Processing of Sub-Bottom Profiler Data using Seismic Migration Methods

Abstract

This master thesis on processing of Sub-Bottom Profiler data has the intention to improve imaging and detection of sub-bottom structures and anomalies. Two seismic migration methods have been exploited to reposition the reflections to their correct location. Time domain based summation migration was preferred due to its ability to allow for inexact recording positions and implementation of additional factors. The migration aperture is determined by the maximum dip of an event or the Sub-Bottom Profiler beam width. The second method was a modified version of phase-shift migration; downward continuation. Compared to summation migration the downward continuation is considerably faster. A time-consuming processing step was omitted, since the wave propagation is limited by beams. Both migration programs produce acceptable results. However, the positioning of traces has been demonstrated to vary from the GPS-positions and the vessel speed. Therefore, the trace spacing had to be adjusted. More exact positioning of traces is required in future surveys. Deconvolution with the purpose of eliminating the source contribution in the echo signal has been proved to be of minor importance. Subsequent to match filtering the contribution from the source is significantly reduced, and deconvolution is a redundant process. Calculating the envelope of each trace together with lateral filtering procedures, may improve the imaging. Migration should be performed in advance to envelope detection. The last part of the report deals with the performance of a Sub-Bottom Profiler at location in the northern part of the North Sea. Drilling result are provided by Norsk Hydro and used to estimate the absorption coefficient. A modified version of the SONAR equation is utilized to calculate the maximum penetration depth. Due to lacking knowledge of the absorption coefficient, the results of the calculations are uncertain and cannot be used to accurately predict the penetration depth. The other way around, it may be possible to estimate the absorption through a Sub-Bottom Profiler survey in the area.