Summary

The main objective of ODP Leg 200 was to drill a hole at the location of the H2O geophysical observatory on the eastern Pacific plate. Two of the three primary scientific objectives of this leg are to document the in-situ physical and chemical properties of fast spreading oceanic crust of the eastern Pacific plate; and to characterize this site and its vicinity for preparation of the ocean seismic network installation at a later time.

To help achieve these scientific objectives, downhole measurements were made in Hole 1224F (141^o58.7567' W, 27^o53.3634' N) at the H2O site, after completion of RCB coring to a total depth (TD) of 174.5 meters below the sea floor (mbsf). The water depth was 4978 meters below rig floor (mbrf) and the sediment thickness is ~28 m at Hole 1224F. Two wireline tool strings, Triple Combo (density, porosity, and resistivity) and FMS/Sonic, were run. The 3-component Well Seismic Tool (WST-3) was also tested as the third run. The logs run during Leg 200 provided high-quality results and can be used to significantly enhance our understanding of the geological settings and the shallow structure of the upper ocean crust at this strategic ODP site in this Pacific region.

Preliminary analysis of the logs and core descriptions clearly shows that Hole 1224F consists of definable alternative layers of fresh and fractured and/or altered basalts (breccia and pillows) that correlate to changes in the measured log properties. The shallow oceanic crust penetrated in Hole 1224F consists of five distinctive units: above 45 mbsf; 45-63 mbsf; 63-103 mbsf; 103-142 mbsf; and below 142 mbsf. These layered formations can be distinguished using the continuous electrical resistivity, density, sonic, neutron porosity, magnetic field, and possibly spectral gamma ray logs. The existence of a conduit or large-scale fracture between 138 and 142 mbsf was detected by all the log tools including the temperature tool. In addition, the temperature tool reveals that the "hot" fluid had a temperature of 4.6 ^oC at the time of the logging. The vicinity of this conduit is much more highly altered than other rocks penetrated by the hole, indicated by the gamma ray logs. In the logged intervals where all the tools overlapped, they provide consistent information to support the layered structural units based on these geophysical properties. This five-layer structure can also be identified on the preliminary high-resolution seismic data processed by the Femarco, Inc., although it cannot be resolved on the original seismic section.

Background

A retired AT&T telephone cable system exists on the seafloor between San Luis Obispo, California, and Makaha, on Oahu, Hawaii, which was originally laid in 1964. Incorporated Research Institutions for Seismology (IRIS) installed a long-term seafloor observatory about half-way along the cable (about 140^oW and 28^oN). The cable was cut at this point and terminated with a seafloor junction box. The location of the junction box on the seafloor defines the location of the Hawaii-2 observatory (H2O), which was named after the original AT&T cable (Figure 1). During ODP Leg 200, Hole 1224D (141^o58.7525' W, 27^o53.3699 N) was drilled and cored. The hole was drilled at the location of the H2O site near the junction box for a later downhole installation of a broadband seismometer as a part of the global Oceanic Seismic Network.

In order to characterize the H2O site by providing ground-truth information on the physical and chemical properties of the drilled oceanic crust and to delineate the seismic sequence boundaries, Hole 1224F (141^o58.7567 W, 27^o53.3634 N) was drilled for logging to a total depth of 174.5 meters below the sea floor (mbsf), adjacent to both Hole 1224D and the H2O. It was cored and then logged using a nearly complete suite of wireline logging tools, including nuclear, acoustic and shear, and electrical and magnetic properties. In addition, borehole electrical image and temperature logs were also acquired.

The oceanic crust at the ODP H2O site has an age of about 45-50 Ma with a spreading rate of 140 mm/yr (full rate). This site poses a unique strategic position among all the holes drilled in the 30 years of the Ocean Drilling Program. It is the deepest hole drilled on "normal" oceanic crust on the Pacific plate with an age <100 Ma and basement penetration >10 m, excluding holes drilled in seamounts, plateaus, aseismic ridges, and fracture zones. Furthermore, there are no boreholes off-axis in "very fast" spreading crust, except this site. Therefore, it is very important to document fully and analyze accurately the logging measurements of structural, lithological, and petrophysical properties of the oceanic crust penetrated in this hole. This will provide a reference site of "normal" ocean crust to constrain geophysical, hydrothermal, and geochemical models of crustal evolution in the large.

It has been known for more than 40 years that fast spreading crust is both simple and uniform, based on the seismic structure resolved using 1960's technology (e.g., Raitt 1963, Menard, 1964). However, preliminary analysis of log data shows that all logging measurements of different formation properties indicate a consistent multi-layered structure within the 140 m crustal basement penetrated by the borehole. Hole 1224F consist of five distinctive units: above 45 mbsf; 45-63 mbsf; 63-103mbsf; 103-142 mbsf; and below 142 mbsf (Figure 2).

Such a layered structure and the physical changes within it are no simpler than what we have known from the ODP legacy hole 504B and others. It is rather qualitatively similar to Hole 504B and/or 395A. A small portion of the seismic site survey data around the site was processed by Femarco, Inc. by request prior to drilling, to help real-time drilling and logging operation. This five-layer structure can be identified on the preliminary high-resolution seismic data processed by the Femarco, Inc. (Figure 3), although basement crustal structure appears to be a simple and uniform layer on the original seismic section (Figure 4; Stephen et al., 1997). In both Figures 3 and 4, the vertical axis is the recorded relative two-way travel time in millseconds with an offset of about 5 seconds from the actual two-way travel time. It should be also mentioned that the seismic line does not go over the drill site. It is offset about 2km. The association of the log-identified 5-layer structure with the seismic data was done through a quick depth-time conversion of the sonic log data. The quantitative log-seismic integration using synthetic seismogram was not able to be performed during leg operations, it will be done as post-cruise research.

In addition, the existence of a hydrothermal conduit or large-scale fracture between 138 and 142 mbsf was detected by all the logging tools including the temperature tool (Figure 5). The temperature tool reveals that the "warm" fluid had a temperature of 4.6^oC at the time of the logging. The vicinity of this conduit is much more highly altered than other rocks penetrated by the hole, indicated by the gamma ray logs (Figure 5).

Results from Preliminary Log analysis

The first tool string (Triple Combo) consists of the accelerator porosity sonde (APS), the hostile environment lithodensity tool (HLDT), and the phasor dual induction-spherically focused resistivity tool (DIT). The hostile environment spectral natural gamma-ray sonde (HNGS) is included at the top of the string, and the temperature, acceleration and pressure tool (TAP) at the bottom. The density (RHOB), neutron porosity (NPHI), natural gamma ray, and temperature logs are shown in Figure 2. The composite log of the gamma ray tools in Figure 5 shows elevated readings of three natural radioactive elements (potassium, thorium, and uranium) indicating an alterated hydrothermal fluid flow zone, together with temperature and other logs. The interval displayed corresponds to the total depth of Hole 1224F, from the total depth (174.5 mbsf) to the base of the pipe (35 mbsf), all in basaltic rocks. The second tool string consists of the Formation MicroScanner (FMS), the general purpose inclinometer tool (GPIT), and the dipole shear sonic imager (DSI). Three passes of this string were run using the DSI in monopole, dipole, and Stoneley-wave recording modes. Switching between recording modes was accomplished while the tool was downhole. Pass 1 was run uphole from the TD to basement contact (27.7 mbsf) to record the data-intensive FMS and DSI Lower Dipole and monopole P-wave and S-wave (P&S) modes. Pass 2 was run uphole from the TD in open hole, then through the pipe to the basement contact, with the data-intensive cross-dipole mode enabled. Pass 3 was run uphole from the TD to the basement contact, with the Stoneley, the Upper Dipole, and the P&S monopole recording modes enabled. The frequency band is from 80 Hz to 5 kHz for dipole measurements and low frequency Stoneley acquisition, and from 8 kHz to 30 kHz from monopole acquisition. Excellent FMS and compressional and shear-waveform data were collected during these three passes. The compressional and shear wave velocity logs from the high-frequency monopole source in the first pass are shown in Figure 2 (VPMP and Vs). As indicated, the shear wave velocity from the lower Dipole is also shown in Figure 2. The difference between the two Vs is most likely due to their difference in source frequency, which needs further analysis. Two caliper logs from the FMS 4 pads are also shown in Figure 2 (Track 1) and illustrate two orthogonal dimensions of the borehole as a function of depth.

The shallow reaching, centimeter-scale FMS images both lithologic contacts and millimeter-scale fractures (Figure 6). The FMS images show considerably greater resolution of the relative conductivity changes over the interval and reflect formation characteristics that are not apparent at the broad scale of the other logs. The alternatively layered structure of fresh and fractured basalts is evident on the continuous image FMS records. For illustration purpose, Figure 6 shows a 5 m interval of the high-quality FMS images from the entire interval of Hole 1224F acquired during FMS/DSI pass 1. Conductive features are imaged with darker color whereas resistive rocks are indicated by lighter color. In this figure, sheeted lava flows occur between 81.5 and 81.8 mbsf, between 82.6 and 82.8 mbsf, and between 84.4 and 84.6 mbsf, for example. The FMS record provides the only means to delineate the continuous structural images of the borehole wall penetrated by Hole 1224F. Above 67 mbsf, the FMS images reveal relatively "smooth" basalts with less fractures and fine structures. Good core recovery of 50% was achieved in this interval, which reveals fresh basalts, whereas the average core recovery is only about 15% below this interval. Below 67 mbsf and above ~103 mbsf, the FMS images show highly fractured fragments intervened with sheeted layers of 10 cm to 0.5 m thick that have high electrical resistivity values. It may be indicated that this interval consists of breccia intruded by sheeted lava flows. FMS images between 100 and 142 mbsf show large blocks of electrically resistive fragments against large blocks of highly conductive area with less fractures and fine structures than the intervals above. This may indicate that the rock formation in this interval mainly consists of less fractured pillows. Below 142 msbf, the rocks become electrically more resistive and contain fewer fractures with greater spacing between fractures than the interval consisting of breccia.

The DSI tool has been a significant addition to the Ocean Drilling Program in the last several years. It can acquire sonic waveforms in a variety of different hard and soft formations to calculate the compressional, shear and Stoneley wave velocities. The sonic waveforms from the cross-dipole mode can be used to study shear wave anisotropy of the formation to infer the stress condition of the borehole and possibly its vicinity. The Stoneley waveforms can be used to estimate in-situ formation permeability. Hole 1224F is the third ODP hole in which complete DSI logs were acquired. The hole condition is good through the entire logged depth interval as indicated by the two FMS caliper logs with the hole diameters <10.7 in (Figure 2). Figure 7 shows two examples of the cross-dipole waveforms in Hole 1224F at depths of 60 and 153 mbsf. There are 32 traces in each panel consisting of 4 blocks of 8 waveforms. From left to right, the blocks are upper dipole inline and crossline and lower dipole inline and crossline waveforms, respectively.

Concluding Remarks

These unexpected findings from preliminary shipboard log analysis and the surprising coincidence of log-derived "lithological" units with high-resolution seismic structures resolved prior to drilling prompt us to raise many fundamental questions about our current understanding and knowledge of ocean crustal structures and hydrothermal regimes and the way in which we derive and define them. Among such fundamental questions: (1) What is the cause for forming this layered structure within only 140 m crust, lithological or structural? (2) If it is structural, is it controlled by porosity change or pore shape and pore size changes? (3) Can this multi-layered structure at the H2O site exist at the regional scale at all? (4) If it can, can we further identify them as Layer 2A, 2B, 2C, ... or find a similarity? Considering the vast difference in thickness, spreading rate, age, seismic resolution scale, and other factors between this H2O site and other sites like 504B and 395A, such similaries, if they exist, can significantly improve our current understanding of the mechanisms of crustal formation and crustal evolution. Further investigation is needed to help address these questions through detailed accurate petrophysical analysis and correlation of log data, compressional and shear wave velocity core measurements under pressure, high-resolution seismic data processing, and their integrated interpretation.

References

Menard, H.W., Marine Geology of the Pacific, McGraw-Hill, New York, 1964.

Raitt, R.W., The crustal rocks, in Hill, M.N. (Ed.), The Sea – Ideas and Observations on Progress in the Study of the Seas (Vol. 3): The Earth Beneath the Sea, Wiley-Interscience, New York, 85-102, 1963.

Stephen, R.A., Swift, S.A., et al., Bathymetry and sediment thickness survey of the Hawaii-2 cable, Woods Hole Oceanographic Institution, 1997.

Yue-Feng Sun: Logging Staff Scientist, Lamont-Doherty Earth Observatory, Columbia University, Borehole Research Group, Palisades NY 10964, USA.

Hartley Hoskins: Seismologist, Computer and Information Services, Woods Hole Oceanographic Institution, MS 46, 360 Woods Hole Road, Woods Hole MA 02543-1542, USA.