CORK Experiments: Long-Term Observatories in Seafloor Boreholes to

Monitor In-situ Hydrologic Conditions and Processes

Keir Becker

Division of Marine Geology & Geophysics, RSMAS, University of Miami, Miami, FL, USA

Earl E. Davis

Pacific Geoscience Centre, Geological Survey of Canada, Sidney, British Columbia, Canada

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Summary: CORK Experiments

A CORK experiment comprises an ODP

reentry hole, cased through the uppermost

sediments, then sealed at the seafloor and

instrumented with a long-term data logger and

sensor string. Over periods of many years, the

CORK monitors in-situ hydrologic conditions

and processes in the open formation of interest

(e.g., basement or decollement) at the base of

the casing. "CORK*1 stands for Circulation

Obviation Retrofit Kit, referring to the fact that

sealing a hole is required to shut off the

hydrologic disturbance which typically results

when a hole is drilled beneath the seafloor.

Sealing the hole then allows slow recovery to

in-situ conditions over periods of several

months after drilling. To date, the downhole

sensor strings have included temperature and

pressure sensors as well as geochemical fluid

samplers, but other sensors such as

hydrophones and seismometers could also be

deployed in future CORKs. Data from CORKs

are usually recovered months to years after

installation, utilizing submersibles or ROV's

for underwater RS-232 connections.

CORKs have been installed at 10 ODP sites

(Fig. 1), in a variety of hydrologically active

seafloor environments. The CORK experiment

was originally developed by a Canadian-

American-ODP team for 1991 ODP drilling at

the sedimented spreading center in Middle

Valley, Juan de Fuca Ridge. Davis et al.

(1992) and Davis and Becker (1993) describe

and illustrate the experiment, and Davis and

Becker (1994) describe initial results from

Holes 857D and 858G. In 1992 and 1994,

CORKs were installed in 4 holes in

accretionary complexes, 2 at Cascadia (Holes

889C and 892B; Davis et alM 1995), 2 at

Barbados (Holes 948D and 949C; Foucher et

al., in press; Becker et al., in press). In 1996,

4 more CORKs were installed in the active

ridge-flank hydrothermal system east of the

Endeavor Ridge (Holes 1024B, 1025B, 1026C,

and 1027B; Davis, Fisher, Firth, et al., in

press), and the 2 original CORKs in Middle

Valley were refurbished.

Examples of pressure and temperatur<

records from CORKs in Middle Valley and th<

Barbados accretionary complex are presentee

herein. These examples document in-situ

formation pressures ranging from 300 kPa

below hydrostatic to as much as IMPa greater

than hydrostatic, and also show temperature

and pressure signals of transient fluid flow

events associated with active hydrologic

processes. In all these examples, the CORK

pressure data show attenuated and phase-lagged

seafloor tidal signals; the attenuation and phase

lag can be used to estimate hydrologic and

elastic properties of the formation behind the

casing (Wang and Davis, 1996). The Barbados

example also illustrates the use of CORKs for

hydrologic testing of the formation at lower

excess fluid pressures than possible using drill-

string packers (Screaton et al., in press).

In 1997, a CORK is planned for installation

in Hole 395A, in young crust west of the Mid-

Atlantic Ridge. Additional CORKs are under

consideration for drilling in western Pacific

sites, particularly the subduction zones off

Japan. These CORKs may require redesign, to

accomodate a more extensive array of

instrumentation, as well as direct connection to

land stations via submarine cables.

Figure 1. Locations of the CORKs deployed through 1996 and planned for 1997. Bathymetric

contours of 200 and 2000 m are shown in addition to coastlines.

Figure 2. Configuration of the two Middle Valley CORKs, as refurbished in 1996. Lines and dots

down the centers of the holes represent thermistor cables and thermistor positions.

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Middle YaUey CORK*

The first two CORKs were deployed in 1991,

during ODP Leg 139, in two reentry holes at

the sedimented spreading center in Middle

Valley, Juan de Fuca Ridge. The puipose of

these CORKs was to study the hydrogeology in

a sedimented ridge-crest hydrothermal system,

with active venting at temperatures of about

260 °C. The two holes, 857D and 858G, are

separated by only 1.6 km, and both are cased

through a continuous sediment cover of several

hundred m (Fig. 2). However, the holes were

drilled into quite different environments, and

encountered quite different basement beneath

the sediments. Hole 858G was drilled to 432 m

depth in the midst of an active vent field, and

penetrates over 100 m into a buried extrusive

edifice that underlies the vent field. Hole 857D

was drilled to 930 m depth away from the vent

field, and penetrates several hundred m of a

sequence of intrusive sills and sediments.

Data from the two CORKs were downloaded

using Alvin roughly 3 weeks after initial

deployment. Longer-term data was recovered

from Hole 858G in 1992 using the ROV

ROPOS, and again in 1993 using Alvin. The

data logger in Hole 857D was damaged during

ODP operations in 1992, and eventually

recovered when both CORKs were refurbished

during Leg 169 in 1996. The recovered logger

yielded more than a year of additional data, up

to the time of damage in late 1992.

Fig. 3 shows the long-term pressure data

from both CORKs, illustrating several

important observations: First, the early periods

of both records are dominated by recovery

from a negative pressure transient in the

borehole, as would be expected after drilling

into a warm formation using cold, dense

seawater as the drilling fluid (see Davis and

Becker, 1994). Second, the record from Hole

857D, the CORK away from the active vent

field, shows a very smooth recovery towards

in-situ pressures, with no change in the

character of the attenuation of the seafloor tidal

signal as seen in the sealed hole. In contrast,

the record from Hole 858G in the midst of the

vent field shows several discrete events, where

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0 100 200 300 400 500 600Time (days)

Figure 3. Long-term pressure records from the

two Middle Valley CORKs.

there are distinct changes in the trend of the

pressure recovery curve as well as the character

of the attenuated seafloor tidal signal. The

causes of these events remain unclear, except

for the final event at about 500 days after

deployment; at that point, pressures dropped

suddenly to the seafloor hydrostatic value, with

a full tidal amplitude, indicating failure of the

seals in the CORK. In fact, when these data

were obtained in 1993, shimmering warm water

was visibly issuing from the CORK, which was

covered with bacterial mat. When the CORK

was refurbished in 1996, the seals were found

to have failed because of high temperatures.

Finally, and perhaps most important, the two

CORKs show very different in-situ formation

pressures. Before the seal failure, Hole 858G

clearly showed a positive pressure, estimated to

extrapolate to an in-situ pressure of 450 kPa

greater than hydrostatic as defined by formation

geothermal conditions. In contrast, the long-

term record at Hole 857D extrapolates to a

strong negative pressure of about 300 kPa,

again relative to hydrostatic as defined by in-

situ geothermal conditions. Such differences in

pressure must reflect subsurface hydrothermal

flow patterns. Assuming that the permeable

uppermost basement in the two holes is

hydrologically connected, the nature of the

pressure difference would initially suggest that

subsurface flow in uppermost basement is

probably directed from the positively-pressured

vent field toward the outlying, negatively-

pressured reference Hole 857D. Providing a

test of the assumption of hydrologic

connectivity between the two sites was one of

the purposes of a hole-to-hole hydrologic

experiment conducted during the refurbishment

of the two CORKs during Leg 169. Data from

this experiment will be recovered using the

ROV JASON during late summer, 1997.

Barbados Accretionary Complex CORKs

In 1994, ODP Leg 156 embarked on a

focused study of the role of fluid pressures and

episodic fluid flow in the Barbados accretionary

prism and their relationship to tectonics.

Critical to this effort was the deployment of

two CORKs in cased holes perforated and

screened across the decollement at the base of

the accretionary prism (Fig. 4). The two

CORK experiments were designed to obtain

long-term (2-3 yr) records of temperatures and

pressures in the sealed holes, to determine

background conditions along the decollement

and to monitor for signs of the episodic fluid

flow events that are now inferred to dominate

the hydrology of accretionary prisms. The Leg

156 CORK program was a joint French-

American-Canadian-ODP effort, culminating

the "ODPNaut" data recovery cruise utilizing

the submersible Nautile in December of 1995

(Becker, Foucher, et al., 1996).

Data recovered from Hole 948D indicated

that the French instrumentation in the hole

worked flawlessly, but the pressure data were

compromised by a failure of the CORK body to

seal in the reentry cone. The CORK sealed

properly in Hole 949C, where American-

Canadian instrumentation had been deployed

(Fig. 5). Pressures in this hole equilibrated to

a value 1 MPa above hydrostatic (Fig. 6),

confirming seismic indications of high fluid

pressures along the decollement. The

overpressure value at Hole 949C is well below

the lithostatic pressure of about 3 MPa, so it is

not particularly significant in tectonic terms,

i.e., in facilitating stress release by fault

motion along the decollement.

Two other aspects of the pressure record in

Hole 949C deserve mention. First, there was

some sort of transient event at 170-250 days,

when the overpressure rose in an irregular step-

wise fashion from a seemingly stable value of

0.9 MPa to a more stable value of 1.0 MPa.

This is the only possible indication in the 17-

month record for any kind of transient fluid

flow event, but temperatures were not

appreciably affected during the same time

period. Second, although the measured

overpressure was otherwise very stable,

indicating a robust seal at the CORK, the tidal

signal seen in the sealed hole had near-full

amplitude and very little phase lag compared to

the signal registered on a seafloor gauge

outside the CORK seal. As described by Wang

and Davis (1996), the attenuation and phase lag

of the tidal signal depend in a complicated way

on elastic and fluid-transport properties of the

sediments; the results at Hole 949C are

consistent with the nature of the muddy

sediments, which have high porosity but

relatively low permeabilty except where

fractures are "opened" by high fluid pressures.

Finally, the ODPNaut experience at Hole

949C also illustrates the use of CORKs for

active hydrologic testing with pumps and/or

flowmeters deployed from a submersible and

connected to the hydraulic sampling valve on

the CORK. This method (also used by Screaton

et al., 1995, at the CORK at Hole 892B) allows

hydrologic testing of the formation isolated by

the CORK or ROV at lower excess fluid

pressures than possible from the drillship using

packers. At Hole 949C, the combination of

packer experiments (Fisher and Zwart, 1996)

and testing during ODPNaut (Screaton et al., in

press) documents an increase of permeability at

the decollement by several orders of magnitude

as fluid pressures vary from hydrostatic to

lithostatic. It is not clear whether the variation

of permeability at the Barbados decollement is

continuous with fluid pressure or occurs as a

discrete change at some critical fluid pressure

value. Additional hydrologic testing to address

this question is planned during a second

ODPNaut cruise planned for 1997.

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Figure 5. Configuration of the CORKinstrumentation in Hole 949C.

Figure 4. Schematic of the Leg 156 drilling program plotted on a line drawing based on a seismic

section across the Barbados accretionary prism. After Shipley, Ogawa, Blum, et al. (1995).

Figure 6. Long-term pressure and temperature

records from the CORK at Hole 949C.

Remarks on Future CORK Prospects

There is considerable international interest in

CORK experiments for drilling now being

planned for the western Pacific. Of special

interest would be possible CORKs for proposed

boreholes in the subduction complexes off the

Japanese coast, particularly the Nankai Trough

and possibly the Japan Trench. The example

CORK data from the Barbados accretionary

complex demonstrate that CORK experiments

in such settings would be important in

documenting in-situ pore-pressure conditions,

in monitoring transient signals due to fluid flow

events, and in determining the relationship

between permeability and fluid pressure along

a decollement. Determining the background in-

situ pore-pressure state and monitoring transient

events at the major fault zones in accretionary

complexes are keys to understanding

earthquakes in this kind of environment and

developing any sort of capability to predict

such earthquakes.

For these future CORKs, there is also

considerable interest in utilizing the holes for

other experiments (e.g., hole-to-hole

tomography), in expanding the range of sensors

(e.g., seismometers and geochemical monitors),

and possibly in linking the experiments to land

yia submarine cables. These factors all argue

for a redesign of the CORK, to reduce the

dependence on the drillship for deployment and

servicing, to avoid diameter limitations on the

sensor strings imposed by the present

deployment method inside the drillpipe, and to

increase access to CORKed holes for other

experiments.

References

Becker, K., Foucher, J.-P., and the ODPNaut

scientific party, 1996, CORK string registers

fluid overpressure, JOl/USSAC Newsletter

9(1), 12-15.

Becker, K., Fisher, A.T., and Davis, E.E., in

press, The CORK experiment in Hole 949C:

long-term observations of pressure and

temperature in the Barbados accretionary

prism, Proc. ODP. Sri. Results. 156.

Davis, E.E., Becker, K., Pettigrew, T.,

Carson, B., and MacDonald, R., 1992,

CORK: a hydrologic seal and downhole

observatory for deep-ocean boreholes, Proc.

ODP. Init.Repts.. 139, 43-53.

Davis, E.E. and Becker, K., 1993, Studying

crustal fluid flow with ODP borehole

observatories, Oceanus. 36(4), 82-86.

Davis, E.E. and Becker, K,, 1994, Formation

temperatures and pressures in a sedimented

rift hydrothermal system: ten months of

CORK observations, Holes 857D and 858G,

Proc. ODP. Sci. Results. 139, 649-666.

Davis, E.E., Becker, K., Wang, K., and

Carson, B., 1995, Long-term observations of

pressure and temperature in Hole 892B,

Cascadia Accretionary Prism, Proc. ODP.

Sci. Results. 146, 299-311.

Davis, E.E., Fisher, A.T., Firth, J., et al., in

press, Proc. ODP. Init. Reports. 168:

College Station, TX (Ocean Drilling

Program).

Fisher, A.T and Zwart, G., 1996, Relation

between permeability and effective stress

along a plate-boundary fault, Barbados

accretionary complex, Geology. 24,307-310.

Foucher, J.-P., Henry, P., and Harmegnies,

F., in press, Long-term observations of

pressure and temperature in ODP Hole

948D, Barbados accretionary prism, Proc.

ODP. Sci. Results. 156.

Screaton, E.J., Carson, B., and Lennon, G.P.,

1995, Hydrogeological properties of a thrust

fault within the Oregon accretionary prism,

Screaton, E.J., Fisher, A.T., Carson, B., and

Becker, K., 1997, Barbados Ridge

hydrogeologic tests: implications for fluid

migration along an active decollement,

Geology, in press.

Shipley, T.H., Ogawa, Y., Blum, P., et al.,

1995, Proc. ODP. Init. Repts.. 156: College

Station, TX (Ocean Drilling Program).

Wang, K. and Davis, E.E., 1996, Theory for

the propagation of tidally induced pore

pressure variations in layered subseafloor

formations, J- Geophva. Res.. 101, 11483-

100, 20025-20035.

11495.

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