In this thesis, I assess the impact of data quality on the use of P-wave seabed seismic recordings for fracture characterisation. A noise removal technique for removing refracted noise from dual-sensor OBC records was developed and tested on 2D data. A practical correlation based approach to dual-sensor summation was devised, tested on real and synthetic data, and compared with existing approaches. A North Sea seafloor seismic dataset was processed into azimuth sector stacks which were used to map amplitude differences between azimuths, highlighting regions of anisotropic variation. Subsequent azimuthal amplitude analysis (AVAz) used the concept of the effective angle of incidence of a stacked reflection to implement surface fitting. The AVOA fits and subsequent fracture interpretation were constrained by modelling a range of fluid-fill and crack properties using information from a nearby well. Maps of fits to the orientation and magnitude of anisotropy produced at each CMP, proved to be biased due to poor signal-tonoise ratios (SIN). To improve the fits analysis was performed over groups of CMPs where the AVOA parameters were expected to be stable, improving the SIN ratio. Testing these AVAz fits against no variation with azimuth showed them to be insignificant, due to the low SIN (3.5) of this data. The impact of different levels of noise on AVAz fitting was determined for the rock properties of this North Sea data, highlighting a requirement for a SIN greater than 10 for meaningful fracture orientation estimates. This was extended to the general case of AVO classes I and III, for a range of azimuth sampling, showing the level of SIN required to reliably fit an AVAz signature. Variations in fold associated with creating restricted azimuth stacks were assessed, as were variations in the effective angle of incidence. Maps of timing errors between azimuth stacks associated with centring on the peak everit were also created. These maps aid interpretation of the AVAz showing regions of low fold, unreliable picks and changes in the vent position with azimuth.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:504740 |
| Date | January 2008 |
| Creators | Harrison, Mark James |
| Publisher | Birkbeck (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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