Systematic detection and correction of instrumental time shifts using crosscorrelations of ambient seismic noise
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Timing errors are a notorious problem in seismic data acquisition and processing. We present a technique that allows such time shifts to be detected and corrected in a systematic fashion. The technique relies on virtual-source surface-wave responses retrieved through the crosscorrelation of ambient seismic noise. In particular, it relies on the theoretical time-symmetry of these time-averaged receiver-receiver crosscorrelations. By comparing the arrival time of the surface waves at positive time to the arrival time of the surface waves at negative time for a large a number of receiver-receiver pairs, relative timing errors can be determined in a least-squared sense. The time-symmetry of the receiver-receiver crosscorrelations, however, is contingent on a uniform surface-wave (noise) illumination pattern. In practice, the illumination pattern is often not uniform. We therefore show that weighting different receiver-receiver pairs differently in the inversion allows timing errors to be determined more accurately. The weights are based on the susceptibility of different receiver pairs to illumination-related travel-time errors. The proposed methodology is validated using both synthetic data and field data. The field data consists of recordings of ambient seismic noise by an array of stations centered around the tip of the Reykjanes peninsula, southwest Iceland (some of these stations exhibit time shifts of an unknown nature).Keywords:
Ambient noise level
Seismic Noise
Seismic interferometry can be used to extract useful information about Earth's subsurface from the ambient noise wave field. It is an important new tool for exploring seismically quiescent areas. The method involves extraction of empirical Green's function from the background ambient vibrations of the Earth, followed by computation of group or phase velocity and tomographic imaging. Here we provide a review of seismic interferometry and ambient noise tomography (ANT) and present an example of the method in south India.
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Cross-correlations between long continuous records of ambient seismic noise at distant stations are investigated. The dominant part of the Green function, namely Rayleigh waves, are reconstructed in a broad period range. This property reminds of the fluctuation-dissipation theorem that relates the random fluctuations of a linear system and the system’s response to an external force. Ambient seismic noise is indeed not a thermal noise but it can be considered as a random and isotropic wave field both because the distribution of the ambient sources responsible for the noise randomizes when averaged over long periods and because of scattering from heterogeneities that occur within the Earth. The dispersion curves of Rayleigh waves for the paths between the stations are measured from the correlations. On paths where direct measurements between earthquake and station are available, we show that they are in good agreement with those deduced from noise correlation. The measurement of correlation along paths crossing different geological structures allows to differentiate them, opening the way for a passive imaging of the Earth structure. The dispersion measurements are applied to seismic tomography at the regional scale. They make it possible to image crustal structures with a resolution higher than conventional techniques.
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ABSTRACT In this study, 20 hours of ambient seismic noise recorded from a small‐scale inter‐station distance was used to obtain near‐surface shear wave velocity structures at a local test site in Tehran (Iran). High‐resolution group velocity dispersion curves using fundamental mode of surface waves were calculated for all possible combinations of station pairs at frequencies ranging from 1 Hz to 25 Hz. Unlike most previous studies regarding ambient seismic noise, which observe very little coherent noise at frequencies larger than 1 Hz, the empirical Green’s functions were extracted using a root‐mean‐square stacking method showing more coherent signals. Our results indicate that ambient seismic noise is a viable technique at a frequency range of 1 Hz–25 Hz even when different sensor types are present. One‐dimensional V SV and V SH models from the near surface were then assessed by inverting the calculated Rayleigh and Love waves’ dispersion measurements. We observed that the calculated shear wave velocity model agrees with the available downhole model and shows three distinct layers in the upper 25 m of the test site.
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Cross-correlation of ambient noise is an effective approach to extract seismic responses between two stations using seismic interferometry. Since we frequently calculate the cross-correlation of the ambient noise assuming homogeneous distribution of ambient noise sources, heterogeneous distribution of the ambient noise sources would interfere in constructing seismic responses in the calculation of the cross-correlation. In this study, we identified the ambient noise sources recorded in a dense seismic array and utilized the information for better subsurface imaging. The seismic array was composed of 50 stations installed in a 480 m ×350 m area in the Itoshima Peninsula, Japan. By analyzing direction of incoming ambient noise, we found that most of ambient noise was generated by traffic from the nearby street. The traffic noise generated surface waves in lower frequencies (< ~10 Hz) and Pwaves in higher frequencies (> ~20 Hz). We also identified high frequency surface (or air) waves generated by a point source at ~60 Hz. This localized noise could be derived from the renovation work because the location of the source was estimated around the renovation site. We then estimated low- and high-frequency surface wave velocities between each station pair. Although we estimated the surface wave velocities in the limited azimuth between the stations due to the localized noise distribution, we estimated reliable surface wave velocities by considering the noise heterogeneity. High resolution maps of the surface wave velocities were tomographically constructed from the surface wave velocities between the station pairs. Thus, identifying the sources of the ambient noise acquired with dense seismic arrays is effective to improve the ability of ambient noise data to image subsurface structures. It also contributes to the design of seismic arrays in further ambient noise surveys.
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Abstract The increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10–20 s period) and secondary (~5–10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical‐ and horizontal‐component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep‐water and near‐coastal source regions.
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A new, cost effective and non-invasive exploration method using ambient seismic noise has been tested at Soda Lake, NV, with promising results. The material included in this report demonstrates that, with the advantage of initial S-velocity models estimated from ambient noise surface waves, the seismic reflection survey, although with lower resolution, reproduces the results of the active survey when the ambient seismic noise is not contaminated by strong cultural noise. Ambient noise resolution is less at depth (below 1000m) compared to the active survey. In general, the results are promising and useful information can be recovered from ambient seismic noise, including dipping features and fault locations.
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Ambient noise tomography (ANT) has been widely used to image crust and upmost mantle structures. ANT assumes that sources of ambient noise are diffuse and evenly distributed in space and the energy of different modes is equipartitioned. At present, the sources of the primary and the secondary microseisms are well studied, but there are only a few on the studies of long-period ambient noise sources. In this study, we study the effects of large earthquake signals on the recovery of surface waves from seismic ambient noise data recorded by seismic stations from the US permanent networks and Global Seismographic Network (GSN). Our results show that large earthquake signals play an important role on the recovery of long-period surface waves from ambient noise cross-correlation functions. Our results are consistent with previous studies that suggest the contribution of earthquake signals to the recovery of surface waves from cross-correlations of ambient noise is dominant at periods larger than 20–40 s.
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In this study, we present an application of the ambient noise tomography (ANT) to study the near-surface geological structures of the metropolitan Tehran/Iran region. Short-period fundamental mode Rayleigh wave Green's functions were estimated using cross-correlations of the vertical component of the ambient noise from 2009 October to 2011 May using a variety of seismic sensors, for example, accelerometers and seismometers, deployed in the Tehran area. Standard common low frequency processing procedures were applied to the cross-correlations, and shorter time-windows comprising 10-min segments were used in the processing step to enhance the time resolution of the signal in the frequency range of interest (1–10 s). Stacking was also conducted using the rms of the estimated empirical Green's functions. Our results demonstrate that ambient seismic noise tomography is a viable technique at periods of 1–10 s in length, even when different sensor types are present. Analysis of the empirical Green's functions indicates that the dominant sources of ambient seismic noise originated from the same origin, and no significant seasonal or spatial variations in the ambient noise sources were observed. Multiple-filter analysis was used to extract the group velocities from the estimated empirical Green's functions, which were then inverted to image the spatially varying dispersion at periods of lengths between 1 and 7 s using tomographic inversion of the traveltimes estimated for each frequency. The resulting group velocity maps show high correlations with known geological and tectonic features of the study region. In general, most of the Tehran basin, with certain exceptions, could be clearly resolved with low group velocities, whereas the mountain ranges were found to be correlated with high group velocities. In the Tehran basin, for 2 and 3 s periods, the low-velocity zone deepens towards the south–southwest, which reflects thicker sediments in the southern part of the basin than in the north. This feature has also been observed in other geological studies. The Vs models also show that bedrock depth varies between 400 and 1400 m from north to south within the Tehran basin. At longer periods main faults are associated with abrupt transitions between regions of high- to low-velocity anomalies. In general, our results indicate that ANT can be a flexible and effective approach for studying near-surface heterogeneity using short-period surface wave data.
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Summary Near-surface imaging is crucial for a broad range of studies, with seismic monitoring becoming an increasingly important tool due to recent developments in Distributed Acoustic Sensing (DAS) and ambient noise interferometry (ANI). Using a dataset acquired on the Rutford Ice Stream, West Antarctic, we develop ANI approach for DAS datasets which retrieves the Rayleigh wave response from the ambient seismic wavefield. We find that a conventional ANI approach, which uses a single DAS channel as a virtual source and the entire continuous dataset is stacked, produces unstable, poor quality interferograms. This is due to coherent instrument noise and an absence of continuous ambient seismic noise. To overcome these issues, we develop an approach based on selective-stacking and hybrid seismic receivers, which significantly improves the quality of the Rayleigh waves retrieved using DAS. Our findings highlight the impacts that coherent DAS instrument noise and the transient nature of seismic noise above 1Hz has on ambient seismic noise studies.
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Passive seismic interferometry is a new promising methodology for seismic exploration. Interferometry allows information about the subsurface structure to be extracted from ambient seismic noise. In this study, we apply the cross-correlation technique to approximately 25 hr of recordings of ambient seismic noise at the Ketzin experimental CO2 storage site, Germany. Common source gathers were generated from the ambient noise for all available receivers along two seismic lines by cross-correlation of noise records. This methodology isolates the interstation Green's functions that can be directly compared to active source gathers. We show that the retrieved response includes surface waves, refracted waves and reflected waves. We use the dispersive behaviour of the retrieved surface waves to infer geological properties in the shallow subsurface and perform passive seismic imaging of the subsurface structure by processing the retrieved reflected waves.
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Ambient noise level
Passive seismic
Vertical seismic profile
Cross-correlation
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