International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania

AUTOMATIC DATA PROCESSING AT BURAR SEISMIC STATION

Daniela Ghica1, Johannes Schweitzer2

ABSTRACT

BURAR seismic data are continuously recorded and transmitted in real-time to the Romanian National Data Centre (RO_NDC), where they are automatically processed using a program developed at NORSAR for detecting and associating seismic signals from regional array data, and applied for BURAR characteristics. Automatic estimates from detections (slowness vector and onset time) were verified with events listed in PDE bulletins (worldwide seismicity) and in Romanian Earthquake Catalogue (local and regional events). Good signal detection capability of BURAR station is emphasized both for teleseismic and regional-local events. Automatic data processing system for BURAR station resulted in a good single-array location capability, mostly for the Northern and Western part of Romania territory. BURAR significantly increases seismic monitoring coverage of Romania's territory with National Seismic Network. Currently, this system provides also essential data to the Antelope data acquisition and processing software, used for the real-time digital seismic network operated by NIEP.

INTRODUCTION

In 2002, the Bucovina Seismic Array (BURAR) was build in the Northern part of Romania, in a joint effort of the Air Force Technical Applications Center, USA, and the National Institute for Earth Physics (NIEP), Romania. BURAR consists of 10 seismic stations located in boreholes and distributed over a 5 km2 area: 9 elements are equipped with vertical short-period (SP) sensors (GS-21, Geotech Instruments) and one station is equipped with a three-component broad band (BB) sensor (KS54000, Geotech Instruments) (Grigore at al., 2004). Seismic data are acquired from individual array elements (digitizers) and transmitted to the Field Site Facility (FSF) located at BURAR site. From there, data are directly forwarded by VSAT in CD1.0 format, or the more recent version CD1.1, to RO_NDC. When arriving at RO_NDC, the data in diskloop are converted to continuous CSS3.0 database format. This database file system is input to the automatic array data processing tasks. All data are constantly archived since BURAR installation, on DVDs-Recordable, stored at RO_NDC. Presently, BURAR seismic monitoring system is operational by continuous recording and transmitting data in real-time to the National Data Center of Romania (RO_NDC), in Bucharest and to the National Data Center of USA (US NDC), in Florida. Figure 1 shows BURAR seismic data flow into acquisition to processing itinerary.

AUTOMATIC DATA PROCESSING

At RO_NDC, the data of BURAR are constantly scanned by automatic detection algorithms and analyzed, using automatic routine processing developed at NORSAR (Fyen 1989, 2001, Schweitzer et al., 2002) and adapted to BURAR array. The procedure applied for associating and locating seismic events is the RONAPP (Regional ON-line Array Processing Package) algorithm (Mykkeltveit and Bungum, 1984).

1 National Institute for Earth Physics, 12 Calugareni St., 077125, PO Box MG-2 Magurele, Romania

Tel.: +40 21 4050670, Fax: +40 21 4050673, e-mail: daniela@infp.ro 2 NORSAR, PO Box 53, N-2027, Kjeller, Norway

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The automatic program is based on the advanced techniques within array data processing and analysis, which proved high efficiency in extracting useful signals from seismograms: beamforming and frequency-wavenumer (f-k) analysis (e.g., Capon, 1969). The automatic array processing used at RO_NDC for BURAR data are divided into three steps (see also Schweitzer et al., 2002):

Seismometer(SP/BB)

Digitizer(SP/BB)

Borehole

Sensor Element

BUR01…BUR09 (SP) BUR31 (BB)

FSFField Site Facility

Bucovina Bucharest

RO_NDC

Intra-site communications

CD1.1

Automatic Data Processing

Diskloop

DatabaseCSS 3.0

Data archive DP/SAP/EP

Data Archiving

dl2db

ss2dl

Inter-site communications

(VSAT)

To Antelope

BURAR site map

Figure 1. BURAR seismic data flow

• Detection Processing (DP) – to define signal triggers using beamforming, filtering and STA/LTA detectors;

• Signal Attribute Processing (SAP) – to estimate the slowness vector using f-k analysis technique, and to determine onset time, signal dominant frequency, amplitude and polarization attributes for each detection;

• Event Processing (EP) – to associate seismic phase arrivals for defining events, based on the analysis of the attributes and the sequence of observed seismic phases. 1. The basic detection algorithm in the DP process is a conventional STA/LTA detector applied to a set of predefined beams with BUR01 as reference site, designed for signal enhancement. Following the “tuning” phase, a list of parameter sets to be used as beam settings (“detector recipe”, Schweitzer et al., 2002) was developed for BURAR: - three different sub-configurations of vertical sensors within the array, adapted to different signal frequencies: ALL (BUR01, …, BUR09) for all signals and frequencies, HIGH (BUR01, BUR02, BUR03 and BUR04) – for local signals with high frequencies, and LOW (BUR01, BUR03, BUR05, BUR06, BUR07, BUR08 and BUR09) – for teleseismic signals (low frequencies); - 9 different, 3rd order Butterworth bandpass filters, in the frequency range 0.5 to 6 Hz; - 133 different slowness vectors (pairs of backazimuth and apparent velocity); 12 different apparent velocities. For the 3C site (BUR31), absolute values of the signal traces for the two horizontal waveforms (after filtering in three different frequency bands) are computed and then summed to obtain three different horizontal beams for detection. The entire beam set obtained with this detector recipe and used for the automatic processing of the data recorded by BURAR, is composed of 673 vertical and three horizontal beams. Four different thresholds are used to define detections with the signal-to-noise (SNR) ratio of the beam. The output of the DP processing of the continuous data stream time is one .DPX file per day, which contains the list of triggers from the detector.

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2. After the DP step, SAP process sequentially reads the detections from the .DPX file and performs for each detection an f-k analysis to estimate apparent velocity and backazimuth (observed slowness). The SAP results are reported in .FKX files; for each trigger in .DPX, the output per processed detection is one entry in .FKX files. CSS 3.0 formatted .arrival files are also produced and sent to Antelope system. 3. EP process sequentially reads all detections from the .FKX file and performs phase association, event localization and seismogram plotting. The single array location procedure RONAPP applies the TTAZLOC algorithm (Bratt and Bache, 1988) and locates events with onset time and backazimuth as input data. Apparent velocities of the detected onsets are only used to identify the different seismic phases (P and S). The travel-times tables used in the automatic location procedure for BURAR data were calculated from the 1-D velocity model for Romania (Oncescu, 1984). In figure 2 the three steps of the automatic processing are shown for an event located with BURAR. The solution is reported with two lines (HYP and EPX): event number (1210), origin time (109:11.39.25.7), latitude (46.797°), longitude (22.307°), ML (3.16), distance in [km] (237.7), backazimuth (248.6°), fixed depth (0F). The associated phases are listed in beneath rows: arrival time, station name, phase name, maximum amplitude in [nm], corresponding dominant frequency in [Hz], SNR, beam name, apparent velocity, backazimuth, and an explanatory code for location (Schweitzer et al., 2002).

DP

SAP

EP

Event location report

Figure 2. Steps of the automatic array processing used for BURAR data.

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RESULTS OF AUTOMATIC DATA PROCESSING

To test the automatic processing program used at RO_NDC for array data, data recorded by BURAR from 01 January 2005 until 31 December 2006 were analyzed. The automatic results of the data processing (f-k analysis results) were verified with seismic events listed in the PDE bulletins (59962 teleseismic events), and in the Romanian Earthquake Catalogue (1785 local and regional events). A program was applied to automatically associate the observed onsets with theoretically estimated onsets calculated for the bulletins (Schweitzer, 2001). Figure 3 shows a map of the events detected by BURAR 2005 – 2006 together with the global seismicity. Teleseismic and regional-local detection statistics are plotted in figure 4.

Figure 3. BURAR detection capability versus global seismicity in 2005-2006.

Figure 4. Detection statistics for BURAR data.

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Figure 5. Seismic events automatically located with BURAR .

Good detection capabilities of BURAR could be especially emphasized for teleseismic events. Additionally, the automatic data processing for BURAR shows good single-array location capabilities mostly for the Northern and Western part of Romania: 946 automatic seismic event solutions (results in .EPX files) were obtained in the years 2005 -2006. Fig. 5 shows a map of these automatic solutions.

DISCUSSION

A program for automatic array-data processing was successfully deployed at RO_NDC, to efficiently exploit the seismic data collected with a high-resolution station: BURAR. This system significantly increases the seismic monitoring coverage of Romania's territory. Many events in the region adjacent the array cannot be located with the National Real Time Network, because a less favorable station coverage for the northern and western parts of Romania. Analysing the BURAR data, an important number of local events (within 5o epicentral distance) could be recorded, identified, and characterized. The location capability for local events critically depends on the identification of secondary seismic phases. The number of S-phases recognized with BURAR is limited by the array configuration: only one three-component element is present. To increase the single-array location capabilities in the future, the installation of more horizontal instruments either in existed sites or at new 3C elements is crucial. Additionally, the location process relies on a correct identification of the local and regional seismic phases, which is very much depending on the knowledge of the local and regional seismic velocities. Developing a suitable seismic model of the local structure around BURAR will result in more consistent event locations. Furthermore, for improving BURAR automatic location capabilities, the array calibration by applying corrections to the measured slowness values is required. Well-defined slowness measurements are very necessary for associating seismic phases to presumed seismic events (Schweitzer, 2001). Presently, at RO_NDC, a near real time system for automatic processing of data recorded by BURAR is running. Phase readings, automatically locations, and additional information are stored in a database. The system provides data in CSS 3.0 format, toward the Antelope acquisition and processing software, to be merged with the information recorded by the real-time digital seismic network of Romania.

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REFERENCES

Bratt S.R. and Bache T.C., 1988. Locating events with a sparse network of regional arrays. Bull. Seism. Soc. Amer., vol. 78, p. 780-798.

Capon, J., 1969. High-resolution frequency-wavenumber spectrum analysis. Proc. IEEE, Vol 57, 1408-1418.

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