About correlation between Frequency and Slowness

[xtyo]

I try to aplicate SPAC in a site. The site is in urban site. The site has 5 stations with apperture approximately 33000 m. the signals are recorded in 1 day, 24 h 0 m 0.06 s. I use SPAC Geopsy, parameters are: time limits from TO to the end (1 day), window length is frequency dependent 50 T, no checked for bad sample tolerance and threshold and anti triggering, frequency sampling from 0.01 till 1.25 Hz, number samples are 200. Before main processing, I tried preliminary with frequency sampling from 0,01 till 20 Hz. I found that there is no correlation at frequencies more than 1.2 Hz. So I make boundary frequency sampling from 0.01-1.25 Hz. The processing needs time 2 hours.
Here I want ask something.
1. Are those my parameters make sense? Is there any missing link to get the right path?
2. At the picture, I find patterns frequencies vs slowness (pointed by eliptics). What do cause that patterns?
3. I try to pick frequencies. I put PREM model (forward modeling using GPDC) to this correlation frequency vs slowness (the region is pointed by square). I get low frequency 0.125-0.2 Hz. What do you think, is that what I do have any wrong? For the information, vicinity of the site has previous experiment using receiver function, I make dispersion curve using receiver function model, plot it to correlation frequency vs slowness, and this is near from what I pick with PREM.