Split ground planes can be put to good use when you also have good control over how currents move between circuit regions outside of ground.
For example, if you have a switching power supply on the PCB then all of the 'forward' current is through a single trace. In this case you put a ground plane under the entire supply and join it with the other ground plane with a single trace directly under (or over) the forward trace. Now all of the currents are moving between circuit regions through a relatively narrow channel that can be well controlled. The current through the channel on one side of the board is exactly matched by the current (in the opposite direction) on the other side of the board.
In your case you have currents moving between regions (outside of ground) through three channels: B+, -12V, and signal. If you split the ground planes then the return ground currents for all of these channels will be forced through a single trace connecting the ground planes. This will likely cause more trouble than it will cure. You might think that joining the ground planes in three places will fix that, but the ground currents won't understand your intentions and will go where they want.
In my opinion, you're better off with one uninterrupted ground plane for the entire circuit.