HYBRID-RAY APPROXIMATION IN ELECTROMAGNETIC SOUNDING

Our purpose is to develop electromagnetic (EM) transient sounding methods that can handle resistivity structures having dipping interfaces. The method is an approximation that uses wave theory for part of the calculation and ray theory for the rest. The approximation is a refinement of Yost's (1952) image method in which the source on a half-space becomes the image beneath the reflecting interface. This approach shows excellent agreement with experiment and promises simple applicaitons. To test the approximation, we compare numerical integration of the fields over a two-layer half-space with approximate values. We separate the Em integrals into integrals for the surfce wave, primary, and multiple reflections. For a current dipole along the x-axis, the electric field Ex is the sum of the transverse electric and transverse magnetic components. For skin depths more than one, the main contributions to the integrals come from directions near the specular direction, and by moving the reflection coefficients at the lower interface outside of the integral, we obtain the hybrid-ray approximation. The transmission coefficients from the source into the earth remain inside because the source is near the interface.-Authors