Rosalena

Imaging Geophysicist

""What I enjoy most working at CGG is the friendly and relaxed environment, the high quality training material and teachers & the new technologies being developed and tested continuously.""

MSc in Exploration and Applied Geophysics, University of Pisa; BSc in Geology, University of Bari.

Current focus: Velocity model building and FWI for proprietary 4D seismic monitoring project.

Recent experience: Time signal processing of 4D proprietary seismic monitoring data.

What is 4D seismic monitoring?

An initial 3D exploration seismic survey is acquired and an image generated. Using this dataset hydrocarbons are discovered and developed. The act of taking oil and gas out of the reservoir causes changes in fluid flow, saturation and pressure and these alter the elastic properties of the affected rocks, which in turn causes measurable changes to the travel-time and amplitude of seismic waves. In order to visualise and quantify these changes a repeat seismic survey using the exact same acquisition parameters (even down to shot location) is acquired. This new survey is known as the monitor and the original is labelled the baseline. This gives us our fourth dimension which is time.

Sometimes the monitor is processed in exactly the same way as the baseline even down to resurrecting old versions of programs used in the original processing; often both surveys are processed with the latest available technologies. The aim of 4D processing is to attenuate the 4D noise caused by changes in acquisition parameters such as more modern recording equipment or from different environmental conditions such as water currents, and to emphasise the actual differences, the 4D signature, caused by the changes in fluid, pressure and stress. A key philosophy in 4D processing is to optimise for the 4D difference. Often if you optimise the processing of each individual dataset you end up with a worse 4D result. 4D cost functions are often used to allow a trade-off between what is best for processing and what gives the best 4D difference. The image above shows the 4D difference at the top of a reservoir into which fluid has been injected in order to extract more oil with the entry point indicated by the strong red area.

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