Specification of Seismograms: The Location Identifier
In order to uniquely identify a seismogram a variety of parameters must be specified. These include the following:
- Network Code
- Station Identifier
- Location Identifier
- Channel Code
- A beginning time and ending time
The Network Code is a 1 or 2 character identifier that is assigned by the Federation of Digital Seismographic Networks (FDSN). Permanent networks are assigned a Network Code that never changes. Temporary networks are assigned a Network Code using the following convention: The Network Code begins with the letters X, Y or Z followed by another letter. These codes are only valid for specific years and can be recycled in subsequent years for another temporary experiment.
Two network codes are reserved for special cases:
SS – used by any institution running a Single Station, but this station should be registered with the FDSN. Care must be given to insure that the Station Identifier (described below) is not identical to another station using the SS network code.
XX – used for an experimental temporary or permanent network. Data collected with this Network Code should be distributed.
A list of FDSN assigned Network Codes can be found at http://ds.iris.edu/stations/networks.txt (permanent) and http://ds.iris.edu/SeismiQuery/tempNets.phtml (temporary).
The National Earthquake Information Center (NEIC) in coordination with the International Seismological Centre (ISC) assigns Station Identifiers. Station Identifiers can be up to 5 characters long and correspond to a specific one-kilometer cube on the earth’s surface. However, a Station Identifier does not uniquely identify a location on the earth and therefor a Network Code is mandatory.
The Channel Code is a 3-character code that specifies the type, bandwidth of the recorded signal, and orientation of the sensor. For instance, a channel code of BHZ is used to indicate the signal came from a broadband (B), high gain seismometer (H) and is recording ground motion in the vertical (Z) direction. A complete summary of channel codes can be found in Appendix A of the FDSN SEED manual
For a variety of reasons some data sources flowing into the IRIS DMC now need an additional parameter to uniquely identify a seismogram. The Location Identifier is a two character code that, when used in conjunction with the other data specifiers, uniquely identifies a data stream. In the case of GSN data the Location Identifier is employed to distinguish between multiple sensors with identical station and channel names. For example, several GSN stations are equipped with both STS-1 and STS-2 broadband high gain seismometers. Both sensors have channel names beginning with BH. Historically this issue of non-uniqueness was handled by specifying non-standard orientation codes. (For instance the STS-1 stream might be named BH followed by N, E or Z and the STS-2 sensor named BH followed by U, V, or W.) Recently support for the Location Identifier was added to replace the orientation code as “unique” identifier.
Historically, within a SEED volume, the Location Identifier was left “blank” (consisted of two spaces). Beginning in 1999, some seismograms at the DMC have non-blank Location Identifiers. Users will need to understand the difference between seismograms that are distinguished by different Location Identifiers.
GSN Use of Location Identifiers
Valid characters for location identifiers are [space, 0-9, A-Z][space, 0-9, A-Z]. (So space-space is a legitimate Location Identifier.) Suggested usage of Location Identifiers for data from the GSN is as follows:
- numbers [0-9][0-9] should be used for multiple instruments at one station. For instance if you have an STS-1 and and STS-2 BHZ channel the location id could be:
- 00 for the STS-1 (primary stream)
- 10 for the STS-2 ( the secondary stream)
- 20 for other sensors, etc
- a [0-9][A-Z] format should be used if one has an instrument that generates multiple data streams with identical channel names, like:
- 0B, etc.
- If it is an array deployment the format should be [A-Z][0-9]:
- A0 perhaps for a central element
- B1, B2, B3 for a first ring in the array
- C1, C2, C3, C4, C5, C6 for a second ring, and so on
Each of these seismograms would still be identified by the same channel code (such as BHZ).
PASSCAL Use of Location Identifiers
For PASSCAL data sets only, the two-character Location Identifier is the data stream number originally used in the REFTEK instrument. There is no special significance of these data stream numbers in terms of the characteristics of the recorded data. To determine which Location Identifier is desired, users will have to query the IRIS DMC database using SeismiQuery (a web-based query tool – http://ds.iris.edu/SeismiQuery) to see what sensor is represented by which identifier.
Location Identifier Specification at the DMC
All of the data request tools (http://ds.iris.edu/data/tutorial.htm) have been updated to allow specification of the Location Identifier. The documentation for the various request tools can be consulted for specific instructions on how to make requests using the various tools.
The DMC has adopted the convention that if the Location Identifier is not specified in a request to the DMC, all data with any location identifiers will be returned. If a data requestor only wishes to receive data that truly has a space in the Location Identifier then this should be specified by placing a dash (-) in one or both of the Location Identifier positions (as needed).
RDSEED and EVALRESP have also been modified to fully support Location Identifiers. RDSEED now produces waveform files with a naming convention such as ANMO.IU.00.BHZ.xxxx or if no location identifier exists in the data ANMO.IU..BHZ.xxxx. RESP files output by RDSEED have adopted this naming convention and EVALRESP requires files with this naming convention.
by Tim Ahern (IRIS Data Management Center)