It’s always helpful to begin with something you know when trying to find something that you don’t know.
Using your ground penetrating radar to record information on a grave that is already marked will help you make interpretations in areas that don’t have markers. Identifying the unique signatures from a cemetery on marked graves will give great foundational information regarding what a burial looks like from a similar time period, date of internment, soil conditions, cultural practices, and climatic factors.
You can then use this information to create a cognitive expectation of what you should see if a grave is present without a marker.
This same practice is useful in utility locating and other applications.