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Observational Experience with Rotational Strong Motion

Observational Experience with Rotational Strong Motion. Robert Nigbor UCLA. Outline. Motivation Early 1993 6DOF deployment 9/93 NPE blast measurement BVDA deployment, 12/93 – present GVDA SFSI Test Structure 6DOF measurements, 2004-present Thoughts on future sensor selection.

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Observational Experience with Rotational Strong Motion

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  1. Observational Experience with Rotational Strong Motion Robert Nigbor UCLA Rotation Workshop 2/16/2006

  2. Outline • Motivation • Early 1993 6DOF deployment • 9/93 NPE blast measurement • BVDA deployment, 12/93 – present • GVDA SFSI Test Structure 6DOF measurements, 2004-present • Thoughts on future sensor selection Rotation Workshop 2/16/2006

  3. Motivations for 6DOF Deployment • Scientific interest per Bouchon & Aki 1982 “Strain, tilt, and rotation associated with strong ground motion in the vicinity of earthquake faults”, BSSA 72 • Sensitivity of conventional strong motion accelerometers to rotational motions Rotation Workshop 2/16/2006

  4. Sensitivity of Linear Accelerometers to Rotational Motions: from IEEE Standard Specification Format Guide and Test Procedure for Linear, Single-Axis, Nongyroscopic Accelerometers Rotation terms Rotation Workshop 2/16/2006

  5. Rotation terms Rotation Workshop 2/16/2006

  6. Rotational Sensor Selection in 1993 • Unfunded “Skunkworks” project, so budget was small • Looked at FOG, RLG’s but cost was too high (and power/reliability was a question) • Systron-Donner QRS11 “Gyrochip” was brand new in 1993, manufacturer was eager to help with potential new applications, gave discount price of $3k for three. • Range (0.2 rad/sec clip, 1e-4 rad/sec stated resolution) seemed right per Bouchon & Aki and other analytical estimations of rotational velocities for strong ground motion Rotation Workshop 2/16/2006

  7. 6DOF Prototype System Rotation Workshop 2/16/2006

  8. Sensor Package (with FBA-23) 3-component rotation sensor 3-component accelerometer (FBA-23) Rotation Workshop 2/16/2006

  9. Aftershock Monitoring • Installed in my garage for a month, no earthquakes (but could observe rotational data during jumping & hammering near the sensor) • Installed in a private home garage north of Landers, 2km from the Landers EQ fault trace, for 2 months. Six small aftershocks recorded, maximum PGA was 0.015g. No rotational motion was resolved above the ~.001rad/sec channel noise level. Rotation Workshop 2/16/2006

  10. NPE “Chemical Kiloton” Event “The DOE conducts a Non-Proliferation Experiment at the Nevada Test Site The U.S. Department of Energy (DOE) detonated approximately 1.29 million kg of a commercial blasting agent, based on ammonium nitrate and fuel oil (ANFO), at the Nevada Test Site (NTS) on September 22, 1993, at 00:01.080 AM Pacific Daylight Time. The blasting agent was emplaced in a cylindrical chamber, approximately 15.2 m in diameter by 5.5 m high, located at 37.20193 N and 116.20986 E in a Rainier Mesa tunnel, 390 m underground. Code-named the Non-Proliferation Experiment (NPE), the explosion had an energy release of approximately 1 kt (1 kiloton = 4.186x1012 joules).” J. Zucca, EOS, Nov. 1993 Rotation Workshop 2/16/2006

  11. 6DOF Accelerograph Installation for the NPE • Opportunity for a piggyback experiment, encouraged and facilitated by Fred Followill & Jay Zucca • 1km from ground zero (blast was 390m underground) • Sensors bolted on a small partially-embedded concrete pedestal at the bedrock siteon top of Rainer Mesa • Unattended triggered recording • No photos… Rotation Workshop 2/16/2006

  12. NPE Results Rotation Workshop 2/16/2006

  13. NPE Waveforms Rotation Workshop 2/16/2006

  14. 1-sec Subset Rotation Workshop 2/16/2006

  15. Displacements Rotation Workshop 2/16/2006

  16. Installation at Borrego Valley Downhole Array (BVDA) • Installed along with FBA-23 at ground level at center of BVDA in Dec. 1993 • Still in place (array is now operated by UCSB & Jamie Steidl) • Free-field site on 220m of stiff sand over granite • Less than 2km from Coyote Canyon Fault at the south end of the Anza seismic slip gap • Hundreds of earthquakes recorded, maximum PGA ~0.2g, no resolvable rotational motions above 0.001 rad/sec yet! Peak velocity contours from 3D FD simulation of M4.9 event; from Olsen, K., Nigbor, R., and T. Konno, “3-D Viscoelastic Wave Propagation in the Borrego Valley, California, Constrained by Borehole and Surface Data,” BSSA 90, 1, 2000 Rotation Workshop 2/16/2006

  17. SFSI Test Structure at Garner Valley Digital Array (GVDA) 3D rotational velocity sensor in center of bottom slab Rotation Workshop 2/16/2006

  18. DAQ is a 24-bit Digitexx System Rotation Workshop 2/16/2006

  19. Small Local EarthquakeRotation:(Ch.8–Ch.10)/374cmpeak base acceleration (cm/s2)= 1.3(NS), 2.0(EW), 0.9(V) Rotation Workshop 2/16/2006

  20. Small Local EarthquakeVertical Bottom Slab Corner Accelerations & Difference Rotation Workshop 2/16/2006

  21. Forced Vibration -Internet Controlled Shaker 5 Acceleration (cm/s2) -5 0 60 Time (s) Rotation Workshop 2/16/2006

  22. GVDA/SFSI Forced VibrationDerived Rotation: Ch. 8 - 10 Rotation Workshop 2/16/2006

  23. SFSI Forced VibrationRotational Velocity:Derived from Av & Measured by ATA ARS-09 Gyro Noise Only Rotation Workshop 2/16/2006

  24. Conclusion Regarding Experience with ATA Gyro at GVDA • Dynamic range of ATA ARS-09 gyro is ~60dB, less than the 100dB in the spec • Resolution is about 5E-3 rad/sec broadband, maybe 1E-3 rad/sec narrowband • Small rotational foundation motion can be resolved using closely-spaced vertical accelerometers if done carefully and with high resolution sensors/digitizers. Ground motion will be tougher, as rotation is proportionally much smaller. • The nees@UCSB SFSI Test Structure at GVDA is a useful (and available) field test facility for rotational sensors. Rotation Workshop 2/16/2006

  25. Future RotationSensor Selection • Cost, reliability, and power consumption are key parameters for 6DOF strong motion systems – the fancy stuff (FOG, RLG) may not be appropriate for most applications. • Linear acceleration cross-axis sensitivity may be critical for our application, and should be experimentally characterized for selected sensors (ARS-09 spec is <0.009 rad/sec per g). • No need to reinvent this wheel: • The IEEE Aerospace and Electronic Systems Society (AESS) has worked through most of the instrumentation issues for gyroscopic sensors • Industry standards and guidelines are available for our use, as they are for accelerometers Rotation Workshop 2/16/2006

  26. IEEE Standards for Gyros Rotation Workshop 2/16/2006

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