E_channel_codes.tex

\chapter{Channel Codes of Seismograms}\label{cha:channel-codes}

Seismic networks, such as the Global Seismographic Network (GSN), generally involve various types of instruments with different
bandwidths, sampling properties and component configurations.
There are standards to name channel codes depending on instrument properties. IRIS (\url{http://www.iris.edu}) uses SEED/FDSN format
for channel codes, which are represented by three letters, such as \texttt{LHN}, \texttt{BHZ}, etc. In older versions of the
SPECFEM package, a common format was used for the channel codes of all seismograms, which was \texttt{LHE/LHN/LHZ}
for three components. To avoid confusion when comparisons are made to observed data, we are now using the FDSN convention
for SEM seismograms. In the following, we give a brief explanation of the FDSN convention and how it is adopted in SEM seismograms.
Please visit \url{http://www.iris.edu/manuals/SEED_appA.htm} for further information.\newline


\noindent
\textbf{\texttt{Band code:}} The first letter in the channel code denotes the band code of seismograms, which depends on the response band and the sampling rate of instruments. The list of band codes used by IRIS is shown in Figure \ref{fig:IRIS_band_codes}. The sampling rate of SEM synthetics is controlled by the resolution of simulations rather than instrument properties. However, for consistency, we follow the FDSN convention for SEM seismograms governed by their sampling rate. For SEM synthetics, we consider band codes for which $dt \leq 1$ s. The FDSN convention also considers the response band of instruments. For instance, short-period and broad-band seismograms with the same sampling rate correspond to different band codes, such as S and B, respectively. In such cases, we consider SEM seismograms as broad band, ignoring the corner period ($\geq 10$ s) of the response band of instruments (note that at these resolutions, the minimum period in the SEM synthetics will be less than $10$ s).
Accordingly, when you run a simulation the band code will be chosen depending on the resolution of the synthetics, and channel codes of SEM seismograms will start with either \texttt{L}, \texttt{M}, \texttt{B}, \texttt{H}, \texttt{C} or \texttt{F}, shown by red color in the figure.\newline

\begin{figure}[ht]
\noindent \begin{centering}
\includegraphics[scale=0.6]{figures/IRIS_band_codes.pdf}
\caption{The FDSN band code convention is based on the sampling rate
and the response band of instruments.
Please visit \url{http://www.iris.edu/manuals/SEED_appA.htm}
for further information. Grey rows show the relative band-code range in SPECFEM,
and the band codes used to name SEM seismograms are denoted in red.}
\label{fig:IRIS_band_codes}
\par\end{centering}
\end{figure}

\noindent
\textbf{\texttt{Instrument code:}} The second letter in the channel code corresponds to instrument codes, which specify the family to which the sensor belongs. For instance, \texttt{H} and \texttt{L} are used for high-gain and low-gain seismometers, respectively. The instrument code of SEM seismograms will always be \texttt{X}, as assigned by FDSN for synthetic seismograms. \newline


\noindent
\textbf{\texttt{Orientation code:}} The third letter in channel codes is an orientation code, which generally describes the physical configuration of the components of instrument packages. SPECFEM uses the traditional orientation code  \texttt{E/N/Z} (East-West, North-South, Vertical) for three components. \newline


\noindent
\textbf{\texttt{EXAMPLE:}} Depending on the resolution of your simulations, if the sampling rate is greater than $0.1$ s and less than $1$ s, a seismogram recorded on the vertical component of station \texttt{AAK} will be named \texttt{IU.AAK.MXZ.sem.sac}, whereas it will be \texttt{IU.AAK.BXZ.sem.sac}, if the sampling rate is greater than 0.0125 and less equal to 0.1 s.