Looping if statement with index and match functions

I'm trying to write a VBA code and I'm at a loss I have a certain survey depth and a range of depths. I need to find the depth interval the survey falls within and then subtract the survey depth from the starting depth. at the next survey, I need to do the same but also add the remainder of the previous survey

Table

Basically what I want to do is:

if S1 > d1 and S1<d2 then s1-d1
if s2>d3 and s2<d4 then (s2-d3)=(d2-s1) 

..and so on. However, the survey depths will not always line up with the depths so i need some type of match function that will correlate the survey with the correct depth range.