Information blocks from Güralp digitizers

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 digitizer’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.

Format of the default 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 digitizer in the lefthand 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.

VPC

The sensitivity of the Z, N/S, and E/W digitizer input channels, in μV per count, separated by commas. These are given on the digitizer calibration sheet.

G

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/m/s). The gain of an accelerometer is expressed in V m^–1 s² (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.

COILCONST

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 CMG-5T accelerometer, the values are all set to unity; use COILCONST=1,1,1 for these.

CALRES

The value of the calibration resistor, in Ω, as given on the sensor calibration sheet. For a CMG-5T, use CALRES=1.

CALVPC

The sensitivity of the digitizer’s calibration channel, in μV per count, as given on the digitizer calibration sheet. Older CMG-5TD instruments do not have calibration input facilities, and thus CALVPC can be omitted.

GRAVITY

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.

RESPONSE

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
CMG-3T (30s–50Hz)	CMG-3_30S_50HZ	Vel
CMG-3T (60s–50Hz)	CMG-3_60S_50HZ	Vel
CMG-3T (100s–50Hz)	CMG-3_100S_50HZ	Vel
CMG-3T (120s–50Hz)	CMG-3_120S_50HZ	Vel
CMG-3T (360s–50Hz)	CMG-3_360S_50HZ	Vel
CMG-3T (120s–100Hz)	CMG-3_120S_100HZ	Vel
CMG-3TB (30s–50Hz)	CMG-3B_30S_50HZ	Vel
CMG-3TB (100s–50Hz)	CMG-3B_100S_50HZ	Vel
CMG-3TB (120s–50Hz)	CMG-3B_120S_50HZ	Vel
CMG-3TB (360s–50Hz)	CMG-3B_360S_50HZ	Vel
CMG-3TB (360s–100Hz)	CMG-3B_360S_100HZ	Vel
CMG-3V (30s–100Hz)	CMG-3V_30S_100HZ	Vel
CMG-40T (1s–100Hz)	CMG-40_1S_100HZ	Vel
CMG-40T (2s–100Hz)	CMG-40_2S_100HZ	Vel
CMG-40T (10s–100Hz)	CMG-40_10S_100HZ	Vel
CMG-40T (20s–50Hz)	CMG-40_20S_50HZ	Vel
CMG-40T (30s–50Hz)	CMG-40_30S_50HZ	Vel
CMG-40T (60s–50Hz)	CMG-40_60S_50HZ	Vel
CMG-40T (100s–50Hz)	CMG-40_100S_50HZ	Vel
CMG-5T (DC–100Hz)	CMG-5_100HZ	Acc
CMG-6T (1s–100Hz)	CMG-6_1S_100HZ	Vel
CMG-6T (2s–100Hz)	CMG-6_2S_100HZ	Vel
CMG-6T (10s–100Hz)	CMG-6_10S_100HZ	Vel
CMG-6T (30s–100Hz)	CMG-6_30S_100HZ	Vel

Example files

The calibration information for a CMG-3T weak-motion velocity sensor might look like the following:

[GURALP-DEMO]
Serial-Nos=T3X99
VPC=3.153,3.147,3.159
G=1010,1007,1002
COILCONST=0.02575,0.01778,0.01774
CALVPC=3.161
CALRES=51000
TYPE=CMG-3T
RESPONSE=CMG-3_30S_50HZ Vel
GRAVITY=9.80122

CMG-5TD accelerometers use 1 Ω calibration resistors, and their coil constant is set to unity. Older CMG-5TD instruments, based on Mk2 digitizer hardware, do not have calibration input facilities, and thus the CALVPC entry is omitted. For example:

[GURALP-CMG5]
Serial-Nos=T5585
VPC=2.013,2.028,2.036
G=0.256,0.255,0.255
COILCONST=1,1,1
CALRES=1
TYPE=CMG-5T
RESPONSE=CMG-5_100HZ Acc
GRAVITY=9.81089