/diy/ - Do It Yourself

If I have a secondary (receiver coil) that I'm supposed to measure voltage across, should I connect one side to the ground or keep it floating?

If I connect one side to the ground I'm just getting short voltage spikes.

If I keep it floating, I'm getting a semblance of the square-ish signal (with the spikes still present) that's driven through the primary (transmitter) coil. The problem is I'm also listening to the 50Hz mains voltage if it's floating.

Pic related is one probe (green) connected to one side of the secondary coil, ground on separate ground. Yellow probe is connected to the other side of the coil, ground also on separate ground.
Middle signal is the difference (Yellow minus Green), which is coincidentally the same voltage I get if one side is grounded and just measure the voltage on the other side.

So my question is, Am I looking at any sort of useful signal in the Yellow and Green signal separately, or is it just crosstalk from the driving signal? Yes I know crosstalk is desirable in some way but I don't really understand if I'm getting any sort of useful information from the those signals, am I?

This system is used for some form of eddy current testing, the primary is the driving coil and the secondary should be the receiver coil