Zuk, Tomasz (2011)
M.Phil. thesis, University of Birmingham.
Alluvial architecture has an inherently three-dimensional character; however, standard methods used within fluvial sedimentology, including ground penetrating radar (GPR) surveys, generally provide only 1-D or 2-D information. Thus data on paleoflow directions, for example, have to be inferred from 2-D profiles. However, full-resolution 3-D GPR data collection and processing is both time consuming and technically challenging and thus has been rarely applied to study fluvial deposits. The primary aim of this thesis is thus to try and bridge this gap and demonstrate that improvements to the processing and visualization of 2-D GPR data, with readily available seismic software, can provide high-resolution 3-D images of fluvial deposits without the need for the application of more technically difficult full-resolution 3-D acquisition and processing. It also tries to find out the extent to which such a method can be used as a standard tool of fluvial sedimentology.
Three grids of very closely spaced GPR lines collected from a meandering and a braided river were processed in 2-D and compiled into 3-D datasets with open-source Seismic Unix. AGC, ‘dewow’ and band-pass filtering were replaced with gain functions and time-variable filtering, which by addressing the fast-alternating GPR signal added up to 40% penetration to the interpretable parts of profiles. Two sets of filter gate values produced lower and higher resolution data. The first one recovers otherwise lost information from the lower parts of the profiles, while the second datasets provides better resolution in the upper parts of the profiles.
The volume display, offered by Kingdom seismic interpretation software, helped to establish the internal organization of sedimentary facies and their bounding surfaces. Time slicing and horizon tracing were used to establish the orientation of the sedimentary structures, to infer changes of the paleocurrent directions and to build 3-D models of the fluvial sedimentary architecture. This enabled features such as bartop hollows, crossbar channels, slipfaces and bar margin reactivation surfaces to be visualized that were not always apparent from 2-D sections alone. Quantitative comparison of interpretations carried out on 2-D profiles and 3-D volumes revealed that the accuracy of 2-D interpretation in relation to the facies recognised in 3-D was 76.0 to 99.9%. The last value does not take into account recognition of less than 20 degree changes of orientation of sedimentary structures which could only be revealed by 3-D volumes. On this basis it is concluded that 2-D data collection and analysis coupled with 3-D visualization may represent a useful compromise between quick 2-D surveys and more challenging fully 3-D methods. The approach outlined in this thesis may thus be especially useful for more complex areas of alluvial architecture. This final point is highlighted with illustrative examples of point bar, recirculation pool, deep scour fill and migrating unit bar deposits.
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