In the processing of land data, static time corrections are used for improving velocity analyses, the enhancement of stacked sections and correcting for long-wavelength structural distortion due to irregular topography. Staties can be split into three categories, namely short-, medium- and long-wavelength, where medium wavelengths are of the order of one spread length. However static corrections are only valid when ray paths are verticaI near the surface. Therefore when the ray paths deviate from the verticaI, owing to steeply dipping reflectors or high velocities in the near surface (such as in permafrost environments), they must be replaced by more accurate wave-equation techniques.