what frequency do you want to pick up????
R u talking about picking up of frequencies below 20Hz or r u talking about picking up of frequencies upto a particular hertz...
generally they are picked up using electrodes... a microphone can pick them but the signal would be very low and need a lot of amplification....
so just if you use a microphone just filter the signal using a filter which would pass your frequency of interest (10Hz)... try having the 3dB point at 10Hz...
Placing a capacitor will be good idea.. It wil filter out sharp edges in the noise and signal.. If u want it to pick up low frequency,choose capacitor accordingly..