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The recent demand for high-resolution broadband seismic data has created a growth in so-called broadband solutions. However, there has been no definition of what actually qualifies as “broadband”. This means that a variety of different

techniques ranging from full equipment,

acquisition and processing solutions to simple data whitening in processing have been bracketed together as “broadband”. We describe a truly broadband, integrated solution, involving equipment, acquisition and processing which enables the effects of both the source and receiver ghosts to be removed to deliver ghost-free data with the broadest bandwidth (in excess of six octaves) and the best signal-to-noise ratio at low frequencies, down to 2.5Hz.

ATTENUATION OF THE RECEIVER GHOST

In conventional acquisition the bandwidth is limited by the source and receiver ghost notches in the amplitude

spectrum, caused by interference between the upgoing and downgoing waves, at frequencies which depend on the source and streamer depths and emergent angles. In this solution, the receiver ghost is tackled by using a variable-depth solid streamer, towed in a proprietary curved configuration, to create ghost notch diversity. This is exploited by advanced deghosting algorithms to produce high-resolution data with bandwidth from about 2.5Hz to the source ghost notch.

A toolbox of different deghosting algorithms, including fully 3D solutions, has been developed for performing Ghost Wavefield Elimination (GWE) that is applied to the data either

pre-imaging (Wang 2012, Poole 2013, Wang 2013) or post-imaging using the joint-decon solution (Soubaras 2010). This will help towards creating an ideal broad bandwidth wavelet – i.e. one with a sharp central peak and minimal sidelobes.

Exceptional low frequencies are recorded using a deep-towed hydrophone; this solution capitalizes on the exceptional low-noise characteristics of truly-solid foam-filled streamers (Dowle 2006), which can be towed deeper than some other streamers, to acquire high-quality signal down to 2.5Hz. Towing at an average depth of 40m provides an 11dB increase in response at 5Hz over towing at 10m, due to the shape of the zero-Hz

ghost notch (see figure 1). This increase is in addition to the improvement in signal-to-noise ratio achieved by towing at depth and avoiding the sea-state noise, and the fact that solid streamers are proven to be 10-15dB quieter than gel streamers below 10Hz in vibration tank tests.

ATTENUATION OF THE SOURCE GHOST

To attenuate the source ghost notch, a similar ghost diversity technique is employed. A synchronized multi-level source is used to defocus the source ghost and generate wavefronts with reduced, unfocused notches in the amplitude spectrum, enabling source

designature for the full bandwidth (Siliqi 2013). The combination of variable-depth receivers and synchronized multi-level sources delivers truly broadband data, limited only by the acquisition sample interval. The different source depths are synchronized so that the deeper array fires at the exact delay of the travel-time between the two levels, so the down-going wavefields are synchronized and produce a focused primary reflection, while their respective ghosts (upgoing wavefield) are unsynchronized and produce unfocused ghost reflections (see figure 2). The optimum depths for the airguns are achieved when the notch caused by one gun level corresponds to the maximum energy of the other. Figure 3 shows the spectral output of a source array with airguns deployed at 6m and 9m depths with a 2ms time delay between them. The ghost notch of a one-layer source array (6m) is filled and the extra low frequencies generated by 9m airguns are visible.

A synchronized multi-level source is a compact solution for acquiring broadband data with variable depth steamers. It can be deployed in the flip-flop mode necessary for 3D broadband surveys and has all the benefits of a standard marine source in terms of robustness, directivity and repeatability. In order to maintain the same directivity created by conventional source arrays, a 3D spatial distribution

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HOW BROAD IS BROADBAND?

By Jo FIRTH and Mark STANLEY, CGG

Figure 1: Zero Hz receiver ghost notch at different streamer depths.

Figure 2: A synchronized multi-level source.

Figure 3. Theoretical sources signatures (right) generated by a conventional source at 6m, a conventional source at 9m and a synchronized multi-level source.

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of airguns across the layer-depths is used and the resulting directivity pattern remains at the same quality range as that for a conventional source, even for frequencies corresponding to the one-layer source notch (see figure 4).

The multi-level source allows defocusing of the ghosts, but does not entirely eliminate them. Nevertheless, the

absence of deep notches in the spectra allows perfect designature of the seismic across the full bandwidth. While the accuracy of pressure field modelling is sufficient for source array design it is not precise enough for building source designature operators. These can be more accurately designed using recorded near-field hydrophone measurements (Ziolkowski 1982, Poole 2013). This

direct measurement approach delivers a more accurate estimation of the bubble effect and low frequencies in general, which are fundamental for broadband acquisition. Figure 5 illustrates the effect of accurate designature on the residual bubble, which generally causes low-frequency artefacts. The technique allows an accurate designature process through the entire bandwidth which

encompasses residual deghosting, de-bubbling and zero-phasing in the same step. Several acquisitions worldwide (including North West Australia) have demonstrated the benefits of this technique in delivering ghost-free images with a full bandwidth from 2.5Hz up to 200Hz, see figure 6.

Improved imaging at depth and facies variation identification are provided by

the lower frequencies whilst the high frequencies clarify the interpretation of thin beds, stratigraphic traps and lateral resolution of channels. For the deeper parts of the section, the highest frequencies will vanish due to the anelastic attenuation of the earth and will not contribute to the image, but the lack of a source ghost still increases clarity as shown in figure 7. The full bandwidth present in the shallow

section enables precise imaging for shallow-hazard mapping and accurate velocity picking.

BENEFITS OF TRULY BROADBAND DATA FOR EXPLORATION

As broadband data provide both low and high frequencies, it is ideal for exploration as potential targets at all

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Figure 4: Comparison of directivity for a conventional and synchronized multi-level source.

Figure 5: Remaining wavelet after designature with far-fields obtained by modeling vs. reconstructed from near-fields.

Figure 6: Example variable depth and multi-layer source data from NW Australia.

Figure 7: Comparison of conventional data and variable-depth streamer data with both a conventional and a multi-layer broadband source.

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to yield accurate impedance and Vp/Vs estimates, even where there is little or no available well data (figure 10). In the 2D example from North West Australia shown in figure 11, the inversion was performed using the same 0-5Hz initial model derived from seismic velocities (Michel 2012). The broadband inversion delivered improved inversion results and well-ties, with a better delineation of the thickness of the gas-bearing

zone to reduce uncertainty in lithology prediction. In areas all over the world, from offshore Brazil (Kneller 2013) to the North Sea (Hake 2013) and Australia, the extended bandwidth has provided significantly improved and more geologically realistic inversion results, with greater dynamic range and more reliable lithology estimates.

Ultra-low frequencies also provide

discrimination of different sedimentary facies. Figure 12 shows an example of 3D frequency decomposition and colour blend from a broadband multi-client dataset in Angola Block 22, where a turbidite channel complex with various phases of sediment fill, meander growth and incisions can be identified clearly. Without the ultra-low frequencies this picture would be incomplete as the red channel would

levels can be imaged. Truly broadband wavelets are characterized by a sharp central peak and minimal sidelobes. The individual peaks or troughs of these seismic wavelets correspond to true stratigraphic horizons, and therefore enable more accurate seismic interpretation, providing clear distinction between stratigraphic packages and high resolution of fine details without the tuning interference experienced with conventional band-limited seismic data. The low frequencies give an envelope to the seismic signal that shapes the larger-scale impedance variations, or in geological terms the major lithology variations. This increases confidence in correlating seismic interpretation across faults and other major structural features, see figure 8.

Three octaves of signal are generally considered necessary for adequate seismic resolution; therefore, in order to achieve sufficient bandwidth in areas where high frequencies are limited, e.g. below salt or basalt where frequencies above 20Hz tend not to penetrate, it is necessary to extend the low frequencies. Lower frequencies are affected less by attenuation and so deliver improved penetration in

these complex areas. Conventional marine streamer acquisition usually lacks sufficient signal-to-noise ratio in the 2-7Hz bandwidth due to streamer depth, streamer tow noise, source array configuration, source depth and source bubble. By using variable-depth streamers, frequencies down to 2.5Hz are routinely recorded, providing three octaves of data below 20Hz.

Variable-depth streamer exploration is not limited to deep waters; data have been acquired using this technique in shallow water areas in the North Sea and offshore Vietnam and China, in water depths as shallow as 37.5m. In these areas the streamer shape is adapted to fit the circumstances, raising the deeper part of the streamer as the water gets shallower. Figure 9 shows data recorded offshore China in water depths varying from 85 – 115m, which provide excellent results, both in the shallow detail and the fractured basement.

LOW FREQUENCY BENEFITS FOR RESERVOIR CHARACTERIZATION

This technology provides true-amplitude, fully 3D de-ghosted gathers,

thus preserving AVO characteristics and providing quantitative inversion attributes. Broadband data provide significant benefits for seismic inversion workflows, especially in terms of low-frequency bandwidth extension. The lack of low frequencies in conventional seismic data means that a low-frequency model must be incorporated in the inversion process in order to recover absolute impedance values. Usually, the low-frequency information is obtained from NMO-derived seismic velocities, which provide frequencies up to about 4Hz, and by interpolating low-passed filtered impedance logs between well locations, using interpreted horizons as a guide, for the 4-10Hz frequencies. If the wells are sparse and the geology complex, the low-frequency model derived from the wells may be inaccurate and tends to yield results that are biased towards the initial model.

The ultra-low frequencies provided by variable-depth streamer broadband data fill in the frequency gap, enabling inversion to be performed using only the seismic velocities without the need for a log-derived low-frequency model,

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Figure 8: Comparison of conventional and broadband data from the Banda Arc Indonesia.

Figure 9: Comparison of conventional (left) and BroadSeis data stacks (right). Variable-depth streamer data show better top of basement, better fault planes, clear indication of stratigraphic unit, and intra-basement reflections. From Yongdeng et al. 2014.

Figure 10: The ultra-low frequencies provided by variable-depth streamer broadband data extend the bandwidth of the seismic reflectivity to fill in the frequency gap, enabling inversion to be performed using only the seismic velocities without needing a log-derived low-frequency model.

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not be visible. This kind of information is key to tracking reservoir sands around potential traps.

BROADBAND SEISMIC THROUGHOUT THE E&P CYCLE

Broadband seismic is increasingly proving its value at all stages of the exploration and production cycle. Sharp wavelets without sidelobes enable accurate interpretation and easier identification of stratigraphic packages to reduce exploration risk. Ultra-low frequencies deliver more detailed facies discrimination, more quantitative

seismic inversion results and therefore more reliable lithology predictions away from existing wells. This leads to more accurate reservoir models and better development plans. Once production is underway the benefits of broadband for 3D surveys are extended into 4D monitoring to provide better dynamic reservoir models to maximise hydrocarbon recovery.

Over the last three years variable-depth streamer acquisition has established a strong position in the marine broadband market with over 40,000 km of 2D and 200,000 km� of 3D, wide- and

full-azimuth data, including 25,000 km� of BroadSource data. With the commercialization of the multi-level broadband source extending the frequency range to the recording interval Nyquist, it continues to set the benchmark in terms of bandwidth, with the optimum signal-to-noise ratio at low frequencies, thanks to the uniquely deep tow, delivering over six octaves of data. This frequency range of physically recorded signal cannot even be approached by solutions which do not record the full bandwidth but rely on processing to extend the frequency range.

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DOWLE, R., 2006 – “Solid streamer noise reduction principles”: SEG Expanded Abstracts 25, 85-89

HAKE, H., AND LOVELOCK, C., 2013 – “Interpretation of broadband seismic over Gannet-B, a Central Graben Tertiary gas field beneath a gas chimney”, DEVEX Conference, Aberdeen

KNELLER, E., FERRER, A., LANGLOIS, J., 2013 – “Benefits of broadband seismic

data for reservoir characterization in the Santos Basin, Brazil”, 13th International Congress of the Brazilian Geophysical Society, Rio de Janeiro.

MICHEL L., LAFET, Y., SABLON, R., RUSSIER, D., AND HANUMANTHA R., 2012 – “Variable Depth Streamer – Benefits for Rock Property Inversion”, 74th EAGE Conference & Exhibition, Copenhagen, Denmark.

POOLE, G., 2013 – “Pre-migration

receiver de-ghosting and re-datuming for variable depth streamer data”, SEG Technical Program Expanded Abstracts 2013: pp. 4216-4220

POOLE, G., DAVISON, C., DEEDS, J., DAVIES K., AND HAMPSON, G., 2013 – “Shot-to-shot directional designature using near-field hydrophone data”, SEG Technical Program Expanded Abstracts 2013: pp. 4236-4240.

SILIQI, R., PAYEN, T., SABLON, R., AND

DESRUES, K., 2013 – ”Synchronized multi-level source, a robust broadband marine solution”, SEG Technical Program Expanded Abstracts 2013: pp. 56-60.

SOUBARAS, R., 2010 – “Deghosting by joint deconvolution of a migration and a mirror migration”, SEG Technical Program Expanded Abstracts 2010: pp. 3406-3410.

WANG, P., AND PENG, C., 2012 – “Premigration

deghosting for marine towed streamer data using a bootstrap approach”: SEG Technical Program Expanded Abstracts 2012: pp. 1-5.

WANG, P., RAY, S., PENG, C., LI, Y., AND POOLE, G., 2013 – “Premigration deghosting for marine streamer data using a bootstrap approach in Tau-P domain”, 75th EAGE

ZHENBO, Z., SHOUJUN, W., DONGFANG, L., RUILIANG, W., SUN, J., YONGDENG, X.,

YONGHE, G., JUDD, C., 2014 – “Huizhou and Liwan blocks in South China Sea”, accepted for 76th EAGE Conference Amsterdam

ZIOLKOWSKI, A., PARKES, G.E., HATTON, L. AND HAUGLAND, T., 1982 – The signature of an airgun array: computation from near-field measurements including interactions. Geophysics 47, 1413-1421.

REFERENCES

Figure 11: 2D inversion comparison from NW Australia.

Figure 12: Frequency decomposition of the ultra-low frequencies provided by BroadSeis delivers details of the channel systems for this fast track data from our offshore Angola multi-client survey. Without the ultra-low frequencies (2.5Hz) there would be no red on this image so the channel architecture could not be delineated properly.