From exploration to production through development, reliable seismic data can play a key role in de-risking and making better informed strategic decisions. Indeed, having reliable seismic data at hand is essential for revealing the finest details of the geological structure, predicting reservoir lithology and fluid content and updating reservoir models.

But how do we get reliable seismic data for reservoir characterization in onshore environments such as the Middle East that are known to be particularly challenging for seismic data quality? To address this question, CGG has developed UltraSeis, an integrated solution delivering high-density broadband seismic data that address the near-surface effects that have plagued onshore data and enable reliable quantitative interpretation and more accurate modeling of onshore reservoirs.

Increasing trace density
For land seismic data, limited bandwidth and the significant noise contamination have for a long time been major obstacles to the delivery of reliable reservoir-quality onshore seismic data. The answer to tackling this significant noise contamination lies mainly in addressing the near-surface effects that encompass all of the undesirable effects on the signal.

Trace density has a major role to play in addressing these near-surface effects. Over the last decade, experience gained from the high channel-count surveys deployed in the Middle East has identified trace density as a key quality metric. Trace density is the ratio between the fold and bin size which, when increased, clearly shows signal-to-noise ratio (S/N) improvement and subsequently enhances the quality of seismic images (Figure 1). Increasing the number of traces per surface area unit allows a better sampling of the noise contamination generated by the near-surface effects, which leads to a more efficient removal of the undesirable noise.

The advantage of being able to better resolve (and therefore compensate for) the near-surface effects leads to a significant uplift in imaging resolution, both laterally and vertically. Further investigation has demonstrated that, beyond the already dramatic uplift in structural imaging, mitigating the near-surface effects by increasing trace density is also of benefit to the derivation of more reliable quantitative reservoir information.

Matching density, productivity rates
Building on the understanding of how trace density impacts the quality of reservoir attributes from seismic inversion, multidisciplinary teams are able to propose survey design and acquisition solutions that meet the dual challenge of offering the appropriate trace density that will deliver reservoir-quality seismic while optimizing operations in the field for efficient rollout of the survey. This leads to new trends in acquisition design and layout of equipment.

Traditionally, the equipment was deployed on 3-D sparse geometries featuring large intervals between lines, long arrays for both receivers and sources and a tendency to use powerful but heavy sources. This resulted in coherent noise being filtered directly in the field through the receiver arrays and each source point benefitting from high source energy, both through the use of a large number of vibrators per array and the use of long sweep times. Addressing the need for 3-D high-density surveys, the seismic industry therefore first moved to geometry designs featuring smaller intervals between lines for both receivers and sources. Smaller source and receiver arrays can be deployed using geophones with a very low-frequency response and high S/N, a new generation of vibrators delivering broadband sweeps have been introduced and new recording systems that support the high channel count and high productivity operations requirements are available. Although the reduction in the number of vibrators along source arrays can generate a decrease in the source energy, this is not a trade-off since the outstanding effects on the seismic image S/N brought by the higher trace density greatly benefit the quality of the seismic in terms of better resolution both laterally and vertically.

Onshore reservoir characterization
The outstanding benefits of broadband data for reservoir characterization have been widely demonstrated over the last years, with most of the emphasis being given to offshore examples. Nevertheless, the advantage of broadband data is equally important for onshore reservoir characterization, allowing better wave penetration and deep target illumination thanks to lower frequency content and sharper wavelets, leading to more reliable horizon interpretation and better lithology prediction. Increasing trace density is intrinsically an enabler for broadband data, and when combined with technologies that allow contractors to efficiently emit a broadband signal, record it with integrity and process it reliably, land seismic data with a bandwidth in excess of 6 octaves and frequencies as low as 1.5 Hz can be achieved. This combination takes land seismic into the new era of broadband data and promises to unlock onshore reservoir knowledge.

The ability to emit and record a broad frequency bandwidth is undoubtedly a major step forward for onshore reservoir-quality seismic. However, if not handled properly during the processing phase, the lowest frequencies can be easily damaged or lost. New processing workflows and quality controls performed on frequency bands (octave) are being progressively adopted to make the most of the low-frequency content. Figure 2 shows the uplift from conventional 4-octave data to 6-octave broadband data facilitated by the use of new processing workflows.

UltraSeis offers a solution for the delivery of more accurate structural information and more quantitative and reliable reservoir attributes for reservoir modeling. It does this through the efficient delivery of high-density broadband seismic, data that address
undesirable near-surface effects that commonly obscure reservoir information in the seismic data. The success of the solution resides in a good understanding of the geological and geophysical challenges of the survey area and the appropriate responses to them through an integrated workflow that spans survey design, acquisition, subsurface imaging and reservoir characterization.

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