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PBO Borehole Response to
Atmospheric Pressure
Strain-tilt Short Course
June 2008
Surface Load From Atmospheric Pressure
Changes
Features in 4-16 day band
• No tidal signal expected
• Long-term trends removed
• Expect mainly atmospheric pressure unlesssome other form of strain is being imposed
• Atmospheric pressure has equilibrated overthe ocean
– Distribution of loading should be uniform withfrequency in this band
Atmospheric pressure load is fairly
uniform spatially
-10
-5
0
5
10
15
7/30/06 8/13/06 8/27/06 9/10/06 9/24/06 10/8/06
PBO BSM BarometersBandpassed 4-16 Days
b004_baro_pltp403_baro_pltb005_baro_pltb007_baro_pltb009_baro_pltb012_baro_pltb018_baro_pltb022_baro_plt
hPa
Example:
B004,
Hoko Falls
• Entire band 4-
16 days
• 4 channels
track pressure
• Similar
amplitudes all
4 channels
B004, Hoko Falls: Sub-bands, Apr & May 2007
• Elliptical paths indicate phase lags
– At these periods, seems unlikely to be an instrument effect
• Relative gains of gauges vary between bands
B004, Hoko Falls: Sub-bands, June & July 2007
• Gain differences not same as Apr&May
• Relative amplitudes vary between bands
B004
Pressure
Response vs
Dates
B005, Shores 1: Sub-bands, Apr & May 2007
• Phase lags seem smaller than at B004
B005, Shores 1: Sub-bands, June & July 2007
• Gain differences similar to Apr&May
B005 Pressure
Response vs
Dates
B005: Selection of Data• B005 turns out to
have a bigseasonal signal,especially onCH1
• This is why onedoes not get goodtidal analyses onarbitrary 90-daystretches of data
49985000
49990000
49995000
50000000
50005000
50010000
50015000
7/1
/05
0:0
0:0
0
1/1
/06
0:0
0:0
0
7/1
/06
0:0
0:0
0
1/1
/07
0:0
0:0
0
7/1
/07
0:0
0:0
0
1/1
/08
0:0
0:0
0
B005_CH1
B
A
3/22/06
11/8/06
9/17/05
11/20/05
6/13/06
6/21/06
6/9/07
10/21/07
11/20/06
1/5/07
Example:
B009,
PGC 1
• Entire band 4-16
days
• 4 channels track
pressure
approximately
but not in detail
• All coastal
stations seem to
behave this way
B009, PGC1: Sub-bands, June & July 2007
• This behavior would seem likely to impede accurate determinationof atmospheric loading effects
• Are long-period signals in the tide gauge data?
B035 and B036 Grants Pass OR
• These two sites about 100 m apart behave
similarly in that they do not track atmospheric
pressure much of the time
B035, Grants Pass1: Sub-bands, Aug-Sep 2007
• There seems to be a strain signal in the 13-15 day band that doesnot come from the atmospheric pressure
– It could be a shear strain signal because CH1 and CH3 are mirror images
B036, Grants Pass2: Sub-bands, Aug-Sep 2007
• In 13-15 day band there is a signal similar to B035
– As at B035, CH1 and CH3 are nearly opposite
• Behavior in all bands resembles B035
B035, Grants Pass1: Sub-bands, Feb-April 2007
• All 4 channels track atmospheric pressure well at this time
• Barometer now has lots of energy in 13-15 day band, and strain channels track it
• No B036 data for this time period
B035 and B036, Grants Pass OR
• Time series plots show a similar signal for B035 and B036 from about 12 July to 1 August on the EN-shear
• Both stations have longer-period variations in the data that are not “self-consistent”
– Probably due to recency of installation at B036 (8 May 2007)
Summary,1• In the 4-6 day passband
– amplitudes of the response change with time at somestations
– relationship between the responses to atmosphericpressure of the 4 different gauges does not chane much
– Implies the product of relative gauge gain andatmospheric pressure-induced strain is approximatelytime invariant in this passband
– Strongly suggests time stability of individual gaugegains
• For passbands at longer periods, the relationshipamong the 4 gauges is not as repeatable with time.
• May be due to difficulty of finding longer periods of datawithout other extraneous signals
Summary,2• Azimuthal variation of atmospheric pressure-
induced strain accounts for little, if any, of thevariation among the gauges.
• For the stations where lab measurements of therelative gains are available, the relative responsesto atmospheric pressure do not correlate with theselab measurements.
• The ratios of the atmospheric pressure coefficientsare not just relative gauge gains - they don’timprove fits to the tidal analysis
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