Studying the Relationship of Earthquakes and Groundwater ... · temperature in hot, deep boreholes...
Transcript of Studying the Relationship of Earthquakes and Groundwater ... · temperature in hot, deep boreholes...
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Studying the Relationship of Earthquakes and Groundwater Levels in Long Valley caldera, California: Results from a 3-km
Deep Borehole and High-temperature-capable Monitoring Tools
E.Roeloffs1, C.D. Farrar1, J. A. Henfling2, J.Howle1, R.D. Jacobson2, and M. Sneed1
1U.S. Geological Survey, 2Sandia National Laboratories
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Goals of our workInstrumentation Goal:
Develop and test instruments to measure pressure and temperature in hot, deep boreholes over long periods of time (months to years).
Scientific Goal:
Understand the mechanisms of earthquake-induced fluid pressure changes at Long Valley caldera, and use this knowledge to help understand seismic and volcanic processes as well as remotely triggered seismicity.
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Reasons to monitor fluid pressure in hot deep wells
Earthquakes nucleate at depths of several km below the earth’s surface
Future plans for drilling in seismically active areas include deep boreholes
Hydrothermally active areas may also be good places to study the behavior of fault zones at depth
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Long Valley Caldera: A Seismically and Volcanically Active Area in Eastern California
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Resurgent Dome is Actively Uplifting
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Water Level and Strain Changes Are Triggered by
Distant and Local Earthquakes
Persistent water level drops occur in non-thermal wells on and north of the caldera’s resurgent dome (e.g., LKT)
Fluid level increases occur in the caldera’s hydrothermally active south moat (e.g., CW-3).
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Hypothesis about Causes of Fluid-Pressure
Changes:
17 -7 3 13 23
-0.00 0.10 0.20 0.30Volumetric Strain, x10^-6
10 km
POPA
BS01
LKT
SFCH10BCW3RD08
MX01
Earthquake-induced water-level drops on and north of the resurgent dome may be caused by incremental dome inflation
Fluid-pressure increases in thermal wells are due to hot material rising from depth
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LVEW: A 3-km Deep Borehole on the Resurgent Dome
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High-temperature Capable Pressure-Temperature Monitoring Tools
Utilizes downhole silicon-on-insulator electronics
Designed for T=150 C and P=5000 psi (35 Mpa)
Includes:
Quartz pressure-temperature transducer
Strain gage pressure transducer
RTD temperature sensor
Digital (RS232) output
Sampling interval 2.5 s
Built at Sandia National Laboratories
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Earthquake-induced pressure drops occur more quickly in LVEW
Minimum water level is reached in 10 days at LKT
In LVEW, time to reach minimum was <6 hours for Hector Mine and about 2 days for the Denali fault earthquake
Shorter time for LVEW is consistent with a source beneath the resurgent dome (closer to LVEW)
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LVEW Pumping Tests
With shallow transducer, drawdown appears to decrease due to temperature changes
Accurate drawdownsare measured with transducer at2.6 km depth (temperature 102º C, pressure 22.3 MPa).
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Test modeling using Gringarten fracture solution
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Comparison of high-temperature and conventional
transducer
Tidal variations are 1.93 kPa peak-to-peak, and ratio of pressure change to strain based on the M2 earth tide constituent is 30.3kPa/microstrain.
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Cable Problems
After one month, problems developed with the data from the LVEW, because the cable developed a leak and a short circuit formed.
The instrument itself was at high temperature (100C) and pressure (22 Mpa) for 7 months. It is still in good condition and was recently deployed in well 28-34 at Long Valley.
A stainless steel tube with conductors inside is expected to survive better than logging cable at high temperature and pressure and this is currently being tested by Sandia Labs.
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Water-Level Rises in CW-3:
Do not appear to be caused by strain:-well CW-3 has no tidal response, so very large contractionalstrain would be required to produce observed water-level rises
-CW-3 is on CASA-HOT geodetic line, which is extending, not contracting
Alternate hypothesis:
-seismic waves stimulate localized upward movement of hot, pressurized fluid in vicinity of CW-3
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Scientific Questions
Are the fluid pressure increases observed in CW-3 detectable at other sites?
What is the reason for these pressure increases?
Are the pressure increases accompanied by temperature increases?
Are there seismic oscillations that occur before the pressure increases?
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Long Valley Hydrothermal System
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Geothermal observation well 28-34
near CW-3 also exhibits very similar
fluid pressure increases caused by earthquakes
Wells further west do not show the pressure increases…so we believe the pressure increase is localized between CW-3 and 28-34 (the “Fumarole Valley”area)
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Pressure Increases Caused by Landers Earthquake
Increases occurred in wells CW-3 and 28-34 , and stage rose in the Hot Bubbling Pool about 100m from CW-3.
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Need for Temperature Measurements
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Example of Data
22
22.02
22.04
22.06
22.08
22.1
22.12
22.148/11/03 12:00:00 8/12/03 12:00:00 8/13/03 12:00:00 8/14/03 12:00:00
CW-3 Pressure
15.4
15.42
15.44
15.46
15.48
15.5
psi
m of w
ater
50.5
50.55
50.6
50.65
50.7
50.75
8/11/03 12:00:00 8/12/03 12:00:00 8/13/03 12:00:00 8/14/03 12:00:00
CW-3 Temperature
Tem
pera
ture
, Deg
rees
C
Local Time
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SummaryIt is possible to record high-resolution water pressure data at intervals as short as 2.5 s to temperatures as high as 150C and pressures as high as 35 Mpa.
Main challenge is corrosion of the cable, which can probably be avoided using a steel tube to house the conductors.
We characterized the permeability, fracture dimensions, and tidal response of the 3 km deep LVEW to help understand whether earthquake-induced dome inflation is causing post-earthquake water-level drops.
We are recording high-resolution data in two thermal wells in which pressure increases occur following earthquakes.