DISCover Technique Records Data Below Normal Seismic Bandwidth | Schlumberger
Case Study
Location
Australia, Oceania, Offshore
Details

Challenge: Image a large-scale event (poorly defined on shallow array data) to overcome the lack of low frequencies recorded by conventional marine seismic acquisition techniques in a 33 x 7-km survey that was acquired on the Northwest Shelf of Australia.

Solution: Deploy the DISCover broadband deep interpolated streamer coverage seismic technique to improve deep imaging, help define large-scale features, and improve inversion accuracy.

Results: The DISCover technique delivers enhanced continuity, especially within the deeper part of the section. Low-frequency seismic signal is improved with no loss of high-frequency content, giving higher resolution and deeper penetration.

DISCover Technique Records Data Below Normal Seismic Bandwidth

Geological mapping and inversion quality improved

Increase low-frequency data for improved imaging

In a 33 x 7-km survey acquired on the Northwest Shelf of Australia, a large-scale event (imaged using conventional marine acquisition techniques) was poorly defined on shallow array data. A study was conducted to determine the value of the enhanced low-frequency content provided by the DISCover acquisition technique.

DISCover acquisition spread

A sparse “deep” 3D marine acquisition spread was used in combination with a conventional “shallow” 3D spread. The conventional spread provided optimal mid-to-high frequencies, while the deep 3D spread is designed for optimal low-frequency recording and was more sparsely sampled.

The acquisition parameters were six shallow cables at 6-m depth and 100-m separation and two deeper cables at 20-m depth and 300-m separation.

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The basic layout of the streamer spread used in the DISCover method, with a conventional 3D spread of shallow cables and a smaller number of deeper cables (six and two respectively in this case).
Two charts, top and bottom, that show by combining the spectra from the shallow (blue) and deep (red) streamer arrays, we can fill in as much as possible of the low-frequency “notch” present in conventional data. This combination (black) optimizes the signal-to-noise ratio over the entire seismic bandwidth.

By combining the spectra from the shallow (blue) and deep (red) streamer arrays, we can fill in as much as possible of the low-frequency “notch” present in conventional data. This combination (black) optimizes the signal-to-noise ratio over the entire seismic bandwidth.

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Images left: Relative acoustic impedance. Left: shallow-cable data only, right: DISCover tech-nique data, circled: a large-scale feature that can only be clearly observed on data from the DISCover technique. Graph right: Spectral analysis of shallow-cable data and data gained with the DISCover technique. The ultra-low frequency range provided by seismic velocities overlaps the low-frequency cutoff from the deep cable.

Generate a low-frequency model for inversion

A deterministic inversion was applied to the final fully processed zero-phase data volumes to generate acoustic impedance for both the shallow array data (equivalent to conventional recordings with the Q-Marine point-receiver seismic system) and the combined shallow/ deep array data obtained with the DISCover technique. Well data was not used in the inversion—only the information contained within the seismic wavelet was used, and the low frequencies were imposed by the seismic data itself.

A well located 4.5 km to the north was used to verify the inversion. It was projected onto the survey area and the acoustic impedance log (a sonic density product) was compared with inverted impedance. The wavelet assumptions appeared to be valid—good ties were observed between the relative acoustic impedance derived from the well data and the seismic data over the inverted frequency range from 3 Hz to 80 Hz.

DISCover data below normal seismic bandwidth mapped

A large-scale event that is poorly defined on the shallow array data is clearly visible on data recorded with the DISCover technique. Such thick, low-frequency, large-scale geological features are often present but never mapped on conventional seismic data—they lie below the normal seismic bandwidth. Therefore, the DISCover technique improves both geological mapping and the quality of any inversion for rock properties performed on the data.

DISCover technique provides higher resolution data

The DISCover technique delivers encouraging results which show enhanced continuity, especially within the deeper part of the section. Low-frequency seismic signal recordings are enhanced with no loss of high-frequency content, giving higher resolution and deeper penetration. This not only results in a much clearer seismic section, but also images events that cannot be clearly seen on conventional seismic data.

The DISCover technique also enables much improved inversion in the absence of a reliable low-frequency model and sparse well control, and, as a result, can be used as an enhanced exploration tool.

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