MIN ZHOU'S THESIS

NEW IMAGING AND FILTERING METHODS: PRIMARY ONLY IMAGING CONDITION AND INTERFEROMETRIC MIGRATION

(Ph.D. Dissertation)

Min Zhou, University of Utah


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ABSTRACT

Two new imaging and filtering methods are formulated and applied to both synthetic and field data. One of the methods, denoted as primary only imaging condition (POIC) migration, discriminates primary reflections from multiple reflections and migrates only the primary reflections. The other method is interferometric migration (IM), which can redatum the traces below salt without knowing the velocity in or above the salt body.

Traveltime tomography is an important imaging tool in oil and gas exploration, and earthquake seismology. However, the spatial resolution of traveltime tomograms are inherently restricted because of the finite-frequency effects in the data and the high-frequency approximation in ray-based tomography. In this dissertation, the generalized Radon transform (GRT) is used to derive the resolution limits for wavepath traveltime tomography, and a new imaging algorithm is developed based on the resolution formula. In addition, I develop and test a new imaging algorithm, early arrival waveform tomography (EWT) algorithm and show that it combines good convergence with high resolution capabilities. The POIC migration and its implementations are presented and applied to both two-dimensional (2-D) synthetic and field marine data for multiple suppression. The key idea is that both the take-off and incidence angles are picked from the seismic data and rays are traced from both the source and the receiver positions along these picked angles. An event is a primary reflection if the intersection point of the rays has a reflection traveltime equal to the observed traveltime. Test results show that POIC is effective in suppressing the migration artifacts generated by multiple energy in the far-offset data.

The POIC method predicts the far-offset multiples and guides the muting of near-offset multiples in the $\tau-p$ domain with a parabolic Radon transform. Synthetic results indicate that the approach is effective in separating the primary reflections from multiple events in the near offset traces. The POIC method is further combined with the wave-equation based reverse-time datuming (RTD) to better suppress multiples and image the reflectivity distribution below the salt body. Synthetic results show that the hybrid approach can eliminate most of multiple artifacts in the subsalt image.

The IM and reduced time migration (RTM) are developed and applied to the CDP data. Both IM and RTM use the $natural$ arrival times associated with the reflections from a reference reflector which can be picked from the data. The RTM shifts the data with the time difference between the $calculated$ and $natural$ arrival times of a reference reflection, while IM extrapolates the data to the reference datum with the $natural$ arrival times and then migrates the redatumed data below salt. Synthetic test results show the effectiveness of RTM and IM compared to the standard Kirchhoff migration. Results with a Gulf of Mexico (GOM) data show that RTM can mitigate the defocusing in the migration image due to timing errors associated with the statics and shallow structures.