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  • Writer's pictureAlastair MacLeod

Solved! Mysteries of the Witch Ground Basin

A case study into the importance of optimised processing of sub-bottom profiler (SBP) seismic data.

Often sub-bottom profiler (SBP) datasets are used to solve the mystery of what is hidden in the very shallow sands and muds deposited within marine infrastructure development sites. Depending on the source type, the top few metres can sometimes be completely obscured by ringing, which is not very helpful to those tasked with solving the subsurface mysteries.

This was the challenge faced by our client, Global Maritime who needed to interpret the top of the Witch Ground Formation, a very soft clay that is widespread beneath the North Sea. However, the interface between the seabed sediments and the very soft clay below was mysteriously hidden within the original data.

Beneath the Green Volt Floating Wind Farm

The site being developed was the Green Volt floating wind farm, the most progressed commercial-scale floating wind farm in Europe, awarded to Flotation Energy and Vårgrønn during Crown Estate Scotland’s recent INTOG leasing round. More information on INTOG can be found via the Crown Estate Scotland website.

Global Maritime were provided pre-existing sub-bottom profiler data (SBP) to interpret the shallow soils at this location (Figure 1). However, the dataset was in envelope form and contained a large amount of ringing at the seabed and major reflective units below, obscuring much of the key subsurface information including the important interface between seabed sediments and the very soft Witch Ground formation.

Sub-bottom profiler data in envelope format showing long, ringing source wavelet.
Figure 1: Sub-bottom profiler data in envelope format showing long, ringing source wavelet.

Envelope data

Here at RockWave, there have been occasions where we have been delivered raw seismic data in envelope format, rather than full-waveform. Envelope data can be verified by looking at the individual samples within a seismic trace. Full-waveform seismic data will have positive and negative amplitude values, whereas envelope data will only have positive and zero amplitudes as illustrated by schematic in Figure 2.

The reason for acquiring data in envelope form rather than full-waveform is unclear, but we suspect the practice was more commonplace on vessels using legacy recording systems, where the acquired data was printed on paper sections for the onboard geophysicists to interpret. If envelope data was easier to interpret on paper during the acquisition, then perhaps it was also assumed that saving the digital file in envelope form would also be suitable.

We are limited in what we can achieve without the full-waveform data. Improvements are largely cosmetic as you cannot recover what has already been entirely removed. In the case of the Green Volt SBP dataset, another approach was necessary…

Reprocessing Full-Waveform

Thankfully, most acquisition firms are contracted to keep a copy of the raw data acquired for a minimum period of five years from completion of the project. This allowed the data owners to return to the acquisition contractor to obtain a copy of the raw, full-waveform data that existed in archive.

Our team of geophysicists were confident we could uncover the shallow subsurface mysteries hidden within the raw, full-waveform data. The team, led by Sandeep Bhamber, tested and optimised a processing sequence that would maximise the interpretability of the shallow soils. Attention to detail was given to collapsing the long, ringing source signature, which would be so valuable in revealing the Witch Ground top, whilst also improving the continuity of deeper horizons with adequate denoise solutions and amplitude recovery.

Upon completion, we provided the RockWave-reprocessed data (Figure 3) to Jack Turner of Global Maritime, enabling him to accurately map the top of the Witch Ground Formation. Turner remarked that the new data was, “unrecognisable compared to what we were previously working with.”

pinger seismic data processed by RockWave
Figure 3: Processing of sub-bottom profiler (SBP) seismic data at Green Volt floating offshore wind farm by RockWave

This before-and-after showcase of SBP data serves as a compelling reminder to ensure that when acquiring subsurface data with the aim of imaging directly below the seabed, data processing optimised for imaging the same depths is equally crucial. If done correctly, the mysteries of the Witch Ground Basin may reveal more than you bargained for...

optimised SBP data processing
GIF 1: Original envelope SBP data compared to the reprocessing

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