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This paper will discuss the successful completion of a 3-D seismic survey in a very difficult area in Western Portugal. The survey area has many operational problems caused by small farms and vineyards, hilly terrain and many small roads and houses. A survey had been attempted previously using a cabled recording system but was abandoned before comp...
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Instrumental seismic observations in northern Finland started in the 1950s.
They were originally initiated by the Institute of Seismology of the
University of Helsinki (ISUH), but the staff of Sodankylä Geophysical
Observatory (SGO) and later geophysicists of the University of Oulu (UO) were
involved in the development of seismological observations and research in
northern Finland from the very beginning. This close cooperation between
seismologists and the technical staff of ISUH, UO, and SGO continued in many
significant international projects and enabled a high level of seismological
research in Finland. In our paper, we present history and current status of
seismic observations and seismological research in northern Finland at the UO
and SGO. These include both seismic observations at permanent seismic
stations and temporary seismic experiments with portable seismic equipment.
We describe the present seismic instrumentation and major research topics of
the seismic group at SGO and discuss plans for future development of
permanent seismological observations and portable seismic instrumentation at
SGO as part of the European Plate Observing System (EPOS) research
infrastructure. We also present the research topics of the recently organized
Laboratory of Applied Seismology, and show examples of seismic observations
performed by new seismic equipment located at this laboratory and selected
results of time-lapse seismic body wave travel-time tomography using the data
of microseismic monitoring in the Pyhäsalmi Mine (northern Finland).
During the time taken for seismic data to be acquired, reservoir pressure may fluctuate as a consequence of field production and operational procedures and fluid fronts may move significantly. These variations prevent accurate quantitative measurement of the reservoir change using 4D seismic data. Modelling studies on the Norne field simulation model using acquisition data from Ocean Bottom Seismometer (OBS) and towed streamer systems indicate that the pre‐stack intra‐survey reservoir fluctuations are important and cannot be neglected. Similarly, the time‐lapse seismic image in the post‐stack domain does not represent a difference between two states of the reservoir at a unique base and monitor time, but is a mixed version of reality that depends on the sequence and timing of seismic shooting. The outcome is a lack of accuracy in the measurement of reservoir changes using the resulting processed and stacked 4D seismic data. Even for perfect spatial repeatability between surveys, a spatially variant noise floor is still anticipated to remain. For our particular North Sea acquisition data we find that towed streamer data are more affected than the OBS data. We think that this may be typical for towed streamers due to their restricted aperture compared to OBS acquisitions, even for a favourable time sequence of shooting and spatial repeatability. Importantly, the pressure signals on the near and far offset stacks commonly used in quantitative 4D seismic inversion are found to be inconsistent due to the acquisition timestamp. Saturation changes at the boundaries of fluid fronts appear to show a similar inconsistency across sub‐stacks. We recommend that 4D data are shot in a consistent manner to optimise aerial time coverage, and that additionally, the timestamp of the acquisition should be used to optimise pre‐stack quantitative reservoir analysis.
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The 3D seismic survey design optimization is a relevant and contemporary subject, open to contributions. Improvements in seismic survey design, the increasing quality demands of the petroleum industry and technical, operational and financial requirements must be integrated to determine the optimum acquisition geometry and its parameters. The existing works, regarding the determination of 3D acquisition parameters, address the question through local or global optimization a methods, and do not consider in the objective function a term associated with the illumination quality.
We have developed a new algorithm for 3D land seismic survey acquisition design using mixed-radix representation. This new methodology allows us to search and evaluate all possible solutions of the parameter space that is normally used in the oil industry, with high search resolution, an improvement from previous works. The mathematical functions that describe the parameters of the geometry as well as the defined objective function are expressions that do not demand significant computational cost, allowing the search and organization of the best performances in a fast and efficient way. As a result the local or global optimization methods may no longer be necessary for this kind of problem.
We applied the inversion algorithm to a synthetic case and to real data, in the three- dimensional survey of Fazenda Alvorada, Rio Itariri and Rio do Bu Fields (3D FAV/RI/RU). Not only do we obtained the optimum acquisition geometry, but also a set of very good acquisition geometries by focusing our efforts on the geological and geophysical requests and on the best operational practices.
Using the state-of-the-art of target oriented illumination analysis, we design a seismic experiment to define a quantitative measure associated with the acquisition design. We use the Finite Difference Method to solve the complete two-way wave equation and to generate, for each illumination point, a wavefield energy distribution at the acquisition surface. Then, we simulate the seismic acquisition itself for the set of geometries validated in the previous step, obtaining for each one its illumination energy. We inserted this quantitative criterion in the optimization process, combining technical and operational requirements with an estimation of the image quality during the acquisition design process.
We re-evaluated the set of geometries resulting from the inversion step using the mixed- radix algorithm, obtaining the final ranking of the best designs. We show that the use of the illumination analysis has a strong influence on the choice of the best geometries, giving weight to their illumination energies as a measure of the success of the acquisition in imaging the target points.
We completed the study with the evaluation of the complexity of the velocity model in the results of energy distribution and in the acquisition design. We have shown that, in the 3D FAV/RI/RBU, there was no significant variation in the final ranking of the best geometries and, therefore, even less complex velocity models could be used as input for target-oriented illumination analysis.
It is important to notice that the development of fast and efficient methods to 3D seismic survey design optimization should not imply pure and simply entail automating the process. On the contrary, it requires to the acquisition and decision teams more integration and refinement when choosing the target values, the constraints to be applied and all the necessary technical inputs. So that we can concentrate our efforts on choosing the best geometry designs, on the geological and geophysical understanding of the area, on the innovations in modeling and inversion, on the best operational practices and on the search of the lowest possible operational and financial cost. That is the direction we intend our work have contributed with.
Instrumental seismic observations in northern Finland started in 1950’s. They were originally initiated by the Institute of Seismology of the University of Helsinki (ISUH), but the staff of the Sodankylä Geophysical Observatory (SGO) and later, geophysicists of the University of Oulu (OU) were involved in development of seismological observations and research in northern Finland from the very beginning. This close cooperation between seismologists and technical staff of ISUH, OU and SGO continued later in a number of significant international projects and enabled a high level of seismological research in Finland. In our paper we present history and present state of seismic observations and seismological research in northern Finland at the OU and the SGO. This includes both seismic observations at permanent seismic stations and temporary seismic experiments by portable seismic equipment. We describe the present seismic instrumentation and major research topics of seismic group at the SGO and discuss the plans for future development of permanent seismological observations and portable seismic instrumentation at the SGO as a part of the EPOS-European Plate Observing System research infrastructure. We also present the research topics of recently organised Laboratory of Applied Seismology of the SGO, show examples of seismic observations performed by new seismic equipment of the Laboratory and selected results of time-lapse seismic body wave travel time tomography using the data of microseismic monitoring in the Pyhäsalmi Mine (northern Finland).
