Is GPR the only investigation method that you have available at this site? I would recommend, at a minimum, at using a Geoprobe to determine the soil stratigraphy. As you are aware, a better option would be to hire a drilling contractor to perform a boring at the site (now of course, the cost of mobilization alone might deter you from performing one boring).
Civil PE Exam Success: Strategy, Sample Exam, Terminology by Michael Frolov, P.E.