Earthquake Prediction Research in a Natural Laboratory

Subproject 4: Borehole Monitoring of Fluid-Rock Interaction

Contractor: F. Roth, Section "Rock Mechanics and Stress Field of the Earth's
                              Crust",
                     Div. "Geomechanics and Management of Drilling Projects",
                     GeoForschungsZentrum Potsdam (GFZ),
                     Telegrafenberg A34, D-14473 Potsdam, Fed. Rep. of Germany
                     Tel.: +49-331-288-1530, Fax: +49-331-288-1534,
                     roth@gfz-potsdam.de

Subcontractor: V. Stefa'nsson, Orkustofnun (OS; National Energy Authority),
                               Grensa'svegur 9, IS-108 Reykjavi'k, Iceland
                               Tel.: +354-569-6063, Fax: +354-568-8896,
                               vs@os.is

Report 1

In the framework of the EU-project "Earthquake Prediction Research in a
Natural Laboratory", a pilot study has started to obtain a time series of logs
in the South Iceland Seismic Zone (SISZ).
An 1100 m deep borehole (LL-03; "Nefsholt") inside the zone (63.92 N, 20.41 W,
7 km south of the seismological station SAU)
is used and provides the unique opportunity to perform measurements much nearer
to earthquake sources than usual - the hypocenter depths at that location range
between 6 and 9 km.
Moreover, data can be obtained for a depth interval of more than 1000 m,
uninfluenced by the sedimentary cover and less disturbed by surface noise.

In the preparational phase of an earthquake, stress accumulation is expected
to be connected with the creation of borehole breakouts, changes in the number
and size of cracks, a possible variation of the stress direction etc.
Therefore, the following set of geo-parameters is monitored:
+ p-wave travel time,
+ electrical conductivity,
+ water content and porosity,
+ stress information from borehole breakouts (orientation and size),
+ crack density, crack opening.

This is achieved by repeated logging with tools as:
- sonic log (BCS),
- dual induction/latero log (DIL),
- neutron log,
- four-arm-dipmeter (FED) and
- borehole televiewer (BHTV).
The neutron log is run with the logging equipment of OS, the rest with the
Halliburton logging truck of GFZ.

Temporal changes visible in these logs will be correlated with data obtained
by other methods used in the whole project, as there are: seismicity,
anisotropy observed in S-waves, crustal deformation, gravity etc.


During winter and spring 1996, the BCS, DIL and neutron tools were checked for
azimuthal isotropy in their sensitivity, as it cannot be guaranteed that the
tools follow the same spiral path through the well in each log run.

Task 1:
Between April and June, OS checked the condition of the borehole selected for
repeated logging - at first this was well NG-01 -, organised and supervised
cleaning of the drillhole, and also arranged the opportunity to log in other
wells on Iceland.

The data of the selected borehole are:

Borehole:          LL-03 at Nefsholt, South Iceland Lowlands,
                         inside the South Iceland Seismic Zone, provided from OS
Drilled:           1977
Position:          63.92 N, 20.41 W
Depth:             1108 m (originally drilled down to 1309 m)
Casing:            0 - 28 m, 12.5" diameter
Uncased:           28 - 1108 m, 8.5" diameter
Used section:      80 - 1108 m, i.e. 1028 m
                   with basaltic lava flows and interbedded hyloclast (tuff)
Mud density:       1.0 kg/dm**3 (water)
Max. temperature:  105 C

Task 2 to 3:
Two field campaigns took place in July 1996, a third in October '96.
More (Task 4) are to follow in 1997.

Moreover, additional logs could be run by us in July '96 in 5 other boreholes
on Iceland to add new information to previous results on the regional stress
field.

The following table gives an overview on the logging activities:

Name     Location      max. Depth  Logged Depth Interval      Tools used
                                       about

NG-01   O'lafsvellir     1070 m      180 - 1070 m        FED, GR, 3-Arm-Caliper,
        (inside SISZ)                                    16"- & 64"-Resistivity,
                                                         SP

HS-36   Reykjavi'k        980 m      330 -  980 m        BHTV, BCS, GR

LPN-10  Laugaland         890 m       80 -  880 m        BHTV, BCS, DIL, GR
        near Akureyri,
        North Iceland

LJ-08   Sydra Laugaland  2740 m      120 - 1890 m        FED, BCS, DIL, GR
        near Akureyri                120 - 1330 m        BHTV

TN-02   Ytri-Tjarnir     1370 m      260 - 1370 m        BCS, GR
        near Akureyri

LL-03   Nefsholt         1309 m       80 - 1100 m        BHVT, BCS, DIL, GR,
        (inside SISZ,                                    Neutron-Neutron,
        site of repeated                                 X-Y-Caliper, SP,
        logging)                                         16"- & 64"-Resistivity
----
Remarks:
GR indicates Gamma-Ray-Log, SP stands for Spontaneous Potential.
As borehole NG-01 partly collapsed between log runs, the hole was abandoned
and well LL-03 was chosen for repeated logging.
For technical reasons, no FED run was possible in LL-03,
                       no BHTV run was possible in LJ-08 below 1330 m depth.
Due to the limited availability of a crane, no BHTV or FED run were possible
in TN-02.
The deepest parts of wells LJ-08 and LL-03 were not accessible anymore.
-----------------------------------------------------------------------


Task 5:
The processing of the data has begun, even though until now (Nov. '96) no
qualified scientist could be hired for the project. All logs add up to about
46 km logged intervals. This is due to the number of holes, the number of tools
used, the number of repetitions of logs in LL-03 and the fact that in LL-03
several logs of the same kind were performed immediately one after the other.
The latter is done to get information on the scattering of data in short time
periods during which no changes due to tectonic processes are expected to have
occurred.

The preprocessing of the data, especially merging of segments and depth
matching, is done for all data. All BHTV data were converted and are being
plotted.

Wellbore breakouts were found in LL-03 and LJ-08 at least. In NG-01, there are
sections with cavities, where the large diameter indicates a breakout, but the
small diameter does not have bitsize. This has to be checked more carefully to
decide whether these cavities may be considered as breakouts induced by
anisotropy in tectonic stresses.

Figures: Yet to be decided


Publications: A report on the outline of the project was given at a German
              workshop on borehole geophysics and rock physics. An extended
              abstract of six pages on this presentation is submitted for
              publication in a special issue of the proceedings of the
              German Geophysical Society.

Meetings of V. Stefa'nsson with F. Roth took place in April and September
         in Reykjavi'k and in September in Potsdam.
Meetings of P. Einarsson with V. Stefa'nsson and F. Roth took place in April and
         September in Reykjavi'k.

                                   ---...---

New proposal:

Subproject 4: Borehole Monitoring of Fluid-Rock Interaction

Contractor: F. Roth, Section "Rock Mechanics and Stress Field of the Earth's
                              Crust",
                     Div. "Geomechanics and Management of Drilling Projects",
                     GeoForschungsZentrum Potsdam (GFZ),
                     Telegrafenberg A34, D-14473 Potsdam, Fed. Rep. of Germany
                     Tel.: +49-331-288-1530, Fax: +49-331-288-1534,
                     roth@gfz-potsdam.de

Subcontractor: V. Stefa'nsson, Orkustofnun (OS; National Energy Authority),
(Assoc. Contractor)            Grensa'svegur 9, IS-108 Reykjavi'k, Iceland
                               Tel.: +354-569-6063, Fax: +354-568-8896,
                               vs@os.is


Application for a second phase,     2 years, from 3/1998 to 2/2000


Objectives

Although much progress has been made during the past decades in investigating
the nature of active faults, most of this progress has involved kinematics
issues. These can be observed directly using numerous geological, geophysical
and geodetic techniques. The dynamics of the processes at faults are far more
elusive and difficult to characterise using the established methods of earth
sciences. Neither the applied stresses nor the rheological response to these
stresses are observable using surface-based instruments or techniques because
of the depths within the lithosphere at which critical processes occur. The same
applies to pore fluids, their presence and temperature, their composition, their
physical and chemical behaviour, their pressure and the rock permeability
in situ.

Key questions in earthquake prediction research (EPR) are still unanswered,
for instance
- What forces, or stresses, are required to cause fault slip?
- Are active fault zones weak? If so, why?
- What factors determine whether a fault is seismically active or aseismic?
- What is the role of fluids in fault processes and where do they originate?
- How does fault zone behaviour change with depth?
- How do geophysical observations relate to fault zone properties?
- Are there fundamental differences between faults in oceanic versus
  continental settings? If so, what causes these differences?


In the framework of the first phase of the EU-project
"Earthquake Prediction Research in a Natural Laboratory",
a pilot study has started in spring 1996 to obtain a time series of logs in the
South Iceland Seismic Zone (SISZ).
An 1100 m deep borehole (LL-03; "Nefsholt") inside the zone (63.92 N, 20.41 W,
7 km south of the seismological station SAU)
is used and provides the unique opportunity to perform measurements in a fault
zone, much nearer to earthquake sources than usual -
the hypocenter depths at the location range between 6 and 9 km.
Moreover, data can be obtained for a depth interval of more than 1000 m,
uninfluenced by the sedimentary cover and less disturbed by surface noise.

In the preparational phase of an earthquake, stress accumulation is expected
to be connected with crustal deformation, the creation of borehole breakouts,
changes in the number and size of cracks,
movement of fluids combined with heat transport and poro-/thermo-elastic
stresses, a possible variation of the stress direction etc.
Therefore, the following set of geo-parameters is monitored:
+ p-wave and s-wave travel times,
+ electrical conductivity,
+ water content and porosity,
+ stress information from borehole breakouts (orientation and size),
+ crack density, crack opening.

This is achieved by repeated logging with tools as:
- sonic log (BCS),
- dual induction/latero log (DIL),
- neutron log,
- four-arm-dipmeter (FED) and
- borehole televiewer (BHTV).
The neutron log is run with the logging equipment of OS, the rest with the
Halliburton logging truck of GFZ.

Emphasis is laid on the detection of changes in the above mentioned parameters.
Nevertheless, from the logs and from combining information from several log
types, further rock physical parameters can be deduced in several ways under
model assumptions:
density, elastic parameters of the rocks, permeability, layering,
bedding planes, rock types etc.

Presently, in the first phase, logs obtained in the initial logging campaigns
(three up to the end of 1996) are analysed. This includes
> correlation of several log runs in one campaign to obtain a value for the
  precision of the measurements,
> correlation of logs from different campaigns to look for temporal variations,
> search for anomalies via a comparison of different log types and via cross
  checks between the series of logs and data bases or time series obtained in
  other experiments as there are: seismicity, fault plane solutions, shear wave
  splitting, surface deformation, gravity, borehole strainmeter recordings etc.

In the second phase proposed now, the sequence of logs should be continued with
another nine campaigns in 1998 and 1999. In addition, emphasis will be laid on
forward modelling of effects observed. Data on rock types around the borehole
and neighbouring wells are gathered and will be compared to published
laboratory data on physical properties of the rocks (constants of elasticity,
specific resistivity, strength, density, porosity etc.; e.g. Hellwege and
Hellwege in Landolt-Boernstein, 1982, Carmichael in CRC-Handbook of Phys. Prop.
of Rock, 1989, Lockner in AGU-Handbook of Rock Physics, 1995 etc.).
These data will be used with source parameters of earthquakes below the
drillhole and information on pumping in other wells of the area to calculate
effects of natural and man-made influences (changes in temperature, load,
stress, crack density) on the site of the borehole. Results will be compared to
those obtained from the logs.


The project will provide information on the state of stress of the rock
near the borehole and about varying water content in cracks. As part
of the multi-method approach to monitor pre-, co- and postseismic stages in the
SISZ, these experiments are thought to provide essential additional information
on the critical state of processes in the Earth's crust in a seismic cycle.


Contractor:        Frank Roth, Section "Rock Mechanics and Stress Field of the
                   Earth's Crust", Div. "Rock Mechanics and Management of
                   Drilling Projects", GeoForschungsZentrum Potsdam, Germany;
Subcontractor:     Valgardur Stefa'nsson, Orkustofnun (National Energy Authority
                   of Iceland), Reykjavi'k, Iceland.

Keywords: stress, cracks, fluids, failure, physical properties of rocks,
          in-situ monitoring, repeated logging


-------------------------------------------------------------------------------

Work content


The data of the selected borehole are:

Borehole:          LL-03 at Nefsholt, South Iceland Lowlands,
                         inside the South Iceland Seismic Zone, provided from OS
Drilled:           1977
Position:          63.92 N, 20.41 W
Depth:             1108 m (originally drilled down to 1309 m)
Casing:            0 - 28 m, 12.5" diameter
Uncased:           28 - 1108 m, 8.5" diameter
Used section:      1028 m, i.e. 80 - 1108 m
                   with basaltic lava flows and interbedded hyaloclast (tuff)
Mud density:       1.0 kg/dm**3 (water)
Max. temperature:  105 C


To monitor changes in physical rock parameters and the migration of fluids due
to tectonic activities, it is of crucial importance that other changes are as
small as possible or can be discriminated from the interesting ones. As the
borehole is already 19 years old, drilling induced changes in the formation
will have strongly diminished by all experience. Further, without tectonic
activity, one would assume that the borehole will have reached a rather
stationary state with the surrounding rock, concerning for example temperature
or diffusion of the drilling mud into the formation and diffusion of formation
fluids into the borehole, respectively. Logging tools whose signals penetrate
to some depth into the formation, as induction, sonic and neutron log, will
permit to discriminate between deeply reaching effects and those confined to 
the borehole wall.
Concerning the tools, it is important to achieve a high resolution of
1) the measured signals,
2) the logging depth, and
3) the azimuth from where the signals originate.


The logging tools available are:

GFZ: Logging truck, winch with 7000 m cable, Halliburton DDL-1 logging system 
    + Borehole Compensated Sonic tools (BCS)
    + Dual Induction/Latero Log tool (DIL, including Spontaneous Potential tool)
    + Borehole Televiewer (BHTV)
    + Four Electrode Dipmeters (FED: Dipmeter and 4-Arm-Caliper)

OS:  Logging truck, winch with 5800 m cable, domestic logging system
    + Neutron-neutron logging tools
    + Short Normal (16") and Medium Normal (64") Resistivity logging tools

Both logging systems are different, so that only the BHTV with data acquisition
on a PC can be operated with both logging trucks and cables.

With these tools, we intend to monitor the following geo-parameters (tool to be
used)

- p- and s-wave travel time, porosity (BCS)
- resistivity at different distances from the borehole (DIL and
                                                        16"/64" Resistivity Log)
- stress information from borehole breakouts (BHTV, FED)
- porosity / water content (NNL)
- crack density & crack closing/opening (BHTV, FED), and

Concerning the azimuthal sensitivity, two tools, BHTV and FED, are supplied
with a navigation subunit that determines azimuth and inclination of the tools.
The sensitivity of the other tools, DIL, BCS and the Normal Resistivity tools,
is only slightly anisotropic.


The time schedule is as follows:

Cam-      Task                        Tools           Duration   Scheduled
paign                                                  [days]    in month #


7-11   checks for changes             all tools         5x3      3,4,5,6,7

12-15  checks for changes             all tools         4x3     15,16,17,19


Task 1: Logging according time schedule above.

Task 2: Cross correlation of logs from different campaigns.

Task 3: Comparison of changes in logs of different type.

Task 4: Comparison of changes in logs with changes in seismicity, fault plane
        solutions, shear wave splitting, gravity,
        borehole strain meter readings, crustal deformation etc.

Task 5: Forward modelling of effects of pumping hot water from a neighbouring
        well.

Task 6: Forward modelling of effects of stress increase on rock around the
        borehole.

------------------------------------------------------------------------


"Borehole Monitoring of Fluid - Rock Interaction",             including

"Modeling of the Earthquake Related Space-Time Behaviour of the
 Stress Field in the Fault System of Southern Iceland"

Period: 3/1998 - 2/2000, i.e. 2 years

Contractor: Dr. Frank Roth, Section 5.3, Division 5, GFZ Potsdam, F.R.Germany