Low freq noise after Butterworth filter

[ale2599]

Hi, I'm processing some vintage MCS data (160 channel-25m receiver distance) and when I perform an initial Butterworth filter just to remove swell noise and avoid aliasing, I get some kind of low-frequency noise (like a reverberation or bubble) that messes up and worsens the data, instead of cleaning it. I attach the original shot and the shot and the same shot after 2 different Butterworth filters: 1-3-100-120 and 5-7-100-120

I've never had this result before... Some ideas?

Raw shot

After Butterworth 1-3-100-120Hz

After Butterworth 5-7-100-120Hz

[GuyM]

To some extent that can be in the nature of sharp, low cut filters - you can get ringing effects at low frequency.

I'd suggest doing some design work in the appropriate tool (or running some impulse response tests on spikes) to look at how stable the filter is with different parameters (especially low frequency slopes); I'd be cautious about the phase with a Butterworth too - I tend to design a minimum-phase or zero-phase equivalent filter for combined anti-alias and swell noise attenuation.