A new method for crosswell reflector imaging

Abstract

Imaging seismic reflectors with crosshole and VSP tomographic data is only occasionally carried out using Kirchhoff-style migration schemes. We introduce a new kinematic method here for tomographically imaging reflectors. The scheme first picks up traveltimes of each reflected event from common-shot gather tomographic data. It then uses the picked times to image the reflector interfaces with a very important principle: that two reflection points from the same interface that produce reflection pulses on two neighbouring receivers have the same tangent line. The same scheme can be used for surface or crosshole reflection data, but the latter requires some special processing to separate upgoing and downgoing waves. This paper will show how to apply the scheme in tomographic surveys. Traveltimes of each reflection event are initially read from a common-shot gather tomographic seismic section. Then each pair of traveltimes from neighbouring traces is processed in sequence. The two isochronal curves are calculated from the two traveltimes. A common tangent to the two isochronal curves is then found and a reflection segment is defined. In this way, all traveltimes in pairs are processed and a straight or curved interface, corresponding to the locus of tangents, is reconstructed to map the reflection interface. The processing difference between surface seismic reflection data and tomographic (crosshole) seismic reflection data is that the crosshole data contains downgoing waves as well upgoing waves. So the program has to decide if a reflection event comes from above or below the source. Then the program processes the data to obtain the reflector orientation. This imaging method can handle multiple layers and curved interfaces for an arbitrary 2D velocity distribution.