Ok, sand issue solved (almost). I knew I was seeing something circular.
See the last figures of this paper. That lead me to these wiki pages but was not understanding on how to create these
But then I found a clever guy with this Matlab contribution. It’s almost what you want, except the center that is too crowded.
Summarized it in this Julia Notebook.
http://fct-gmt.ualg.pt/lixo/misc_spirals.jl.html
Now Paul can do his gmtmath magick.