The theoretical solutions for the behavior of the sinusoidal electromagnetic fields of a plane wave within a two quarter spaces earth are established. The results for the fields, induced by the lateral contrast of conductivity, are given in the form of inverse Fourier cosine transforms. They have been obtained by solving Helmholtz wave equations subject to the proper boundary conditions. Two parameters, the modulus and the argument of the normalized vertical component of the magnetic field intensity, have been computed for five values of the contrast of electrical conductivity, and as a function of induction number. The analysis of the curves obtained for the two parameters shows that: (i) the modulus may be used in the geological interpretation of structures with one vertical contact, and it presents the advantage of being accurately and rapidly measured;(ii) though the argument does not render any aditional information, it may be combined to the modulus to calculate the tilt angle of the polarization ellipse, which is the parameter normally measured in both the AFMAG and VLF methods surveys.