Waveform Processing

Waveform Preprocessing

Description

Perform standard preprocessing operations on ObsPy Stream/Trace: detrending, demeaning, tapering, filtering, resampling, and windowing.

Main Functions

detrend_stream(st, type="demean")

Remove trend or mean from waveform.

Parameters:

• st : obspy.Stream
• type : str — "demean" (remove mean) / "linear" (remove linear trend) / "constant" (same as demean)

Returns: obspy.Stream (modified in place and returned)

st = read_stream("/data/wave.mseed")
st = detrend_stream(st, type="demean")     # Remove mean
st = detrend_stream(st, type="linear")    # Remove linear trend

taper_stream(st, max_percentage=0.05, type="cosine")

Apply cosine taper window to both ends of waveform to reduce spectral leakage.

Parameters:

• st : obspy.Stream
• max_percentage : float — Taper fraction of total length, default 0.05 (5%)
• type : str — Window type, "cosine" / "hann" / "hamming", etc.

st = taper_stream(st, max_percentage=0.05)

filter_stream(st, filter_type, freqmin=None, freqmax=None, corners=4, zerophase=True)

Apply frequency domain filtering to Stream.

Parameters:

• st : obspy.Stream
• filter_type : str — "bandpass" / "lowpass" / "highpass" / "bandstop"
• freqmin : float — Low cut frequency (Hz), required for bandpass/highpass
• freqmax : float — High cut frequency (Hz), required for bandpass/lowpass
• corners : int — Filter order, default 4
• zerophase : bool — Zero-phase filtering (forward + reverse), default True

Returns: obspy.Stream

st = read_stream("/data/wave.mseed")
st = detrend_stream(st)
st = taper_stream(st)

# Bandpass 1-10 Hz
st_bp = filter_stream(st, "bandpass", freqmin=1.0, freqmax=10.0)

# Lowpass 5 Hz
st_lp = filter_stream(st, "lowpass", freqmax=5.0)

# Highpass 0.5 Hz
st_hp = filter_stream(st, "highpass", freqmin=0.5)

resample_stream(st, sampling_rate)

Resample to specified sampling rate.

Parameters:

• st : obspy.Stream
• sampling_rate : float — Target sampling rate (Hz)

Returns: obspy.Stream

# Downsample to 50 Hz
st = resample_stream(st, sampling_rate=50.0)

trim_stream(st, starttime=None, endtime=None, pad=True)

Extract specified time window.

Parameters:

• st : obspy.Stream
• starttime : obspy.UTCDateTime or str, e.g. "2024-01-01T00:00:00"
• endtime : obspy.UTCDateTime or str
• pad : bool — Pad with zeros if insufficient data, default True

from obspy import UTCDateTime
t0 = UTCDateTime("2024-03-15T08:30:00")
st = trim_stream(st, starttime=t0, endtime=t0 + 60)  # Extract 60 seconds

merge_stream(st)

Merge multiple traces of the same channel (fill gaps).

Returns: obspy.Stream

st = merge_stream(st)

remove_response(st, inventory_or_paz, output="VEL", pre_filt=None)

Remove instrument response and convert to physical quantity (displacement / velocity / acceleration).

Parameters:

• st : obspy.Stream
• inventory_or_paz : obspy.Inventory or PAZ dict
• output : str — "DISP" (displacement) / "VEL" (velocity) / "ACC" (acceleration)
• pre_filt : tuple — Water-level pre-filtering, e.g. (0.005, 0.01, 45, 50)

from obspy import read_inventory
inv = read_inventory("station.xml")
st = remove_response(st, inv, output="VEL", pre_filt=(0.005, 0.01, 45, 50))

Standard Preprocessing Workflow

st = read_stream("/data/wave.mseed")

# Standard four-step preprocessing
st = detrend_stream(st, type="demean")
st = detrend_stream(st, type="linear")
st = taper_stream(st, max_percentage=0.05)
st_filtered = filter_stream(st, "bandpass", freqmin=1.0, freqmax=10.0)

print("Preprocessing complete")
stream_info(st_filtered)

Notes

• Always detrend and taper before filtering, otherwise edge effects will contaminate the spectrum
• zerophase=True introduces no phase shift and is strongly recommended in seismology
• Apply lowpass filtering (cutoff < new sampling rate / 2) before resampling to avoid aliasing