The tiskitpy FIRConverter class

Replacing a stream’s last zero-phase FIR stage by an equivalent minimum phase stage, to improve first arrival time and direction picks.

The fir_converter_sds` console script allows you to apply this to an entire SDS directory.

from tiskitpy import FIRConverter
from obspy.core import read, UTCDateTime

# Create a converter from a zero-phase FIR file
converter = FIRConverter.from_zeros_file('data/cs5322_fir3.json', 0)

# Plot technical figures about the input FIR
print('Plotting Z-plane representation of the input filter')
converter.plot_zplane()

Plotting Z-plane representation of the input filter
../_images/9_firconv_zpoles.png 
print('Plotting decomposition of the input filter')
converter.plot_impulse_parts()

Plotting decomposition of the input filter
../_images/9_firconv_responses.png 
# Apply the converter to the data
datafile = 'data/LSVEI_20150827T0612.mseed'
plotstart = UTCDateTime('2015-08-27T06:12:37.5')
plot_seconds = 0.6

# Read and convert data
st = read(datafile, 'MSEED')
st_min, _ = converter.apply(st, new_loc_code='01')

# Plot
print(f'Plotting Lucky Strike event at {str(plotstart)}.  Locations are:\n'
      '    "00": the original data\n'
      '    "01": converted to minimum phase')
st_compare = st.select(component='Z') + st_min.select(component='Z')
st_compare.resample(500)  # Oversample, to smooth the waveform
st_compare.plot(starttime=plotstart,
                endtime=plotstart + plot_seconds)

Plotting Lucky Strike event at 2015-08-27T06:12:37.500000.  Locations are:
    "00": the original data
    "01": converted to minimum phase
../_images/9_firconv_output.png

## The zero-phase filter file

There are several ways to decribe the zero-phase filter or its conversion. In this example we use a JSON file describing the zero-phase filter and the constructor FIRConverter.from_zeros_file:

The JSON file must contain:

  • type: linear.

  • fir: the linear-phase coefficients.

  • timetag: the sample offset to fir’s peak.

  • decimation_factor: the decimation applied after this filter.

The contents of data/cs5322_fir3.json are:

{
    "type": "linear",
    "fir": [
        -26,    -247,     -822,   -1362,   -839,   1012,    2197,      212,   -3443,   -3077,
       3156,    7168,     -256,  -10709,  -7644,  10713,   18055,    -3873,  -28007,  -11826,
      31641,   35194,   -22177,  -60427,  -5404,  77065,   51056,   -71982, -106905,   33416,
     156296,   43678,  -175718, -152408, 139856, 270573,  -29083,  -360427, -162173,  371807,
     417807, -246840,  -693181,  -78388, 902497, 685231, -865217, -1713558,    -262, 3276208,
    4950471,
    3276208,    -262, -1713558, -865217, 685231, 902497,  -78388,  -693181, -246840,  417807,
     371807, -162173,  -360427,  -29083, 270573, 139856, -152409,  -175718,   43678,  156296,
      33416, -106905,   -71982,   51056,  77065,  -5404,  -60427,   -22177,   35194,   31641,
     -11826,  -28007,    -3873,   18055,  10713,  -7644,  -10709,     -256,    7168,    3156,
      -3077,   -3443,      212,    2197,   1012,   -839,   -1362,     -822,    -247,     -26
    ],
    "timetag": 50,
    "decimation_factor": 2
}