It's relatively trivial to find direction of the 30 Hz sound source, given 1 million budget - you just need to place microphones 11 meters apart and raise them above buildings until you get direct line of sight to the main source.

A million bucks wouldn't get a square city block of what you describe.

A square block? 5-10 microphones is plenty. Your goal is to find a direction from the place you deploy, not to blanket the city. Then you can redeploy somewhere else to get another direction, etc.

That's an example of how poor directonality is at 30 Hz.