic susceptibility measurements

Figure 7
Figure 9. Summary of magnetic susceptibility logs recorded in Hole U1417E.

Expedition 341 marks the first at-sea deployment of the full Magnetic Susceptibility Sonde (officially, MSS-B), built by Lamont-Doherty Earth Observatory between 2010 and 2012 to replace an earlier version of the tool. Comprising both a deep-reading sensor, which was also included in all Triple Combo runs, and a high-resolution sensor, the full MSS tool has the capability to make magnetic susceptibility measurements at 10 and 40 cm vertical resolution.

The MSS tool string was run in Hole U1417E in a limited depth interval due to a borehole obstruction deeper ~200 m WMSF. Comparison of deep-reading sensor data between the Triple Combo and MSS runs, as well as high-resolution data from multiple passes of the MSS tool string, show that magnetic susceptibility measurements were repeatable (Figure 9A). The high-resolution data seem to track gamma ray variations, both of which are likely responding to fine-scale lithology changes. There is also a trend superimposed on all magnetic susceptibility logs that is likely related to internal tool temperature. When the temperature of the MSS increases linearly with depth, for example as observed in the deep-reading MSS data from the Triple Combo, a simple linear correction can be applied (Figure 9B). Non-linear tool temperature effects, such as those observed during the short passes of the MSS tool string, cannot be corrected so simply and post-cruise investigation will be required to better understand tool temperature responses. 

Core-log seismic integration

Figure 7
Figure 10. Core-log seismic integration, Site U1421.

Linking the sedimentary stratigraphy of the Gulf of Alaska with high-resolution borehole data to establish age models for glacial sedimentation is a major objective for Expedition 341. To achieve this, shipboard physical properties and downhole logging data were used to define ties between various seismic datasets and the five drilled sites. Core-log integration reduces the uncertainty associated with sediment characterization by taking advantage of both the detailed information provided by core samples and continuous, in situ measurements provided by downhole logging. Then, to integrate the core and logging data with seismic profiles, key lithostratigraphic and logging boundaries were converted from depth in meters to two-way traveltime using velocity data from cores, logs, and VSPs. Figure 10 illustrates the results of core-log-seismic integration at Site U1421, providing a preliminary link between borehole data and the broader seismic stratigraphy in the region.

    Angela Slagle: Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964 USA.

    Laureen Drab: Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964 USA.