|
The Butterworth filter is a type of digital filter that engineers use to clean up signals. It is often called a “maximally flat” filter because its passband (the frequency range you want to keep) is very smooth — no ripples, bumps, or distortions in amplitude.
This filter is used since it does not introduce strange bumps or oscillations in the useful frequency range. Butterworth filter is used widely for ground motion filtering, acceleration → velocity conversions, and removing sensor drift.
When applying a Butterworth filter in QuakeManager, the user usually need to define the following:
Band type:
•Low-pass: Keeps low frequencies, removes high ones.
•High-pass: Keeps high frequencies, removes low ones.
•Band-pass: Keeps a middle range, removes low and high.
•Band-stop: Removes a middle range, keeps low and high.
Order (integer number, e.g., 2, 4, 8):
•Higher order = steeper transition between “kept” and “cut” frequencies.
•Trade-off: sharper edges may add more delay or ringing.
Critical frequencies (in Hz):
•These are the cutoff points where filtering begins.
•Example: if you want to remove noise above 20 Hz, you set a low-pass cutoff at 20 Hz.
.png)
Figure: Butterworth dialog
.png)
Figure: FFT Spectrum of B-ICC record before (red) and after (blue) applying Butterworth filter
|