Hello,
You cannot derive soil moisture using just a single SAR image. Most of the studies of soil moisture retrieval conducted using time series data (as described in the article I have posted above). This procedure is called change detection and it is widely used.
To derive soil moisture from SAR, to be more precise, relative soil wetness you can do the following steps:
-
Data collection. A series of SAR data collection within a period of time -
Image pre-processing: Each SAR image should be pre-processed seperately (speckle noise suppression, radiometric calibration, incidence angle normalization if needed and terrain correction). -
Change detection: To perform change detection you need to subtract a SAR image acquired under dry weather conditions from a SAR image acquired under wet weather conditions. You should follow the equations described in the article i have posted.
These equations are straightforward to be applied. SO, you can use the band math in SNAP to apply the equations on your image.
Things to consider
Relative soil wetness from SAR is not possible if your AOI is covered by a thick vegetation. The microwave signal interacts with the leaves and doe not penetrate through to reach the soil.
NDVI index should be calculated using optical data. Then, this layer can be used to mask out the vegetated area in your image.
I hope that hekps