Suppose I have two sensors $s_1$ and $s_2$. $s_1$ measures the desired heart signal (with most of the frequency content below 100 Hz), and the other is a reference sensor picking mainly background noise.
Due to the pumping of the blood in the heart, there is also low frequency vibrations, mainly below 10 Hz that contaminates both sensors. The sensors are spatially separated, making direct cancellation unfeasible.
Nonlinear denoising is effective in removing mains hum without compromising the signal. However, it seems to be ineffective in removing the low frequency vibrations. The figure below shows the effect of applying it to $s_1$
My question is how can these vibrations be removed from $s_1$, without causing too much distortion to the desired signal?