Hi,
in Most of the studies, backscatter analysis is used on identifying water-logged areas, hence, GRD data. Water-logged areas produce higher backscatter (due to water content) compared to dry areas.
There are few studies conducted using Interferoemtry to detect soil moisture (which is similar to what you try to do)
Here is an article that summarizes some of the techniques used: soil moisture retrieval_current techniques.pdf (425.2 KB)