Ignoring reflection at the air-soil boundary, if the amplitude of a 3-GHz incident wave is 10 V/m

Question:

Ignoring reflection at the air-soil boundary, if the amplitude of a 3-GHz incident wave is 10 V/m at the surface of a wet soil medium, at what depth will it be down to 1 mV/m? Wet soil is characterized by μr = 1, r = 9, and 5 = 10 = 4 S/m.