NEW SEISMIC NOISE MODEL FOR ROMANIA

Abstract. Icequakes are the result of processes occurring within the ice mass or between the ice and its environment. Studying icequakes provides a unique view on ice dynamics, specifically on the basal conditions. Changes in conditions due to environmental or climate changes are reflected in icequakes. Counting and characterizing icequakes is thus essential to monitor them. Most of the icequakes recorded by the seismic station at the Belgian Princess Elisabeth Antarctica Station (PE) have small amplitudes corresponding to maximal displacements of a few nanometres. Their detection threshold is highly variable because of the rapid and strong changes in the local seismic noise level. Therefore, we evaluated the influence of katabatic winds on the noise measured by the well-protected PE surface seismometer. Our purpose is to identify whether the lack of icequake detection during some periods could be associated with variations in the processes generating them or simply with a stronger seismic noise linked to stronger wind conditions. We observed that the wind mainly influences seismic noise at frequencies greater than 1 Hz. The seismic noise power exhibits a bilinear correlation with the wind velocity, with two different slopes at a wind velocity lower and greater than 6 m s−1 and with, for example at a period of 0.26 s, a respective variation of 0.4 dB (m −1 s) and 1.4 dB (m −1 s). These results allowed a synthetic frequency and wind-speed-dependent noise model to be presented that explains the behaviour of the wind-induced seismic noise at PE, which shows that seismic noise amplitude increases exponentially with increasing wind speed. This model enables us to study the influence of the wind on the original seismic dataset, which improves the observation of cryoseismic activity near the PE station.

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Frequency limits for seismometers as determined from signal-to-noise ratios. Part 1. The electromagnetic seismometer

Bulletin of the Seismological Society of America ◽

10.1785/bssa0820021071 ◽

1992 ◽

Vol 82 (2) ◽

pp. 1071-1098 ◽

Cited By ~ 3

Author(s):

Peter W. Rodgers

Keyword(s):

Operational Amplifier ◽

Seismic Noise ◽

Low Noise ◽

Noise Model ◽

Noise Voltage ◽

Signal To Noise ◽

Noise Current ◽

System Noise ◽

Large Gain ◽

Total Noise

Abstract The range of frequencies that a seismometer can record is nominally set by the corner frequencies of its amplitude frequency response. In recording pre-event noise in very quiet seismic sites, the internally generated self-noise of the seismometer can put further limits on the range of frequencies that can be recorded. Some examples of such low seismic noise sites are Lajitas, Texas; Deep Springs, California; and Karkaralinsk, U.S.S.R. In such sites, the seismometer self-noise can be large enough to degrade the signal-to-noise ratio (SNR) of the recorded pre-event data. The widely used low seismic noise model (LNM) (due to Peterson, 1982; Peterson and Hutt, 1982; Peterson and Tilgner, 1985; Peterson and Hutt, 1989) is used as representative of the input ground motion acceleration power density spectrum (pds) at such very low noise sites. This study determines the range of frequencies for which the SNR of an electromagnetic seismometer exceeds 3 db (a factor of 2 in power and 1.414 in amplitude). In order to do this, an analytic expression is developed for the SNR of a generalized electromagnetic seismometer. The signal pds using Peterson's LNM as an input is developed for an electromagnetic seismometer. Suspension noise is modeled following Usher (1973). In order to determine the electronically caused component of the self-noise, noise properties are compared among three commonly used amplifiers. The advantages and disadvantages of the inverting and noninverting configurations in terms of their SNR are discussed. In most cases, the noninverting configuration is to be preferred as it avoids the use of the large gain setting resistances required in the inverting configuration to avoid loading the seismometer output. A noise model is developed for a typical low noise operational amplifier (Precision Monolithics OP-27). This noise model is used to numerically compute the SNRs for the three electromagnetic seismometers used as examples. The degradation in SNR caused by large gain setting resistances is shown. Numerical examples are given using the Mark Products L-4C and L-22D and the Teledyne Geotech GS-13 electromagnetic seismometers. For each of the example seismometers, the calculated range of frequencies for which their SNR exceeds 3 db is as follows: the GS-13, 0.078 to 56.1 Hz; the L-4C, 0.113 to 7.2 Hz; and the L-22D, 0.175 to 0.6 Hz. For the GS-13, the calculated lower and upper frequencies at which the SNR is 3 db are 0.078 and 56.1 Hz. This compares with the values 0.073 and 59 Hz measured in the noise tests on the vertical GS-13. Expressions for the total noise voltage referred to the input of an operational amplifier are developed in Appendix A. It is shown that in the inverting configuration, although no noise current flows in the input resistor, the noise current appears in the expression for the total noise voltage as if it did. In Appendix B, it is shown that any noise current flowing through an electromagnetic seismometer having a generator greater than several hundred V/m/sec generates a back emf that adds significantly to the noise of the system. This implies that system noise tests that substitute a resistor at the noninverting input of the preamplifier or clamp the seismometer mass will tend to underestimate the system noise.

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A Seismological Study of the Sos Enattos Area—the Sardinia Candidate Site for the Einstein Telescope

Seismological Research Letters ◽

10.1785/0220200186 ◽

2020 ◽

Vol 92 (1) ◽

pp. 352-364

Author(s):

Matteo Di Giovanni ◽

Carlo Giunchi ◽

Gilberto Saccorotti ◽

Andrea Berbellini ◽

Lapo Boschi ◽

...

Keyword(s):

Seismic Noise ◽

Velocity Structure ◽

Group Velocity Dispersion ◽

Low Noise ◽

Noise Model ◽

Peak Strain ◽

Observation Distance ◽

Candidate Site ◽

Einstein Telescope ◽

The Future

Abstract The recent discovery of gravitational waves (GWs) and their potential for cosmic observations prompted the design of the future third-generation GW interferometers, able to extend the observation distance for sources up to the frontier of the Universe. In particular, the European detector Einstein Telescope (ET) has been proposed to reach peak strain sensitivities of about 3×10−25 Hz−1/2 in the 100 Hz frequency region and to extend the detection band down to 1 Hz. In the bandwidth [1,10] Hz, the seismic ambient noise is expected to represent the major perturbation to interferometric measurements, and the site that will host the future detectors must fulfill stringent requirements on seismic disturbances. In this article, we conduct a seismological study at the Italian ET candidate site, the dismissed mine of Sos Enattos in Sardinia. In the range between few mHz to hundreds of mHz, out of the detection bandwidth for ET, the seismic noise is compatible with the new low-noise model (Peterson, 1993); in the [0.1,1] Hz bandwidth, we found that seismic noise is correlated with sea wave height in the northwestern Mediterranean Sea. In the [1,10] Hz frequency band, noise is mainly due to anthropic activities; within the mine tunnels (≃100 m underground), its spectrum is compliant with the requirements of the ET design. Noise amplitude decay with depth is consistent with a dominance of Rayleigh waves, as suggested by synthetic seismograms calculated for a realistic velocity structure obtained from the inversion of phase- and group-velocity dispersion data from array recording of a mine blasting. Further investigations are planned for a quantitative assessment of the principal noise sources and their spatiotemporal variations.

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Frequency limits for seismometers as determined from signal-to-noise ratios. Part 2. The feedback seismometer

Bulletin of the Seismological Society of America ◽

10.1785/bssa0820021099 ◽

1992 ◽

Vol 82 (2) ◽

pp. 1099-1123

Author(s):

Peter W. Rodgers

Keyword(s):

Seismic Noise ◽

Transfer Functions ◽

Low Noise ◽

Noise Model ◽

Power Density Spectrum ◽

Signal To Noise ◽

Density Spectrum ◽

Velocity Feedback ◽

Noise Models ◽

Feedback Seismometer

Abstract The range of frequencies that a seismometer can record is nominally set by the corner frequencies of its amplitude frequency response. In recording pre-event noise in very quiet seismic sites, the internally generated self-noise of the seismometer can put further limits on the range of frequencies that can be recorded. Some examples of such low seismic noise sites are Lajitas, Texas; Deep Springs, California; and Karkaralinsk, U.S.S.R. In such sites, the seismometer self-noise can be large enough to degrade the signal-to-noise ratio (SNR) of the recorded pre-event data. The widely used low seismic noise model (LNM) (due to Peterson, 1982; Peterson and Hutt, 1982; Peterson and Tilgner, 1985; Peterson and Hutt, 1989) is used as representative of the input ground motion acceleration power density spectrum (pds) at such very low noise sites. This study determines the range of frequencies for which the SNR of a feedback seismometer exceeds 3 db (a factor of 2 in power and 1.414 in amplitude). Analytic expressions for the SNR are developed for three types of feedback seismometers. These are the displacement feedback, velocity feedback, and coil-to-coil velocity feedback seismometers. It was found that the analytic SNRs of the displacement and velocity feedback seismometers are identical and that the SNRs for the coil-coil feedback seismometer and the electromagnetic seismometer are also the same. The signal pds using Peterson's LNM as an input is developed for each of the three types of feedback seismometers. Suspension noise is modeled following Aki and Richards (1980). In order to model the electronically caused component of the self-noise, the electronic noise properties of two commonly used operational amplifiers (Precision Monolithics OP-27 and the Burr-Brown OPA2111 FET) are described. Using these, noise models are developed for a synchronous demodulator and a chopper-stabilized amplifier. These noise models are used to numerically compute the SNRs for the two feedback seismometers used as examples, which are the Guralp Systems CMG-3ESP and Sprengnether Instruments SBX-1000 feedback seismometers. For each of the example seismometers, the calculated range of frequencies for which their SNR exceeds 3 db is as follows: the CMG-3ESP, 0.025 to 13.3 Hz; the SBX-1000, 0.098 to 11.3 Hz. The calculated and measured SNRs for the CMG-3ESP are compared. The calculated upper frequency for a SNR of 3 db was 13.3 Hz compared with 18.4 Hz measured in the noise tests. The calculated lower frequency for a SNR of 3 db was 0.025 Hz, whereas the measured value was 0.047 Hz. The difference is most likely due to the fact the CMG-3ESP is cut off at 0.1 Hz. Formulas are developed in Appendix A for calculating the SNR and self-noise of identical, colocated seismometers from their recorded outputs. The analytic transfer functions, midband gain, upper and lower corner frequencies, and bandwidths for the three types of feedback seismometers are given in Appendix B for comparison with the frequency limits set by the SNR.

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THEORETICAL PREDICTION OF SEISMIC NOISE IN A DEEP BOREHOLE

Geophysics ◽

10.1190/1.1439408 ◽

1964 ◽

Vol 29 (5) ◽

pp. 714-720 ◽

Cited By ~ 4

Author(s):

Robert L. Sax ◽

Royal A. Hartenberger

Keyword(s):

Free Surface ◽

Surface Wave ◽

Energy Density ◽

Seismic Noise ◽

Shear Velocity ◽

Noise Model ◽

Noise Attenuation ◽

Deep Borehole ◽

Energy Fraction ◽

Model C

Seismic noise attenuation as a function of depth below the earth’s surface is described in terms of surface wave modes. The energy density of the noise, which often represents a combination of modes, is computed as the arithmetic sum of the energy densities of all contributing modes. Three models are used to determine the fractional contribution of each mode. Model A stipulates that each mode contributes equally to the energy of the noise; Model B specifies that the energy fraction contributed by each mode is dependent upon the energy in those beds in which the shear velocity of the bed exceeds the phase velocity of the mode; and Model C prescribes that all modes have equal energy density at the free surface. A comparison of observations with attenuations based on the three models reveals that Model B is the best.

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