How to assess the quality of the unwrapped phase

s1tbx Interferometry
#1

Hello everyone,

I am working on land deformation over a mining site using SBAS (MintPy). I have created 120 unwrapped interferograms where I am visually assessing them trying to discard the low quality ones (e.g phase jump). Below we can see the coherence, wrapped and unwrapped phase of the AOI where numbers correspond the the following dates. 1). 2021/01/29 - 2021/02/10, 2). 2021/01/29 - 2021/02/22, 3). 2021/01/29 - 2021/03/06

Coherence

coherence
coherence804×570 157 KB

Wrapped phase

wrapped
wrapped792×575 79.2 KB

Unwrapped phase

unwrapped
unwrapped806×587 212 KB

Looking at the unwrapped phase of (1) it looks good to me but for the other two, the unwrapped interferogram is completely different despite the fact hey share the same reference date. the unwrapped phase of number (2) is above -10 radians for the whole scene (it does not look normal to me) and for the number (3) we see lots of variation between -10 to 10 radians. Can this be due to tropospheric noise ?

My first though is to discard number (2) and (3) from SBAS processing in MintPy. I do not understand why there is such a huge difference amongst all three unwrapped interferograms. Does this has to do with 1) the low coherence of the area , 2) atmospheric noise or anything else?

There are quite a few interferograms in my analysis that look like the ones above. Am I right to assume that there is something wrong with those unwrapped phases and need to be discarded ?

One last thing, Does the positive values on the unwrapped phase indicate subsidence (signal travelling longer distance) and negative ones uplift? or is it the other way round

I would appreciate any comments or thoughts on this, especially by people that are more experienced in that filed such as @EJFielding @ABraun @qglaude

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#2

You are correct to be suspicious of the unwrapping of interferograms 2 and 3. The problem here is coherence is very low and degrades quickly with the time interval length, which is a typical situation in areas of significant vegetation cover. Your interferogram 1 has a 12-day interval and acceptable coherence so the unwrapping is reasonable except in a few small areas. Interferograms 2 and 3, with intervals of 24 and 36 days have very low coherence, and the unwrapped phase is not usable. You can see that the wrapped phase looks like almost pure noise, so the unwrapping has no information. It seems that only the 12-day intervals in this area are usable. You can try to make a MintPy time series with only the 12-day pairs, but it will not be very accurate because there is only one pair connecting each date.

Because the interferograms have low coherence, it is difficult to see whether atmospheric effects are having any effect.

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#4

Thank you very much for your response.
I suspected that coherence might be the major issue on getting good quality interferograms. I suppose, there is no much I can do given that I cannot collect sentinel-1 data more frequently than every 12 days
for my AOI.

#5

You may be able to extract some pixels with high coherence with one of the persistent scatterer InSAR (PS-InSAR) methods. I have not done that processing myself, but there are many who do that. The SNAP stack processing can be loaded into StaMPS that is one of the PS-InSAR systems.

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