Daniel Alejandro Rosales Roche

Imaging of converted-wave Ocean-bottom seismic data.

Abstract

Converted-wave data can be imaged with several methodologies. The transformation of data into the image space, is defined by an imaging operator, the simplest of which is normal moveout correction plus stack. This is valid for a velocity model that only varies with depth and flat layers. Depending on the degree of geological complexity there are more advanced transformation, such as dip moveout, poststack and prestack time migration, and postack and prestack depth migration.

This thesis uses wave-equation-based prestack migration operator for imaging. The output for this operator is in depth, image-midpoint, and subsurface offset. I present a method to transform the subsurface offset axis into the angle domain to form converted-wave angle-domain common-image gathers (PS-ADCIGs). This method exploits the robustness of computing 2-D isotropic single-mode ADCIGs, and incorporates the P-to-S velocity ratio ($\gamma$), with the local image-dip field. The PS-ADCIGs can also be mapped into two complementary ADCIGs, the first one is function only of the P-incidence angle, the second ADCIG is function of the S-reflection angle. The method to obtain PS-ADCIGs is independent of the migration algorithm implemented, as long as the migration algorithm is based on wavefield downward-continuation, and the final prestack image is a function of the horizontal subsurface offset.

A partial-prestack migration operator to manipulate multicomponent data, called converted-wave azimuth moveout (PS-AMO), transforms converted-wave prestack data with an arbitrary offset and azimuth to equivalent data with a new offset and azimuth position. This operator is a sequential application of converted-wave dip moveout and its inverse. As expected, PS-AMO reduces to the known expression of AMO for the extreme case when the P-velocity is the same as the S-velocity. Moreover, PS-AMO preserves the resolution of dipping events and internally applies a correction for the lateral shift between the common midpoint and the common reflection/conversion point. An implementation of PS-AMO in the frequency-wavenumber log-stretch domain is computationally efficient. The main applications for the PS-AMO operator are: 1) geometry regularization; 2) data-reduction through partial stacking; and 3) interpolation of unevenly sampled data.

The converted-wave common-azimuth migration operator (PS-CAM) and the single mode common-azimuth migration operator share the advantage that they need a 3-D prestack cube of data with four dimensions, instead of the entire five dimensions. The five dimensions can be reduced to four dimensions through different processes. The last portion of this thesis compares two images of the PS data set from the OBS acquisition on the Alba oil field in the North sea. The two images correspond to the following processes: 1. The common-azimuth migration of the data regularized using Normal Moveout. 2. The common-azimuth migration of the data regularized using PS Azimuth Moveout. The final results show that the image from the data regularized using PS Azimuth Moveout is significantly better than the images obtained the image from the data regularized using Normal Moveout.