About the Project
The Palaeozoic and Mesozoic basins of the North Sea have for decades been the focus of significant seismic reflection interest. The hydrocarbon prospectivity of the region, coupled with the varying structural styles of hydrocarbon-bearing structure, has prompted significant development of novel seismic acquisition, processing and interpretation techniques. While hydrocarbon basins are the priority targets of these acquisitions, there is the potential to reprocess and reinterpret them to reveal deep crustal structure. However, for such interpretations to be possible, significant effort is required to boost signal-to-noise ratio. A major component of seismic signal loss is the process of frequency-dependent attenuation. This project is a study of the methods by which attenuation mechanisms are manifest in seismic data, and how they are based compensated, to facilitate enhanced seismic interpretation across the whole depth range of the recorded archive.
Frequency-dependent attenuation, quantified by the Quality Factor Q, arises because subsurface materials are imperfectly elastic: an amount of the elastic wave energy is lost as the wave propagates: the wave is ‘damped’. While methods for measuring attenuation are well-established, the quality factor expressed in seismic data is a combination of intrinsic and extrinsic effects. Intrinsic effects include the conversion of seismic energy to heat across grain boundaries that are loosely-bonded or fractured, and the intra-pore fluid flow that occurs as the seismic wavelet propagates; extrinsic effects include the interference of the wavelets at between closely-spaced reflective interfaces. Intrinsic and extrinsic effects are difficult to separate using just one survey type, but for meaningful Q results as a rock property, and for robust Q-compensation and improvement of signal-to-noise ratio, an assessment must be made of the relative contribution of intrinsic and extrinsic attenuation mechanisms.
Industry partners TGS have very recently developed a tool for semi-automated estimation of measured Q from post-stack seismic reflection data: it shows considerable promise (Figure 1) and now merits refining. Furthermore, estimates of extrinsic attenuation can be made from well log analysis (e.g., Shapiro and Zien, 1993), allowing these Q estimates to be validated. TGS are ideally placed to collaborate in this project, because you will work with TGS’s extensive North Sea 2D marine seismic archive (Figure 3), and also their associated comprehensive suite of borehole logs – thereby facilitating, for the first time, building regional 3D models of measured, extrinsic, and hence intrinsic Q for the North Sea. You will investigate the interpretative advantages of robust Q-compensation to a suite of structural styles ranging from crustal architecture to shallow compaction related structures. A successful demonstration of the applicability of these approaches will motivate reprocessing of other seismic archives, thereby improving the understanding of deep basin processes on the European continental shelf.
Besides contributing additional funding to the NERC student stipend, TGS provides access to their seismic reflection data archive from the North Sea, together with financial support for a 3-month internship for the successful student in their Bedford office. The benefit to TGS is a refined means of measuring the seismic quality factor, including validation of an automated Q-compensation algorithm to improve the resolution of North Sea hydrocarbon targets at depth.
OBJECTIVES
In this project, you will work with leading scientists in the University of Leeds’ Institute of Applied Geoscience (IAG) and its industrial partners at TGS. You will use 2D seismic reflection profiles, spanning the North Sea, to characterise seismic Q, correct it for extrinsic effects using well-log data, and then apply Q-compensation algorithms to the seismic archive. The uplift in resolution will then facilitate a wide range of possible benefits for structural interpretation. Objectives for the studentship include, but are not limited to:
1. Derivation of accurate Q estimates from post-stack seismic reflection data, including resolving the contributions of intrinsic and extrinsic Q components by incorporation of well-log results;
2. Relating the suite of intrinsic Q values to known lithologies and petrophysical properties;
3. Develop a suite of case studies across the North Sea that quantitatively demonstrates the enhancement of imaging and its interpretative implications. These studies will include: deep crustal architecture and basin-controlling fault geometry; pre-Jurassic faulting; sub and intra-salt; differential compaction and top seal integrity."
http://www.nercdtp.leeds.ac.uk/projects/index.php?id=470
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