ABSTRACT: Applications of physical modeling and localized slant stacking to a seismic study of subsea permafrost

CGRG Bibliography of Canadian Geomorphology
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Abstract

The distribution of ice-bearing permafrost in the Canadian Beaufort Sea shelf has been determined primarily by methods of reflection seismic imaging. A number of factors, including wide-angle acquisition geometries, laterally discontinuous velocity structures and coherent noise, contribute to difficulties in the processing and interpretation of multichannel reflection data from areas affected by permafrost. This thesis concerns the study of problems related to permafrost occurrence, with the aim of improving high-resolution reflection imaging. Two methods were developed in conjunction with the permafrost study. Firstly, a physical modeling system was constructed to conduct scale-model seismic experiments using an ultrasonic wavefield. The system has been used to study the acoustic response of velocity models representative of permafrost, to optimize acquisition patterns and to generate data sets for testing data processing and numericalmodeling techniques. Secondly, a method of data processing based on localized slant stacking was developed on a supercomputer. The processing method involves a localized slowness decomposition, combined with mean and median filtering. The technique was used to combine several data processing functions, including receiver-array beam steering, coherent-noise suppression, velocity analysis and time domain moveout corrections, into a single processing step. The physical modeling system and the data processing method were applied to a study of ice-bearing permafrost. Several data sets, including fixed-offset and multichannel sections, were recorded with the modeling system. Models of water and plexiglas were used to represent the alternating low- and high-velocity layers related to unfrozen and frozen sediments. The fixed-offset example illustrates the effects that shallow discontinuous permafrost bodies have on the reflection image of deeper horizons. Theseeffects include multiply reflected diffractions from the edges of the shallow bodies that appear as diffraction apices coincident with deeper reflections. The multichannel data sets revealed that coherent noise related to mode conversions occurs within the apparent-velocity range of primary reflections, even at offset distances less than 400 m. The reflections from the base of permafrost layers are typically limited to similarly short offset distances, and may be overwhelmed on CMP stacked sections by coherent noise with larger amplitudes at longer offsets. The localized slant stacking method is effective for suppressing linear coherent noise, and improves the interpretability of conventional processing displays such as constant-velocity moveout-corrected gathers. The velocity analysis and moveout corrections performed using localized slant stacking illustrate the difficulty of determining a reliable velocity function in the presence of coherentnoise with moveout patterns similar to primary reflections.

Bibliography of Canadian Geomorphology