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.

Figure 1: Measured angles (gray circles) corresponding to apparent incident angles of (a) P (θv) and (b) S (ϕv), and (c) acute P-horizontal angle with respect to the north (θh) at station TKTH. The data are shown as a function of time from 2004 to April of 2016. The different shades of gray represent the robustness of each measurement. The circles with red and magenta outlines are those identified to have instrument issue in the vertical and horizontal component, respectively, in the first step. The blue lines with plus signs are amplitude ratio between horizontal and vertical (a) and between the two horizontal (c) components obtained based on the Hi-net calibration pulses.

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