This paper gives an account of the work related to the development of the NORESS concept of a regional array. The array design considerations and objectives are reviewed, and a description is given of the NORESS and ARCESS array facilities in Norway with their field installations, data transmission lines, and data-receiving center functions.
The automatic signal detection processing of NORESS data applies multiple narrow-band frequency filters in parallel and forms array beams from selected subgeometries. The detection algorithm is based on computing the STA/LTA ratio for each beam individually, and a detection is declared whenever this ratio exceeds a given threshold. It is explained how the beam deployment and the individual threshold values can be tuned to ensure that the interesting phase arrivals are not missed, but at the same time how to avoid coda detections.
For each detected signal, frequency-wavenumber analysis is invoked to determine arrival azimuth and apparent velocity. Currently, a broadband estimator is used, and it is demonstrated that the use of this algorithm increases the stability of the azimuth and apparent velocity estimates, relative to narrow-band methods. Local and regional events are automatically located on the basis of identification and association of P- and S-wave arrivals. The uncertainty in the arrival azimuth is the limiting factor in accurately determining single-array event locations, and it is shown that this uncertainty is as large as 10° to 15° for Pn phases from certain regions.
In order to further investigate the potential of the NORESS concept, work was initiated toward installing a network of regional arrays in northern Europe. This involved the development of the ARCESS array in northern Norway and the installation of the FINESA array in Finland in cooperation with the University of Helsinki. Data from these three arrays have been used jointly in a location estimation scheme. It is shown that, for events in the Fennoscandian region of magnitude typically around 2.5 and for which at least one phase is detected by each array, location estimates can be obtained automatically that deviate from published network locations by only 16 km on the average.
In the future, it is anticipated that additional arrays and single stations in northern Europe will contribute real-time data to NORSAR for analysis jointly with existing arrays. The first additional data to become available will be from the GERESS array, which will be established in the Federal Republic of Germany in 1990. Future perspectives also include the use of expert system technology in the data analysis, and the IMS system already in operation represents the initial attempt in this regard. A summary is given of problem areas where further work is needed in order to fully exploit the regional array concept.