Doubt on the fog generation

[Abhishek0075]

This is the clear image :

This is the depth image created :

This is the foggy creation :

As you can see in the fog image there is a ring like structure on the top this is on most of the images created why is it so

[Abhishek0075]

The problem arises due to attenuation map created

I am not familiar with these I would appreciate it if someone could help me to clear this