A better understanding of the marine source delivers better results

by CGG

All seismic imaging begins with the release of acoustic energy into the water, continues with the propagation of this energy through the subsurface and ends with the harvesting of encoded messages from the reservoir. To get the required response from the reservoir you have to start with the right sound, at the right level, from the right angle.

The image of the subsurface and the attributes of the reservoir have to be deconvolved from any imprint of the acquisition system. CGG has done extensive work to eliminate the echo of the water surface from the image and attributes. The company’s advanced deghosting technologies from acquisition to processing have delivered the expected accuracy on both the source and receiver sides of the Marine Acquisition System.

Bubble rising after source firing behind the Oceanic Sirius (image courtesy of CGG)

However, their ambition is to go deeper into resolution. The reality of acquisition means that the marine source is moving continuously. From a distance it appears to be a single point, quiet and static, but from close up and shallow, each element of the 250 m2 source array can be distinguished. Removing the illumination imprint of this structure requires an accurate estimation of the seismic signature of each element at its true location.

CGG’s Full Source Characterization™ technology enables the source signature to be estimated for each element in any direction (azimuth) at any distance (offset) and for each shot. A full acquisition system records the sound propagation of each individual source element at near-field, mid-field and far-field, while dedicated real-time software performs joint inversion of these measurements. The complexity of the generated wavefield, including the actuated bubbles, their interactions, and their reflection from the sea surface, is then revealed. The contribution of each element at any location and any depth can be identified. This makes the removal of the imprint of the marine source system from the images and attributes of the subsurface possible, regardless of depth. 3D source deghosting and designature, critical for shallow targets, can be performed accurately to complement 3D receiver deghosting. The imprint of the bubble on the low frequencies can be removed, to deliver stable and accurate low-frequency data. Monitoring the stability of the source signal for all azimuths and offsets highlights the areas where shot-by-shot deconvolution would be recommended.

In 4D acquisition the expectations of 4D source repeatability are very high. Any source imprint, even on only some azimuths or offsets, reduces the detectability threshold of the 4D signature. With Full Source Characterization technology, CGG moves forward, in combination with 4D illumination monitoring, to extend the assessment of repeatability beyond the simple geometrical repeatability of source and receiver positions.

CGG’s improvements to resolution are not only based on better engineering of marine sources. Identifying the expectations of their clients helps them to define the right strategy. By understanding the requirements of processing technologies, they can design the right sensing of their sources to deliver the right measurements for the inversion algorithms to deliver stable and accurate high-resolution solutions.

By distributing sources with depth, CGG can remove the imprint of sea surface interference (ghosts) with BroadSource™, its broadband marine seismic source. By measuring each single element of the source, the imprint of the size and geometry of the source array (Full Source Characterization) can be removed. Increasing the density of the sources in space and time provides better sampling, in terms of longer records and increased fold, with improved efficiency and accuracy using Simultaneous Source technology. Continuous recording is a reality today, paving the way for the continuous generation of acoustic sound through Marine Vibrators tomorrow.

From shallow to deep water, CGG has the equipment, talent and techniques to successfully complete the most complex Marine projects.

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