# What's the physical quantity of waveforms

**URL:** https://discourse.obspy.org/t/whats-the-physical-quantity-of-waveforms/898
**Category:** Users Mailing List Archive
**Tags:** archive
**Created:** [May 30, 2020, 4:00pm UTC](https://discourse.obspy.org/t/whats-the-physical-quantity-of-waveforms/898 "2020-05-30T16:00:18Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![Feng\_Xuping](https://discourse.obspy.org/letter_avatar_proxy/v4/letter/f/54ee81/32.png) [@Feng\_Xuping](https://discourse.obspy.org/u/Feng_Xuping)
#### Post date: [May 30, 2020, 4:00pm UTC](https://discourse.obspy.org/t/whats-the-physical-quantity-of-waveforms/898/1 "2020-05-30T16:00:18Z")

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Hello,

&nbsp;&nbsp;&nbsp;&nbsp;I used the Python tools of Library Obspy (obspy.clients.Client) to download seismic waveforms from IRIS but encountered that the SAC header variable "IDEP" is "-12345".

So I want to know what's the physical quantity of the waveforms, displacement, velocity or acceleration? Can you help me? Thanks!

Kind regards  
Xuping

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### Author: ![Jonathan\_K\_Maccarthy](https://discourse.obspy.org/user_avatar/discourse.obspy.org/jonathan_k_maccarthy/32/266_2.png) [@Jonathan\_K\_Maccarthy](https://discourse.obspy.org/u/Jonathan_K_Maccarthy)
#### Post date: [May 30, 2020, 5:14pm UTC](https://discourse.obspy.org/t/whats-the-physical-quantity-of-waveforms/898/2 "2020-05-30T17:14:13Z")

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Hi Xuping,

If you used the FDSN client, without applying any corrections, I think the units would be in counts (see [NSF SAGE: Frequently Answered Question](https://ds.iris.edu/ds/support/faq/6/what-is-a-count-in-timeseries-data/)). To convert to another unit, you would need the channel response information from the “client.get\_stations” method of your client.

Best,  
Jon