The detection of low magnitude seismic events using array-based waveform correlation

Abstract

SUMMARYIt has long been accepted that occurrences of a known signal are most effectively detected by cross-correlating the incoming data stream with a waveform template. Such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori. Repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co-located events at lower magnitudes than would be possible using traditional power detectors. Even greater improvement in signal detectability can be achieved using seismic arrays; running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain. If two events are co-located, the time separating the corresponding patterns in the wavetrain as indicated by the cross-correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not. We illustrate the power of array-based waveform correlation using the 16 August 1997 Kara Sea event. The weak event which occurred four hours after the main event was barely detected using an STA/LTA detector on the SPITS array but is readily detected by signal matching on a single channel. The main event was also recorded by the far more distant NORSAR array but no conventional detection can be made for the second event. A clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array. We estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise. We show that, for this case, waveform correlation using a single channel detects signals of approximately 0.7 orders of magnitude lower than is possible using an STA/LTA detector on the array beam. Waveform matching on the full array provides an additional improvement of approximately 0.4 magnitude units. We describe a case study in which small seismic events at the Barentsburg coal mine on Spitsbergen were detected using the signals from a major rockburst as master waveforms. Many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wavefronts. We demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency-wavenumber analysis upon the set of individual correlation coefficient traces.

Creep and slip: Seismic precursors to the Nuugaatsiaq landslide (Greenland)

“…To quantify the evolution of precursory signals (Figure 2), I exploit their similarity and use a coherence-based method [Gibbons and Ringdal, 2006] aimed at counting how many events are occurring in the hours before landslide. To that end, I arbitrarily select a reference three-component waveform at station NUUG (Figure 2d) and correlate it against 24 h of seismic data.…”

“…Given the similarities between the newly detected events, they can be stacked to improve the signalto-noise ratio [Brown et al, 2008] and to define a new reference trace. Using this new reference signal, I run the coherence-based method [Gibbons and Ringdal, 2006;Brown et al, 2008] for the second time. This second stage provides 95 detections (Figures 3 and 4).…”

Probabilistic infrasound propagation using realistic atmospheric perturbations

“…Locations and origin times are obtained from ground truth or GT events. A seismic signal with a known source is unambiguously associated with GT events using correlation detectors [Gibbons and Ringdal, 2006] and Empirical Matched Field Processing [Harris and Kvaerna, 2010], which are so-called pattern detectors. The uncertainty in the source location is limited to the dimensions of the mine (of the order of a couple of kilometers) [Evers et al, 2012], and the origin time of each explosion can be determined to within approximately 1 s. All explosions take place at approximately 19:30 central European time (CET).…”

Sedimentary basins investigation using teleseismic P‐wave time delays

“…This approach has been applied in several studies and gives stable results under different conditions of event magnitude, network geometry and tectonic settings, from local to global scale (e.g. Gibbons and Ringdal 2006;Schmandt and Humphreys 2010). However, two main pitfalls arise in the VDC90 method.…”