4. A low frequency plane wave traveling in water is given by E = x[5e-0.2z el(-0.22+)] V/m. Assume the water surface is located at z = 0 and σ = 4 S/m. Note that you may use the good conductor approximation that σ/(sw) >> 1. (a) Find the average power density in terms of z. (2 pts.) (b) Determine the depth z at which the power density has been reduced by 40 dB. (1 pt.)