去均值#
去均值是从每个数据点中减去所有数据点的平均值（也称直流分量），这一操作旨在消除仪器零点漂移，一般是数据处理的第一步。
我们以 2022 年 9 月 22 日发生在墨西哥的一个 Mw 6.8 级地震为例，该地震的详细信息见 https://earthquake.usgs.gov/earthquakes/eventpage/us7000ia36。
使用 ObsPy 提供的 Client.get_waveforms() 方法下载
该地震在 ANMO 台站的事件波形资料。
from obspy import UTCDateTime
from obspy.clients.fdsn import Client
import matplotlib.pyplot as plt
client = Client("IRIS")
# 下载 2022 年墨西哥 Mw 6.8 级地震在 ANMO 台站的波形数据
starttime = UTCDateTime("2022-09-22T06:18:00")
endtime = starttime + 720 # 下载 12 分钟数据
st = client.get_waveforms(
network="IU",
station="ANMO",
location="00",
channel="BHZ",
starttime=starttime,
endtime=endtime,
)
st.plot();
---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
Cell In[1], line 1
----> 1 from obspy import UTCDateTime
2 from obspy.clients.fdsn import Client
3 import matplotlib.pyplot as plt
File ~/micromamba/envs/seismo-learn/lib/python3.13/site-packages/obspy/__init__.py:44
41 msg = ('pkg_resources is deprecated as an API')
42 warnings.filterwarnings(
43 'ignore', message=msg, category=DeprecationWarning, module='obspy')
---> 44 from obspy.core.utcdatetime import UTCDateTime # NOQA
45 from obspy.core.util import _get_version_string
46 __version__ = _get_version_string(abbrev=10)
File ~/micromamba/envs/seismo-learn/lib/python3.13/site-packages/obspy/core/__init__.py:120
12 """
13 obspy.core - Core classes of ObsPy
14 ==================================
(...) 117 .. _NumPy: http://www.numpy.org
118 """
119 # don't change order
--> 120 from obspy.core.utcdatetime import UTCDateTime # NOQA
121 from obspy.core.util.attribdict import AttribDict # NOQA
122 from obspy.core.trace import Stats, Trace # NOQA
File ~/micromamba/envs/seismo-learn/lib/python3.13/site-packages/obspy/core/utcdatetime.py:21
18 import warnings
20 import numpy as np
---> 21 from obspy.core.util.deprecation_helpers import ObsPyDeprecationWarning
24 # based on https://www.myintervals.com/blog/2009/05/20/iso-8601, w/ week 53 fix
25 _ISO8601_REGEX = re.compile(r"""
26 ^
27 ([\+-]?\d{4}(?!\d{2}\b))
(...) 40 $
41 """, re.VERBOSE)
File ~/micromamba/envs/seismo-learn/lib/python3.13/site-packages/obspy/core/util/__init__.py:22
20 # import order matters - NamedTemporaryFile must be one of the first!
21 from obspy.core.util.attribdict import AttribDict
---> 22 from obspy.core.util.base import (ALL_MODULES, DEFAULT_MODULES,
23 NATIVE_BYTEORDER, NETWORK_MODULES,
24 NamedTemporaryFile, _read_from_plugin,
25 create_empty_data_chunk, get_example_file,
26 get_script_dir_name, MATPLOTLIB_VERSION,
27 SCIPY_VERSION, NUMPY_VERSION,
28 CARTOPY_VERSION, CatchAndAssertWarnings)
29 from obspy.core.util.misc import (BAND_CODE, CatchOutput, complexify_string,
30 guess_delta, score_at_percentile,
31 to_int_or_zero, SuppressOutput)
32 from obspy.core.util.obspy_types import (ComplexWithUncertainties, Enum,
33 FloatWithUncertainties)
File ~/micromamba/envs/seismo-learn/lib/python3.13/site-packages/obspy/core/util/base.py:26
23 from pathlib import PurePath
25 import numpy as np
---> 26 import pkg_resources
27 from pkg_resources import get_entry_info, iter_entry_points
29 from obspy.core.util.misc import to_int_or_zero, buffered_load_entry_point
ModuleNotFoundError: No module named 'pkg_resources'
ObsPy 提供了 obspy.core.trace.Trace.detrend() 方法可以实现去均值操作。
tr = st[0]
print(f"Mean value before demean: {tr.data.mean()}")
# 去均值处理
tr.detrend("demean")
print(f"Mean value after demean: {tr.data.mean()}")
虽然原始波形在视觉上观察不到显著的直流分量，但是去均值（demean）操作成功地 将原始波形的均值从 1060.42 变为 4.04e-12，表明已经完成了去均值操作。