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Journal ArticleDOI

Ratio‐to‐moving‐average seismograms: a strategy for improving correlation detector performance

01 Jul 2012-Geophysical Journal International (Oxford University Press)-Vol. 190, Iss: 1, pp 511-521
TL;DR: A correlation detection calculation is demonstrated, which fails due to the presence of a high-amplitude signal interfering with part of the correlation window, but which succeeds when RMA seismograms are useddue to the diminished influence of the interfering signal.
Abstract: SUMMARY Correlation detectors are becoming a standard method for identifying seismic signals from repeating sources. These highly sensitive, source-specific detectors frequently facilitate a reduction in the detection threshold by around an order of magnitude. Detections are typically declared when the value of the correlation coefficient (CC), or a related statistic, exceeds significantly some measure of the variability of values over a longer time window. The performance of correlation detectors is often compromised by the presence of short duration, high-amplitude signals, which influence excessively the value of the CC. We suggest replacing the original seismograms with waveforms in which the value of each sample is replaced by the ratio of that value to a centred moving average of absolute values of the original waveform. These ratio-to-moving-average (RMA) seismograms are relatively featureless over long time intervals, but resemble greatly the original waveforms over short time windows and hence still capture the characteristic seismic fingerprint of a given source. We demonstrate a correlation detection calculation, which fails due to the presence of a high-amplitude signal interfering with part of the correlation window, but which succeeds when RMA seismograms are used due to the diminished influence of the interfering signal. We also demonstrate an example from an aftershock sequence where the CC traces are heavily modulated due to the high dynamic range of the original waveforms. This makes the setting of detection thresholds difficult and results in multiple peaks, which do not correspond to events in the vicinity of the master event. Repeating the calculation using RMA seismograms results in CC traces with a more well-defined detection threshold and most of the spurious detections are lost. The ability to set lower detection thresholds without increasing greatly the number of false alarms facilitates the robust detection of lower magnitude events.
Citations
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Journal ArticleDOI
Xibing Li1, Xueyi Shang1, Zewei Wang1, Longjun Dong1, Lei Weng1 
TL;DR: In this paper, a discrete wavelet transform (DWT) and short time average to long time average (STA/LTA)-based Kurtosis algorithm (W-S/L-K method) is proposed to pick the arrival time of the P-phase; this method advantageously combines the outstanding ability of retrieving the P -phase arrival information from wavelet coefficients at high resolutions with inherent reliability in obtaining the P −phase arrival time using the STA/L TA picking method.

45 citations

Journal ArticleDOI
TL;DR: In this paper, an empirical mode decomposition (EMD)-cosine function denoising-based Akaike Information Criterion (AIC) picker (ECD-AIC picker) is proposed for picking the P-phase arrival time.

27 citations

Journal ArticleDOI
TL;DR: This study proposes a method for picking the arrival of the P-wave in locally stationary seismic background noise that can detect and pick changes both in frequency and amplitude and provides robust detection and picking of P-phase onsets even when the signal-to-noise ratio is low.

26 citations


Cites background or methods from "Ratio‐to‐moving‐average seismograms..."

  • ...This hindrance restricts their use to only repeating sources [23,28,30]....

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  • ...The cross-correlation technique is becoming a standard method for identifying seismic signals from repeating sources [23]....

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Journal ArticleDOI
TL;DR: The results of the study suggest that doing broad regional monitoring using historical and real‐time‐generated templates is feasible and will increase detection capabilities.
Abstract: Waveform correlation is garnering attention as a method for detecting, locating, and characterizing similar seismic events. To explore the opportunities for using waveform correlation in broad regional monitoring, we applied the technique to a large region of central Asia over a three‐year period, monitoring for events at regional distances using three high‐quality stations. We discuss methods for choosing quality templates and introduce a method for choosing correlation detection thresholds, tailored for each template, for a desired false alarm rate. Our SeisCorr software found more than 10,000 detections during the three‐year period using almost 2000 templates. We discuss and evaluate three methods of confirming detections: bulletin confirmation, high correlation with a template, and multistation validation. At each station, 65%–75% of our detections could be confirmed, most by multistation validation. We confirmed over 6500 unique detections. For monitoring applications, it is of interest that a significant portion of the Comprehensive Nuclear‐Test‐Ban Treaty Organization’s Late Event Bulletin (LEB) catalog events was detected and that adding our confirmed detections for the LEB catalog would more than double the catalog size. Waveform correlation also allows for relative magnitude calculation, and we explore the magnitudes of detected events. The results of our study suggest that doing broad regional monitoring using historical and real‐time‐generated templates is feasible and will increase detection capabilities.

24 citations


Cites methods from "Ratio‐to‐moving‐average seismograms..."

  • ...Another method of validation which might help screen the as-yet unvalidated events is doing postdetection frequency– wavenumber (f-k) screening on the correlation traces (Gibbons and Ringdal, 2006; Gibbons et al., 2012)....

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Dissertation
01 Jan 2013
TL;DR: Carmichael et al. as mentioned in this paper use seismic monitoring to detect and locate the creation and growth of some of these hydraulic pathways by monitoring their seismic emissions, or icequakes.
Abstract: Melt-Triggered Seismic Response in Hydraulically-Active Polar Ice: Observations and Methods Joshua D Carmichael Chair of the Supervisory Committee: Professor Ian Joughin Applied Physics Laboratory Glacier ice responds to environmental forcing through changes in its sliding speed and mass balance. While these changes often occur on daily time scales or longer, they are initiated by brittle deformation events that establish hydrological pathways in hours or seconds and allow meltwater access to englacial or subglacial depths to facilitate ice motion. In this thesis, we (various contributing authors including myself) use seismic monitoring to detect and locate the creation and growth of some of these hydraulic pathways by monitoring their seismic emissions, or icequakes. More specifically, we address (1) what seismic observables, unavailable from other sensing methods, indicate an initial glaciogenic response to meltwater input and (2) if these comprise evidence of feedbacks that may destabilize polar ice under a warming climate. Supplemental to our scientific contributions, we advance statistical processing methods that demonstrably improve the capability of digital detectors at discriminating ice quakes from astationary noise. We begin by interpreting geophysical observations collected from a dry-based, subfreezing (−17◦C), polar glacier environment (Taylor Glacier, ANT). By implementing a calibrated surface energy balance model, we estimate the timing and volume of surface meltwater generated during the collection of seismic data from a six-receiver geophone network. This comparison illustrated that any effectively nonzero meltwater triggered large, repeating ice quakes localized near a deep, supraglacial-to-subglacial crack within a meltwater catchment region. The focal mechanisms of these icequakes are consistent with an expansive growth within the crack. Their occurrence at night suggests that this expansion was accommodated by volumetric straining of confined, re-freezing meltwater. These cracks likely sustained their surface-to-bed hydrological connection, in the absence of melt-assisted basal sliding. Further, this appears to be the first report attributing seismogensis in glacial ice to fracturing induced by phase change. We proceed by contrasting these response characteristics with geophysical observations following an early (spring) supraglacial lake drainage within the lake-forming ablation zone of the Western Greenland Ice Sheet. The subglacial drainage system present during this time was considered incapable of efficiently draining large fluxes of meltwater input and therefore likely to undergo transient motion. Using measurements from a ∼ 5km-aperture geophone network, we find that the anticipated post-drainage icequakes are diurnally responsive, largely surficial in origin, and indicative of tensile fracturing from shallow cracks in the ice. The creation of the lake-drainage moulin appears to coincide with a shift in mean icequake source locations, and an increase in icequake occurrence at night relative to that in the day. Contrary to our expectations, we find that the timing of GPS-derived surface speeds do not clearly indicate this seismic activity on any given day. Rather, these icequakes are best explained by peaks in localized strain gradients that develop at night when decreased subglacial water flux likely increases variability in basal traction. Additionally, our results appear comprise the first detailed seismic observations targeted at an actively draining lake. Our last study addresses the apparent deficiency in observed basal icequakes detected from Greenland lake site. To explain the lack of deep icequakes, we compute thresholds on the magnitude of detectable basal events within the network and thereby illustrate that surficial icequakes with similar magnitudes and spectral content are more likely to be observed. By restricting our attention to seismic events that produce lower frequency waveforms, we find a population of nearly monochromatic, sub-1Hz, large magnitude (Mw ≤ 3) seismic events borne from remote glaciogenic sources. In contrast to surficial icequakes, these events occur without significant bias between day and/or night periods and are best explained as glacial earthquakes generated by sliding episodes or iceberg calving events in the vicinity of Jakobshavn Glacier. These events occur daily and not correlate with the presence of local, surficial seismicity. We conclude with three general assertions regarding melt-triggered response characteristics of polar ice. First, hydraulic connections established by fracture events do not necessarily result in seismogenic basal stick slip, and therefore cannot necessarily be observed with conventional GPS monitoring. This was demonstrated at Taylor Glacier. Here, meltwater input to a hydraulic pathway led to fracture growth deep within a cold glacier without any change in surface speed. Second, the presence of melt-triggered basal sliding does not necessarily induce a clear seismogenic basal response in the lakes regions. This was demonstrated on the Greenland Ice Sheet. Seismogenesis may instead be more clearly reflected by surficial strain gradients established by variability in basal traction, suggesting these feedbacks are secondary rather than primary. The response is therefore not clearly indicated from day-to-day timing of GPS-observations. Third, the absence of an observed local response does not necessarily indicate the absence of a local physical response. This was also illustrated in Greenland. Here, deep local icequakes are likely muted by noise, waveform-attenuating ice, and viscous basal rheology. Magnitude thresholds suggest that (Mw ≤ 2) for consistent recording of local, non-surficial sources. In contrast, remote, low frequency seismic events were clearly observed, and attributed to activity within ice catchments along the western edge of the ice sheet or Jacobshavn glacier. Finally, we assert that early-indicators of melt-triggered glacial response include components of spatially localized, brittle deformation that is most suitable to seismic observation. Critically-stable regions along mass-balance equilibrium lines constitute potential sites for newly forming surface-to-bed hydraulic connections in a warming climate, and likewise, a potential target for future seismic experiments.

14 citations

References
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Journal ArticleDOI

2,001 citations


Additional excerpts

  • ...For example, some authors (e.g. Vandecar & Crosson 1990) advocate applying the Fisher transformation (Fisher 1915) to map the correlation coefficient traces, bounded in the interval [-1:+1], to normally distributed functions....

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Journal ArticleDOI
11 Mar 2005-Science
TL;DR: Cross-correlation of 1 month of ambient seismic noise recorded at USArray stations in California yields hundreds of short-period surface-wave group-speed measurements on interstation paths that are used to construct tomographic images of the principal geological units of California.
Abstract: Cross-correlation of 1 month of ambient seismic noise recorded at USArray stations in California yields hundreds of short-period surface-wave group-speed measurements on interstation paths. We used these measurements to construct tomographic images of the principal geological units of California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges. This method can improve the resolution and fidelity of crustal images obtained from surface-wave analyses.

1,912 citations


Additional excerpts

  • ...…that high-amplitude transients (e.g. from the presence of earthquake signals) were likely to bias the results and it became standard practice to perform so-called “one-bit correlations”, only considering the sign of each sample (e.g. Campillo & Paul 2003; Larose et al. 2004; Shapiro et al. 2005)....

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Journal ArticleDOI
24 Jan 2003-Science
TL;DR: This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation and has potential applications in other fields.
Abstract: The late seismic coda may contain coherent information about the elastic response of Earth. We computed the correlations of the seismic codas of 101 distant earthquakes recorded at stations that were tens of kilometers apart. By stacking cross-correlation functions of codas, we found a low-frequency coherent part in the diffuse field. The extracted pulses have the polarization characteristics and group velocities expected for Rayleigh and Love waves. The set of cross-correlations has the symmetries of the surface-wave part of the Green tensor. This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation. Because it relies on general properties of diffuse waves, this result has potential applications in other fields.

1,139 citations


Additional excerpts

  • ...…that high-amplitude transients (e.g. from the presence of earthquake signals) were likely to bias the results and it became standard practice to perform so-called “one-bit correlations”, only considering the sign of each sample (e.g. Campillo & Paul 2003; Larose et al. 2004; Shapiro et al. 2005)....

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Journal ArticleDOI
TL;DR: In this article, a computer program was developed for the automatic detection and timing of earthquakes on a single seismic trace, which operates on line and is sufficiently simple that it is expected to work in inexpensive low-power microprocessors in field applications.
Abstract: A computer program has been developed for the automatic detection and timing of earthquakes on a single seismic trace. The program operates on line and is sufficiently simple that it is expected to work in inexpensive low-power microprocessors in field applications. In tests with analog tapes of earthquakes, the program correctly identified and timed to within 0.05 sec about 70 per cent of the events which would normally be timed in operation of a network. The program evaluates the accuracy of its picks, and its estimates appear to be quite reliable. The algorithm is working at present in a 16-bit minicomputer and appears to be compatible with presently available microprocessors.

938 citations


"Ratio‐to‐moving‐average seismograms..." refers background in this paper

  • ...At the opposite extreme, at which we know nothing about the signal waveform, the most typical procedures detect increases in signal amplitude (e.g. Allen 1978, 1982; Withers et al. 1998) with the most common trigger being the signal-to-noise-ratio (SNR) defined as the ratio between a…...

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Journal ArticleDOI
15 Mar 2007-Nature
TL;DR: It is demonstrated that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface.
Abstract: Extended-duration seismic signals occur episodically on some major faults, often in conjunction with aseismic or 'slow-slip' earthquake events. The mechanism underlying this tremor and its relationship to the aseismic slip are as yet unresolved. David Shelley et al. demonstrate that tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Non-volcanic tremor is a weak, extended duration seismic signal observed episodically on some major faults, often in conjunction with slow slip events1,2,3,4. Such tremor may hold the key to understanding fundamental processes at the deep roots of faults, and could signal times of accelerated slip and hence increased seismic hazard. The mechanism underlying the generation of tremor and its relationship to aseismic slip are, however, as yet unresolved. Here we demonstrate that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process.

835 citations


"Ratio‐to‐moving‐average seismograms..." refers methods in this paper

  • ...Correlation methods have been used increasingly and extensively in the identification of low frequency earthquakes (LFEs) in non-volcanic tremor, partly due to the difficulties in applying classical seismological methods to these signals (e.g. Shelly et al. 2007; Ohta & Ide 2011)....

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  • ...Significant CC values can be detected using a power detector (e.g. Gibbons & Ringdal 2006; Schaff 2008) or by considering statistical outliers to the distribution of the CC-traces over long time intervals (e.g. Shelly et al. 2007)....

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Trending Questions (1)
How often should you replace CO detectors?

The performance of correlation detectors is often compromised by the presence of short duration, high-amplitude signals, which influence excessively the value of the CC.