SAR processing (or diffraction stacking migration) is an important signal processing method for ground penetrating radar (GPR) in many application case including landmine detection. It can improve signal-clutter ratio and reconstruct subsurface image, summing amplitudes along the hyperbolic trajectory that is Huygens surface (or diffraction travel time surface). For SAR processing, the travel time surface generally is smooth spherical surface in the case of zero-offset data and smooth ellipsoidal surface in the case of nonzero-offset data whose curvature is governed by the velocity function. But when the height of ground surface varies largely in the very rough ground area, for example mound, the travel time surface will be affected by the ground surface for the synthetic aperture - ground penetrating radar (SAR-GPR). In this paper we will consider the effect of the ground surface into the migration processing for the multi-offset CMP SAR-GPR data set. Firstly, using the SAR-GPR CMP data set, we will build the 3D velocity model including the ground surface topography and the subsurface velocity. Then, depending on the 3D model, we can do the ray tracing and compute the travel time between transmitter, receiver and each subsurface scattering point. At last, using the travel time, we can build the Huygens surface (or diffraction travel time surface) for each scattering point. The Huygens surface is the best migration trajectory. Depending on the Huygens surface and the migration trajectory of the SAR processing, we can discuss the migration aperture for SAR processing in rough ground area.