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Expressing response information in SEED files

This page is retained for historical reference only. The techniques described here have been superseded. Modern dataless SEED volumes are typically generated using software such as Nexus.

The Standard for the Exchange of Earthquake Data (SEED) is an international standard format for seismological data developed by IRIS. A feature of SEED is that it allows you to document the exact sequence of filters used in the digitising process, so that others can work back to the original ground movement if necessary.

This document explains how to construct SEED headers for seismic data from Guralp instruments.

SEED files use control headers containing sets of blockettes to describe the response characteristics for each stream. In SEED, a blockette consists of a type code, its length in bytes, and a series of fields in a specified order. You can have several blockettes of the same type.

Data in the blockettes is given in ASCII format, so the integer 100 is expressed as the three characters 100. To represent a floating-point number, SEED uses 12-character records consisting of:

Thus the value 45.5 could be expressed in 12-character form as “ 4.55000E+01” (note the space at the beginning).

If you have saved your data in SEED format from a compatible program, the file should already include 050 Station Identifier and 052 Channel Identifier blockettes. Response data for each channel should go immediately after the 052 Channel Identifier blockette.


You will need to refer to the 034 Units Abbreviation blockettes in the file, to find out the code for the units you will be using. If you open the file in a text editor, this blockette will look like

0340044701M/S~Velocity in Metres Per Second~

The three figures 701 immediately before the unit description M/S~Velocity in Metres Per Second~ are the relevant code, and you should use it whenever you need to refer to a velocity later in the file. If there is no 034 Units Abbreviation blockette for velocity in the file, you can add this one (changing the 701 to some other unique code if necessary.) Other common 034 Units Abbreviation blockettes are:

0340031702M~Position~in~Metres~ 0340059703M/S**2~Acceleration in Metres Per Second Squared~ 0340020704V~Voltage~ 0340019705C~Counts~

The sensor

The first stage in the process of converting ground motion to the final data stream is the response of the sensor. This is described in two blockettes:

  1. the transfer function, described in a 053 Response (Poles & Zeros) blockette, and
  2. the sensor sensitivity, described in a 058 Channel Sensitivity/Gain blockette.

Poles and zeros

The transfer function of the instrument is expressed in terms of its poles and zeros in an 053 Response (Poles & Zeros) blockette:

0530382B 1701704 7.87395E+00 5.00000E-02 3 0.00000E+00 0.00000E+00 0 .00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+0 0-1.27000E+01 0.00000E+00 0.00000E+00 0.00000E+00 4-1.96418E-03 1.96 418E-03 0.00000E+00 0.00000E+00-1.96418E-03-1.96418E-03 0.00000E+00 0 .00000E+00-6.23500E+00 7.81823E+00 0.00000E+00 0.00000E+00-6.23500E+0 0-7.81823E+00 0.00000E+00 0.00000E+00

This can be broken down into the following fields:

Sensor sensitivity

The next blockette required is an 058 Channel Sensitivity/Gain blockette to describe the sensitivity of the sensor:

0580035 1 3.00000E+03 1.00000E+00 0

Here the fields in use are:

The digitiser

In a SEED file, the conversion of analogue voltage into a digital signal is described in three blockettes:

  1. a transfer function, described in a 054 Response (Coefficients) blockette,
  2. a sampling process, described in a 057 Decimation blockette, and
  3. an output gain, described in a 058 Channel Sensitivity/Gain blockette.

Response coefficients

The digitising process is accomplished entirely in the time domain, and thus it has a flat frequency response. The corresponding 054 Response (Coefficients) blockette contains a single coefficient with a value of 1, denoting that the signal is passed through unchanged:

054004D 2704705 1 1.00000E+00 0.00000E+00 0

See below for a detailed explanation of the 054 Response (Coefficients) blockette.

Sampling and decimation

A 057 Decimation blockette is used to give the initial sample rate of the digital data. Since the SEED standard defines the input sample rate of the stage, the decimation factor of the digitiser is set at 1:

0570055 2 2.56000E+05 1 0 0.00000E+00 0.00000E+00

See below for a detailed explanation of the 057 Decimation blockette.

Output gain

A 058 Channel Sensitivity/Gain blockette is used to provide the gain of the digitiser.

0580035 2 7.87401E+03 1.00000E+00 0

See above for a detailed explanation of the 058 Channel Sensitivity/Gain blockette.


The digitiser’s initial signal is not output directly. Instead, data undergoes a series of decimation filters to reduce its rate and focus on the frequency range of interest.

Guralp digitisers allow you to select up to four data streams from each component, with different sample rates. Each of the digitiser’s output points is called a tap. Before each tap, the digitiser applies up to two FIR filters which reduce the rate by a factor of 2, 4 or 5. In addition, the initially-sampled data has undergone several stages of decimation before it reaches the taps. To provide a full description of the final tap, you will need to describe all the decimation stages in turn—which may be as many as 12.

The exact filter configuration depends on the digitiser you are using. This information is available in a separate article.

In a SEED file, each stage of decimation is represented by three blockettes:

  1. the coefficients of the filter, described in a 054 Response (Coefficients) blockette,
  2. the decimation itself, described in a 057 Decimation blockette, and
  3. an output gain, described in a 058 Channel Sensitivity/Gain blockette.

As well as being in order, each of these blockettes includes a sequence number showing which filter stage it applies to. Stages 1 and 2 have already been used for the sensor and digitiser, so the filter stages will begin with stage 3.

Filter coefficients

The 054 Response (Coefficients) blockette provides the coefficients for a FIR filter.

0540336D 3705705 13 2.44000E-04 0.00000E+00 2.93000E-03 0.00000 E+00 1.61130E-02 0.00000E+00 5.37110E-02 0.00000E+00 1.208500E-0 1 0.000000E+00 1.933590E-01 0.00000E+00 2.255860E-01 0.00000E+00 1.93359E-01 0.00000E+00 1.208500E-01 0.00000E+00 5.371100E-02 0 .00000E+00 1.611300E-02 0.00000E+00 2.93000E-03 0.00000E+00 2.4 40000E-04 0.000000E+00 0

This can be broken down into the following fields:

A single SEED blockette cannot contain more than 414 filter coefficients, since the maximum length allowed is 9999 characters. To express filters with more coefficients than this, you should place as many as possible in the blockette, and start a new blockette immediately after with the same sequence number. Thus, a 1000-point FIR filter would be expressed as a sequence of blockettes as follows:

0549984D 3705705 415 … first 415 coefficients … 0000 0549984D 3705705 415 … next 415 coefficients … 0000 0544104D 3705705 170 … last 170 coefficients … 0000

Be sure to include the 0000 (or 0) at the end of each blockette, which tells SEED that blockette contains no denominators. The length of each blockette is given by 24 + ( number-of-coefficients × 24).


The 057 Decimation blockette describes the input sample rate and the decimation factor provided by the filter:

0570055 3 2.56000E+05 8 0 0.00000E+00 0.00000E+00

Here the fields in use are:

Output gain

A 058 Channel Sensitivity/Gain blockette is used to provide the gain of the filter. This is always 1.

0580035 3 1.00000E+00 1.00000E+00 0

See above for a detailed explanation of the 058 Channel Sensitivity/Gain blockette.