Evaluation of the National Seismic Station

Abstract

Abstract : The resolution in A/D conversion when recording waveforms of night- time (low) noise on four different NSS channels at CPO is 0.3 to 1% of the RMS noise power. Clipping of the middle period (MP) and long period (LP) channels in A/D conversion was observed in the case of an M sub s = 7.6 shock at a distance 22 deg, and signal amplitudes up to 30% of the clipping level were observed on the short-period channel when recording nondestructive local shocks (up to 1 deg). In one such case a probably spurious waveform was observed on the vertical MP and LP channels, suggesting non-linear behavior of the analog system at large voltages. A similar phenomenon has been observed at an SRO station. Coherencies computed between the SDCS and NSS channels, i.e., between independent sets of instruments both located at CPO, indicate that when recording night-time (low) noise, the earth-motion/system-noise ratio is good ( 4) up to 60% of the folding frequency for the LP and MP channels. The short-period channel of the Model I NSS seems to have extraneous noise above 4-5 Hz, and it is able to record earth noise down to about 0.15 Hz. The vertical HF channel seems to have system noise at frequencies around 2 Hz which, in some cases, obscures the earth noise. SNR computations suggest that regional phases (P and Lg) can be easily detected above 5 Hz; Lg waves from an NTS shot at a distance of 22 deg had their maximum SNR at a period of 4 seconds. P-waves from teleseismic events generally had SNR maxima at 1 Hz, poor SNR above 5 Hz, and a deep minimum around periods of 4-5 seconds.