DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY

Transcription

1 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 1 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY R.A. Stephen 1, J.H. Natland 2, R. Butler 3, K. Becker 2, A.D. Chave 1 and F.K. Duennebier 4, April 4, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida Incorporated Research Institutions for Seismology, 1616 Fort Myer Drive (Suite1050), Arlington, VA Department of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, Hawaii

2 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 2 Table of Contents TABLE OF CONTENTS...2 INDEX OF FIGURES ABSTRACT OVERVIEW OF SCIENTIFIC OBJECTIVES...5 A) BASEMENT DRILLING ON THE PACIFIC PLATE...5 B) THE OCEAN SEISMIC NETWORK BACKGROUND INFORMATION AND DESCRIPTION OF GEOLOGICAL SETTING...7 A) THE H2O PROGRAM...7 B) GEOLOGICAL SETTING...8 C) THE CABLE SURVEY CRUISE IN AUGUST D) REGIONAL MAP SHOWING BATHYMETRY, LATITUDE AND LONGITUDE, NEAREST LAND AREAS AND PROPOSED SITE LOCATIONS...10 E) TRACK CHARTS...10 F) SEISMIC REFLECTION AND 3.5KHZ PROFILING LINES...10 G) IDENTIFICATION OF IMPORTANT REFLECTIONS REGIONAL CLATHRATE AND HYDROCARBON OCCURENCES PREVIOUS SAFETY AND POLLUTION PREVENTION STUDIES CURRENT SPEED AND DIRECTION POTENTIAL MAN-MADE HAZARDS DRILLING STRATEGY OUTLINE (SITE-BY-SITE DESCRIPTION)...12 A) NOTES ON DRILL SITES...12 B) NOTES ON CRUISE LENGTH...13 C) NOTES ON CASING SIZES...14 D) WATER DEPTH...15 E) SITE LOCATION ON A REGIONAL MAP...15 F) REGIONAL SEISMIC LINE CROSSING THE SITE ADDITIONAL GRAPHIC ITEMS...15 A) ADDITIONAL SEISMIC PROFILES...15 B) ALL AVAILABLE BATHYMETRIC DATA...15 C) SEISMIC REFRACTION, GRAVITY, AND MAGNETIC DATA...16 D) CRUSTAL AGE

3 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY ACKNOWLEDGEMENTS REFERENCES...16 Index of Figures Figure 1: The Hydrosweep bathymetry around the H2O 19 Figure 2: The repeater locations of the Hawaii-2 cable 20 Figure 3: The location of the Hawaii-2 observatory on Hydrosweep bathymetry 21 Figure 4: The bathymetry from the NGDC - DBDB5 data 22 Figure 5: The H2O location with respect to the track lines for the Revelle during the site survey in Figure 6: The actual site is bracketed by two survey lines. 24 Figure 7: The track lines, annotated in SCS shot 25 Figure 8: The track lines, annotated in Julian 26 Figure 9: The latest 3.5KHz example from the line north of the H2O site 27 Figure 10: The latest 3.5KHz example from the line south of the H2O site 28 Figure 11: The unmigrated SCS profile for the line 29 Figure 12: The migrated SCS profile for the line 30 Figure 13: The unmigrated SCS profile for the line south 31 Figure 14: The migrated SCS profile for the line 32 Figure 15: During installation of the junction boxes cable "wuzzles" were 33 Figure 16: By drilling beyond 1km from the site under the seismic lines, we are assured not to encounter the wuzzles or the cable. 34 3

4 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 4 Figure 17: The unmigrated single channel profile for the south line is shown at larger scale. 35 Figure 18: The migrated single channel profile for the south line is shown at larger. 36 Figure 19: The unmigrated single channel profile for the north line is shown at larger. 37 Figure 20: The migrated single channel profile for the north line is shown at larger. 38 Figure 21: Bathymetry along the Hawaii-2 cable from the NOAA 39 Figure 22: The regional swath 40 Figure 23: Local bathymetry around the H2O site is shown superimposed on the track lines annotated in Julian day. 41 Figure 24: Local bathymetry around the H2O site is shown superimposed on the track lines annotated in shot number. 42 Figure 25: Raw 3.5 KHz data is shown for the north line. 43 Figure 26: Raw 3.5 KHz data is shown for the south line. 44 Figure 27: Satellite gravity data is shown at global scale. 45 Figure 28: Satellite gravity data is shown at regional scale. 46 Figure 29: Satellite gravity data is shown at local scale. 47 Figure 30: Crustal age 48 4

5 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 5 1. Abstract 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 (Chave, Butler et al. 1990), the Ocean Seismic Network (OSN) group (Purdy 1995),, and International Ocean Network (ION) group (Montagner and Lancelot 1995). Drilling at H2O will also provide useful background information for the Borehole Observatories, Laboratories, and Experiments (BOREHOLE) group (Carson, Becker et al. 1996) and the oceanic lithospheric processes community (Dick and Mével 1996). We propose drilling one re-entry hole within 2km of the Hawaii-2 Observatory (H2O) junction box in the Eastern Pacific at ' N, ' W in 4979 m water depth (see Figures 1 and 2) (Butler 1995). 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 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.) a) Basement Drilling on the Pacific Plate 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 (Pringle, Sager et al. 1993). 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 5

6 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 6 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 fastspread crust is both simple and uniform, certainly so in terms of seismic structure (Raitt 1963; Menard 1964). 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 Hole# Leg# Age Location Basement Sediment Comments (Ma) Penetration Thickness N 150 W 18m 176m N 106 W 29m 118m N 106 W 29m 85m 429A N 107 W 21m 31m N 121 W 58m 391m foot of Patton Escarp. 470A N 118 W 48m 167m N 112 W 82m 741m N 114 W 25m 112m 597B S 130 W 25m 48m 5.5cm/yr 597C S 130 W 91m 53m Re-entry cone 599B S 120 W 10m 41m 843B N159 W 71m 243m OSN-1 Hole 6

7 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 7 b) The Ocean Seismic Network 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) (Butler 1995; Butler 1995; Purdy 1995). 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 (Forsyth, Dziewonski et al. 1995). 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 (Butler 1995). 3. Background Information and Description of Geological Setting a) The H2O Program 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 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 7

8 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 8 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 (Chave, Green et al. 1995), a hydrothermal observatory (Davis, Becker et al. 1992; Foucher, Harmegnies et al. 1995), or a broadband borehole seismic observatory (Orcutt and Stephen 1993), 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 (Sutton, McDonald et al. 1965; Sutton and Barstow 1990). For the foreseeable future the most practical method for acquiring real time, continuous data from the seafloor will be over cables (Chave, Butler et al. 1990). 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. b) Geological Setting The Hawaii-2 cable runs south of the Moonless Mountains between the Murray and Molokai Fracture Zones (Figure 2) (Mammerickx 1989). 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 (Winterer 1989). Tectonically, the cable runs across the 'disturbed zone' south of the Murray Fracture Zone, between magnetic isochrons 13 and 19 (Atwater 1989; Atwater and Severinghaus 1989). 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 8

9 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 9 Pacific and Farallon plates under 'normal' spreading conditions at a 'fast' half-rate of about 7cm/yr (Atwater 1989; Cande and Kent 1992). 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 (Leinen 1989). 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 (McManus, Burns et al. 1970). Site 39 is north of the cable at latitude N with an age of 60Ma. It has a sediment thickness of only 17m. Sites 40 and 41 are near the same latitude at 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 ( N, W) at an estimated crustal age of 35Ma (Kulm, von Huene et al. 1973). Sediment thickness above the basaltic basement was 24m. The sediment thickness from seismic reflection profiles had been interpreted as m. The discrepancy was attributed to reverberations and thin sediment cover. c) The Cable Survey Cruise in August 1997 In August 1997 we carried out a survey of the Hawaii-2 cable between 140 W and 143 W (Stephen, Swift et al. 1997). 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 reentry cone, to be in relatively undisturbed 'normal' crust in a plate tectonic sense and to optimize drilling penetration by selecting sites with wellconsolidated 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. 9

10 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 10 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 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 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. 10

11 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 11 g) Identification of important reflections 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. 4. Regional Clathrate and Hydrocarbon Occurences There are no regional clathrate or hydrocarbon occurences. This is an abyssal plane environment at about 40Ma age with very thin sediment cover. 5. Previous Safety and Pollution Prevention Studies There have been no previous safety or pollution prevention studies. 6. Current Speed and Direction 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. 11

12 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 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). 8. Drilling Strategy Outline (Site-by-Site Description) a) Notes on Drill Sites We have a specific site for the observatory at ' N, ' 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 (Figures 7 and 8). The locations are: Site Site Site Site 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 reentry 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). 12

13 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 13 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. b) Notes on Cruise Length 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" reentry 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 " casing to get to basement seems prudent, but we would also need the time the under-reaming requires. 13

14 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 14 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. c) Notes on Casing Sizes On the OSNPE the casing size was 11.75". (This is nominal OD, with a nominal ID of 10.8" and a "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 " casing requires a 16" hole which needs an under-reamer which is expensive, difficult and risk prone " 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 " 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", " 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, 14

15 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY deg 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. d) Water Depth The water depth at the junction box is 4979 m. The maximum relief between sites is 40m. e) Site Location on a Regional Map See Figures 7 and 8. f) 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 Additional Graphic Items a) Additional Seismic Profiles Unmigrated and migrated single channel profiles for both the north and south lines are shown at larger scale ( secs as opposed to secs) in Figures 17 to 20. b) All Available Bathymetric Data Bathymetry along the Hawaii-2 cable from the NOAA ETOPO5 (5 minute gridded base) is shown in Figure 21. 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. 15

16 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 16 c) Seismic Refraction, Gravity, and Magnetic Data Satellite gravity data is shown at global, regional and local scales in Figures 27, 28 and 29. d) Crustal Age Crustal age is shown in Figure Acknowledgements This material is based upon work supported by the National Science Foundation (NSF Grant Number OCE ) and JOI Prime Contract OCE JOI Budget Code J Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or JOI. 11. References Atwater, T. (1989). Plate tectonic history of the northeast Pacific and western North America. The Eastern Pacific Ocean and Hawaii. E. L. Winterer, D. M. Hussong and R. W. Decker. Boulder, Geological Society of America. N: Atwater, T. and J. Severinghaus (1989). Tectonic maps of the northeast Pacific. The Eastern Pacific Ocean and Hawaii. E. L. Winterer, D. M. Hussong and R. W. Decker. Boulder, Geological Society of America. N: Butler, R. (1995). The Hawaii-2 Observatory: A deep-ocean geoscience facility reusing the Hawaii-2 telephone cable. Broadband seismology in the oceans - Towards a five-year plan. G. M. Purdy and J. A. Orcutt. Washington, D.C., Ocean Seismic Network, Joint Oceanographic Institutions: Butler, R. (1995). Proposed station locations and rationale for the OSN component of GSN. Broadband seismology in the oceans - Towards a five-year plan. G. M. Purdy and J. A. Orcutt. Washington, D.C., Ocean Seismic Network, Joint Oceanographic Institutions, Inc.: Cande, S. C. and D. V. Kent (1992). A new geomagnetic polarity time scale for the late Cretaceous and Cenozoic. Journal of Geophysical Research 97: 13,917-13,

17 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 17 Carson, B., K. Becker, et al., Eds. (1996). BOREHOLE - A plan to advance post-drilling, sub-seafloor science. Washington, D.C., Joint Oceanographic Institutions / U.S. Science Advisory Committee. Chave, A. D., R. Butler, et al., Eds. (1990). Workshop on scientific uses of undersea cables. Washington, D.C., Joint Oceanographic Institutions, Inc. Chave, A. D., A. W. Green, Jr., et al. (1995). Report of a workshop on technical approaches to construction of a seafloor geomagnetic observatory. Woods Hole, MA, Woods Hole Oceanographic Institution. Davis, E. E., K. Becker, et al. (1992). CORK: A hydrologic seal and downhole observatory for deep-ocean boreholes. Proceedings of the Ocean Drilling Project (Initial Reports) 139: Dick, H. J. B. and C. Mével (1996). The oceanic lithosphere and scientific drilling into the 21st century. Woods Hole, MA, ODP-InterRidge- IAVCEI. Forsyth, D., A. Dziewonski, et al. (1995). Scientific objectives and required instrumentation. Broadband seismology in the oceans. G. M. Purdy and J. A. Orcutt. Washington, D.C., Ocean Seismic Network, Joint Oceanographic Institutions: Foucher, J.-P., F. Harmegnies, et al. (1995). Long-term observation and testing in boreholes: The ODP Leg 156 experiment. Multidisciplinary observatories on the deep seafloor. J.-P. Montagner and Y. Lancelot. Marseille, INSU/CNRS: Kulm, L. D., R. von Huene, et al. (1973). Initial Reports of the Deep Sea Drilling Project. Washington, D.C., U.S. Government Printing Office. Leinen, M. (1989). The pelagic clay province of the North Pacific Ocean. The Eastern Pacific Ocean and Hawaii. E. L. Winterer, D. M. Hussong and R. W. Decker. Boulder, The Geological Society of America. N: Mammerickx, J. (1989). Large-scale undersea features of the northeast Pacific. The Eastern Pacific Ocean and Hawaii. E. L. Winterer, D. M. Hussong and R. W. Decker. Boulder, The Geological Society of America. N: McManus, D. A., R. E. Burns, et al. (1970). Initial Reports of the Deep Sea Drilling Project. Washington, D.C., U.S. Government Printing Office. Menard, H. W. (1964). Marine geology of the Pacific. New York, McGraw-Hill. 17

18 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 18 Montagner, J.-P. and Y. Lancelot, Eds. (1995). Multidisciplinary observatories on the deep seafloor. Marseille, INSU/CNRS, IFREMER, ODP-France, OSN, USSAC, ODP-Japan. Orcutt, J. A. and R. A. Stephen (1993). OSN seismograph system is underway. Seismic Waves, OSN Newsletter. 2: 3-5. Pringle, M. S., W. W. Sager, et al., Eds. (1993). The Mesozoic Pacific: Geology, tectonics, and volcanism. Geophysical Monograph Series. Washington, D.C., American Geophysical Union. Purdy, G. M. (1995). A five-year plan. Broadband seismology in the oceans. G. M. Purdy and J. A. Orcutt. Washington, D.C., Ocean Seismic Network, Joint Oceanographic Institutions: Raitt, R. W. (1963). The crustal rocks. The Sea. M. N. Hill. New York, Wiley. 3: Stephen, R. A., S. A. Swift, et al. (1997). Bathymetry and sediment thickness survey of the Hawaii-2 cable. Woods Hole, MA, WHOI. Sutton, G. H. and N. Barstow (1990). Ocean-bottom ultralow-frequency (ULF) seismo-acoustic ambient noise: to 0.4 Hz. Journal of the Acoustical Society of America 87(5): Sutton, G. H., W. G. McDonald, et al. (1965). Ocean-bottom seismic observatories. Proc. IEEE 53: Winterer, E. L. (1989). Sediment thickness map of the Northeast Pacific. The Eastern Pacific Ocean and Hawaii. E. L. Winterer, D. M. Hussong and R. W. Decker. Boulder, The Geological Society of America. N:

19 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 19 Figure 1) The Hydrosweep bathymetry around the H2O observatory that was acquired from the R/V Thompson in September The site is on a relatively benign ribbon of "normal oceanic crust. 19

20 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 20 Figure 2) 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. 20

21 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 21 Figure 3) 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. 21

22 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 22 Figure 4) The bathymetry from the NGDC - DBDB5 data base on a regional 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. 22

23 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 23 Figure 5) The H2O location with respect to the track lines for the Revelle during the site survey in

24 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 24 Figure 6) The actual site is to the southwest of a well surveyed block but is bracketed by two parallel single channel seismic lines. 24

25 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 25 Figure 7) The track lines, annotated in SCS shot numbers, are shown 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. 25

26 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 26 Figure 8) The track lines, annotated in Julian time, are shown 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. 26

27 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 27 Figure 9) The latest 3.5KHz example from the line north 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. 27

28 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 28 Figure 10) The latest 3.5KHz example from the line south 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. 28

29 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 29 Figure 11) The unmigrated SCS profile for the line north of this site is shown. A tenth of a second two-way time corresponds to about 75m 29

30 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 30 Figure 12) The migrated SCS profile for the line north of this site is shown. A tenth of a second two-way time corresponds to about 75m 30

31 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 31 Figure 13) The unmigrated SCS profile for the line south of this site is shown. A tenth of a second two-way time corresponds to about 75m. 31

32 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 32 Figure 14) The migrated SCS profile for the line south of this site is shown. A tenth of a second twoway time corresponds to about 75m. 32

33 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 33 Figure 15 ) During installation of the junction boxes cable "wuzzles" were formed within 200m of the site. 5" of longitude at this latitude corresponds to about 136m. 33

34 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 34 Figure 16) By drilling beyond 1km from the site under the seismic lines, we are assured not to encounter the wuzzles or the cable. 34

35 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 35 Figure 17) The unmigrated single channel profile for the south line is shown at larger scale ( secs as opposed to secs). 35

36 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 36 Figure 18) The migrated single channel profile for the south line is shown at larger scale ( secs as opposed to secs). 36

37 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 37 Figure 19) The unmigrated single channel profile for the north line is shown at larger scale ( secs as opposed to secs). 37

38 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 38 Figure 20) The migrated single channel profile for the north line is shown at larger scale ( secs as opposed to secs). 38

39 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 39 Figure 21) Bathymetry along the Hawaii-2 cable from the NOAA ETOPO5 (5 minute gridded base) is shown in black and white. 39

40 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 40 Figure 22) The regional swath bathymetry acquired from the R/V Revelle in August 1997 is shown. 40

41 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 41 Figure 23 Local bathymetry around the H2O site is shown superimposed on the track lines annotated in Julian day. 41

42 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 42 Figure 24 Local bathymetry around the H2O site is shown superimposed on the track lines annotated in shot number. 42

43 Figure 25) Raw 3.5 KHz data is shown for the north line. DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 43 43

44 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 44 Figure 26) Raw 3.5 KHz data is shown for the south line. 44

45 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 45 Figure 27) Satellite gravity data is shown at global scale. 45

46 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 46 Figure 28) Satellite gravity data is shown at regional scale. 46

47 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 47 Figure 29) Satellite gravity data is shown at local scale. 47

48 DRILLING AT THE H2O LONG TERM SEAFLOOR OBSERVATORY 48 Figure 30) Crustal age is shown. 48