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Effects of fractures on seismic velocities in fault zones

Subsurface sequestration of CO2 has been developed into a viable technique for mitigating global emissions. Natural CO2 seepage system (Utah, USA) can be used to investigate the geological footprint and geophysical signatures of an active CO2 seal bypass systems and improve our understanding of how geological and geomechanical properties control flow paths within a fault zone.

Vertical fractures in the fault damage zone are identified as important fluid conduits and the fractures can be observed both in the field and within available core samples from the Little Grand Wash Fault. Fracture geometrical properties like aperture, roughness and contact ratio as well as their failure mode and mineral precipitation is important for their geophysical signature.

A better understanding of how the fracture geometric parameters and seismic velocities are related is needed.          

Aim of the thesis:

Contribute to a better understanding of the link between fracture geometrical properties and seismic velocities to be expected within the fault zone.

Objectives:

  1. Characterize geometrical properties for selected fractures using CT imaging at NGI
  2. Detailed interpretation of existing geophysical data (velocities) from the NGI laboratory with and without fractures
  3. Discuss the effects of fracture geometry on velocities and application for field observations

Research method and dataset:

The student will work on samples collected from the Little Grand Wash fault (figure 1). The work will be based on CT scans and existing experimental data measuring velocities for various samples both with and without fractures. The dataset may be extended or focused with new field data or experimental data depending on interest from the student. Tools for interpretation of experimental data and CT image characterization will be available at NGI.

The student will learn:

  • Get a unique experience working with experimental data combining geology, geophysics and geomechanics at different scales
  • Independent research, academic writing and presentation skills

Background for the project:

This MSc work is part of the COTEC research project at UiO focusing on CO2 containment and monitoring techniques. The student will get a chance to work in a cross-disciplinary team taking part in a collective effort on understanding how CO2 migrates through fractures and faults and its application for CO2 monitoring and risk assessment.

Field campaigns for data collection in Utah are planned within the project and provide possibilities for the collection of additional data for this study.

Figure 1 Samples with 25 mm diameter used in the experimental work on unaltered reddish siltstone and bleached, altered siltstone. Intact unaltered host rock sample LGW2 (a), fractured unaltered sample, LGW1 (b), CT image of unaltered fractured sample LGW1 (c), intact bleached sample LGW8 (d), fractured altered host rock (LGW7) (e) and CT image of altered fractured sample LGW7 (f). In the CT images, white is air and dark is dense material. A dark fracture-fill identified as oxides is observed mainly for the altered fracture in LGW7(e and f). Click here for a bigger picture (opens in a new window).

 

Published Sep. 10, 2019 12:56 PM - Last modified Nov. 13, 2019 2:19 PM

Scope (credits)

60