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

Example:

B005,

Shores 1

• Entire band 4-16

days

• 4 channels track

pressure

• Different

amplitudes

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