Question: A couple of questions:

1) It looks to me that the three-prong ground connection in the bottom right of this schematic is the only ground connection. I used the GNDPWR symbol in my KiCad schematic to represent this ground connection. Can (or should?) this GNDPWR be connected to the "normal" GND which I am using on the jack sockets which connect to the inputs/outputs?

2) Why is the -12v connected directly to this GND? Is this just saying that the GND reference is -12v? But because I am using op-amps and a dual-rail powersupply, don't I also need a 0V GND? Won't it cause a short-circuit if I connected the -12v GND to the 0v GND? I do not understand.

3) Can I use any op-amp for U1-U4?

4) For the op-amps should I use a +/-15V power rails? Or should I use +/-12V?

5) What does the test point 4 mean (near the +12v)? Should I just ignore that?

[![enter image description here](https://i.stack.imgur.com/6CLXU.jpg)](https://i.stack.imgur.com/6CLXU.jpg)

Answer: 
I suspect that this circuit is intended to use a single 12 volt power supply, with the negative terminal of the supply connected to the circuit ground. The power input labelling is misleading - it appears to imply a total 24 volt supply (+12 and -12) rather than a single 12 volt supply.

Test point 4 and the test point connected to Ground are just handy spots to connect your meter to measure the supply voltage - you can ignore them.

Does the place you found this circuit recommend any particular op-amp? If not, I expect any op-amp that will work from 12 volts will do.

When you want to ask questions about a circuit you found somewhere, you should include links to the source (and you should look around the source - it might answer your questions...)
The most important assumption not shown is what voltages are used by the OA V+,V-.

The certainty is that the midpoint between the external , DC V+,V- inputs shown will become the DC output of all Op Amps with a null input. The input and the notch out are AC coupled, the latter of which is dubious for the reason of blocking DC only on this output.

The missing assumptions for 0V and OA Supply and lack of specs , makes this schematic incomplete.