Model Based Signal Processing

We demonstrate that several techniques based on cross correlation are able to significantly reduce the detection threshold of seismic sources worldwide and to improve the reliability of IDC arrivals by a more accurate estimation of their defining parameters. More than ninety per cent of smaller REB events can be built in automatic processing while completely fitting the REB event definition criteria. The rate of false alarms, as compared to the events rejected from the SEL3 in the current interactive processing, has also been dramatically reduced by several powerful filters. The principal filter is the difference of arrival times between the master events and newly built events at three or more primary stations, which should lie in a narrow range of a few seconds. Two effective pre-filters are f-k analysis and F prob based on correlation traces instead of original waveforms. As a result, cross correlation may reduce the overall workload related to IDC interactive analysis and provide a precise tool for quality check for both arrivals and events. Some major improvements in automatic and interactive processing achieved by cross correlation are illustrated by an aftershock sequence of a large continental earthquake. Exploring this sequence, we describe schematically the next steps for the development of a processing pipeline parallel to the existing IDC one in order to improve the quality of the REB together with the reduction of the magnitude threshold. The current IDC processing pipeline should be focused on the events in areas without historical seismicity which are not properly covered by REB events.

The waveform cross correlation technique is used to detect signals from aftershocks of the ML = 6.1, August 14, 2016, Sakhalin earthquake using regional seismic network data. Regular P-and S-wave phase arrivals are associated with seismic sources; the resulting cross correlation bulletin is compared with a standard seismic bulletin for the same aftershock sequence. An advantage of the cross correlation bulletin is that it has more detected seismic events tied to three or more stations and a lower catalog completeness threshold; even the epicenters of the weakest aftershocks are located with greater accuracy. The exact aftershock locations tend to cluster along a narrow zone related to the Central Sakhalin Fault.

The multi-master method based on waveform cross-correlation has found several aftershocks of the third (DPRK3) and the fourth (DPRK4) announced underground nuclear tests conducted by the DPRK on 12.02.2013, and 06.01.2016, respectively. The second DPRK test had no reliable aftershock hypotheses at the level of the method sensitivity and resolution. The closest to the test site array station USRK was not operational before 20.12.2008 and the first DPRK test cannot be analyzed by the multi-master method based on waveform templates from two IMS stations-USRK and KSRS. The largest aftershocks of the DPRK3 and DPRK4 are close in magnitude to the first aftershock of the DPRK5 on 11.09.2016, and many aftershocks of the DPRK6. Three big aftershocks of the DPRK3 occurred on 25.05.2014, i.e. 467 days after the mainshock. The DPRK3 aftershock found at 6:43:03 UTC had a relative magnitude of 2.91. There were found two aftershocks of the DPRK4: on 19.02.2016 (0:28:07 UTC) and on 02.07.2016 (19:52:27 UTC). The largest aftershocks of the DPRK3 and DPRK4 could be interpreted as related to the cavity collapse process likely followed by a chimney collapse, not reaching the free surface, however, as no noble gases and/or radionuclide particles were detected. Seismic signals from all events found before the DPRK5 (09.09.2016), had a high level of cross-correlation with the waveform templates of the DRPK5 and DPRK6. These signals prove the shape similarity, and thus, the close spatial proximity of the seismic events after DPRK3/DPRK4 to the DPRK5/DPRK6 aftershocks. There were no other events found by the multi-master method since 01.01.2009, even when the highest resolution regime was used. The resolution and sensitivity of the multi-master method were tuned and tested using the data after 09.09.2016.

The fundamental and practical problem of passive localization in range and depth, of an acoustic underwater source is addressed, with application to an at-sea experiment. We propose and try a new matching method based on a metric called as Hausdorff distance as a cost-function to be minimized, in order to perform the localization inversion. The data set analyzed here was collected during the DGA campaign ALMA 2015, which took place in a shallow water environment of the southern coast of France. Acoustic data were measured over a 10m-high vertical linear array (VLA), composed of 64 hydrophones. The 2-D localization, in range and depth, is performed by matching the patterns of time difference of arrival (TDOA), between respectively observed and modeled sequences. Several variants of the Hausdorff Distance are applied, firstly separately in each single hydrophone, and then combined in order to improve the localization accuracy, reducing the ambiguity either is depth and in range. The performance is evaluated in terms of the localization accuracy of the proposed method, in a context of passive localization with a cooperative system considering a motionless target. Very satisfactory performance and accuracy are obtained.

We apply cross correlation between multichannel seismic waveforms as a technique for signal detection and automatic event building at the International Data Centre (IDC). This technique allows detecting signals with amplitudes by at least a factor of two lower than those found in the current version of IDC processing. Previously, we processed with a cross correlation detector aftershock sequences of a large earthquake with thousands of aftershocks detected by the International Monitoring System (IMS) and a middle-size earthquake (hundreds of aftershocks). Our study has revealed that the official Reviewed Event Bulletin (REB) of the IDC misses from 50% to 70% valid seismic events. Since the IDC is a major contributor to the International Seismological Centre (ISC) these extra events together with the associated arrivals are missing from the ISC bulletin which is an open data source for the broader seismological and geophysical community.Here, we assess the ultimate resolution of the ...

We apply cross correlation between multichannel seismic waveforms as a technique for signal detection and automatic event building at the International Data Centre (IDC). This technique allows detecting signals with amplitudes by at least a factor of two lower than those found in the current version of IDC processing. Previously, we processed with a cross correlation detector aftershock sequences of a large earthquake with thousands of aftershocks detected by the International Monitoring System (IMS) and a middle-size earthquake (hundreds of aftershocks). Our study has revealed that the official Reviewed Event Bulletin (REB) of the IDC misses from 50% to 70% valid seismic events. Since the IDC is a major contributor to the International Seismological Centre (ISC) these extra events together with the associated arrivals are missing from the ISC bulletin which is an open data source for the broader seismological and geophysical community.Here, we assess the ultimate resolution of the ...