Advanced Seismic Data Acquisition and Processing
The course deals with advanced methods of seismic acquisition and processing. It will be taught not only by explaining the methods, but above all by applying the theory in mainly Excel based assignments. Seismic data is one of the main sources of information on the subsurface. We not only need to obtain the structure that could contain hydrocarbons, but also the rock properties so we can decide on whether we are dealing with reservoir rocks (sandstone, carbonates, even shales), sealing rocks (shales, salt) or source rocks (shales, coal). It is not only important to know what type of rock is present, but also what its porosity and permeability is: how easy do the hydrocarbons flow through the rocks. To obtain the best image of the subsurface we first need optimum acquisition. Optimum means fit for purpose. There are several criteria that need to be satisfied. First of all, the area covered during acquisition should be the prospect area extended sufficiently to provide fold-fold and fully migrated data. An acquisition principle that should be adhered to as much as possible is symmetric sampling, which means equal shot and receiver spacing and equal in-line and cross-line distances (for a 3D). A noise spread (trial acquisition with closely spaced receivers and shots) is acquired in each new area to determine the needed shot and receiver intervals, the bandwidth, etc. The shot and receiver station spacing should be such that no spatial aliasing of the data occurs. Surface and subsurface diagrams are useful to see what CMP spacing and offsets in each CMP gather result from the surface geometry of shots and receivers. The data recorded is the ground motion which gives a continuous (analogue) signal in time which needs to be digitized for the processing. This digitization needs to be done so that neither temporal nor spatial aliasing occurs. Namely, by aliasing information will be lost. Hence, the complete wave-field which arrives at the surface must be faithfully represented by the discrete/digital data. Although all the information is present in the so-called shot or field records, processing is needed to make them accessible for interpretation. In interpretation, we try to obtain a true image of the “geology” of the subsurface. Processing can be divided into a) signal processing steps and b) wave propagation based processing steps. Signal processing steps are, for example, static corrections, removal of shot-generated noise by velocity filtering, shortening of the wavelet by de-convolution, NMO correction, etc. The wave-propagation part consists of migration or imaging. For wave propagation we need, in principle, to use equations describing full elastic wave propagation in an inhomogeneous, anisotropic, visco-elastic earth (as that is what really happens in the subsurface). However, these equations would lead to very complicated and computer intensive processing algorithms. So, we usually simplify our description of the wave propagation. What we do is to use, as phrased by Ian Jones and others, "appropriate approximations". The one most commonly used is the one-way acoustic wave equation which describes only a single reflection per reflection ray-path and ignores density. It only uses a velocity depth model and only considers P-wave propagation. This will provide us, for example, with migration algorithms/operators (for time- as well as depth migration) that will still do a reasonably correct summation of acquired data. It will give a migration output that still shows, maybe not correctly, the results of anisotropy, attenuation, wave conversions, shear velocities, etc. Despite the use of this acoustic approximation in our processing, amplitudes can be used (can they?) to determine pore-fluids and pre-stack migrated data that can be used in AVA analysis for deriving shear wave properties. But note that if we model, as in inversion, a synthetic geophysical quantity, say related to amplitudes, such as the reflection coefficient we need (do we?) to include densities across the interface and for AVA we need to include density and shear velocity to interpret the pre-stack seismic amplitudes (as the effect of these properties is contained in the observed data). All of this will be treated in this course.
At the end of the course participants will have a good understanding of what information seismic data can give and for what purposes in Exploration and Production it can be used. This will enable them to specify the requirements for a survey, either done by themselves of by a special service provider. Other benefits include: • Place and value geophysical activities in a multi-disciplinary context • Judge the merits of various seismic geophysical techniques • Better liaise and collaborate with staff in related disciplines • Recognise artefacts and direct hydrocarbon indications on seismic • Value novel developments such as time lapse methods for hydrocarbon reservoir monitoring
• Part 1: The role of seismic in the Exploration and Production of Hydrocarbons • Part 2: Seismic Acquisition Strategies • Part 3: Seismic processing Strategies • Part 4: Time-to-Depth conversion, Direct Hydrocarbon Indicators • Part 5: Value of Information: How much to spend on new acquisition and/or new processing
The course is designed for geophysicists involved in designing and supervising seismic acquisition and processing, and for those involved in specifying/supervising the acquisition and processing done by service companies.
Participants should have a basic understanding of seismic acquisition and processing and general knowledge of the role of seismic in exploration and production of hydrocarbons.
• An Introduction to Geophysical Exploration, Keary, Brooks & Hill, ISBN-0-632-04929-4 • Looking into the Earth, Mussett & Aftab Khan, ISBN-0-521- 78574-X • Fundamentals of Geophysics, Lowrie, ISBN-978-0-521-67596-3 • The Art of Being a Scientist, Snieder & Larner, ISBN-978-0-521- 74352-5 • 52 Things you should know about Geophysics, Hall & Bianco, ISBN-978-0-9879594-0-9
About the Instructor
Dr Jaap C. Mondt obtained a Bachelors degree in Geology at the University of Leiden followed by a Masters degree in Theoretical Geophysics and a PhD on “Full wave theory and the structure of the lower mantle” at the University of Utrecht. Dr Mondt then joined Shell Research in The Netherlands to develop methods to predict lithology and pore-fluid based on seismic, petrophysical and geological data. Subsequently he worked at Shell Expro in London to interpret seismic data from the Central North Sea Graben. After his return to The Netherlands, he headed a team for the development of 3D interpretation methods using multi-attribute statistical and pattern recognition analysis on workstations. After a period of Quality Assurance of “Contractor” software for seismic processing, he became responsible for Geophysics in the Shell Learning Centre. During that time he was in addition part-time professor in Applied Geophysics at the University of Utrecht. From 2001 till 2005 he worked on the development of Potential Field Methods (Gravity, Magnetics) and EM methods (CSEM) for detecting oil and gas. After his retirement from Shell, he founded his own company (Breakaway), specialised in courses on acquisition, processing and interpretation of geophysical data (seismic, gravity, magnetic and electromagnetic data). In addition to providing support to the Shell Learning Centre, he gives his own courses to International as well as National energy companies.