Transcript PowerPoint Presentation - 12.540 Principles of the Global

GLOBK Velocity Solutions
Thomas Herring
Overview
• Basic velocity solution
• Generating realistic velocity sigmas (eariler
lecture)
• Accounting for earthquakes and site offsets
• Pre-stacking and speeding up results
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Basic velocity solution
• Basic process is:
– Make a file with list of binary h-files (usually .glx
files). These hfiles span multiple years normally
– glist can be used to see list of stations and
durations and check for large differences in
coordinates.
– globk command file add:
• apr_neu all 10 10 10 1 1 1
Assigns 1 m/yr uncertainty to velocities at all sites
– glorg command file add:
• rate_org xtran ytran ztran <xrot> <yrot> <zrot> <scale>
• Entries depend on if rotations and scale are estimated in
globk
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Basic solution: Orbits
• Considerations in globk:
– If ‘Choice of Experiment = RELAX’ in sestbl. then orbits
needed to accounted for.
• Merge with global orbit files (available from MIT as combined
binary h-files)
• Constrain or fix the orbits. Post-2000 IGS orbits can generally be
fixed,
• There are advatages to generated updated h-files with orbits
contsrained or fixed. (Speeds up velocity solution because
apr_svs command not needed in velocity solution).
– If ‘Choice of Experiment = BASELINE’ then orbits are fixed so
do not need to be treated specially.
• With older data (pre-2000) still useful to estimate
orbits. Also depends on next size (<1000km).
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Basic solution: EOP
• When RELAX mode, EOP parameters (apr_wob and
apr_ut1) are estimated; when BASELINE mode used,
they are not estimated in gamit and therefore
orientation of system is fixed.
• If apr_wob/apr_ut1 used, then mar_wob and mar_ut1
should also be used to allow these to vary with time.
• For long baselines and BASELINE mode, these
should be included in command file.
• For short baselines (<1000km) and RELAX mode,
they can be not included (not further constraint), or
constrained (even F is OK).
– When tightly constrained (<0.2 mas), xrot yrot zrot should be
included in pos/rate_org commands
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Basic Solution: Scale
• Treatment of scale is unclear as to best
approach
– For long baseline and long time series, apr_scale
and mar_scale probably should be used until reprocessed IGS products are available.
– Shorter baseline, probably don’t need scale
estimated.
• Suggestion is to try both ways and see impact
on results (good use of OPTION string in
globk and glorg command files).
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Accounting for earthquakes and site
offsets
• Earthquakes and offsets are accounted for with the
eq_file.
• Generally, after and earthquake there is post-seismic
motion which can be accounted for with the eq_post
process noise or eq_log log estimates.
– Parameters for some earthquakes are given in distributed
tables directory. Ideally for other earthquakes, model for
coseismic motion can be used to determine parameters,
Postseismic is often assumed to 10-20% of coseismic.
• The glorg commends equate and eq_dist can be used
to make velocities the same before and after offsets.
Apriori velocity be the same; FIXA org_opt can bs
used but should be checked.
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Offsets
• The RNRP org_opt can be used to get an
estimate of the offset magnitude to see if it is
significant.
• The script sh_exeqs can be used to extract
equate lines from glorg output. These can be
used then in glorg to apply equates.
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Pre-stacking and speeding up results
• With large numbers of days of data and
stations, velocity solutions can take a long
time. Some methods of speeding up runs:
– The globk command use_num can specify the
minimum number of times a site is used. Also glist
output can be used for this (with grep and awk)
– The command decimate to process every n’th file.
With process noise, decimation make little
difference to results. (decimate 7 or even 30 works
well with continuous data).
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Pre-stacking
• Pre-stacking hfiles also works well
• Several approaches here:
– Combination of local and global hfiles to get orbits:
• Orbits loose in combination and then not used afterwards (I.e., no
further constraints)
– Combined hfiles or individual hfiles can be combined in time to
form n-day averages (7 to 30 days work fine).
• With process noise used: 7-30 day combination should estimate
only position and not velocity. MIDP org_opt will refer result to
center of data span.
• Same process noise model should be used when making 7-30
day combination and when combining the combined h-files.
(Statistically, the results are same as direct solution)
• Care needed in duration of combinations during times of large
post-seismic deformation (use shorter combinations)
• With time-stacks, the chi^2 increments will be greater
than 1 because random walk is too step a spectrum.
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Extracting and displaying results
• There are shell scripts to extract results (e.g.,
sh_exglk) and shell scripts to plot velocity field
(e.g., sh_plotvel)
• velview and tsview can be used to display
velocities and time series.
• tsview outputs rename files that can be used
in globk. It can also read an eq_file from
globk to apply breaks and earthquakes.
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Iteration of results
• Common approach is to:
– Run velocity solution and extract coordinates for new apriori
coordinate file.
– Use this new apr_file and “good” stations to generate time
series with glred.
– Use sh_gen_stats to get process noise on the sites and get
lists fo stabile sites. Command sig_neu can used to account
for occasional error such as campaign with bad height,
– Use these estimates to re-estimate velocity and iterate the
system
– Time series can be used to detect outliers (use rename to
<name>_XCL or <name>_XPS in eq_file) and breaks (can
rename in eq_file to <name>_?PS where ? Is letter or
number)
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Summary
• Basic velocity solution: Useful when small
number of days and stations.
• Accounting for earthquakes and site offsets:
– Can be complex with large data sets
– Use source command to allow reading in of
standard values. Runstring option can be used to
have same command file for velocity and
repeatability runs
• Pre-stacking and speeding up results: Useful
with large continuous data solutions. Multiple
techniques can be used.
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