Calibration of Guralp Seismometers
Objective
- Estimate transfer functions of Guralp A ( near ETMX) and Guralp B ( near ETMY)
- Calibrate the instruments by estimating Velocity Sensitity Parameter
- Convert previously measured Voltage Spectrum to Velocity Spectrum
Instruments Used
- Guralp CMG-40 T Seimometers : Guralp A (Serial Number: T4Q17)
- Guralp CMG-40 T Seimometers : Guralp B (Serial Number: T4157)
- Guralp Handheld Control Unit (HCU)
- FFT Spectrum Analyzer: Model SR785: 2 Channel Dynamic Signal Analyzer
- Oscilloscope: TDS 3014B
- Function Generator: DS 345
Procedure & Results
Sinusoidal current of known frequency and amplitude was injected to the Seismometer calibration coil using signal generator and handheld control unit & corresponding Magnitude and Phase response were recorded. For Guralp B, system response was also estimated with a FFT Spectrum Analyzer.
 
Frequnecy Range: 0.1 Hz to 45 Hz.
Equivalent Input Velocity was derived from the Input Voltage measurements using the relation: v = V/ (2*pi*f*R*K) , V is the peak to peak Calibration Signal voltage, f is the calibration signal frequency, R is the calibration resistor and K is the feedback coil constant. [See Appendix for R & K values]
Velocity Sensitity at the required frequency is obtained by dividing the Output Response Voltage by the Equivalent Input Velocity.
 
The obtained Velocity Sensitivity is used to convert the recorded Volatge PSD to Velocity PSD as shown below. The obtained results are compared to gloabl high noise model [NHNM] and USGS New Low Noise Model [NLNM,Peterson 1993] which gives the lowest observed vertical seismic noise levels across the seismic frequency band. Plot legend NLNM shows both the high & low levels.
Guralp A [X Arm] Low Velocity Output
  
Guralp B [Y Arm] Low Velocity Output
  
DTT Power Spectrum
Both the Seismometers were connected to the 40 M Control and Data Acquisition System (CDS) and Power Spectrum was estimated for the Vertical, North/South & East/West Channels using Diagnostic Test Tool (DTT) software.
Comments
- The transfer function from Guralp A [ETMX] looks similar to that of Guralp B [ETMY] in both magnitude and phase but with a lower gain.
- Velocity Sensitivity of Guralp A is comparable to the value provided in the Calibration Data Sheet [~ 400] for all the channels [Vertical, North/South, East/West] after 1 Hz. For Guralp B, Velocity Sensitivity is a factor of 2.5 higher [all channels] than the specification [~ 400] after 1 HZ.Below 1 Hz Sensitivity drops down for both sensors. I am not ruling out a missing common factor in the calculation, but anyway, test shows that Guralp B has ~2.5 times better Velocity Sensitivity than Guralp A.
- The Calibrated Seismic Velocity Spectrum for Guralp B is within the Globally Observed High and Low Noise Seismic Spectrum while Guralp A's Spectrum is more noisier above 1 Hz [Anthropogenic Activity normally contributes the most in 1 Hz to 10 Hz frequency band].
- Concurrently acquired Power Spectrum using DTT [Diagnostic Test Tools] shows that Guralp A Spectrum behaves rather strangely. The system response seems to be completely different from the one we obtained locally using signal generator. While Guralp B functionality seems normal. One reason for this erratic beahvior might be faulty cables used for data acquisition from Guralp A. This needs to be verified.
Appendix
CMG-40T Guralp A Calibration Sheet
| |
Velocity Output: V/m/s (Differential) |
Mass Position Output (Acceleration Output) : V/m/s^2 |
Feedback Coil Constant : Amp/m/s^2 |
| Vertical |
2 x 400 |
19 |
0.00397 |
| North/South |
2 x 398 |
23 |
0.00486 |
| East/West |
2 x 401 |
23 |
0.00479 |
Calibration Resistor: 51000
CMG-40T Guralp B Calibration Sheet
| |
Velocity Output: V/m/s (Differential) |
Mass Position Output (Acceleration Output) : V/m/s^2 |
Feedback Coil Constant : Amp/m/s^2 |
| Vertical |
2 x 401 |
19 |
0.00408 |
| North/South |
2 x 400 |
20 |
0.00421 |
| East/West |
2 x 404 |
22 |
0.00466
|
Calibration Resistor: 51000 |