Need help in Segment tree

I am learning about Segment tree topic.

Let’s say you are given an array ar whose size is n and you build a segment tree for range minimum query. The segment tree size would be less than 4n.

for eg:
given array : 1 23 4 51 6 7 with size of n=6

in the given array, element 51 is present at position 4 (1-based indexing).
Now after building segment tree (which is stored in array of 1-based indexing), how can I get the index of element 51 from the segment tree? At what index would it be stored in the segment tree?