Calculate stream sensitivity

To determine the magnitude correctly you have to configure the sensitivity of all streams of a station correctly. This information is saved at $SH_INPUTS/sensitivities.txt.

Seismic Handler expects this value in unit (nm/s)/count (velocity per count). This data yields the features of the complete recording chain:

   |    |                       ,---.
   |    |        ,----.         |   |
seismometer   pre-amplifier   digitizer

Because you are able to combine any suitable devices, the resulting gain factor must be calculated from these three input values, which can be found in the devices manuals:

  1. seimometer's generator constant or output sensitivity - V/(m/s) (volts per velocity)
  2. factor for pre-amplification for adopting different voltage ranges of sensor and digitizer (unit-less)
  3. digitizer's bit-weight - V/count (volts per count)

To get the total gain value needed for SH/SHM, simply divide the bit-weight by generator constant (convert to V/nm/s before!).

Now let's look at two examples using a STS/2 seismometer:

  • generator constant: 1500 V/(m/s); sometimes given as 2x 750 V/(m/s) - you should use the actual value from the calibration sheet.
  • This value would be altered if a pre-amplifier is used: an attenuation by a factor of two will result in 750 V/(m/s), an amplification by a factor of 2 results in 3000 V/(m/s). Our examples will not use any of them, so the generator constant is not changed.
  1. STS/2 seismometer and Reftek 130 digitizer:

The value of bit-weight can be found in the manual; it is 1.589e-06 V/count.

Dividing bit-weight by generator constant (converted to V/nm/s) results in the total gain:
1.589e-06 / 1500e-09 = 1.0593

  1. STS/2 seismometer and Quanterra Q330:

There is no bit-weight available from the manual. But it can be calculated from the differential input range (Q330: -20 to +20 volts (peek to peek; p-p) resulting in 40 volts total) and the bit resolution (24 bit):

bit-weight = 40 V / 2^24 = 2.3841e-06 V/count

As before we can divide bit-weight by generator constant:
2.3841e-06 / 1500e-09 = 1.5894

Using this recipe it's possible to configure any seismometer-digitizer combination for usage in Seismic Handler.