Proximity Sensor

Since I'm waiting for my convocation, I have some me time . I'm currently browsing some metal detector schematics and found a lot, including Gary's (Chemelec) being most popular, the Hammerhead, and recently this one:**broken link removed**.The circuit's incredibly simple for the implementation via this IC.

I'd just like to ask: is there a way to make the range of detection farther than it could be? Since the datasheet mentioned the range like this:

**broken link removed**

Looks pretty juggled to me. Anyone?