New Prototype Detector - Coil Questions

[Sun-Boy]

Hi Folks, I'm developing a new VLF analog prototype detector and am using a concentric coil I've seen in the book "Inside the Metal Detector", it has the TX coil in the center with upper and lower RX coils that are balanced. So I have some questions for you experienced folks: Does anyone know if this style of coil is still used? If not, does anyone know why it fell out of favor? Any pro's and con's? Thanks for your help, I'm considering showing some of the pic's as I progress, I have the next revision pcb on order, we'll see how it goes.