Non-tomographic Resolution of Small-scale Velocity Anomalies Offshore Netherlands

Features with a lateral extent much shorter than the recording cable length, because the recorded seismograms to exhibit highly irregular moveout behaviour in preSDM CRP gathers during the initial phases of velocity model update. If these features are only a few hundred metres wide, then it is unlikely that conventional ray-based tomography will be able to invert for them. In addition, if the autopicker being used to supply residual moveout behaviour to the tomographic inversion scheme is using a parametric picker, then the complex moveout behaviour will not be adequately described, and the tomographic inversion may not converge.

Here we show one such example of this problem, for a structure offshore Netherlands, and describe a piecemeal solution using the measured autopicking error distribution to guide a local vertical velocity update. This approach helps to resolve the shallow velocity anomaly, permitting conventional tomographic update for the deeper section to proceed successfully.

Further, this approach will be contrasted with that of a non-parametric (generalised moveout) picking approach wherein the assumption of 2nd or 4th order parametric moveout is relaxed, so as to capture the more complex moveout behaviour in the CRP gathers so as to enable the subsequent tomographic inversion to better resolve small-scale structure.