Information blocks from Güralp digitisers
The DM24 has a 1kB buffer inside its firmware, which can be used to store information about attached sensors. Users are free to store any data they wish in the digitiser’s information block.
Typically, this is used to store calibration parameters, poles and zeroes, etc. Newer digital sensors have their calibration information pre-loaded into the information block, in the format described below. DM24mk3 instruments use this information to perform their strong motion calculations.
Information on viewing and uploading digitiser information blocks with Scream! can be found here.
Information on viewing and uploading digitiser information blocks with a Platinum device, such as a DM24SnEAM, an EAM or an instrument with an integrated EAM, such as a 40TDE, can be found here.
Format of the information block
The calibration information begins with the line [instrument-id]. The instrument-id is formed from the system ID and stream ID (serial number), separated by a dash, e.g. GURALP-DEMO. It is also the string which identifies the digitiser in the left-hand pane of Scream!. This information is not actually used by the digitiser or by Scream!, but is included for consistency with the calvals file and other utilities.
After the identifier, calibration information is stored in the form FIELD=VALUE, with one field on each line. The fields used are:
- Serial-Nos [optional]
- An optional serial number to be displayed in the title of Scream! graphs.
- The sensitivity of the Z, N/S, and E/W digitiser input channels, in μV per count, separated by commas. These are given on the digitiser calibration sheet.
- The gain of the Z, N/S, and E/W sensor components, separated by commas. For velocity sensors, the gain, or sensitivity, is given on the sensor calibration sheet in V m–1 s (V/ms-1 or V/m/s). The gain of an accelerometer is expressed in V m–1 s2 (V/ms-2 or V/m/s2) and given in these units on the calibration sheet. These are given on the sensor calibration sheet. Because the outputs are differential, these are given on the calibration sheet as “2 × single-ended-sensitivity”. Most setups (including all those using DM24mk3 and CD24 digitisers) will use the doubled value, but older integrated instruments combining a DM24mk2 with a sensor will use only the single ended sensitivity value.
- The coil constant for the Z, N/S and E/W sensor components, in A m–1 s2, separated by commas. These are given on the sensor calibration sheet. For a 5T accelerometer, the values are all set to unity; use COILCONST=1,1,1 for these.
- The value of the calibration resistor, in Ω, as given on the sensor calibration sheet. For a 5T, use CALRES=1.
- The sensitivity of the digitiser’s calibration channel, in μV per count, as given on the digitiser calibration sheet. Older 5TD instruments do not have calibration input facilities, and thus CALVPC can be omitted.
- The local acceleration due to gravity at the installation site, in m s–2. This field is used by Scream! to display streams in physical units. By default, a standard average g value of 9.80665 m s–2 is used, but this field can be changed for greater accuracy as necessary.
- TYPE [optional]
- The model number of the instrument, used for display purposes.
- A string describing the theoretical response of the instrument, in the form response-type unit. See the table below for the correct values.
Response codes and units
|Instrument description||Response type||Unit|
|3T, 3ESPC or 3EX (30s–50Hz)||CMG-3_30S_50HZ||Vel|
|3T, 3ESPC or 3EX (60s–50Hz)||CMG-3_60S_50HZ||Vel|
|3T, 3ESPC or 3EX (100s–50Hz)||CMG-3_100S_50HZ||Vel|
|3T, 3ESPC or 3EX (120s–50Hz)||CMG-3_120S_50HZ||Vel|
|3T, 3ESPC or 3EX (360s–50Hz)||CMG-3_360S_50HZ||Vel|
|3T, 3ESPC or 3EX (120s–100Hz)||CMG-3_120S_100HZ||Vel|
|5T and 5TC (DC–100Hz)||CMG-5_100HZ||Acc|
The calibration information for a 3T weak-motion velocity sensor might look like the following:
5TD accelerometers use 1 Ω calibration resistors, and their coil constant is set to unity. Older 5TD instruments, based on Mk2 digitiser hardware, do not have calibration input facilities, and thus the CALVPC entry is omitted. For example:
When working with six-channel digitisers, you have a choice:
You can keep a single serial number for both instruments. In this case, the VPC, G and COILCONST lines need to have six comma-separated values each and the CALRES and RESPONSE entries also need double-values; or
You can configure a second serial number into the digitiser for use with SENSOR B. In this case two separate sets of information are required; one for each serial number.
A second serial number can be configured using the web interface of a Güralp EAM (or other Platinum device) or, from the command line of the digitiser, using the SERIAL2 command.
The effects of both methods are identical so you should decide based on whether having two serial numbers appears natural in your application or not.
For more information about working with six-channel digitisers, please see the Serial Numbers for six-channel digitisers page.