OBS for Land Seismologists II

Transcription

1 OBS for Land Seismologists II Anne Sheehan, University of Colorado at Boulder Input from Josh Stachnik, Zhaohui Yang, Ball, Dan Zietlow, Martha Savage, Fan- Chi Lin, John Collins, Monica Kohler, Spahr Webb - Overview of New Zealand MOANA OBS experiment - Data quality noise spectra, best and worst parts of data set - Orienta@on of horizontal components - Examples of analyses - Lessons learned

2 MOANA OBS Seismic Experiment, New Zealand Marine Observations of Anisotropy Near Aotearoa

3 MOANA OBS Seismic Experiment, New Zealand Marine Observations of Anisotropy Near Aotearoa R/ V Ro g e r R e v e ll e C ruis e 38 S S S S S S 162 E 164 E 166 E 168 E 17 E 172 E 174 E 176 E Deployment offshore South Island of New Zealand - January 29 February 21-3 SIO OBSIP Broadband OBS - 5 sps sample rate - Water depths 55 m 47 m - Recovery: 28 by R/V Revelle (2 lassoed), 1 by trawler, 1 washed ashore - DPG data fine for all, 1 bad OBS

4 SIO Short Period Deployment

5 OBS Specifications Broadband Seismometers: 24 seconds to 5 Hz (SIO); 12 s to 5 Hz (WHOI); Short-Period Sensors (4.5 Hz) : ~1 Hz to tens of Hz Hydrophone: ~1 Hz to >1 Hz DPG: ~1 seconds to a few Hz APG: DC to 2 Hz 24 bit A/Ds. SIO and LDEO use their own designs. WHOI use Q33. Slide courtesy John Collins, WHOI OBSIP

6 Seismometer & Enclosure The sensor ball contains a 3- componet seismometer, fixed in an auto- leveling gimbaled system, and enclosed within two vacuum sealed aluminum hemispheres. (Right & Below) Slide courtesy Mar;n Rapa and SIO OBSIP The sensor ball is temporarily coupled to the OBS arm with a mechanical advantage leaver, which is held together with two magnesium couplers. (Below)

7 Differential Pressure Guage DPG measures p Pass band 2Hz to 5 s l Chambers filled with oil Plas@c p1 diaphragm Advantages: No specific orienta@on needed Useful for tsunami waves Slide courtesy Mar;n Rapa and SIO OBSIP 7 '' diameter Aluminum cylinder p2 Needle regulates p in inner chamber Ambient pressure = p Cox, C., T. Deaton, and S. Webb (1984), J. Atmos. Oceanic Technol., 1,

8 Seismology on the Ocean Floor: Challenges Power; Timing; Coupling Seismometer to Seafloor (Cannot build vault). Lithium battery cost for a one-year deployment is ~$4-5K. Shiptime: ~$35K/day for a ship with deck space to stage 3-4 OBS; ~$25K/day for intermediate-class vessel. No GPS on the seafloor. Low-power (~1 mw) clocks have drift rates of a 2-5 ms/day. Coupling is a huge challenge. Drop and Pray. Slide courtesy John Collins, WHOI OBSIP

9 Determining Instrument Location

10 Instrument Recovery

11 Display of background noise spectra for OBS and land stations - Plots made using the program PQLX (PASSCAL Quick Look extended) Based on algorithm of McNamara and Buland, BSSA, 24

12 Display of background noise spectra for OBS and land stations - Power spectra determined for one-hour segments of the entire year of data - Probability density function used to display the thousands of spectra - Blues and greens represent frequent values, light purple represents less frequent values (following method of McNamara and Bulland, 24).

13 Interpretation of Noise Spectra Tool for Data QC McNamara and Buland, BSSA, 24

14 PDF Detail View PQLX.ppt

15 Display of background noise spectra Power spectral density PSD Probability density function PDF Earthquakes Microseismic peak High noise model Infragravity waves Low noise model = good, quiet

16 Display of background noise spectra Power spectral density PSD Probability density function PDF Earthquakes Microseismic peak High noise model Infragravity waves Low noise model = good, quiet Body waves

17 Display of background noise spectra Power spectral density PSD Probability density function PDF Earthquakes Microseismic peak High noise model Infragravity waves Low noise model = good, quiet Local earthquakes Body waves Surface waves

18 Comparison of spectra from deep water, shallow water, and land a) Shallow OBS Horizontal 1 Horizontal 2 Vertical Shallow OBS < 1 km water depth b) Deep OBS Deep OBS > 4 km water depth c) Land Land Station, New Zealand Zhaohui Yang et al., submi\ed to G- cubed, 212 Figure 2

19 Comparison of spectra from deep water, shallow water, and land a) Shallow OBS Horizontal 1 Horizontal 2 Infragravity wave peak is depth dependent Vertical Shallow OBS < 1 km water depth b) Deep OBS Deep OBS > 4 km water depth c) Land Land Station, New Zealand Zhaohui Yang et al., submi\ed to G- cubed, 212 Figure 2

20 Comparison of spectra from deep water, shallow water, and land a) Shallow OBS Horizontal 1 Horizontal 2 Vertical Long period horizontals noisy for all Shallow OBS < 1 km water depth b) Deep OBS Deep OBS > 4 km water depth c) Land Land Station, New Zealand Zhaohui Yang et al., submi\ed to G- cubed, 212 Figure 2

21 Comparison of spectra from deep water, shallow water, and land Horizontal 1 a) Shallow OBS Horizontal 1 Horizontal 2 Horizontal 2 Vertical Vertical Shallow OBS < 1 km water depth b) Deep OBS Deep OBS > 4 km water depth c) Land Land Station, New Zealand Zhaohui Yang et al., submi\ed to G- cubed, 212 High frequencies noisier on land Figure 2

22 Must determine orientation of horizontal components. Stachnik et al, SRL, in press 212

23 Must determine orientation of horizontal components. Body waves: minimize ratio of amplitudes of transverse energy to radial energy for known events Before Ajer

24 Must determine orientation of horizontal components. Challenges In our case BH2 and BH1 were mislabeled (by us) Finding enough good 3C body waves to do the body wave Measurement No Measurement > 25km 25 5km 5 3km >3km 8 Magnitude (Mb) A good one! P Magnitude v. Distance 8.5 NZ21 BH1 NZ21 BH2 NZ21 BHZ NZ2 BH1 NZ2 BH2 NZ2 BHZ NZ22 BH1 NZ22 BH2 NZ22 BHZ NZ24 BH1 NZ24 BH2 NZ24 BHZ NZ23 BH1 NZ23 BH2 NZ23 BHZ NZ25 BH1 NZ25 BH2 NZ25 BHZ NZ29 BH1 NZ29 BH2 NZ29 BHZ NZ27 BH1 NZ27 BH2 NZ27 BHZ NZ28 BH1 NZ28 BH2 NZ28 BHZ NZ26 BH1 NZ26 BH2 NZ26 BHZ 17:54: Distance (km) :55: :56: :57: :58: S 17:59: :: :1: :2: :3: Filter: None, Amp: Auto BRTT dbpick: testdb plotfile.ps zietlow Mon Jun 11 15:3: /29/9 M 8.1 Samoa earthquake. Unfiltered. Epicentral distance ~3 degrees.

25 Must determine orientation of horizontal components. Surface waves: We utilize cross correlation between the vertical and Hilbert transformed radial components X - 12 STACHNIK ET AL.: OBS ORIENTATION NZ19 orid1113 2/18/ :53: Hz lat lon km Δ=17. seaz a BHZ C*zr = 163 (12.8 ) Czr = 167 (124.8 ) Low Zero cross 73 (12.8 ) BH2 BH1 22:1:37. b Azimuth ( ) 36 C*zr c Time (sec) Rayleigh waves exhibit ellip@cal par@cle mo@on theore@cally observed only on the ver@cal and radial components of an instrument. - It is easier to asses linearity. Thus, instrument orienta@on is determined by cross correla@ng the ver@cal component with the Hilbert transformed radial component. - Polariza@on analysis included: 1) Predic@ng a 4. km/s phase arrival. 2) Applying a bandpass Bumerworth filter from.2-.4 Hz. 3) Compu@ng the radial component for a range of backazimuths. 4) Cross correla@ng the Hilbert transformed computed radial component with the ver@cal component. Codes available at h\p://research.flyrok.org/ socware.html Czr Stachnik et al, SRL, in press 212 Upper right corner shows summary of analysis. seaz is great circle arc azimuth from stationfollowing to algorithm of Baker and Stevens, 24 Figure 2. Surface wave polarization analysis for an earthquake recorded on station NZ19. epicenter, values in parentheses are residuals. Note strong Love wave arrival on BH1 component

26 Must determine orientation of horizontal components. The body wave and surface wave methods are in good agreement Component azimuth (Rayleigh wave) ( ) Component azimuth (P wave) ( ) Figure 5: Stachnik et al, SRL, in press 212

27 Ambient Noise Cross Correlations Can be used to check clock drift NZ24 NZ28 BHZ BHZ 333 km 259 deg (a) broadband (b) 2.8 sec (c) 12.3 sec (d) 6.1 sec 29#12#1 29#11#1 29#1#1 29#9#1 29#8#1 29#7# Lag time, s 29#6#1 29#5#1 29#4#1 29#3#1 29#2#17 #2 #1 time (sec) 1 2

28 Surface waves Ambient Noise Group Velocities (6 s 27 s) Ballistic Phase Velocities (25 s) Signal quality issues > 4s Use Ms > 6 for OBS, Ms >5 for land Sensitivity to water layer Josh Stachnik Fan- Chi Lin

29 Shear Wave Splitting Challenging to find good events. 1 N Event: 1-Aug-29 (222) 19: N 92.89E 5km Mw=7.5 Station: NZ3 Backazimuth: Distance: 93.6 init.pol.: 17.5 Filter:.1Hz -.1Hz SNR : SC E Rotation Correlation: -39< -27 < -18.8<.9s<1. Minimum Energy: -48< -31 < -17.8<.9s<1.1 Eigenvalue: -54< -21 < -3.8<.9s<1.4 Quality: good IsNull: No Phase: SKKS corrected Fast ( ) & Slow(-) 1 corrected Q( ) & T(-) 5 Particle motion before ( ) & after (-) 9 Map of Correlation Coefficient 6 3 fast axis S - N Rotation-Correlation Inc = W - E corrected Fast ( ) & Slow(-) corrected Q( ) & T(-) sec Energy Map of T Particle motion before ( ) & after (-) 9 3 fast axis S - N Minimum Energy W - E Dan Zietlow 4sec

30 Receiver Functions Not promising, but huge community interest in overcoming the issues. Ultra-low Vs sediments, can t trust P-S times Water layer reverberations Noise often totally obscures arrivals Poor azimuthal coverage, few good events See IRIS workshop poster by Jus;n Ball

31 Local Earthquakes Pn Local earthquake Shear wave splinng Looks good! Magnitude 3.7 earthquake from the Chatham rise. Seismograms filtered from 2-1 Hz.

32 Seafloor Pressure Data (Differential Pressure Gauge DPG or Absolute Pressure Gauge APG) Seafloor Compliance Tsunamis Infragravity waves Slow Slip Seafloor Geodesy Tool to improve signal quality on OBS

33 Seafloor Compliance ~ 1 km Wavelength in Compliance Band ~ 1 mm Ocean Surface Displacement Ocean surface infragravity waves present varying pressure load to the seafloor. The seafloor s resul@ng displacement is a func@on of its s@ffness. Defined as the inverse of s@ffness, seafloor compliance depends on the suboceanic shear modulus. Lower forcing frequencies lead to greater depth sensi@vity. [1] ~ 1 µm Seafloor Displacement D(t) Measured by OBS Onboard DPG measures pressure P(t) D(t), P(t) measured. Essen@ally, compliance is the transfer func@on between seafloor displacement and pressure: D(ω)/P(ω) See IRIS workshop poster by Jus;n Ball Modified from h\p://

34 Tsunami Signals on Seafloor Pressure Guages July 15, 29 South Island Mw ~7.5 Arrives at OBS ~ 1 hour to 1.5 hours ajer the earthquake took place. Z. Yang

35 Seafloor Pressure Instrument Response Absolute Pressure Gauge APG Flat from DC to ~2 Hz Pressure Gauge DPG Rolls off at long periods > ~ 3s.1 Hz.1 Hz.1 Hz.1 Hz 1 Hz 1 Hz.1 Hz.1 Hz 1 Hz 1 Hz 1 Hz

36 Summary Noise spectra give you a good idea of what to expect. Teleseismic body waves are challenging. Local events fine, surface waves good (both earthquake and ambient noise). IGW strong at long periods. PQLX a powerful tool for data QC and identifying sources of spectral energy. Horizontal component rotation Stachnik codes (perl) at Still a need for well-understood, easy-to-apply instrument response functions New areas tremor, slow slip, seafloor geodesy Sense of discovery - I didn t anticipate how fun this data would be to work with or how interesting the pressure data would be!

37 Extra slides

38 Earthquakes PQLX.ppt

39 Bad Units Instead of Velocity Input Units Extra Zero in Response File Results in Tilted PSD: Low Amps at Low Period High Amps at High Period Corrected Response PQLX.ppt

40 Bad Units Displacement Instead of Velocity Input Units Missing zero in Response File Results in Tilted PSD: High Amps at Low Period Low Amps at High Period Corrected Response PQLX.ppt

41 Bad STS2 Gain Used 2, counts/volt Instead of 15 counts/volt in low corrected Amplitude across spectrum. Corrected Response PQLX.ppt

42 Mass Re-Centering PQLX.ppt