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). Sea water is characterized by r =72, μr =1, and =4 (S/m). The plane is trying to communicate a message to a submarine submerged at a depth d below the water 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?