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Logging Summary
IODP Expeditions 309 & 312:

Superfast Spreading Rate Crust 2 and 3

Expedition 309 and Expedition 312 Scientific Parties
Introduction
    Figure 1. Age map of the Cocos-, Pacific-, and Nazca Plates with isochrones at 5-Ma intervals. The locations of deep drill holes into the oceanic crust of Hole 1256D and Site 504 are shown.

    Integrated Ocean Drilling Program (IODP) Expeditions 309 and 312 were part of a three-component programs with the objective to deepen Hole 1256D initiated during Ocean Drilling Program (ODP) Leg 206. Hole 1256D is located in the eastern equatorial Pacific (Figure 1) and was drilled into 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (>200 mm/y).

    Wireline operations during Leg 206 provided high-quality data (Pezard and Anderson, 1989) on the in situ physical properties of the upper part of the oceanic crust combined up to a depth of ~752 mbsf. Expeditions 309 and 312 were highly successful continuations of this drilling effort with the agenda to provide further constraints on the physical properties in deeper sections of the oceanic crust. The primary logging objectives were to refine the volcanic stratigraphy, eruptive morphology, and variations in seawater-basalt alteration as a function with depth at a superfast spreading centre and in particular of the sheeted-dikes to gabbro transition. Hole 1256D was extended to 1255 mbsf during Expeditions 309 and finally deepened to 1507 mbsf during Expedition 312.

Logging Tools
    Figure 2a. Schematic illustration of wireline tool string configurations used at Hole 1256D during Expeditions 309 and 312
    Figure 2b. Schematic illustration of the Versatile Seismic Imager tool used during Expedition 312.

    The logging program on Expeditions 309 and 312 were designed to obtain data needed to illuminate the physical properties of the drilled rocks and shed light on the structure of the oceanic crust formed at a superfast spreading center. Standard wireline tool strings -- the Triple Combo, the Formation MicroScanner (FMS)/Sonic, and Well Seismic Tool (WST), -- were deployed during Exp 309 (Figure 2a).

    In addition to the standard wireline tool strings the Versatile Seismic Imager (VSI) and the Temperature Acceleration Pressure (TAP) combine with the Dual LateroLog (DLL) and Environmental Mechanical Sonde (EMS) were deployed during Exp 312 (Figure 2b).

    Details on standard wireline tools can be found here.

     

     


Logging Operations & Technical Highlights

    Figure 3. Logging operations at Hole 1256D during Expeditions 309 and 312. Depths are shown in meters below seafloor (mbsf). HNGS = Hostile Environmental Gamma Ray Sonde, APS = Accelerator Porosity Sonde, HLDS = Hostile Environmental Lithodensity Sonde, DLL = Dual LateroLog, TAP = Temperature-Acceleration-Pressure tool, SGT = Scintillation Gamma Ray Tool, DSI = Dipole Sonic Imager, GPIT = General Purpose Inclinometer Tool, FMS = Formation MicroScanner, UBI = Ultra Sonic Borehole Imager, VSI = Versatile Seismic Imager, EMS = Environmental Mechanical Sonde.

    Expedition 309 pre- and post-drilling logging operations

    Logging during Exp 309 was split into pre- (phase 1) and post-drilling (phase2) operations (Figure 3) using standard tool strings (Figure 2). The primary purpose of the two pre-drilling logging deployments were to check the condition of ODP Hole 1256D and identify borehole wall breakouts, and variations in hole diameter. Post-drilling logging operations (Figure 3) were dedicated to provide constraints on the physical properties of the newly drilled sections of the oceanic crust and determine in as much coring influenced borehole conditions. Despite several attempts a fifth logging run including the WST could not be deployed and the logging run was abandoned. All successfully deployed logging operations provided high quality data overlapping data previously collected during Leg 206.

    Expedition 312 logging operations

    Prior to Exp 312 logging operations the bit was placed in the open hole at ~20 m below the 16-inch casing shoe at a depth of ~290 mbsf (Figure 3). The hole was successfully logged with six different tool strings: the triple combo; VSI; FMS with Scintillation Gamma Ray Tool (SGT) and Dipole Sonic Imager (DSI), Ultrasonic Borehole Imager (UBI) with the General Purpose Inclinometer (GPIT), SGT, and DSI tools; FMS with SGT only; and the TAP, DLL, SGT, and EMS. The Triple Combo made two passes, from 1440 to 343 mbsf and from 1438 to 1080 mbsf. A check shot experiment using the VSI was conducted at 58 stations ~22 m apart from a maximum depth of 1383 mbsf. The UBI with the GPIT, SGT, and DSI tools logged from 1430 to 1099 mbsf, followed by a repeat pass covering the interval from 1433 to 1089 mbsf. The Formation MicroScanner was combined with the SGT and logged the hole from 1437 to 1089 mbsf and from 1436 to 1101 mbsf. A last logging suite was made up of the TAP, DLL, and SGT and logged the hole from 1440 mbsf to 290 mbsf.
Logging Results

    Wireline logging operations during Expeditions 309 and 312 built on the success of Leg 206, and provided for the first time in the history of DSDP, ODP and IODP in situ physical properties of a complete section of the oceanic crust including the sheeted dikes–gabbro transition.

    Expedition 309 results

    Wireline operations during Expedition 309 provided high-quality data on the in situ physical properties of the upper part of the oceanic crust combined up to a depth of ~1220 mbsf (Figure 4a, 4b, 4c, and 4d).

    Figure 4. Summary of the wireline logging results obtained at Hole 1256D during Expeditions 309 and 312. (A) Depth interval 250-500 mbsf; (B) Depth interval 500-750 mbsf; (C) Depth interval 750-1000 mbsf; (D) Depth interval 1000-1250 mbsf; (E) Depth interval 1050-1500 mbsf. Note: A-D represents Expedition 309 data only, whereas in E Expeditions 309 and 312 data are displayed. Circles represent bulk density and averaged compressional velocities (x, y, and z components) measured on discrete sample cubes from Leg 206, Expeditions 309 and 312.

    Caliper readings derived from triple combo and FMS-sonic tool strings show generally good borehole conditions. The average hole diameter measurements from the FMS/sonic calipers are 11.25 inches for C1 and 10.90 inches for C2; this slight difference is the result of an elliptical borehole between 807 and 966 mbsf. Wide sections (>13 inches) are particularly common in this interval, as well as between 1048 and 1060 mbsf. Comparison of the caliper data from the pre- and post drilling operations of the upper 500 m shows that the borehole is being progressively enlarged with continued drilling. The excellent hole conditions over the rest of the interval resulted in good measurements by these contact tools, particularly for the lowermost 300 m. Triple combo data is of high data quality and there is an excellent overlap with the previous logging runs. The FMS and UBI provided high quality data (Figure 5a, 5b, and 5c. Note: Only FMS images are shown). However, because the UBI was deployed very slowly (120 m/hr), incomplete heave compensation and sticking of the tool influence the data quality. Whereas the FMS images can be corrected with confidence, the UBI images still show artifacts of sticking. In most intervals the coverage of the borehole wall by the two FMS passes is good and is complemented by the UBI images.

    Figure 5. Formation MicroScanner (FMS) resistivity images (static normalization) of depth intervals 345 - 355 mbsf, 470 - 480 mbsf, and 645 - 655 mbsf recorded during Expedition 309. Natural radioactivity, electrical resistivity (LLD: LateroLog Deep, LLS: LateroLog Shallow), density, photoelectric effect (PEFL), neutron porosity and capture cross-section (sigma) are reported on the right columns. (A) Transition between the lava pond (Unit 1) and thin flows (Unit 2) at 348 mbsf. This transition is characterized by a strong decrease in the electrical resistivity. (B) Transition between a thin flow unit and a massive unit at 473 mbsf. (C) Massive unit displaying a marked increase of the natural radioactivity at 648 mbsf.

    Principal rock types distinguished during Expedition 309 were sheet flows or brecciated basalts as the most common, followed by massive units and pillow basalts. Pillow basalts were only described in the upper borehole section between ~365 and 375 mbsf (Figure 4a). It is evident that highly fractured lithologies like pillow and brecciated basalts display higher natural radioactivity compared to massive units. These fractured units are also characterized by variable porosities and densities with values well above 5 % and below 2.9 g/cm3, respectively. Compressional velocities for these units vary from 3.2 to 5.5 km/s. Pillow basalts may be distinguished from brecciated lithologies by resistivities lower than or equal to ≤10 Ωm. However, a clear discrimination between these units using well-logging data alone remains uncertain. Examples for massive units are found in depths intervals 316–338 mbsf, ¬472–490 mbsf, 819–833 mbsf, and 1120– 1140 mbsf. These units are clearly separated from the previous described lithologies by high compressional velocities (>5.5 km/s) and densities (~2.7 g/cm3) and increased resistivity (usually >100 Ωm), and correlate with low porosity (< 12%) and natural gamma ray emissions (<4 gAPI).

    The most compelling change in log response is observed below the transition zone (~1060 mbsf) in the sheeted dikes. Natural radiation in these rocks remains relatively constant with values generally below 3 gAPI. This constant value may reflect a change in stability of K-bearing minerals (e.g., saponite), which is essentially the main carrier of the naturally occurring radioactivity in these rocks. Increased bulk density, compressional velocity and electrical resistivity demonstrate a clear change in lithology and show the highest values obtained in Hole 1256D. Resistivity data recorded with the Dual LateroLog tool (DLL) demonstrate a strong decoupling between the shallow (LLS) and the deep (LLD) resistivity below 1080 mbsf. Shallow LateroLog measurements have the same vertical resolution as the deep LateroLog but respond more strongly to that region around the borehole affected by invasion. Caliper readings from 1080 mbsf to 1211 mbsf are on average 10.98 inches (± 0.5 inch) indicating good borehole conditions and the shallow resistivity measurements are consequently less influenced by fluid invasion. It is therefore unlikely that fluid invasion is solely responsible for the observed decoupling of both resistivity measurements. Pezard and Anderson (1989) described this difference between the shallow and deep resistivity in ODP Hole 504B and attributed this to an anisotropic distribution of pore space in the rock. In the case of a subvertical network of conductive fractures the value of the shallow resistivity is affected more and consequently more reduced than the deep resistivity. It is very likely that the resistivity data obtained in Hole 1256D also indicate a dominant presence of vertical features in the sheeted dikes.

    Expedition 312 results

    Expedition 312 downhole measurements in Hole 1256D were conducted from a depth of 1440 mbsf, ~67 m above the total cored depth (Figure 3 and Figure 4e). Borehole conditions were good during the six logging runs and provided high quality data with an excellent overlap of logging results from Expedition 309 (Figure 4a, 4b, 4c, and 4d). Overall results obtained during Expedition 312 support the division of the lithology based on core description from recovered sample material (see: http://iodp.tamu.edu/publications/PR/312PR/312PR.html for more details). The overall total gamma-ray is relatively constant and well below 4 gAPI in the logged sections. The net measured formation resistivity increased with increasing depth but this trend is interrupted at several depth intervals (Figure 4e). Strong decoupling between the shallow and deep resistivity measurements described at the top of the sheeted dikes continues to TD. Values for the shallow and deep resistivity measurements are in the range of 500 –140000 Ωm. The resistivity data observed in the sheeted dike complex suggests that the lithostratigraphy may be divided into four sections (1060–1155 mbsf, 1155–1275 mbsf, 1275–1350 mbsf, and 1350–1407 mbsf) based on variability and magnitude of the electrical resistivity.
    Figure 6. Velocity-depth plot of Hole 1256D showing wireline sonic and check-shot interval velocities from Expedition 312 and Leg 206. Logging and core bulk density data from Hole 1256D are also shown. The increase in velocity in the sheeted to granoblastic dike boundary to values around 7.0 km/s is apparent.
    Figure 7. Formation MicroScanner (FMS) resistivity and sonic Ultrasonic Borehole Imager (UBI) images (static normalized) showing the depth range 1402–1409 mbsf covering the sheeted dike-gabbro transition described on recovered samples. FMS data (static normalized grey scale) obtained during logging pass 2 (see Figure 3) are overlain the UBI image for comparison.
    Figure 8. Temperature profile of Hole 1256D recorded by the Temperature Acceleration Pressure (TAP) and Environmental Mechanical Sonde (EMS) tools during Expeditions 309 and 312. Excursions between 900 and 950 mbsf, and 1350 and 1400 mbsf are evident, as is the temperature increase by nearly 18ºC from beginning to end of the Expedition 312 bottom hole temperature measurement. Also shown are caliper data indicating good correlation between enlarged borehole diameter and negative temperature excursions in some parts of Hole 1256D (e.g., ~950 mbsf).

    Although, overall density and neutron porosity range from 1.5–3.1 g/cm3 and 2–75 %, respectively, the variation remains small in the newly cored section of Hole 1256D. The average densities of the sheeted dike complex and the granoblastic dikes are 2.89 g/cm3 and 2.99 g/cm3, respectively. Density drops to an average density of 2.95 g/cm3 in Gabbro 1. A similar drop occurs at a depth of 1407 mbsf where the density decreases from 3.10 g/cm3 to only 2.93 g/cm3. This change in density is accompanied with a decrease in compressional velocity from 6.2 km/s to 4.6 km/s observed both in wireline and discrete cube measurements. However, post-cruise examination of the wireline compressional velocity data acquired below 1300 mbsf show discrepancies between the 3 logging runs. This may be related to hole conditions and/or tool movement in the hole and requires careful re-processing of the obtained data prior to detailed interpretation.

    The VSP was shot in Hole 1256D to determine interval velocities and to record seismograms for further analysis of the seismic properties of upper ocean crust. In general, the VSP interval velocities parallel trends in the sonic log and the shipboard velocity measurements on recovered rock samples (Figure 6). Although the velocity magnitude differs among the various methodologies due to different frequencies of sound and the different confining pressures, the trends with depth are similar. This similarity demonstrates the fundamental dependence of velocity fluctuations in uppermost crust on the primary eruptive process and the increase in velocity with depth in ocean velocity layer 2 on the increasing density of the rocks due to progressively higher temperature alteration and metamorphism. However, there are two unusually high interval velocities of 7.6 km/s between 1339-1361 mbsf and a velocity of 6.5 km/s at 880-903 mbsf that are not matched by low velocities at neighboring stations.

    Preliminary analysis of the resistivity and sonic image data (Figure 7) indicates that directly above the boundary the formations are characterized by randomly oriented fractures, whereas the fractures in the gabbroic section are regular oriented.

    Features observed in the UBI image at 1402 mbsf and 1409 mbsf have a north-east oriented plunge and an approximate dip between 35 and 40 degrees and may represent fractures. The same features are also evident on the resistivity image where they represent zones of high conductivity. Bottom hole temperature was recorded three times (Figure 8) and an increase from 64.24 ºC to 67.90 ºC, and 86.5 ºC observed in a time frame of ~5 hrs and 68 ½ hrs, respectively. Perturbations are visible between 900–950 mbsf and 1350–1400 mbsf with negative deviation from the temperature profile. These negative temperature anomalies indicate a slower return to equilibrium temperatures and may be due to a higher influx of seawater invasion during the drilling process.
References

    Pezard, P.A., and Anderson R.N., 1989. Proc. ODP, Sci. Results., 111: College Station, TX (Ocean Drilling Program).


    Florence Einaudi: Expedition 309 Logging Staff Scientist, Laboratoire de Géophysique et d'Hydrodynamique en Forage, ISTEEM, cc 056, 34095 Montpellier Cedex 5, France

    Marc Reichow: Expedition 312 Logging Staff Scientist, University of Leicester, Borehole Research, Department of Geology, University of Leicester, University Road. Leicester, LE1 7RH, United Kingdom.