Phase velocity and source direction estimation using collocated measurements of rotational and translational motions from ambient seismic noise Peter Gaebler, Joachim Wassermann and Heiner Igel Department of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich Contact: [email protected] 1 Abstract We introduce a study, in which the possibility of determining surface wave phase velocities and source directions from collocated measurements of translational and vertical rotational motions excited by ambient seismic noise sources is investigated. Assuming plane, horizontally polarized shear waves, the information about the local phase velocity is contained in the amplitude ratio of transversal acceleration and rotation rate. F The first set of data consists out of data recorded at the geodetic observatory in Wettzell using the high quality rotational motion ringlaser sensor and the collocated STS2-broadband seis- mometer. Data analysis shows the possibility of determining local seismic phase velocities from ambient seismic noise in the primary and secondary oceanic microseisms frequency band. Furthermore it is possible to estimate the backazimuthal direction of the generating areas of oceanic microseisms. F An additional data set was acquired using the portable angular motion sensor R1 and a Trillium compact broadband seismometer in field operation. Data analysis in the microseisms frequency band clearly showed the necessity of higher quality and more sensitive portable angular motion sensors. The analysis results of rotational and translational ambient noise data in the frequency band above 10Hz were compared with results derived from seismic array measurements. A clear consistency in the phase velocity estimation is visible. 2 Physical background An application of using data from collocated measurements of rotations and translations is the possibility of determining seismic wave phase velocities. This method was already described by Igel et al. [2005] and Kurrle et al. [2010]. For a plane, horizontally polarized shear wave in a homogenous medium, it is theoretically expected, that collocated measurements of rotation and translation, should show a very high correlation. Ferreira and Igel. [2009] showed, that this fact is also true for a layered halfspace, that means it also holds for surface waves. Under the assumption of a horizontally polarized, plane wave, the vertical rotation rate ˙ Ω z should at all times be in phase with the transversal acceleration a T . The ratio between a T and ˙ Ω z is then given as twice the horizontal phase velocity c. a T ˙ Ω z = -k 2 c 2 Asin(kx-kct) 1 2 k 2 cAsin(kx-kct) = -2c 3 Processing method ? Data pre-processing ? Source backazimuth and phase velocity estimation → Instrument corrections → Compute transversal accelerations → Bandpass filtering → Cross-correlate transversal accelerations and rotation rate → Sliding windows → Store source backazimuth (Set cross correlation threshold) → Compute phase velocity for stored backazimuth 4 Ringlaser experiment In the ringlaser experiment section local seismic phase velocities, as well as the direction of the source, are determined, using ambient seismic noise generated by oceanic microseisms. Small oscillations of the ground, recorded on land as continuous seismic background signals, are referred to as microseisms (Kedar et al., 2008). This high level of noise, clearly visible in the frequency band from around 0.04 to 0.2 Hz (5-25s period), is probably generated by ocean wave energy induced by atmospheric disturbances, coupling into the motion of the Earth. Following figure shows the processing results for day 2009/11/07 in the primary microseisms band. Figure: The record duration is 24h (1440 windows with length 1 minute). Shown is the vertical rotation rate (grey), the correlation coefficient between best fitting transversal acceleration and the rotation rate (blue dots), as well as the corresponding source backazimuth (green dots). The computed phase velocity is plotted in the bottom trace (black crosses), horizontal black line represents a sliding average (10 minutes), vertical dashed lines indicate high amplitudes in the rotation rate signal induced by primary microseisms. Calculated phase velocities during the time of high primary microseisms activity are in the range of 2500m/s. A computed backazimuth of around 280 ◦ on a spherical surface from Wettzell points towards the channel region, respectively to the southern part of the British Isles. This observation is supported by a study of Friedrich et al. (1998), who stated those regions as possible sources of oceanic microseisms noise. Following figure shows an example time window of 60 seconds length in the primary microseisms band on 2009/11/07, 11:11-11:12. The resulting phase velocity is 2600m/s, the corresponding backazimuth is 260 ◦ . The correlation between transversal acceleration and rotation rate is 0.94. Figure: Top trace shows the rotation rate (grey), middle trace shows the best fitting transversal aceleration (dashed black). The bottom trace shows the best fit transversal acceleration, as well as the rotation rate scaled with twice the computed phase velocity. 5 R1 experiment For field measurements the portable R1 angular motion sensor and a Trillium broadband seismometer were used. Following figure shows the processing results in the frequency band from 13-15Hz, probably associated with cultural noise. Figure: Record duration is 60 minutes. Shown is the vertical rotation rate (grey), the correlation coefficient between best fitting transversal acceleration and rotation rate (blue dots), as well as the corresponding source backazimuth (green dots). The computed phase velocity for each window is given in the bottom trace (black crosses). The average computed phase velocity in the frequency band from 13-15Hz was 150m/s, with an average correlation coefficient of 0.61. The computed phase velocity is confirmed by seismic array measurements carried out very close to the position of the collocated measurements of rotation rates and transversal accelerations. 6 Conclusions F Ringlaser experiment ◦ Possibility to estimate seismic phase velocities from primary and secondary microseisms ◦ Estimation of the backazimuth of the microseisms generation areas ◦ Extreme sensitivity of the ringlaser system to local wind field influences F R1 experiment ◦ No clear analysis results in the microseisms frequency band (due to eigen noise, low sensitivity) ◦ Promising results, confirmed by array measurements, in the frequency bands above 10Hz → Collocated measurements of rotations and translations might become a reasonable alternative to costly array measurements → Possibility of determining seismic phase velocities, source directions and estimating the seismic wave content (polarization filter) 7 References and annotations Ferreira, A. and Igel, H. (2009). Rotational Motions of Seismic Surface Waves in a Laterally Heterogeneous Earth. Bulletin of the Seismological Society of America, 99(2):1429-1436. Friedrich, A., Kr¨ uger, F., and Klinge, K. (1998). Ocean-generated microseisms noise located with the Gr¨ afenberg array. Journal of Seismology, 2:47-64. Igel, H., Schreiber, U., Flaws, A., Schuberth, B., Velikoseltsev, A., and Cochard, A. (2005). Rotational motions induced by the M8.1 Tokachi-oki earthquake, September 25, 2003. Geophysical Research Letters, 32(8):1-5. Kedar, S., Longuet-Higgins, M., Webb, F., Graham, N., Clayton, R., and Jones, C. (2008). The origin of deep ocean microseisms in the North Atlantic Ocean. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 464(2091):777-793. Kurrle, D., Igel, H., Ferreira, A. M. G., Wassermann, J., and Schreiber, U. (2010). Can we estimate local Love wave dispersion properties from collocated amplitude measurements of translations and rotations? Geophysical Research Letters, 37(4):1-5. This study was carried out as part of a Master’s thesis project at the LMU Munich in 2010 and was supervised by Dr. J. Wassermann and Prof. H. Igel.
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Phase velocity and source direction estimation using collocated measurementsof rotational and translational motions from ambient seismic noise
Peter Gaebler, Joachim Wassermann and Heiner IgelDepartment of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich
Contact: [email protected]
1 Abstract
We introduce a study, in which the possibility of determining surface wave phase velocities andsource directions from collocated measurements of translational and vertical rotational motionsexcited by ambient seismic noise sources is investigated. Assuming plane, horizontally polarizedshear waves, the information about the local phase velocity is contained in the amplitude ratio oftransversal acceleration and rotation rate.
F The first set of data consists out of data recorded at the geodetic observatory in Wettzell usingthe high quality rotational motion ringlaser sensor and the collocated STS2-broadband seis-mometer. Data analysis shows the possibility of determining local seismic phase velocities fromambient seismic noise in the primary and secondary oceanic microseisms frequency band.Furthermore it is possible to estimate the backazimuthal direction of the generating areas ofoceanic microseisms.
F An additional data set was acquired using the portable angular motion sensor R1 and a Trilliumcompact broadband seismometer in field operation. Data analysis in the microseisms frequencyband clearly showed the necessity of higher quality and more sensitive portable angular motionsensors. The analysis results of rotational and translational ambient noise data in the frequencyband above 10Hz were compared with results derived from seismic array measurements. A clearconsistency in the phase velocity estimation is visible.
2 Physical background
An application of using data from collocated measurements of rotations and translations is thepossibility of determining seismic wave phase velocities. This method was already described byIgel et al. [2005] and Kurrle et al. [2010].For a plane, horizontally polarized shear wave in a homogenous medium, it is theoreticallyexpected, that collocated measurements of rotation and translation, should show a very highcorrelation. Ferreira and Igel. [2009] showed, that this fact is also true for a layered halfspace,that means it also holds for surface waves.Under the assumption of a horizontally polarized, plane wave, the vertical rotation rate Ωz shouldat all times be in phase with the transversal acceleration aT .The ratio between aT and Ωz is then given as twice the horizontal phase velocity c.
aTΩz
=−k2c2Asin(kx−kct)
12k
2cAsin(kx−kct) = −2c
3 Processing method
? Data pre-processing ? Source backazimuth and phase velocity estimation→ Instrument corrections → Compute transversal accelerations→ Bandpass filtering → Cross-correlate transversal accelerations and rotation rate→ Sliding windows → Store source backazimuth (Set cross correlation threshold)
→ Compute phase velocity for stored backazimuth
4 Ringlaser experiment
In the ringlaser experiment section local seismic phase velocities, as well as the direction of thesource, are determined, using ambient seismic noise generated by oceanic microseisms. Smalloscillations of the ground, recorded on land as continuous seismic background signals, are referredto as microseisms (Kedar et al., 2008). This high level of noise, clearly visible in the frequencyband from around 0.04 to 0.2 Hz (5-25s period), is probably generated by ocean wave energyinduced by atmospheric disturbances, coupling into the motion of the Earth.Following figure shows the processing results for day 2009/11/07 in the primary microseisms band.
Figure: The record duration is 24h (1440 windows with length 1 minute). Shown is the vertical rotation rate (grey), the correlation coefficient between best
fitting transversal acceleration and the rotation rate (blue dots), as well as the corresponding source backazimuth (green dots). The computed phase velocity
is plotted in the bottom trace (black crosses), horizontal black line represents a sliding average (10 minutes), vertical dashed lines indicate high amplitudes in
the rotation rate signal induced by primary microseisms.
Calculated phase velocities during the time of high primary microseisms activity are in the rangeof 2500m/s. A computed backazimuth of around 280 on a spherical surface from Wettzell pointstowards the channel region, respectively to the southern part of the British Isles. This observationis supported by a study of Friedrich et al. (1998), who stated those regions as possible sources ofoceanic microseisms noise.Following figure shows an example time window of 60 seconds length in the primary microseismsband on 2009/11/07, 11:11-11:12. The resulting phase velocity is 2600m/s, the correspondingbackazimuth is 260. The correlation between transversal acceleration and rotation rate is 0.94.
Figure: Top trace shows the rotation rate (grey), middle trace shows the best fitting transversal aceleration (dashed black). The bottom trace shows the best
fit transversal acceleration, as well as the rotation rate scaled with twice the computed phase velocity.
5 R1 experiment
For field measurements the portable R1 angular motion sensor and a Trillium broadbandseismometer were used. Following figure shows the processing results in the frequency band from13-15Hz, probably associated with cultural noise.
Figure: Record duration is 60 minutes. Shown is the vertical rotation rate (grey), the correlation coefficient between best fitting transversal acceleration and
rotation rate (blue dots), as well as the corresponding source backazimuth (green dots). The computed phase velocity for each window is given in the bottom
trace (black crosses).
The average computed phase velocity in the frequency band from 13-15Hz was 150m/s, with anaverage correlation coefficient of 0.61. The computed phase velocity is confirmed by seismic arraymeasurements carried out very close to the position of the collocated measurements of rotationrates and transversal accelerations.
6 Conclusions
F Ringlaser experiment
Possibility to estimate seismic phase velocities from primary and secondary microseisms
Estimation of the backazimuth of the microseisms generation areas
Extreme sensitivity of the ringlaser system to local wind field influences
F R1 experiment
No clear analysis results in the microseisms frequency band (due to eigen noise, low sensitivity)
Promising results, confirmed by array measurements, in the frequency bands above 10Hz
→ Collocated measurements of rotations and translations might become a reasonable alternativeto costly array measurements
→ Possibility of determining seismic phase velocities, source directions and estimating the seismicwave content (polarization filter)
7 References and annotations
Ferreira, A. and Igel, H. (2009). Rotational Motions of Seismic Surface Waves in a Laterally Heterogeneous Earth. Bulletin of the Seismological Society ofAmerica, 99(2):1429-1436.
Friedrich, A., Kruger, F., and Klinge, K. (1998). Ocean-generated microseisms noise located with the Grafenberg array. Journal of Seismology, 2:47-64.
Igel, H., Schreiber, U., Flaws, A., Schuberth, B., Velikoseltsev, A., and Cochard, A. (2005). Rotational motions induced by the M8.1 Tokachi-oki earthquake,September 25, 2003. Geophysical Research Letters, 32(8):1-5.
Kedar, S., Longuet-Higgins, M., Webb, F., Graham, N., Clayton, R., and Jones, C. (2008). The origin of deep ocean microseisms in the North Atlantic Ocean.Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 464(2091):777-793.
Kurrle, D., Igel, H., Ferreira, A. M. G., Wassermann, J., and Schreiber, U. (2010). Can we estimate local Love wave dispersion properties from collocatedamplitude measurements of translations and rotations? Geophysical Research Letters, 37(4):1-5.
This study was carried out as part of a Master’s thesis project at the LMU Munich in 2010 and was supervised by Dr. J. Wassermann and Prof. H. Igel.
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