9C 2D Piceance survey: Near‐surface velocity model building and tomostatics solution

An accurate near-surface velocity model is critical for weathering statics correction and initial model building for depth migration and full-waveform inversion. However, near-surface models from refraction inversion often suffer from errors in refraction data, insufficient sampling, and over-simplified assumptions used in refraction algorithms. Errors in refraction data can be caused by picking errors resulting from surface noise, attenuation, and dispersion of the first-arrival energy with offset. These errors are partially compensated later in the data flow by reflection residual statics. Therefore, surface-consistent residual statics contain information that can be used to improve the near-surface velocity model. We have developed a new dataflow to automatically include median and long-wavelength components of surface-consistent reflection residual statics. This technique can work with any model-based refraction solution, including grid-based tomography methods and layer-based methods. We modify the cost function of the refraction inversion by adding model and data weights computed from the smoothed surface-consistent residual statics. By using an iterative inversion, these weights allow us to update the near-surface velocity model and to reject first-arrival picks that do not fit the updated model. In this nonlinear optimization workflow, the refraction model is derived from maximizing the coherence of the reflection energy and minimizing the misfit between model arrival times and the recorded first-arrival times. This approach can alleviate inherent limitations in shallow refraction data by using coherent reflection data.

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Selected aspects of modern seismic imaging and near-surface velocity model building in the area of Carpathian fold and thrust belt

Geology, Geophysics and Environment ◽

10.7494/geol.2021.47.2.71 ◽

2021 ◽

Vol 47 (2) ◽

Author(s):

Andrzej Michał Dalętka

Keyword(s):

Model Building ◽

Geophysical Methods ◽

Velocity Model ◽

Hydrocarbon Exploration ◽

Surface Velocity ◽

Structural Representation ◽

Near Surface ◽

Velocity Model Building ◽

Orogenic Wedge ◽

Refraction Tomography

Despite the increasing technological level of the reflection seismic method, the imaging of fold and thrust belts remains a demanding task, and usually leaves some questions regarding the dips, the shape of the subthrust structures or the most correct approach to velocity model building. There is no straightforward method that can provide structural representation of the near-surface geological boundaries and their velocities. The in-terpretation of refracted waves frequently remains the only available technique that may be used for this purpose, although one must be aware of its limitations which appear in the complex geological settings. In the presented study, the analysis of velocity values obtained in the shallow part of Carpathian orogenic wedge by means of various geophysical methods was carried out. It revealed the lack of consistency between the results of 3D refraction tomography and both the sonic log and uphole velocities. For that reason, instead of the indus-try-standard utilization of tomography, a novel, geologically-consistent method of velocity model building is pro-posed. In the near-surface part, the uphole velocities are assigned to the formations, documented by the surface geologic map. Interpreted time-domain horizons, supplemented by main thrusts, are used to make the velocity field fully-compatible with the litho-stratigraphic units of the Carpathians. The author demonstrates a retrospective overview of seismic data imaging in the area of the Polish Carpathian orogenic wedge and discusses the most recent global innovations in seismic methodology which are the key to successful hydrocarbon exploration in fold and thrust regions.

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