Safety Package for Leg 200

(with additional figures for SSP)

DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY

Co-Proponents: R.A. Stephen ¹, J.H. Natland ², R. Butler ³,
K. Becker ², A.D. Chave ¹, and F.K. Duennebier ⁴,

Proposal Number: 500 - Full 2 - PPSP

April 4, 2000

1 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

2 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149

3 Incorporated Research Institutions for Seismology, 1616 Fort Myer Drive (Suite1050), Arlington, VA 22209

4 Department of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, Hawaii 96822

      The H2O long term observatory site satisfies three scientific objectives of crustal drilling: i) It is located in one of the high priority regions for the Ocean Seismic Network; ii) Its proximity to the Hawaii-2 cable and H2O observatory make it a unique site for real time, continuous monitoring of geophysical and geochemical experiments in the crust; and iii) It is on fast spread Pacific crust (7cm/yr) which represents one end-member for models of crustal generation and evolution and crust/mantle interaction. This is a multi-disciplinary proposal which primarily represents the interests of the JOI/IRIS Steering Committee for Scientific Use of Submarine Cables , the Ocean Seismic Network (OSN) group , , and International Ocean Network (ION) group . Drilling at H2O will also provide useful background information for the Borehole Observatories, Laboratories, and Experiments (BOREHOLE) group and the oceanic lithospheric processes community . We propose drilling one re-entry hole within 2km of the Hawaii-2 Observatory (H2O) junction box in the Eastern Pacific at 27° 52.916' N, 141° 59.504 ' W in 4979 m water depth (see Figures 1 and 2) . The site is roughly half-way between California and Hawaii. A limited geophysical survey of the cable, including single channel seismics, was carried out from R/V Revelle in August 1997.

2. Overview of Scientific Objectives

      This proposal addresses directly the second of three initiatives outlined in the ODP Long Range Plan: "In situ monitoring of geological processes" (pages 49-51) and it represents an initial step in accomplishing the oceanic crustal component of the third initiative: "Exploring the deep structure of continental margins and oceanic crust" (pages 52-54). The drilling is intimately tied to the use of "seafloor observatories" (page 63) and represents the partnership of ODP with OSN, ION and BOREHOLE (page 74). (Page numbers refer to pages in the Long Range Plan.)

      There are no deep boreholes (>100m) in the Pacific plate, the largest modern tectonic plate. Table 1 summarizes the boreholes that have been drilled on 'normal' crust on the Pacific plate which have more than 10m of basement penetration and with crustal ages less than 100Ma. Holes in sea mounts, plateaus, aseismic ridges and fracture zones have not been included. Holes with crustal ages greater than 100Ma are not included since they would be affected by the mid-Cretaceous super plume . In 30 years of deep ocean drilling and over 1,000 holes world wide there have been only 12 holes with greater than 10m penetration into 'normal' igneous Pacific plate: only one hole on ODP, no holes with greater than 100m penetration, and no holes in crust with ages between 29 and 72Ma. Furthermore there are no boreholes off-axis in 'very fast' spreading crust. At the latitude and age of the H2O site the spreading rate was 140mm/yr (full rate). Having a reference station in 'normal' 45-50Ma ocean crust will constrain geochemical and hydrothermal models of crustal evolution.

      Although fast-spreading ridges represent only about 20% of the global ridge system, they produce more than half of the ocean crust on the surface of the planet, almost all of it along the East Pacific Rise. Most ocean crust currently being recycled back into the mantle at subduction zones was produced at a fast-spreading ridge. If we wish to understand the Wilson cycle in its most typical and geodynamically significant form, we need to examine ocean crust produced at fast-spreading ridges.

      We have also known for a long time, more than 40 years, that fast-spread crust is both simple and uniform, certainly so in terms of seismic structure . Successful deep drilling of such crust at any single location is thus likely to provide fundamental information which can be extrapolated to a significant fraction of the Earth's surface.

Table 1: Summary of holes drilled in 'normal crust' on the Pacific Plate with an age less than 100Ma and penetration into basement greater than 10m

      Drilling at the H2O site would address both teleseismic, whole earth seismic studies and regional studies. The site is located in a region on the earth's surface where there is no land in a 2000km square area. For uniform coverage of seismic stations on the surface of the planet, which is necessary for whole earth tomographic studies, a seafloor seismic observatory is required. This site is one of three high priority prototype observatories for the Ocean Seismic Network (OSN) . Global seismic tomography (GST) provides three dimensional images of the lateral heterogeneity in the mantle and is essential in addressing fundamental problems in sub-disciplines of geodynamics such as: mantle convection, mineral physics, long wavelength gravimetry, geochemistry of ridge systems, geomagnetism and geodesy. Specific problems include: the characteristic spectrum of lateral heterogeneity as a function of depth, the anisotropy of the inner core, the structure of the core-mantle boundary, the role of oceanic plates and plumes in deep mantle circulation, and the source rupture processes of southern hemisphere earthquakes which are among the world's largest .

      The culturally important earthquakes in California are only observed at regional distances on land stations in North America which restricts the azimuthal information to an arc spanning about 180°. To observe California earthquakes at regional distances to the west requires seafloor stations. Regional observations are used in constraining earthquake source mechanisms. Since the H2O data will be available in real time, data from H2O will be incorporated into focal mechanism determinations within minutes of California earthquake events. Other problems that can be addressed with regional data from Californian and Hawaiian earthquakes are: the structure of the 400km, 525km and 670km discontinuities in the northeastern Pacific, the variability of elastic and anelastic structure in the Pacific lithosphere from Po and So and pure-path oceanic surface wave studies, and improved locations for Juan de Fuca Ridge earthquakes from T-phase arrivals .

      The Hawaii 2 Submarine Cable system is a retired AT&T telephone cable system between San Luis Obispo, California and Makaha, on Oahu, Hawaii. The cable system was originally laid in 1964. IRIS (Incorporated Research Institutions for Seismology) has installed a long term seafloor observatory about half-way along the cable (about 140°W and 28°N). The cable has been cut and terminated with a seafloor junction box. The junction box has eight underwater make-break connections. About 500W of power is available from the junction box and there is ample capacity for two-way real time communications with seafloor instruments. Data channels from the seafloor can be monitored continuously via the Oahu end of the cable to any lab in the world. (The California end of the cable cannot be used because it has been cut and removed from the continental shelf.) There is a shallow buried broadband seismometer operating at the site. Other seafloor observatories, such as a geomagnetic observatory , a hydrothermal observatory , or a broadband borehole seismic observatory , can be installed at the site as funding becomes available.

      Within the ODP and marine geology and geophysics communities there has been considerable interest in the past few years in long term seafloor observatories which include a borehole installation. Prototype long term borehole and seafloor experiments almost exclusively use battery power and internal recording. The data is only available after a recovery cruise. One exception to this is the Columbia-Point Arena ocean bottom seismic station (OBSS) deployed by Sutton and others in the 1960's . For the foreseeable future the most practical method for acquiring real time, continuous data from the seafloor will be over cables . The H2O project provides this opportunity.

      On an SD cable like Hawaii-2 there are repeaters every 20nm to compensate for attenuation on the cable. The repeater boxes are about 0.2m in outside diameter and a meter long. On the H2O site has been located between two of these repeater boxes. The location of the junction box defines the H20 seafloor observatory. Experiments should be carried out within about a kilometer or two of the junction box. The borehole should be no closer than about 500m.

      The Hawaii-2 cable runs south of the Moonless Mountains between the Murray and Molokai Fracture Zones (Figure 2) . Between 140°W and 143°W, water depths along the cable track are typical for the deep ocean, 4,250-5,000m; the crustal age varies from 45Ma to 50Ma (Eocene); and the sediment thickness to within the available resolution is about 100m or less. Prior to our cable survey cruise in August 1997, sediment thickness particularly was not well resolved along the track .

      Tectonically, the cable runs across the 'disturbed zone' south of the Murray Fracture Zone, between magnetic isochrons 13 and 19 . In the disturbed zone substantial pieces of the Farallon plate were captured by the Pacific plate in three discrete ridge jumps and several propagating rifts. To avoid this tectonically complicated region and to be well away from the fracture zone to the south of the disturbed zone the H2O observatory was situatedm west of isochron 20 (45Ma) at about 140°W. The crust west of 140°W was formed between the Pacific and Farallon plates under 'normal' spreading conditions at a 'fast' half-rate of about 7cm/yr . At the time this crust was formed the Farallon plate had not split into the Cocos and Nazca plates and the ridge that formed this crust was the same as the present day East Pacific Rise.

      Between 140°W and 143°W the Hawaii-2 cable lies in the pelagic clay province of the North Pacific . The sediments here are eolian in origin consisting primarily of dust blown from Asia. They are unfossiliferous, red clays. DSDP Leg 5 drilled a transect of holes in the pelagic clay province along longitude 140°W . Site 39 is north of the cable at latitude 32°48.28’N with an age of 60Ma. It has a sediment thickness of only 17m. Sites 40 and 41 are near the same latitude at 19°50’N with an age of about 67Ma.. Site 40 was drilled in an area of ponded sediments at the base of a large abyssal hill. Basement was not reached. Drilling terminated at a chert bed at 156m. The acoustic basement, the deepest horizon identified on the seismic reflection profiles, corresponded to the chert beds. Site 41 was drilled 15km from Site 40, but was considered to be more representative of the sediments in the general area. Basaltic basement was encountered at 34m BSF but there were no cherts. Site 39 is north of the Murray Fracture Zone and Sites 40 and 41 are south of the Molokai Fracture Zone. The actual ‘ribbon’ of crust on which the cable lies is between the two fracture zones and was not drilled on Leg 5.

      Site 172 was drilled on Leg 18 between the Molokai and Murray Fracture zones but east of 140°W in the ‘disturbed’ zone (31°32.23’N, 133°22.36’W) at an estimated crustal age of 35Ma . Sediment thickness above the basaltic basement was 24m. The sediment thickness from seismic reflection profiles had been interpreted as 90-105m. The discrepancy was attributed to "reverberations and thin sediment cover".

      In August 1997 we carried out a survey of the Hawaii-2 cable between 140°W and 143°W . Our survey strategy consisted of two phases: First we collected Seabeam bathymetry, magnetics and single channel seismics along the cable track starting at 140°W and heading west. Our site criteria were to have 100m of sediment thickness for setting the re-entry cone, to be in relatively undisturbed 'normal' crust in a plate tectonic sense and to optimize drilling penetration by selecting sites with well-consolidated basement, not rubble or highly altered zones. As a second phase we carried out a survey in a 20x20km area around each of three drill sites to map bathymetry, sediment thickness, basement morphology and magnetics in the vicinity.

 

d) Regional map showing bathymetry, latitude and longitude, nearest land areas and proposed site locations

      Figure 1 shows the Hydrosweep bathymetry around the H2O observatory that was acquired from the R/V Thompson in September 1999. The site is on a relatively benign ribbon of "normal oceanic crust.

      Figure 2 shows the repeater locations of the Hawaii-2 cable, the location of the H2O observatory and the location of previous drilling on Legs 5 and 18 of DSDP.

      Figure 3 shows the location of the Hawaii-2 observatory on Hydrosweep bathymetry acquired during the site survey in August 1997 as well as the location of the repeaters (ATT waypoints) on the cable.

      Figure 4 shows the bathymetry from the NGDC- DBDB5 data base on aregional scale from the observatory. There is no land within 2000km of the site. All drill sites are within 2km (about 1nm) from the H2O observatory.

e) Track Charts

      Figure 5 shows the H2O location with respect to the track lines for the Revelle duirng the site survey in 1997. The actual site is to the southwest of a well surveyed block but is bracketed by two parallel single channel seismic lines (Figure 6).
Figures 7 and 8 show the track lines, annotated in SCS shot numbers and Julian time respectively, for the single channel seismic and 3.5 KHz data. Circles at 1, 2 and 3km radius from the site and the specific proposed drill locations are also indicated. Although cross-tie seismic lines are not available the parallel seismic lines are sufficiently close together that contiguous structure can be identified across the lines.

f) Seismic reflection and 3.5KHz profiling lines

      Figures 9 and 10 are the latest 3.5KHz examples from lines north and south respectively of the H2O site. This 3.5 data was acquired on the R/V Revelle in August 1997 at the same time as the SCS data and is available in SEG-Y files.
Unmigrated and migrated SCS profiles from this site are shown in Figures 11 and 12 respectively for the north line
and in Figures 13 and 14 respectively for the south line. A tenth of a second two-way time corresponds to about 75m.

      We know there are chert layers in this part of the Pacific from early drilling on DSDP (Legs 5 and 18). On these 3.5KHz records there is a clean single pulse followed 10msec later by a diffuse event. Our interpretation is that the clean event is the seafloor and that the diffuse event is the chert layer. 10msec of 2-way time corresponds to about 8m thickness of soft sediments. 3.5KHz sees nothing coherent below the 'chert layer'. This was also the experience in the 1960 surveys where "acoustic basement" was chert.

      There is a continuous mid-sediment reflector at bout 0.3 secs or about 25m depth, which does not correspond to the chert layer identified on the 3.5 records. If we interpret the diffraction events at about 0.6secs in the SCS data as occurring at the sediment-basement boundary we get a very uniform sediment thickness of about 50m. This may get as thick as 75m in some areas but in no area did we identify 100m of sediment.

      We need to discuss with the ODP drilling engineers their thoughts on drilling a re-entry hole in 50-75m of sediment. The sediments (red clays) here may be more rigid (they are certainly less transparent) than the nannofossil ooze found in other areas. Also, since in this area it seems that we do not have much choice, it could be argued that we should at least try a re-entry cone and casing in thin sediment.

i) Sketch of major structural elements (N/A)

      There are no regional clathrate or hydrocarbon occurences. This is an abyssal plane environment at about 40Ma age with very thin sediment cover.

      There have been no previous safety or pollution prevention studies.

      The drill site at about 28°N and 142°W is situated between the eastward flowing North Pacific Current and the westward flowing North Equatorial Current. These are both deep water, open ocean currents. We estimate that the current direction and speed will be variable depending on the direction and constancy of the wind. Typical speeds should be less than 1knot.

7. Potential Man-Made Hazards

      There is of course a cable near the observatory site. Simple extrapolation of the ATT repeater positions places the cable directly under the North Line (intentionally), but the cable was actually recovered about 1.5km southwest of the line. During installation of the junction boxes cable "wuzzles" were formed within 200m of the site (Figure 15). In Figure 15, 5" of longitude at this latitude corresponds to about 136m. By drilling beyond 1km from the site under the seismic lines we are assured not to encounter the wuzzles or the cable (Figure 16).

      We have a specific site for the observatory at 27° 52.916' N , 141° 59.504 ' W. Although it is not within any of the large 20km by 20km blocks that we surveyed in 1997 we do have two parallel single channel lines that run within 1.5km of the site. The structure is sufficiently smooth that it should be OK to assume that it is continuous between the lines. The site itself is in a "contiguous block" of crust within at least 2km. The SCS data is laterally homogeneous over this block and the quality of the migrated and unmigrated data is similar. At this stage we have no new information on sediment thickness. It is no thicker than 100m; but it is interpreted to be at least 50m. We won't know for sure until we drill.

      The specific, main objective of the proposal is to drill a borehole that can be used for an Ocean Seismic Network permanent seismic observatory. The major issue is whether there will be enough sediment at the site to set the re-entry cone. Four sites have been identified as indicated on track charts and seismic profiles. The locations are::

Site 1 -141.9924° 27.8988°

Site 2 -141.9967° 27.8667°

Site 3 -141.9857° 27.8733 °

Site 4 -142.0076° 27.8883°

      Our strategy will be to probe the sediment at each site in an attempt to locate the site with the deepest sediment above the chert. We will also wash or drill through the chert layer to determine how solid it is. In some areas chert layers are rubble zones which can be washed through when installing the re-entry cone. At the site with the deepest sediment above chert we will wash down to basement and drill a few meters to identify drillability. The deepest sediment may be in a fault zone where the basement is highly fractured (for example Site 3 on Figure 10 or Site 1 on Figure 9) and it may be difficult to drill. The best strategy if we have the option may be to select a site high on the block (say Site 4 in Figure 9) or a mid-block site (such as Site 2 on Figure 10).

      At a minimum we require one hole with a re-entry cone and cased to basement. Ideally the hole would penetrate about 400m into basement to acquire good quality basalt samples for geochemical studies, adequate penetration into layer 2 for paleomagnetic analyses and good hole conditions for in situ experiments. A full suite of downhole logs should be acquired while the drill ship is on site and before setting the final casing string. The final casing string will be cemented in place. A cement bond log should be run in the final casing string to ensure that the casing is well coupled to the surrounding basalt.

      The H2O site has considerable potential for lithospheric, broadband seismic and crustal process studies. There is adequate scientific justification to install a junction box on the cable even without a borehole. A relatively shallow (about 100m) hole may be sufficient for an OSN broadband borehole seismic installation provided the crust is sufficiently consolidated. A deeper hole (say about 500m or more) would be extremely useful for temperature and hydrothermal circulation studies. However if drilling proves facile at the site this location would be ideal for deep crustal penetration (3km or more).

      If the OSN re-entry hole is completed before the end of the allotted time on site there are a number of options depending on how much time is left. If we have less than 24 hours I suggest repeating the VSP in the cased hole. If we have a few days left I suggest we drill further single bit holes at the alternate sites around the observatory to characterize the lateral heterogeneity of the sediment and to confirm the depths to basement. This information would be useful in interpreting seafloor heat flow measurements that may be made at the site in the future. It would also provide useful background information for further drilling at the site in the future. If we have a week left, I would suggest setting a second re-entry cone and starting a hole that could be used with wireline re-entry for other long-term downhole experiments such as flow, permeability, pore water chemistry, etc.

My thoughts on the scheduling for this leg are based on a 36 day cruise with about 14 days transit and 22 days on site. It seems to me that three weeks on site would be a minimum given the notorious drilling history for young Pacific crust. Also H2O is sufficiently far north that we can expect some rough weather in December, and contingency time for weather (in transit as well as on site) would be prudent. Planning based on "normal" re-entry operations is optimistic and cutting back to 15days of on-site operations seems too short.

      Even drilling OSN-1, which was in much better drilling conditions, better weather, and shallower water (4407m), took 17days on site. At Hole 843B (OSN-1) there was only 30m of 16" casing, even though there was 240m of sediment. At H20 we could expect a similar penetration of 16" casing to a chert layer but much less sediment (50 to 80m max) with which to support the BHA for the hard basement drilling. Having options for the 13.375" casing to get to basement seems prudent, but we would also need the time the under-reaming requires.

      In the drilling proposal (#500-Rev) we had requested 400m of basement penetration. This was a conservative estimate to get into consolidated basalts based on the drilling experience at 504B. At 504B sonic logs and resistivity measurements indicate poorly consolidated basalt down to 600m. So if we are fortunate enough to set a re-entry cone and casing and get good drilling into basement, it would do no harm to continue drilling to ensure that we are in consolidated basalt.

      On the Ninety-East Ridge drilling (Leg 179), for example, there was some question whether the hole (at 122m below the first basalt contact) had reached true basement because of possible inter-layering of basalts and sediments. The drilling plan for NERO was to drill 200m into basement. This was a site with ample sediment thickness (371m) and they knew from previous drilling on Leg 121 how much 16" casing to use (48.8m) so a pilot hole was not necessary. The water depth was only 1660m so round trips took less time. Although they were only on site for 6 days, they were rushed; they did not have time to core; and they did not have time to carry out the downhole measurements program or offset VSP.

      So based on prior experience with OSN-1 and NERO, the weather conditions that we can expect in the North Pacific in December, as well as the drilling record for young Pacific crust, it would seem that three weeks on site is not unreasonable.

      On the OSNPE the casing size was 11.75". (This is nominal OD, with a nominal ID of 10.8" and a 10.724"drift. "Drift" means that they can actually pass a test cylinder of a given length of this diameter through the casing.) For a variety of reasons the ODP engineers would prefer to use 10.75" casing on the H2O hole. (This has a nominal ID of 9.95" and a drift of 9.795".) [1. Smaller holes are more stable and easier to drill 2. Their new "standard" is 10.75" since this is all you need for a 9 7/8 RCB bit. 3. 11.75" casing requires a 16" hole which needs an under-reamer which is expensive, difficult and risk prone. 10.75" casing can be placed in a 14.75" hole which can be drilled with a roller cone bit which is cheaper, simpler and more robust (for hard rock). 4. They don't have 11.75" casing on the ship, they may have sold it already and they may not have the tools for it. 5. If we use the 11.75" we would not have the option of three casing strings (there is an intermediate size casing at 13.375" that we may need to use to get to basement.)]

      At OSN-1 we had two casing strings: 16" and an 11.75" (30m and 250m long respectively). On H2O we would like the option of three casing strings (16", 13.375" and 10.75") because drilling is going to be much more difficult. Crust in this part of the Pacific has not been drilled since Legs 5 and 18, which stopped at either basement or chert layers with hole depths of 17m, 24m, 34m, and 156m (compared to a sediment thickness at OSN-1 of 242m). The deepest holes in basement in Pacific crust of comparable age (29Ma versus 46Ma at H2O) were only 25m and 91m at Site 597 (18deg 48'S, 129deg 46'W) with sediment thicknesses of 48 and 53m respectively. But these sites are a long distance away from H2O and may not be representative. In over 1000 sites drilled on the DSDP and ODP programs there are no holes on Pacific crust deeper than 91m in basement and younger than 100Ma. There are only four holes deeper than 50m, three of them on much older crust than H2O. Since Leg 54 there has been a phobia of drilling on young Pacific crust, perhaps well deserved.

      a) Water Depth

      The water depth at the junction box is 4979 m. The maximum relief between sites is 40m.

      b) Sediment Thickness

      See above.

      c) geologic characteristics

      See above.

       i) structure N/A

       ii) reservoirs N/A

       iii) seals N/A

       iv) hydrocarbons N/A

      a) site location on a regional map

      See Figures 8 and 9.

      b) regional seismic line crossing the site

      See Figures 11 and 12 (North Line) for Sites 1 and 4 and see Figures 13 and 14 (South Line) for Sites 2 and 3.

      c) seismic section crossing the site N/A

      d) 3 page safety review check sheet - Attached in 5 page format

10. Additional Seismic Profiles

      Unmigrated and migrated single channel profiles for both the north and south lines are shown at larger scale (6.5-7.3secs as opposed to 6.55-6.75secs) in Figures 17, 18, 19, and 20.

      a) all available bathymetric data

      Bathymetry along the Hawaii-2 cable from the NOAA ETOPO5 (5 minute gridded base) is shown in Figure 21 color or 21 black and white.
The regional swath bathymetry acquired from the R/V Revelle in August 1997 is shown in Figure 22.

Local bathymetry around the H2O site is shown superimposed on the track lines in Figure 23 (annotated in Julian day) and Figure 24 (annotated in shot number).

Raw 3.5 KHz data is shown in Figures 25 and 26 for the north and south lines respectively.

      b) track charts with locations of geological, geophysical, and geochemical data; seismic lines to be reviewed; site locations

      c) Structure maps, sedimentary thickness maps and maps of estimated depth to the base of clathrate horizons

      d) seismic reflection data sufficient to defend the safety of each site

      e) seismic refraction, gravity, and magnetic data

      Satellite gravity data is shown at global, regional and local scales in Figures 27, 28 and 29.

      f) hydrocarbon occurrences at near-by boreholes and wells

      g) international jurisdiction and extent of oil leases

      h) core and dredge descriptions

      i) regional geologic maps and cross-sections

      j) interval velocity information

      k) crustal age

      Crustal age is shown in Figure 30 and 31.

12. Acknowledgements

13. References