Detection of Instrument Gain Problem Based on Body-Wave Polarization
Monitoring instrument performance and assessing the instrument response are vital for various seismological analyses that utilize the seismic signal recorded by the instrument. One of the important components of the instrument response is the gain or the amplification factor, which is essential for determining the absolute amplitude, or relative amplitude between different components or instruments.
We introduce a new method to detect problems in the gain of three-component instruments by examining the body-wave polarization measurements. The measurements are made using the same algorithm developed in Park & Ishii (2018). Based on the measured angles, a two-step procedure is applied: (1) identifying time windows with extreme values of measured angles and (2) isolating individual anomalous measurements using cluster analysis. The method has been applied to the Hi-net stations and over 200 stations are identified with issues at various time periods. For example, station TKTH has problem in vertical gain from the late 2009 to about the first half of 2010, and in east-west component from about 2006 to late 2007 (Figure 1). The identified problems generally coincides with anomalous relative amplitude calculated based on the daily calibration pulses of Hi-net, confirming the efficacy of the method. The method also detects problems that calibration pulses cannot detect, which may be linked to the imperfect ground coupling. This techniue is an effective way to detect instrument gain problems without physically accessing the instrument, which is advantageous for instruments such as borehole or ocean-bottom sensors that cannot be accessed easily.
