Delay correction

As written in the tutorial, stage-level correction s are calculated by OBSINFO using the filter-level offset, stage-level delay and datalogger-level delay_correction fields:

  • offset can be set at the Filter level. If not, it is set to zero. It is a required field for FIR filters (why not the other digital filters?)

  • delay can be set at the Stage level. If it is not specified, it is set to the Stage Filter’s offset divided by the Stage’s ‘input_sample rate. This cannot be done when the Stage is first read, because the Stage’s input_sample_rate usually depends on the preceding Stages.

  • correction depends on the value of datalogger.delay_correction:

    • if datalogger.delay_correction is None (i.e., not specified in the information file, correction = delay in every stage

    • otherwise, correction = datalogger.delay_correction in the last stage and correction = 0 in every other stage.

    this calculation can only be made after delay is calculated

The process for calculating delay and ` correction is as follows:

  1. An attribute dictionary is passed to Instrument._init(), which

    1. creates Sensor, Datalogger, and PreAmplifier properties using their constructors

      • each one creates/contains a ResponseStages() property, but only the Datalogger object gets a delay_correction` value

      • The ResponseStages constructor creates a list of Stage s. Each Stage:

        • contains a delay (usually not specfied in the information file, in which case delay = None)`

        • contains a Filter` that contains an offset (set to 0 if not specified)

    2. calls self.combine_response_stages(), which combines the Sensor, PreAmplifier and DataLogger stages into a single, ordered list (Sensor stages, then Preamplifier and finally Datalogger)

    3. calls self.integrate_response_stages(), which loops through each stage, checking/creating continuity, calculating delay (in Stage.calculate_delay()) and setting correction = delay if correction is None

    4. creates a list of obspy stages and incorporates them into an obspy Response object (including calculating Sensitivity)