# 3-D seismic acquisition geometry design and analysis: Investigation of the requirements to include illumination from all multiples

A Kumar

Research output: ThesisDissertation (TU Delft, preparation external)

## Abstract

A seismic survey should be designed such that imaging of the acquired data leads to a sufficiently accurate subsurface image. For that purpose, methods for acquisition geometry analysis and design are available. These methods are used to judge whether an acquisition geometry is suited for the specified objectives. Conventional 2D and 3D acquisition geometry analysis methods are largely based on common-midpoint processing that assumes a horizontally layered earth model. Consequently, the influence of the inhomogeneity of the subsurface on the data quality and image quality is not taken into account. However, in practice it has been shown that the data and image quality can suffer significantly from complex geology. Therefore, the inhomogeneity of the subsurface must be taken into account in the acquisition analysis methods. This can be done by the use of a macro model of the subsurface. Additionally, recent developments in seismic imaging and reservoir characterization use the multiple-reflections in the data to extend the illumination in areas that cannot be reached by the primary-reflections. The use of multiples yields a better vertical resolution as well as to suppress migration artefacts caused by crosstalk of multiple-reflections. The seismic value chain suggests that the new developments in seismic imaging and reservoir characterization should lead to new opportunities in the seismic acquisition. Therefore, the goal of this research is the development of a method that meets all the above mentioned requirements. The method presented in this thesis is based on the previously developed $\it{focal}$ $\it{beam}$ $\it{analysis}$ concept. This concept emphasizes the separate analysis of the source geometry and the detector geometry, leading to two outputs: the focal source beam and the focal detector beam. This gives the opportunity to separately judge and adjust the configuration of the sources and the configuration of the detectors. These beams are inspected in the space-frequency domain and in the Radon-frequency domain. In the spatial domain, it is visible whether the wavefield has properly been focused into a point. In the Radon domain, it is visible how the angle-dependent amplitudes are affected by the acquisition geometry and overburden structures. This approach provides thorough understanding of the cause of image deficiencies. The source and detector beams can be multiplied to compute a migrated image. The multiplication can be carried out in two domains to assess the different quality parameters of a seismic image: • Multiplication of the source and detector beam in the space-frequency domain yields the resolution function, which represents the image of a unit point diffractor. • Multiplication in the Radon-frequency domain yields the AVP-function, which is the angle-dependent image of one reflection point on an angle-independent reflector. However, so far this method assumed primary reflections only as signal, leaving out multiple reflections as noise. In this thesis, I discussed that multiples can be considered as signal in the seismic imaging if they are handled correctly. Therefore, I extended this focal beam method to the multiples as well. In the extended focal beam method, the full wavefield propagation between a subsurface point and the acquisition surface is simulated using a recursive full wavefield modeling engine. It uses a macro velocity model for the wavefield extrapolation from one depth level to another depth level and a reflectivity model to include all the reflection and transmission properties related to the same depth level. Subsequently, the data are focused to the target point using the source and the detector geometry. This full wavefield data is a complex wavefield which includes the effects of all multiples (i.e., propagation as well as reflection and transmission effects). Therefore, the focusing is achieved by a minimization scheme. I have demonstrated with the help of some examples, that the gap in the Radon transformed focal source beam due to sparse sampling issues can be filled with the use of all multiples. It means more angle-dependent information can be obtained. Imaging of subsalt sediments is challenging in practice, because of the high velocity contrasts and irregular shapes. My analysis shows that in such situation, image quality varies strongly with the position of the target point with respect to salt. In that case, even if one has a perfect source distribution, primary illumination may be limited due to geology. The image quality cannot be improved any further by adjusting the acquisition geometry in this case. I show that utilizing all multiples may be the part of the solution. The illumination from below is important in such cases. The method presented in this thesis offers opportunities for the investigation of the added value of the surface-related and the internal multiples. To summarize, this new method meets all the following criteria: • Dealing with the complexity of the subsurface model. • Considering all the multiple reflections as useful information. • Illumination from below in the complex sub-salt scenarios. • Development of geophysical-based infill specifications to assess the impact of coverage holes on data quality
Original language English Doctor of Philosophy Delft University of Technology Wapenaar, C.P.A., SupervisorBlacquière, G., Advisor 9 Dec 2015 Delft A. Kumar 978-94-6203-962-9 https://doi.org/10.4233/uuid:392ff377-0133-473c-9796-32ce61535aca Published - 2015

## Keywords

• seismic acquisition
• imaging
• illumination
• multiples