Y-Axis plot values

Usage Questions
#1

Hello,

I’m 100% new to ObsPy and sorry in advance if the question is stupid.
After going through tutorials, I wanted to start to get my hands on real data. So I created a client to fetch particular waveform data.

t_start = UTCDateTime("2021-01-07T00:30:00.000Z")
st = client.get_waveforms("CR", "ZAG", "*", "BH?", t_start, t_start+30*60) 
st.plot()

However, the Y-Axis doesn’t have “normal” scale. 0 in the middle and than corresponding + and - areas above/below.

Screen Shot 2021-01-09 at 09.33.58
Screen Shot 2021-01-09 at 09.33.581558×1410 198 KB

Could someone explain me why are y-axis values as they are? I know that this isn’t ObsPy problem, but the data is returned like this upon download.

Thank you in advance and sorry for this stupid question

Regards,
Neven

#2

So it seems that I forgot to remove instrument response in the process.

#3

Hi Neven1986,
I similar like you were being new to ObsPy and got a similar issue. The data for the event I am interested in have very high values. You commented you forgot to remove the instrument response. Ca you explain more in detail, please?

Screen Shot 2021-01-30 at 6.48.15 PM
Screen Shot 2021-01-30 at 6.48.15 PM767×237 11.8 KB

Also, I am trying to find a way to extract the actual data series: acceleration time series. Do you maybe tried to do that and have an idea.

Sorry for those stupid questions. I would really appreciate your help.
Cheers,
Anna



#4

You could try to remove the mean by using the detrend command.

https://docs.obspy.org/packages/autogen/obspy.core.trace.Trace.detrend.html

#5

Hi @Ania_K,

sorry for late response… Wasn’t so much active in last time… :slight_smile:
Once I understood from where this high numbers on y-axis come, it was easier to wrap my mind around it. Y-axis is actual seismograph “voltage” representation when movement ocurrs. In order to get graph which makes sense to us e.g. vibrations towards + and - area with 0 as reference value, we need to include seismograph response when plotting the graph.
You can think of it as sort of coefficent(s) which is/are applied to actual plotting values to get the plot starting at reference value of 0. This “response” is different among seismographs and this values need to be included when pulling waveforms, like this (mind “attach_response” in the end):

st = client.get_waveforms("CR", "ZAG", "*", "BHN", t_start, t_start+30*60, attach_response=True)

Now you have everything you need to get your desired graph. First copy your array to another array and apply “remove_response” method.

st_rem = st.copy()
st_rem.remove_response(output = "VEL")

You only need to decide which output is relevant for you; velocity, acceleration or displacement.
Now you can plot your waveform.

Hope this helps,
Neven