Noise Performance of IRIS/IDA Broadband Seismic Stations AAK and TLY in the USSR

Abstract : Averaged ambient ground noise power spectra are found two broadband IRIS/IDA seismic stations deployed at Talaya (TLY) near Lake Baikal in Russia and Ala-Archa (AAK) near Bishkek in Kirghizia, central Asia. Site descriptions are also provided for these two stations, as well as major episodes up to mid 1991 in their operational history that are relevant to potential data users. Findings can be summarized as follows: AAK shows among the lowest average absolute nighttime noise levels above I Hz documented to date for IRIS/IDA stations in the former USSR; its night-averaged noise levels above 1 Hz are very similar to those observed at GAR. Ground noise increases during the day over night levels at AAK, with the maximum increase (7-9 dB) occurring between 2-3 Hz. Below .7 Hz, day and night noise levels are the same at AAK. TLY average nighttime ground noise levels are about 6-10 dB higher than AAK levels above 1 Hz; its night-averaged noise levels above 1 Hz are very similar to those observed at IRIS/IDA station KIV. Below .6 Hz, nighttime levels at AAK and TLY are comparable, except that TLY has lower horizontal noise levels (4-5 dB) at periods longer than 25 s. Almost no difference between night and day noise levels was observed at TLY; in this sense it is unique among the IRIS/IDA broadband stations in the USSR. microseism peaks at both stations are comparable (between - 135 to - 140 dB relative to (1 m/s2)2/Hz) at both stations. High- frequency noise levels appear to be fairly constant between 10 - 50 Hz, at between - 150 to - 140 dB relative to (I m/s2)2/Hz.