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voremargot / Seismic-Tremor-Reveals-Spatial-Organization-and-Temporal-Changes-of-Subglacial-Water-System

The code used to analyize the seismic data used in Vore et al. in Journal of Geophysical Research: Earth Surface [ based on Park et al. (1987) paper]
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Very few northerly azimuths #1

Open tbartholomaus opened 4 years ago

tbartholomaus commented 4 years ago

Hey @voremargot,

I know you identified this issue previously, but can't remember what you figured out. Do you know why essentially no backazimuths are identified coming from the north?

Thanks BBWL_Day184

voremargot commented 4 years ago

I know it doesn’t work for E or W because you can’t divide by zero in the calculation for the back azimuth (eq. 11 in our paper) and the opposite tan of 90 degrees is infinity (this is the case I saw in my work) .

A_E in equation 11 is never zero as there is always three components as an input (eq. 7 always has three components). I am wondering if there is always a little bit of E-W motion in the signal. Maybe there is a threshold for the E-W vs N-S ratio where you start seeing near zero backazimuths?

Looking at the equations, there is nothing else that immediately jumps out as an explanation. Hopefully this is a bit helpful!

Margot

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I know you identified this issue previously, but can't remember what you figured out. Do you know why essentially no backazimuths are identified coming from the north?

Thanks [BBWL_Day184]https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fuser-images.githubusercontent.com%2F24902730%2F83470365-dc557a00-a436-11ea-8b19-e7ad208ea524.png&data=02%7C01%7C%7C8af0a5f0d5de43473ae108d807563983%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637267414191092972&sdata=hF5YwDKY8Tw2mcn3osm1VXAV8IY7qiV5pI4bd1RIeYk%3D&reserved=0

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tbartholomaus commented 4 years ago

Thanks @voremargot- that makes sense. I'm wondering now if maybe this has something to do with the averaging of spectral covariance matrices that is in the code. Could that be leading to numbers with greater absolute values than they ought to have? Like you write, having r_x or r_y components be close to zero is the key to producing waves that are coming in from the E-W, or N-S. Averaging might inflate the values of r_x and r_y, and push them away from zero? I'll look into this.