AVO analysis by offset-limited prestack migrations of crustal seismic data

The physical causes of reflections from the crystalline crust are still a matter of debate. The polarity of the wavelet and the amplitude-versus-offset (AVO) dependence of seismic reflection data can help to reveal the nature of a reflector. In contrast to seismic data from sedimentary environments, however, it is very difficult to get polarity and AVO information from crystalline basement data. Reflections in prestack data are typically weak, incoherent and often obscured by numerous diffractions. There is, however, a better chance of obtaining reliable AVO information from prestack-migrated data after true-amplitude processing, as the migration process unravels the jumble of reflections and diffractions and improves the signal/noise ratio. Important steps in this processing sequence are careful editing followed by surface-consistent amplitude corrections for source and receiver coupling to the ground. Migration-velocity models can be obtained from refraction seismics and/or from migration-velocity analyses. The migration itself is of Kirchhoff-type and applied to COF gathers (COF = Common OFfset) to get one migrated section for each COF. These COF-migrated images can then be used to get AVO functions for specific reflections. Examples from the DEKORP near-vertical reflection line KTB8502 show some strong reflections, from beneath a granite body, at about 9 to 11 km depth. Both the positive and negative reflection coefficients of the reflectors are consistent with an interval velocity model obtained from migration-velocity analyses. AVO functions from these reflections include offsets from zero to 12 km and reflection angles from zero to about 30°. Absolute reflection coefficients are surprisingly small with maximum values of 0.015. Two-layer modelling has been done to match the AVO dependence of these reflections.