P‐S converted‐wave AVO

Abstract

The advent of new tools for the acquisition and processing of multicomponent seismic data has made substantial improvements in the quality of modern shear-wave measurements. With this improved quality comes the opportunity to assess the information that the amplitudes of these seismic records impart. Aki and Richards (2002) derived a theory that describes the contribution of different rock properties to the P-S seismic amplitude response. In particular, they show that the amplitudes recorded in the PS converted-wave gathers are related only to the density and shear reflectivities. The significance of this result is that it is viable to estimate the density of the rock in a reservoir directly from amplitude preserved P-S seismic gathers using the P-S Amplitude Versus Offset (AVO) technique. The advantage of this approach over the P-wave AVO method is that the density effect is measurable at much shorter shot-receiver offsets because the converted Swave is reflected at a much sharper angle than the corresponding P-wave. In fact, the required P-S shotreceiver offsets are typically less than two-thirds of the corresponding P-wave offsets. This presentation shows the results of applying the P-S AVO method to both real and synthetic P-S seismic gathers. It shows that reliable estimates of the density and shear reflectivities can be derived from modern P-S converted-wave data shot in the Western Canadian Basin by comparing the results of the P-S AVO analysis to density and dipole shear logs acquired at the same location. INTRODUCTION The means of doing P-S AVO has been around since 1980 when Aki and Richards published an approximation to the Zoeppritz (1919) equations. This was done for P-S data as well as for the more familiar P-P approximation and for all other combinations of P, SV and SH waves. A major change in recent years has been the improvement in the data quality observed in P-SV or 3C (Three-Component) seismic data. These changes have come about due to vast improvements in acquisition technology (e.g. VectorSeis TM phones) and in processing algorithms for these data. The result of these advances is that 3C data is now of such quality that we can consider extracting amplitude information from it. To do this, Aki and Richards’ equation can be inverted for the parameters contained in it, which are the reflectivities of shear-wave velocity (β), and density (ρ) or any combination thereof. In this presentation, this is done for both synthetic seismic data and seismic data acquired over the Long Lake Project, a Nexen/OPTI synthetic oil joint venture in North-Eastern Alberta. The P-S AVO results from the synthetic data show that the equations, as implemented generate the correct response. The P-S AVO results from the Long Lake Project indicate that they might provide a solution to one of the most significant problems in this heavy oil reservoir: the detection of shale plugs that interfere with the SAGD (Steam-Assisted Gravity Drainage) production method being used here. THEORY Aki and Richards (2002) show AVO approximations for the Zoeppritz (1919) equations, which are derived based on the assumptions of small contrasts in elastic properties between two similar half-spaces. The more familiar is the P-P approximation, but approximations were also derived for all combinations of down-going and up-going P, SV and SH waves. This paper examines what can be done using the P-SV approximation, which is appropriate for modern 3C methods that produce P-SV “converted-waves”. This equation takes the following form, using Aki and Richards' (2002) notation,       ∆       − − ∆       + − − = β β