The capability of three component substation FIA1 at local and regional distances. Comparisons with FINESA and Helsinki bulletins

The automatic analysing capability of the three component substation called hereafter also FIA1 (coordinates: 61.4444°N, 26.0793°E) is studied. The detections and daily bulletins of FINESA (renamed as FINESS since August 1993) are used as a basis of the study. At the three component substation FIA1 a detector bulletin producer of type Husebye Ruudwas used to detect events and after forming a single station daily bulletin the common detections with FINESA were taken into a more detailed examination. From 689 detections of FINESA (also referred to as FIAO) the three component substation could associate 258 events. The main part of events were mining and quarry explosions in Estonia, Russia, north of St. Petersburg and Finland, at distances up to 250 km. The diurnal distribution of events was studied and the connections to certain mines were attempted to be determined. It is found that certain mining areas have very specified shooting times, thus making it possible to monitor this kind of areas under predefined procedure. The median difference of azimuths obtained from the three component station compared with FINESA azimuths was 50 and the median difference of distance was as small as 6.2 km. In these comparisons the siting of the three component sensor was not taken into account, even though it is not located in the centre of the array. The main sources of location differences are found to be the errors of azimuth and veiled later phases. Sometimes the phase pickings of the two different methods did not give any coinciding results, even though the P detections occurred simultaneously. Also, the deviation of azimuths under poor SNR circumstances caused clear location biases. To investigate the detection and locating performance of the three component sub station FIA1 at local and regional distances, the results were compared with the preliminary weekly analysis and Helsinki bulletins of the Finnish National Data Centre (FNDC), which handled 480 events during the test period. Altogether 205 events located by FIA1 could easily be connected with the results of the Finnish National Data Centre. The median errors in azimuth and distance of these connected events were quite small, 8.9° and 9.1 lan, respectively.