Imaging primaries and multiples simultaneously with depth-focusing

Huazhong Wang

wang@sep.stanford.edu

ABSTRACT

Seismic imaging amplitudes are extracted with the imaging conditions t=0 and h=0, where t=0 means that the take-off time of the upward-coming wave is zero, and h=0, with h the half-offset between the source and receiver position, means that the downward-going and upward-coming waves meet together during the wavefield extrapolation. However, h=0 makes no sense for multiples imaging. This imaging condition is suitable for imaging the primary, where the source position must be known. I introduce an imaging condition for imaging primaries and multiples simultaneously. The imaging condition, in essence, states that the take-off time of the upcoming wave equals zero, and that the radius of curvature of the wavefront of the upcoming scattered wavefield equals zero. It is known that the primary and multiple scattered waves will be focused during the wavefield depth extrapolation, but the primary and multiple scattered waves at the same depth focus at different times; this is because the traveltimes from the source to the scattering point are different for the primaries and multiples, even for the same scattering point. The focused scattered wave can be picked out, and the image is formed at the focusing point. The advantages of the method are several: the primary and multiples can be imaged simultaneously, only the up-coming wave must be downward extrapolated, all the scattered wavefields in the different shot gathers can be added together and simultaneously extrapolated, and the source position needs not be known. Its disadvantage is that the imaging condition is much more difficult to use.