Vardy, Mark E.
Real–Time Imaging of Decimetre–Resolution 3D Seismic Volumes
University of Southampton, School of Ocean and Earth Science,
The 3D Chirp sub–bottom profiler acquires a true 3D seismic volume with decimetric horizontal and centimetric vertical resolutions, providing an ideal platform for shallow–water engineering, archaeology, military, and geological studies. In this thesis, I show how simple processing flows built around a combination of standard Chirp/Vibroseis techniques and well known industry methods produce effective and impressive results by considering an object identification case study in a shallow–water, harbour setting (Vardy et al., 2008). Both stacked and migrated volumes are used to identify 89 individual buried targets that are correlated with coincident objects. Through subsequent dredging, a 100 % detection success is demonstrated, along with the strong similarity between the migrated reflector morphology and co–incident object shape. However, this processing approach requires extensive manual input and very long processing times (? 1 month).
For this reason, a new method for pre–stack 3D Kirchhoff imaging is developed. Correlation with a series of bandwidth limited theoretical source sweeps is used to frequency decompose the raw traces for pre–stack time migration using a constant velocity. By accommodating dispersion through imaging a series of band limited traces, rather than through Fourier Transform, processing times are reduced from ? 1 month to c. two days for the object detection volume (i.e., approaching real–time application). The effectiveness of this new algorithm is examined using several synthetic volumes, allowing the degenerative effects of gaps in the fold to be explored. Finally, the application of the 3D Chirp system to geological cases is demonstrated through the geomorphological mapping of a sequence of mass movement events in Windermere, UK Lake District. Three mass movement deposits are identified in a 100 m by 400 m survey area. Through mapping of the package distributions and their interaction with the pre–existing sediments stratigraphy, they are identified as Younger Dryas climate amelioration deposits, resulting from the rapid deposition of gravitationally unstable, unconsolidated sediments. A metre–scale structural interpretation allows the depositional regimes (two being debris flow and the third a mass flow deposit) and dominant transport directions to be inferred.
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