There are many
reasons for carrying out the pilot hole project
- Seismic
recording instrumentation deployed in the pilot hole will facilitate
the determination of precise earthquake hypocenter locations that
will guide subsequent SAFOD investigations in the active fault
zone. These subsurface seismic receivers will also record surface
seismic sources and provide depth control for several on-going
and planned crustal imaging experiments, outlined below.
- Downhole
measurements of physical properties, stress, fluid pressure and
heat flow in the pilot hole will characterize the shallow crust
adjacent to the fault zone. These measurements will be used to
help calibrate physical properties inferred from surface-based
geophysical surveys (e.g., seismic velocities, resistivity and
density) and better constrain the thermomechanical setting of
the San Andreas Fault Zone prior to SAFOD drilling.
- Long-term
seismic, pore fluid pressure, strain and temperature monitoring
in the pilot hole will make it possible to assess time-dependant
changes in the physical properties and mechanical state of the
crust adjacent to the fault zone for comparison with similar measurements
to be recorded in the SAFOD hole.
- Approximately
60 m of granite core will be extracted from the bottom of the
pilot hole. The resulting open-hole section (or core hole) will
then be used for downhole measurements of permeability and pore
pressure and obtaining uncontaminated pore fluid samples. Laboratory
studies of these rock and fluid samples will determine the nature
and extent of fluid-rock interaction along the San Andreas Fault
and the sources and transport paths for fault-zone fluids.
- Real-time
seismic monitoring in the pilot hole (and at the surface) during
SAFOD drilling using the drill bit as a seismic source will allow
high-resolution imaging of the San Andreas Fault Zone at depth.
- From a strictly
technological point of view, the pilot hole will provide the opportunity
to obtain information about drilling conditions that will be extremely
valuable in designing and drilling the main SAFOD hole.
Overview
of Operations and Science Plan
Nearly the entire length of the pilot hole will be rotary drilled.
An initial casing will be set at 800 m, after penetrating the
sedimentary section and the uppermost granitic basement. Because
of budgetary constraints, no coring or logging will be done in
this interval. After cementing the 9 5/8" casing,
the hole will be rotary drilled vertically with a 8 3/4"
bit to a depth of 2.1 km (7000'). Again, because of
budgetary constraints, there will be no cores taken in this section
of the hole. However, rock chips (i.e., cuttings) will be continuously
collected, described and logged during rotary drilling.
After drilling to 2.1 km, a fairly complete suite of geophysical
logs will be run, principally by a commercial wireline logging service. These logs will be
supplemented by several geophysical logs collected by the science
team.
All drilling and logging information will be kept in the Drilling
Information System (DIS) data base developed by ICDP and posted
regularly on the SAFOD
website.
After logging, the 2.1 km deep, 8 3/4" hole will be cased
with 7" casing. After this casing has been cemented into place, we plan to collect
~60 m of "HQ" core (6.4 cm diameter) at the bottom of
the pilot hole. A protocol is being developed for how the core
samples will be handled at the site, distributed for study and
archived. We will use the DOSECC top-drive coring
system successfully used in Hawaii and Long Valley.
After coring, logging of the core hole will be carried out with
an ultrasonic borehole televiewer. This will make it possible to
magnetically orient fractures and faults observed in the core.
The core hole will also be used for fluid sampling and measurements
of permeability and the least principal stress.
Upon completion of these measurements, seismic, strain and pore
fluid pressure instrumentation will be deployed in the hole for
continuous monitoring of seismic activity occurring within and adjacent to the San
Andreas Fault Zone.
Geophysical Studies of the Pilot Hole Site
Over the past several years, a wide variety
of geophysical investigations have carried out at and around the
SAFOD site. These studies include:
- Magneto-telluric soundings (Unsworth et al., 2000).
- Gravity and magnetic profiles (Miller et al., 2000).
- High-resolution seismic reflection and refraction profiles (Rymer
et al., 1999; Hole et al., 2000).
- A number of shallow exploration techniques
run at the drill site as part of the NSF-sponsored Parkfield
field camp. Information about the Parkfield field camp is available
at http://www.nicholas.duke.edu/eos/fieldtrips/Parkfield2000/parkfield.htm
.
- major microearthquake experiment – the Parkfield Area Seismic
Observatory (PASO) – is now underway using portable seismic instruments
deployed by Univ. Wisconsin and Rensselaer Polytechnic Institute,
the permanent stations of the USGS Northern California Seismic
Network, and the Parkfield High Resolution Seismic Network run
by U.C. Berkeley. This experiment is described on the web at http://gretchen.geo.rpi.edu/roecker/paso_home.html
.
- Monitoring of the Parkfield region
by the USGS and U.C. Berkeley
as part of the Parkfield Earthquake Experiment continues, with networks of
borehole strainmeters, global positioning system (GPS) receivers, water
wells, creepmeters, magnetometers, high-gain seismometers and strong motion
accelerometers. Work is presently underway to expand the continuous GPS network. Information
about deformation monitoring at Parkfield is available at http://quake.usgs.gov/research/deformation/parkfield/index.html
.
The next phase of the geophysical exploration
of the fault zone and surrounding crust is planned for the fall of 2002, after the completion of the pilot
hole:
- John Hole (Virginia Tech) has been funded by NSF to shoot a 50-km-long reflection/wide-angle refraction
profile at right angles to the fault through the SAFOD site.
His plan is to use conventional and turning-ray reflection methods
and refraction methods to image the P-velocity structure of the
fault and nearby crust. Trond Ryberg (GFZ, Germany) and Claus Prodehl (U. Karlsruhe, Germany)
will expand this active source experiment to image the S-wave velocity
structure.
- Peter Malin (Duke) will instrument the pilot hole with a vertical array of 3-component
geophones that will be used to record both the artificial sources
and nearby earthquakes.
- Cliff Thurber (Univ. Wisconsin) and Steve Roecker (Rensselaer Polytechnic Institute) will set off a series of
calibration shots at the sites of their surface stations to be recorded by
the seismic receivers within the pilot hole in order to test and calibrate
their 3-D seismic velocity model.. By traveltime reciprocity,
this will create a "virtual earthquake" at the bottom
of the pilot hole (Ellsworth, 1996) that will be used to refine
double-difference and tomographic earthquake locations.
This comprehensive suite of geophysical investigations in and around the pilot hole
will achieve a number of critical milestones. These include determination of the
absolute locations of the repeating microearthquakes we will target with the
main SAFOD hole and better defining the overall structure and geophysical
setting of the San Andreas Fault Zone at Parkfield.
Scientific opportunities
The pilot hole project will present opportunities for research in
three general areas:
Downhole Measurements Due to budgetary constraints, only
a modest number of downhole measurements are currently planned for
the pilot hole.
A suite of open-hole geophysical logs will be conducted prior to setting the final casing string. This
will include resistivity, density, porosity, dipole sonic and borehole
imaging logs (both acoustic and electrical) and will provide the
information needed to characterize variations in physical properties,
fracture geometry and stress directions at depths of 0.8 to 2.1
km.
We welcome ideas by interested investigators for additional downhole
measurements after the hole is completed and the drill rig moves off
site, or to conduct detailed analyses of the geophysical logs that
we already plan to collect in this hole. Other already planned downhole
measurements include: 1) repeated temperature measurements (coupled
with thermal conductivity measurements) for heat flow, 2) a vertical
seismic profile (VSP) in the cased and cemented pilot hole to allow
seismic properties measured during geophysical logging and on the
core to be "scaled up" and extrapolated away from the borehole,
and 3) permeability measurements and a single hydraulic fracturing
stress test in the core hole.
Monitoring The plan for completion of the pilot
hole first calls for temporary installation of a 40-level
3-component array of high-frequency geophones (~8 Hz) that will be in place to
record surface sources during the seismic surveys mentioned above. Similar
arrays have been installed for use in the petroleum industry,
and Peter Malin has been working closely with industrial partners to design
and deploy such a system in the pilot hole.
After these surveys are completed, this seismic string will be temporarily
removed and fitted with additional sensors, including:
- Pore pressure monitoring in the uncased core hole.
- Installation of strainmeters and / or tiltmeters.
- Installation of broad-band seismometers and accelerometers.
After reinstallation, this array will be permanently cemented in the pilot
hole for long-term monitoring of nearby seismic activity and variations in
fluid pressure and deformation adjacent to the San Andreas Fault Zone.
Analysis of Core, Cuttings and Fluids - Since the pilot hole will be drilled outside
of the San Andreas fault zone, the opportunities for addressing
many of the basic scientific questions pertaining to the mechanics
of faulting and earthquake generation are relatively limited.
However, we anticipate a few key areas in which important scientific
progress can be made through analysis of core, cuttings, and fluids
obtained from the pilot hole. These include:
- Mineralogical, geochemical and microstructural
studies to determine the geometry, chemical zonation and timing
of vein-filling episodes and their possible relation to the earthquake cycle.
- Geochemical and isotopic investigations of pore water and dissolved gasses - using either bulk water
samples or fluid inclusions - to ascertain the origins, pathways
and transport rates of fluids associated with the fault zone
and the nature and extent of water-rock interactions in the country rock.
- Laboratory rock mechanics studies of the strength and transport properties of country rock, to help
in the interpretation of stress-induced borehole failure and
as "boundary conditions" to hydromechanical models
for the San Andreas fault zone.
- Laboratory studies of P- and S-wave velocities, seismic anisotropy, mineral fabric and microcrack
geometry for comparison with physical properties and stress directions
inferred from downhole measurements and surface-based geophysical surveys.
For Instructions to potential PI's see Scientists or click here.
To learn more about References Cited Above see References or click here. |
