8.16 a 0.5 mhz antenna carried by an airplane flying over the ocean surface generates a wave that approaches the water surface in the form of a normally incident plane wave with an electric-field amplitude of 3,000 (v/m). seawater is characterized by r = 72, μr = 1, and σ = 4 (s/m). the plane is trying to communicate a message to a submarine submerged at a depth d belowthewater surface. if the submarine’s receiver requires a minimum signal amplitude of 0.01 (μv/m), what is the maximum depth d to which successful communication is still possible?