Joint ray + Born least-squares migration and simulated annealing optimization for target-oriented quantitative seismic imaging

Abstract

A seismic processing workflow based on iterative ray þ Born migration/inversion and target-oriented postprocessing of the migrated image is developed for fine-scale quantitative characterization of reflectors. The first step of the workflow involves linear iterations of the ray þ Born migration/inversion. The output of the first step is a true-amplitude migrated image parameterized by velocity perturbations. In a second step, postprocessing of the migrated image is performed through a random search with a very-fast simulated annealing (VFSA) algorithm. The forward problem of the global optimization is a simple convolutional model that linearly relates a vertical profile of the band-limited migrated image after depth-to-time conversion to a 1D velocity model composed of a stack of homogeneous layers of arbitrary velocity and thickness. The aim of the postprocessing is to eliminate the limited bandwidth effects of the source from the migrated image for resolution improvement and enhanced geological interpretation of selected targets. The global optimization approach allows for uncertainty analysis required by the intrinsic nonuniqueness of the velocity model output by the postprocessing. The relevance of the convolutional model when applied to the output of the ray þ Born migrated inversion is first illustrated with a one-layer model. The accuracy and the robustness of the workflow to image geologically complicated models are then illustrated with an application to the synthetic Marmousi model. Some practical issues (e.g., the source wavelet estimate and the scaling of the migrated image required by the VFSA optimization) are discussed with an application to a 2D real seismic multichannel reflection data set collected in the Gulf of Guayaquil (Ecuador). The postprocessing is applied to derive the fine-scale velocity structure of a décollement zone on top of the subduction channel. The postprocessing allows for mapping structural variations along different segments of the décollement, which can be associated with changes in fluid content and porosity.

Toward a dynamic concept of the subduction channel at erosive convergent margins with implications for interplate material transfer

“…Seismic images also help in demonstrating the presence of fluids along the plate boundary. In particular, geophysicists interpret high reflectivity, especially when associated with negative polarity, as layers containing high‐pressure fluids with large pore‐volume fractions along the décollement [ Collot et al , 2008b; Ranero et al , 2008; Ribodetti et al , 2011]. Reflectivity patterns indicate that fluid content decreases considerably at the updip limit of the seismogenic zone [ Bangs et al , 2004b; Ranero et al , 2008].…”

“…Reflectivity patterns indicate that fluid content decreases considerably at the updip limit of the seismogenic zone [ Bangs et al , 2004b; Ranero et al , 2008]. Overpressured layers may be thin (≪100 m, too thin for seismic resolution), and irregularly distributed along the plate boundary interface [ Bangs et al , 2004a, 2009; Ribodetti et al , 2011]. Furthermore, porosity values calculated from seismic velocities indicates that the underthrust sediment contains a large volume fraction of overpressured fluids [ Sage et al , 2006; Calahorrano et al , 2008].…”

The South Ecuador subduction channel: Evidence for a dynamic mega-shear zone from 2D fine-scale seismic reflection imaging and implications for material transfer

“…Prestack depth‐migrated (PSDM) MCS line SIS‐72: (a) PSDM image based on the Kirchhoff algorithm and a 2D velocity model obtained by focusing analysis and iterative velocity picking; TOC = Top of Oceanic Crust reflector; TSC = Top of Subduction Channel reflector [after Calahorrano et al , 2008]. (b) PSDM calibrated image obtained by Iterative ray + Born waveform migration/inversion [ Ribodetti et al , 2011]. Note the difference in thickness and dip of the subduction channel and the finer resolution in the channel and accretionary wedge.…”

Seismic Exploration of the Deep Structure and Seismogenic Faults in the Ligurian Sea by Joint Multi Channel and Ocean Bottom Seismic Acquisitions: Preliminary Results of the SEFASILS Cruise

“…After a careful binning of the MCS data using shot and receiver navigation files, the postcruise processing sequence, applied to both reflection profiles using CGG's Geovation ® software (version 6401) includes (1) field data transcription, (2) 2.5 Hz low-cut filter, (3) spherical divergence compensation, (4) noise attenuation in f-x domain, (5) trace editing, (6) cable and source corrections, (7) resampling from 2 to 4 ms, (8) first pass velocity picking, (9) multiple attenuation with 2D-SRME method (two passes) followed by Radon demultiple, (10) second pass velocity picking, (11) deconvolution, (12) third pass velocity picking, and (13) pre-stack Kirchhoff time migration. Prestack depth migration is also performed using a Ray+Born imaging technique [16], Common Image Gather analysis [17], as well as slope tomography being tested to build suitable velocity models for migration [18].…”