Seismic velocity update for anisotropic depth migration

A method for updating, in a CPU and without access to well log data, anisotropic parameters from near-offset prestack P-P and P-S seismic data, and from P- and S-wave velocity models for velocity ray tracing, which comprises the steps:

• determining prestack depth migration of said near-offset prestack P-P and P-S seismic data to form P-P and P-S depth image gathers;
• selecting corresponding depth consistent image gathers from said P-P and P-S depth image gathers;
• performing joint velocity inversion in depth on said depth consistent image gathers to estimate vertical P-wave velocity vpo, vertical S-wave velocity vso, and vertical depth Zo;
• determining P-P isotropic depth migration of P-P depth consistent image gathers comprising said depth consistent image gathers to estimate isotropic depth zpp;
• calculating estimate of an anisotropic parameter δ based upon said isotropic depth zpp;
• calculating an anisotropic parameter σ based upon said anisotropic parameter δ;
• and
• repeating the above steps until estimates of said vertical P-wave velocity vpo and said vertical S-wave velocity vso remain substantially unchanged.

A seismic data processing system, which comprises:

• a seismic source means for providing P-P and P-S seismic waves reflected by a subsurface layer in presence of VTI anisotropy;
• and
• processing means for receiving said P-P and P-S seismic waves and selecting therefrom near-offset data, for forming P-P and P-S depth consistent image gathers from said near-offset data, for performing a joint velocity inversion in depth on said P-P and P-S depth consistent image gathers to estimate vertical velocities and vertical depth of said subsurface layer;
• for determining isotropic depth migration of P-P depth consistent image gathers comprising said P-P and P-S depth consistent image gathers based upon said vertical velocities and said vertical depth to estimate a vertical isotropic depth of said subsurface layer, and for calculating anisotropic parameters from said vertical velocities and said vertical isotropic depth to update said vertical velocities.

A method of processing in a CPU both P-P seismic data and P-S seismic data received from a seismic source to obtain accurate post stack images of subsurface layers in presence of VTI anisotropy, which comprises:

• operating upon first near-offset data of said P-P seismic data to determine P-P wave prestack time migration, and upon second near-offset data of said P-S seismic data to determine P-S wave prestack time migration;
• performing a post stack joint velocity inversion upon said P-P wave prestack time migration and said P-S wave prestack time migration to obtain interval velocities in depth;
• creating initial P-P and P-S velocity models from said interval velocities in depth;
• creating P-wave and S-wave velocity models from said initial P-P and P-S velocity models;
• determining P-P prestack depth migrations and P-S prestack depth migrations from said P-wave and S-wave velocity models, said first near-offset data, and said second near off-set data;
• forming P-P depth image gathers from said P-P prestack depth migrations, and P-S depth image gathers from said P-S prestack depth migrations;
• selecting depth consistent image gathers from said P-P depth image gathers and from said P-S depth image gathers;
• performing a joint velocity inversion in depth on said depth consistent image gathers to obtain estimates of vpo, vso, and zo;
• performing an isotropic depth migration by scanning focus velocities of P-P depth consistent image gathers comprising said depth consistent image gathers to obtain an isotropic depth zpp;
• calculating estimates for anisotropic parameters δ and σ based upon zo, said isotropic depth zpp, vpo, and vso;
• updating said P- and S-wave velocity models based upon vpo, vso, zo, said isotropic depth zpp, and said anisotropic parameters δ and σ;
• and
• repeating the above steps beginning with the step of creating P- and S-wave velocity models until vpo and vso remain substantially unchanged.