PBO Borehole Strainmeter Workshop Bozeman MT 24-26 Sept 2007

1. PBO Borehole Strainmeter WorkshopBozeman MT 24-26 Sept 2007 Summary by Evelyn Roeloffs and Kathleen Hodgkinson

2. Participants • UNAVCO/PBO: • Kathleen Hodgkinson, Dave Mencin, Wade Johnson, Brent Henderson, Sara Venator • Mick Gladwin, GTSM Technologies • Amy Day-Lewis, Stanford University • From PBOSC: • Evelyn Roeloffs (USGS) • Herb Dragert (GSC) (joined for 30 minutes via teleconference) • University of Utah: Bob Smith and Taka’aki Taira • PBO Data Products Committee: John Langbein (USGS) • Wendy McCausland (USGS)

3. Topics • Long-term trends in borehole strainmeter data • Poroelastic model (Amy Day-Lewis) • Current PBO practice (Kathleen Hodgkinson) • Montserrat dilatometer observations (Taka’aki Taira) • Strainmeter recordings of teleseisms (Wendy McCausland) • Strain data in 5-16 day period band (Evelyn Roeloffs) • Strainmeter performance and installation practices (entire group) • Tidal band (Kathleen Hodgkinson) • Estimating trends and offsets in correlated data (John Langbein) • Yellowstone: field trip to drilling locations • Special considerations for Yellowstone • Questions/Concerns from Herb Dragert

4. Long-term trends : Poroelastic model • Amy Day-Lewis described her Ph.D. thesis work: • Physically-based model • Attributes trends to poroelastic re-equilibration of pressure gradients induced by drilling into a pre-existing stress field • Decomposes the long-term strain of each gauge into • A uniform contraction or expansion common to all gauges, with arbitrary functional form • A long-term variation whose time function is common to each gauge, but whose amplitude and sign vary as cos 2J , where J is the gauge angle relative to maximum horizontal compressive stress • How well does it match the PBO BSM data: • Long term trends at 5 strainmeters are consistent with the model • 8 strainmeters have data problems that preclude fitting the model • 8 work if gauge ordering is changed (further discussed later)

5. Long term trends: Poroelastic model, 2 • Action item: Kathleen and Evelyn will investigate whether non-uniform gauge weightings will allow Amy’s model to fit the data without renumbering the gauges (by November 1) • Action item: Meanwhile Kathleen is double-checking this week to be sure there is no mislabeling of data streams • Action Item:Mick will check labeling of data streams in logger as soon as possible

6. Current PBO detrending • Kathleen Hodgkinson described PBO’s current practice: • fit sum of 2 decaying exponentials to individual gauge data • independent of gauge ordering and mechanisms • Chosen because convenient and effective for pre-PBO borehole strainmeter records • Effectiveness of this method for PBO: • 20 strainmeters exhibit this “classic” type of trend • 19 strainmeters do not exhibit this trend • Alternative approaches for PBO to consider: • A physical model, such as Amy’s • A numerical model such as filtering into separate frequency bands (I.e., PBO-supplied trend would be low-pass filtered data) • PBO does need to estimate a trend, although experts may elect not to use it

7. Action items on PBO detrending: • Choose a subset of the installations to test different methods • Subset should include a well functioning strainmeter (B004), as well as a strainmeter that does not follow the classic trends • Compare residuals using • poroelastic model, • the double exponential (current method) • Filtering into different frequency bands • Need to experiment with filter cutoffs, nominal value is 6 months • Kathleen hopes to complete these by mid-December

8. Strainmeter recordings of teleseisms • Wendy McCausland presented strainmeter recordings of seismic waves from large, distant earthquakes. • 15 Aug 2007, M8.0 Peru earthquake on groups of: • on a well-functioning strainmeter • a nearby strainmeter that is poorly coupled at tidal frequencies • a broadband seismometer • First-order observations: • B001 and B006 record identical strain seismograms on all gauge, implying they may be “hydrologically coupled” • The poorly coupled strainmeters recorded teleseismic waveforms similar to the well-functioning strainmeters • For most strainmeters, the Love wave signals on CH1 and CH3 are equal and opposite, as expected for orthogonal gauges, implying the gauges are in fact properly labeled • P403 is a possible exception

9. BSM recordings of teleseisms, 2 • Strain seismograms were resolved into areal, shear, and differential extension • Love and Rayleigh waves appeared on the proper components • Implies strainmeter orientations not too far off • Non-equal gauge weightings were investigated for B018 • Bench measurements, equal weights, weights based on atmospheric pressure response • Differences between areal strains from 2 orthogonal gauges, vs 3 equally-spaced gauges, were a significant fraction of the areal strain signal for all weights • Differential extensions computed with the two subsets of gauges agreed well for all sets of weights • Problem may be that the areal coupling is low for this instrument (and other northern WA instruments)

10. PBO Borehole Seismometers • Action item for PBO: Determine the orientation of the downhole seismometers • Contract out, if necessary • Data are not easily useable by the community until this is done. • This is more routine than determining strainmeter orientations from data • Kathleen will scope this project and determine by November 1 if it should be contracted out

11. Strain data in 5-16 day period band • Evelyn Roeloffs presented plots of the strainmeter data bandpassed to the 5-16 day band. • Expectation is that all 4 channels should track atmospheric pressure • Amplitude differences due to different gauge gains and/or nonuniform strain caused by atmospheric pressure • Results: • 6 stations track atmospheric pressure as expected • This is a potential source of gauge weightings • Coastal stations generally track atmospheric pressure, but not in detail • Several stations do not track atmospheric pressure in this band • May influence the determination of pressure coefficients

12. Atmospheric Pressure Response: Action Items • Intercomparison of atmospheric pressure coefficients • Evelyn will provide the atmospheric response coefficients from the 5-16 day band (using GTSM barometer) • John will provide coefficients from his method • Kathleen will compare with results from Baytap • Flatness of pressure response across frequencies needs to be checked • Including the 1 sps data from the Setra barometers, for sites where this is available

13. Tidal frequency band • Kathleen summarized behavior of PBO GTSM’s in the tidal band • Time stability of phases: • There is ~ 5 degree variability with time in the M2 areal phase for most gauges of most instruments. • 5 degrees is a good metric for channels that are more than 3 months from installation • Some stations have more variability than this. • B011,B035, B073 • Time stability of amplitudes: • The Vancouver Island sites have a quasi-seasonal variation of amplitudes • Action items: • Kathleen will investigate getting the tides from subsets of 3 out of the 4 gauges, in late December 2007

14. Tidal frequency band, 2 • Kathleen summarized results of fitting PBO BSM tidal observations to tidal models using a fully general matrix of response coefficients • Termed the “Hart et al. Method”, after the JGR 1996 paper comparing the Pinon Flat GTSM with the nearby LSM • Generally Hart et al. Method gives good match to predicted tides, but coupling matrix may be hard to reconcile with published physical model of strainmeter • Where tides cannot be reconciled, this indicates a problem with the instrument, tidal model, or site conditions • Largest differences are for coastal sites in PNW

15. Tidal Band: Self-consistency • Simple physical model of strainmeter is that results from the 2 orthogonal gauges should be consistent with (ie, the same as) those from the three equally spaced gauges • Requiring this equality is in principle a way to determine relative gauge weights • 9 of 23 sites fail consistency for the areal strain M2 tide • Action items: • are these the same sites where Amy found the gauges needed to be reordered? • Try using L1 norm in inverting for gauge weights • Post theoretical amplitudes and phases for each site so all investigators can compare with the same model

16. Calibration • The information required to convert the high-frequency data to physical units will be added to the dataless seed volumes that PBO currently produces • The Hart et al method should be considered as an option for converting instrument strain to formation strain • Almost always yields a calibration • the results may be nonphysical • Presumes the tidal model is the standard for calibration • Action item: Test at Anza strainmeters • tides are known from the PFO LSM • modeled tides are close to correct

17. Questions/Concerns from Herb Dragert • Latency of O&M visits • 3 visits per year would be optimal during the first year after installation, according to Mick Gladwin (for strainmeter only, not including comms) • Thereafter once per year is needed for strainmeters • O&M is done on a resource available basis while the contruction phase is ongoing • PBO will allow PGC to perform some maintenance on the Vancouver Island sites

18. Back-analysis of strainmeter environments • Action item: we should evaluate instrument performance against logging information about the boreholes and cores, where available • In particular, compare Parkfield (poor performance) and Anza (generally good performance) • For example, the logging information could be used to determine rock modulus • Appears to require simple post-processing of the logging data with software PBO already has • We need better high-resolution photos of the cores, where available, and in general we need to utilize the cores to understand the performance of the installations

19. Current Installation Protocol-Pore Pressure • Sites are being installed sans pore pressure infrastructure at the request of the PBO SC and with the concurrence of Mick Gladwin • Sites recently installed in Parkfield without pore pressure infrastructure are not functioning better than those with pore pressure • Yellowstone sites will not have pore pressure due to having tiltmeters and heat flow tubes • If pore pressure infrastructure is to be installed, then the protocol needs to be such that that procedure cannot be mask the cause of the instrument failures • For example, waiting several days before installing the pore pressure • The packer complicates the pore pressure installation and leaving it out, or not inflating it, may be the best approach for now

20. Trend and Offset Estimation in Strainmeter Data • John Langbein described methods he has developed for estimating trends and offsets simultaneously using spectral analysis. • John explained the process of finding and estimating trends in an automated and statistically robust fashion using routines he developed for GPS data. • Techniques are based on spectral analysis and comparison of automatically processed data and carefully manually cleaned data showed little difference. • The advantage of John’s method is that it is an automatic method of estimating offsets which will be an essential process when PBO has data streaming from 412 BSM data channels. • Action item: Kathleen will implement John’s software and compare offsets already estimated by PBO for B012 to those estimated using John’s techniques.

21. Yellowstone Strainmeters: • High profile dictates special considerations • National Park management : • ultra-conservative on possibility of environmental damage • Will monitor progress/success closely; could retract permission • Accustomed to working with Yellowstone Volcano Observatory (YVO), which is mostly U. Utah and USGS • The following people will be interfacing directly with YNP management and need clear and timely information: • Hank Heasler, Park Geologist, YNP • Bob Smith, U. Utah • Taka’aki Taira, U. Utah • Dave Mencin will be the point of contact for Yellowstone strainmeters