GeoStreamer PESA News 042009 Andrew Long

34 | PESA News | April/May 2009

technology focus: streamer

Australian Insights into Streamer Technology InnovationsAndrew Long, Petroleum Geo-Services, [email protected]

Introduction

The evolution of towed seismic streamers has progressed from the development of fluid-filled to solid-

filled streamers, and from sensors that only record the scalar pressure field to combined pressure and particle velocity sensors within the streamer. For many years the primary focus of streamer development was upon reducing weather, swell and operational noise effects recorded within the seismic data. Unfortunately, fundamental physics related to “ghost” notches in the recorded amplitude spectra restricted typical towing depths to 5–9 m. Recent developments with dual-sensor streamer technology, however, have allowed this restriction to be removed. Dual-sensor streamers can be towed very deep (15–25 m), providing greater operational flexibility combined with a step-change in data quality and frequency content. In contrast to the deep towing

been a forced trade-off between towing shallow to record high frequency (but noisy) data at the cost of reduced lower frequencies, or towing deep to record low frequencies at the cost of reduced higher frequencies.

It has long been understood that by recording seismic data from collocated hydrophones and velocity sensors, and by properly combining their signals, ghost reflections can be cancelled. The technology described here uses an efficient architecture with densely sampled groups of collocated pressure and velocity sensors in a low-noise solid-fill streamer with distributed electronics and Ethernet telemetry (Tenghamn et al., 2007). In contrast to pressure sensors, velocity sensors are directional, so the down-going velocity wavefield is measured as having equal polarity to the up-going velocity wavefield. Consequently, as observed in the right side of Figure 1, receiver ghost notches for the pressure and velocity sensors are separated in the frequency domain by regular intervals,

of conventional streamers with the loss of mid and high frequency amplitudes, dual-sensor data processing perfectly recovers all frequencies without penalty.

A pressure sensor in a towed streamer always records two wavefields that interfere with each other. The two wavefields are the up-going pressure wavefield propagating directly to the pressure sensor from the earth below, and the down-going pressure wavefield reflected with opposite polarity downwards from the free (sea) surface immediately above the streamer. Thus, every recorded reflection wavelet from conventional marine streamers is accompanied by a ‘ghost’ reflection from the ocean’s surface. The combination we traditionally measure is thus the ‘total pressure’ wavefield. The reflection wavelet is undesirably elongated, reducing temporal resolution. The consequence, as illustrated in the left side of Figure 1, is that a series of receiver ghost notches are introduced into the frequency spectra. There has historically

Fig. 1. The image on the left shows the receiver amplitude spectra for a pressure sensor towed at 8 m and 15 m depth. The wavefield is assumed to have vertical propagation (zero angle of incidence). Black is 8 m receiver depth, and blue is 15 m receiver depth. The image on the right shows the superposition of the amplitude spectra for both a pressure and velocity sensor at 15 m depth for zero angle reflections. Blue is the pressure wavefield spectra, and red is the velocity wavefield spectra.

35April/May 2009 | PESA News |


but are perfectly offset from each other in terms of complementary peaks and notches. Appropriate processing of the measured pressure and velocity data will completely cancel the amplitude of the ghost event trailing each primary event, and the notches in the amplitude spectra will be removed. This is the case for all angles of incidence, i.e. for all source-receiver offsets.

Historical streamer efforts to remove the receiver ghost

It has been demonstrated that the noise effects of weather-induced surface waves decrease significantly when the tow depth is increased to about 15 m or more. This can be seen in Figure 2, which compares the levels of recorded moderate weather surface-wave noise at tow depths of 7 and 15 m.

Unfortunately, as demonstrated above, the effect of the receiver ghost has prevented conventional streamers from being towed any deeper than about 8 meters. At conventional towing depths the low frequencies required for robust seismic impedance inversion are absent, operations are exposed to surface-related weather, sea swell and other hazards that will create unwanted noise, and high frequencies required for thin bed resolution are also often absent.

Several historical efforts have been made to combine the frequency spectra of streamers towed simultaneously at different depths; variously referred to as ‘twin streamers’ or ‘over-under’ towing. In the simplest approaches taken (Monk, 1990), time shifts and/or phase corrections are used in an effort to align and combine data acquired at different depths. More recent publications from Posthumus (1993) and Özdemir et al. (2008) describe extensions of these ideas to address residual noise from the down-going pressure wavefield, and reduce noise and errors introduced or enhanced via the methodology. On the operational side it is obviously expensive and challenging to tow half of the available streamers at one depth and the other half of the available streamers at another depth. Each streamer spread will be exposed to different local forces and noise effects. In addition to the well-known critical requirements to laterally align the shallow and deep streamers, a lesser-known problem is the sensitivity of the methodology to small errors in streamer depth. As a consequence of vertical streamer separation errors, the up-going pressure data derived from processing becomes less robust at higher frequencies; typically beyond 70 Hz in normal operating conditions. One pragmatic solution is to tow a smaller number of coarsely-spaced streamers at larger depths than the

streamer spread being towed shallow. By grossly relaxing the spatial sampling assumptions, the methodology is less sensitive to streamer depth errors, and the deeper streamers are only used to contribute very low frequencies to the final processed result (e.g. 0–20 Hz). Thus, the trade-off is that mid and higher frequencies retain the down-going pressure wavefield effects (receiver ghosting), and have a signal-to-noise content appropriate to streamers towed closer to the surface.

Many attempts have been made through the years to incorporate vertical particle velocity detectors into the towed streamer, but their response to vibration noise propagating along the streamer always overwhelmed their response to the desired seismic reflection signals. PGS eventually made the required engineering breakthroughs, and introduced a commercial solution (the ‘GeoStreamer’) at the 2007 EAGE international technical conference.

By having collocated pressure and particle velocity sensors, there are no issues related to streamer depth control and the removal of the receiver ghost. In fact, the dual-sensor configuration enables a methodology to accurately identify the streamer depth to within 0.2 m on an offset-dependent and shot-dependent basis. Any errors in receiver ghost removal are thus identified and removed early in processing. By towing the collocated dual-sensors very deep (typically 25 m) the signal-to-noise content across all frequencies after removal of the down-going wavefield is high. This increased “insulation” from the effects of bad weather also increases the operational weather window and enhances

project flexibility. Both low and high frequencies are boosted without compromise in fidelity or accuracy all the way up to the Nyquist frequency. Thus, the only constraints upon the recoverable high frequencies are the effects of the source ghost and the inescapable attenuation effects of the earth. The source ghost is briefly addressed below.

Spectral enhancement in processing of high frequencies lost to attenuation benefits from dual-sensor data acquisition for two main reasons:

1. The high frequencies have signal-to-noise content appropriate to streamers towed very deep; and

2. The removal of the receiver ghost effects have already boosted the signal content of high frequencies relative to conventional streamer data anyway.

Thus, high frequency amplitudes can be robustly recovered before noise levels become unacceptable.

Field tests and data analyses in Australia: The receiver ghost

The test line location

A regional 2D streamer seismic survey was acquired in two regionally overlapping phases over the North West Shelf of Australia during 2007 and 2008 (Figure 3). Water depths vary between 50 and 3,500 m. Phase I was acquired with a 7,950 m fluid-fill streamer at a depth of 7 m, and Phase II was acquired with a 8,100 m solid-fill dual-sensor streamer at a depth of

Fig. 2. Weather noise levels at streamer towing depths of 7 and 15 m. The streamer used was towed to passively record the ambient background energy without any active source firing. It makes obvious sense to tow deep if the receiver ghost can be removed in processing.



15 m. Receiver interval in both phases of the survey was 12.5 m. A symmetric 2,980 in3 source array of Sodera G-guns was towed at a depth of 6 m for both surveys, with a shot interval of 37.5 m. Record length was 12 s. Phase I acquisition and processing was completed in 2007. Phase II (dual-sensor) acquisition and processing was completed in 2008. As observed in Figure 4, the recorded pressure and particle velocity data yield complementary information about the subsurface, and together in processing, result in significant data resolution, quality and deep target imaging improvements.

Typical of the area, processing follows a cascaded flow, incorporating velocity mute in the tau-p domain, SRME and high-resolution radon demultiple. Pre-stack Kirchhoff time migration used an optimised high-order velocity function to accurately incorporate longer offsets into imaging. Specialised pre-processing and wavefield separation used for the Phase II dual-sensor data followed the methods described in Tenghamn et al. (2007) and Carlson et al. (2007). The obliquity term is automatically included during wavefield separation, and amplitudes are equally correct for all incidence angles

and offsets. The resultant up-going pressure wavefield is free of the receiver ghost, and has been extrapolated to an equivalent receiver depth of 7 m for comparison with the existing total pressure (hydrophone) data.

The power of dual-sensor acquisition and processing is demonstrated here on the North West Shelf (NWS) of Australia. A 156 km 2D seismic line in the northern Carnarvon Basin was acquired twice; both in Phase I (conventional streamer) and Phase II (dual-sensor streamer). Thus, a true apples-for-apples comparison was made. The NW-SE line orientation is in the dip direction, and intersects a known gas field associated with a pronounced amplitude anomaly. Water depth exceeds 750 m.

Challenges to seismic penetration and resolution

Much of the North West Shelf is affected by relatively near-surface barriers to deeper target seismic imaging and resolution. Major sequence boundaries in the post-rift section occur in the Turonian (Late Cretaceous) and Base Tertiary. Campanian inversion of transpressional structures was accentuated by regional tectonism in the Neogene, and is associated with trap breach in the area. Complex swarm faulting affects the Tertiary section, and requires rich high frequency content for high resolution seismic imaging. Analysis of the dual-sensor data (refer to Figure 5) shows that brute stack frequencies beyond the source ghost in the up-going pressure wavefield have much stronger amplitudes; up to at least 200 Hz. This particularly benefits thin bed resolution and subtle stratigraphic interpretation in the first second two-way time (TWT) below the water bottom. Although no spectral enhancement processing has been applied to any of the figures here, estimation of the quality factor (Q) is more robust using up-going pressure

Fig. 3. 2D survey location on the North West Shelf of Australia. The survey area is about 1000 km long. Two test lines were acquired in multiple passes to quantify the removal of the receiver and source ghost effects from streamer data. The line used to quantify receiver ghost removal described here lies in the northern Carnarvon Basin; in the southern part of the survey area. The line used to address source ghost attenuation described here lies in the central Browse Basin; in the northern part of the survey area.

Fig. 4. Near-trace gathers for pressure (left) and velocity (right) sensor data from the study area in the northern Carnarvon Basin. The window width is about 75 km, and the vertical extent is 0.0–4.2 s TWT.



data, and linear regressions using the spectral ratio method have good fit down to about 3 Hz.

Carbonates are mapped in the Mid Cretaceous/Aptian, and are more regionally pervasive in the Late Cretaceous and Late Tertiary, producing complex seismic multiple noise events. The seismic multiple problems become more severe to the north and east of the survey area (towards the southern Browse Basin), creating problems for velocity picking and seismic imaging. Production velocity picking was, however, significantly easier on up-going pressure data, in large part because of the characteristic low frequency boost at all TWT (refer also to Figure 7 and discussion below). A clearer primary semblance trend is evident during analysis, and associated improvements in later stage velocity analysis subsequently derive from improved multiple removal.

Fig. 5. Superimposed amplitude spectra from brute stacks. Red is the up-going pressure data from the dual-sensor streamer used in 2008, and black is the total pressure data from the conventional streamer used in 2007. The time window used was 1.5 – 4.0 seconds TWT. Removal of the receiver ghost also benefits amplitudes at frequencies beyond the source ghost at about 125 Hz. Processing sample rate is 2 ms. Fig. 6 demonstrates the combined benefits of higher resolution and high signal-to-noise content.

Fig. 6. Shallow PSTM comparison of total pressure (conventional streamer: left) vs. up-going pressure (dual-sensor streamer: right) stacks that are spatially coincident. The window width is about 38 km, and the vertical extent is 1.3–2.7 s TWT. Over this depth range the up-going pressure data are visibly richer in high frequencies, very clean, and demonstrate enhanced thin bed resolution.

Fig. 7. Stack filter panels for total pressure (conventional streamer: left) vs. up-going pressure (dual-sensor streamer: right). Data stacked at an effective receiver depth of 7 m in both cases. Frequency ranges are as follows: All, 0-5 Hz, 15-20 Hz, 30-35 Hz, 45-50 Hz, 60-65 Hz, 75-80 Hz, 90-95 Hz, 105-110 Hz, 120-125 Hz and 135-140 Hz. Frequencies beyond the source ghost (refer to Figure 5) have been truncated for display purposes. Processing sample rate is 2 ms. Note the low frequency contrast in the second panel.



Although untested as potential hydrocarbon plays, the two strongest seismic markers throughout the survey area correspond to Top Triassic and Callovian events. These two events can be easily confused with each other wherever the Lower Jurassic section thins over Triassic structural highs – typically the top of rotated Triassic fault blocks. Various investigations were made to address potential hydrocarbon-related amplitude anomalies at the top of these Triassic fault blocks. The results from a relative impedance inversion study are discussed below.

The largest gas field in the survey area is hosted below the Valanginian Unconformity, and was probably affected by fault block rotation of the

underlying Triassic sediments during the Early Cretaceous, and again later by the regional Campanian inversion. Post-rift fault reactivation is more clearly interpreted to affect internal reservoir and trap geometry.

Finally, the Triassic Mungaroo Formation is a thick pre-rift section that contains dolerite intrusions. Mapping the intrusives is a key to constraining basin evolution; notably areas of basin weakness during the main Callovian extensional event. Figures 7 and 8 demonstrate the overwhelming improvements in deep target image clarity and character on up-going pressure data. The deeper target velocity field is surprisingly variable throughout the region, and was

considerably easier to pick in poor data areas by examination of the up-going pressure data. Additional low frequency imaging benefits were proven in the Browse basin using a dual-sensor streamer complemented by a source array configured to attenuate the source ghost (see below).

Quantification of signal and noise

It is self-evident that a seismic dataset rich in very low frequency content and with high signal-to-noise ratio across all frequencies and for all two-way time (TWT) provides the best platform for the recovery of elastic rock and fluid properties in a hydrocarbon-saturated reservoir. High frequencies required for thin bed

Fig. 8. Deep PSTM comparison of total pressure (conventional streamer: left) vs. up-going pressure (dual-sensor streamer: right) stacks that are spatially coincident. The window width is about 44 km, and the vertical extent is 5.0 – 6.8 s TWT. Over this depth range the up-going pressure data are visibly richer in low frequencies, very clean, and demonstrate enhanced event continuity, character, and fundamental interpretability.

Fig. 9. Relative pre-stack P-impedance inversion for conventional (top left) vs. dual-sensor data (top right), and S-impedance inversion for conventional (lower left) vs. dual-sensor data (lower right). Note the profound improvements in impedance character, low frequency content, and resolution for both dual-sensor results. A large gas field is evident on the P-impedance sections.



resolution are inevitably and universally lost faster than low frequencies during transmission through the earth. It is only possible to recover some of these high frequency amplitudes at large target depths during data processing if the signal at these depths is demonstrably stronger than the noise component of the seismic wavefield.

After data processing to recover the up-going pressure wavefield, completely free of any receiver ghost effects, a cross-spectral analysis was pursued using methodology influenced by White (1973). The signal and noise components of the up-going pressure wavefield were used to derive frequency spectra for various TWT windows. A systematic set of comparative analyses were then possible.

In comparison to conventional streamer data, the dual-sensor streamer data show significant improvements in low and high frequency signal content, less low and high frequency noise content, greater frequency range for signal at all TWT, and greater signal-to-noise ratio of signal across all frequencies and for all TWT:

■ At 10 Hz, the signal is consistently an additional 12–15 dB (a factor of 4 to 5) stronger than noise for all TWT.

■ At maximum frequency, the signal is consistently an additional 10 dB (a factor of 3 to 4) stronger than noise for the first couple of seconds TWT below water bottom, tapering to lower values as attenuation in the earth affects deeper targets.

■ The frequency range of signal is consistently an additional 55–65% greater for the first couple of seconds TWT below water bottom, tapering to lower values as attenuation in the earth affects deeper targets.

■ The signal-to-noise ratio is greater for all frequencies and for all TWT.

Seismic impedance inversion comparison

A pre-stack relative impedance inversion was done for the 2007 and 2008 surveys using a simultaneous sparse spike algorithm (refer to Figure 9). The wavelet through the target interval was extracted using a Bayesian inversion scheme, but the well log data were not used to derive any low frequency model. Thus, only the seismic data contribute to each impedance inversion. It is demonstrated that the dual-sensor data inversion result has contributed at least half an octave of low frequency impedance information in comparison to the conventional streamer result, and has also yielded more high frequency

impedance information. Thus, a bridge is made in the historical gap between the seismic bandwidth and the low frequency bandwidth derived from well log data and the seismic velocity model.

The next stage of this ongoing investigation (at the time of writing) will be to perform absolute impedance inversions, and calibrate predicted reservoir properties such as porosity between well locations at large separations.

Velocity picking and model building

The survey area is infamous for regional carbonates and severe seismic multiple problems. In fact, many areas of the NWS have historically yielded seismic data that is too poor for even basic exploration efforts. The upper part of Figure 10 demonstrates the characteristic historical problems with ambiguous primary vs. multiple trends on velocity semblance, noisy and weakly coherent energy on image gathers, and the associated poor stacking results. In contrast, the lower part of Figure 10 demonstrates a clearer semblance trend, strong and coherent events on image gathers, and an uplift in the associated stack quality.

Furthermore, it is our observation that dual-sensor streamer data acquired in all global locations to date, demonstrate significant improvements in low frequency content and signal-to-noise ratio. Thus, velocity model building and multiple removal are greatly facilitated from day one of any data processing project when dual-sensor data are available.

It is worth noting that four unique wavefields are created during the wavefield separation in dual-sensor processing: up-going and down-going pressure, and up-going and down-going particle velocity. Whilst the up-going pressure wavefield is the primary product for velocity picking, interpretation and inversion, the other wavefields can be used for various high-end multiple removal and seismic imaging pursuits.

Field tests and data analyses in Australia: The source ghost

A new approach to seismic acquisition has also been tested in the southern Browse Basin during 2008, in an effort to remove both source ghost and receiver ghost effects from towed streamer data. Of particular relevance in the test area is the almost complete absence on conventional streamer data of coherent seismic events at depths greater than about two seconds TWT. A 100 km 2D line was acquired three times in a spatially-coincident manner during 2008: With a conventional

Fig. 10. Comparison of velocity picking panels for a discrete analysis location with conventional (upper) vs. dual-sensor (lower) data. Note the striking improvement in the strength and coherency of events and semblance amplitudes on the dual-sensor panels. These improvements arise from much stronger lower frequency content and higher signal-to-noise ratio on the raw dual-sensor streamer field data.



source array and solid streamer in 2007, with a conventional source array and the dual-sensor streamer in 2008, and then finally with a prototype “multi-level” source array and the dual-sensor streamer. The objectives in this case were to address signal penetration problems by a combination of receiver ghost removal and source ghost attenuation. Receiver ghost removal arises from the dual-sensor streamer processing, and source ghost attenuation arises from the multi-level source array. Again, true apples-for-apples comparisons were possible.

As illustrated schematically in Figure 11, the multi-level source array deploys sub-arrays

of air gun elements at different depths. The shallowest sub-array is fired first, sequentially followed by the firing of successively deeper source arrays. Thus, the source ghost wavefront is decoupled from the primary source wavefront. In the test described here, two sub-arrays were towed in vertical alignment at depths of 12 and 18 m. The deeper sub-array was fired 4 ms after the shallow sub-array, coincident in time with the arrival at 18 m depth of the primary source wavefront from the 12 m sub-array.

Reference to Figure 12 shows that data in the target depth range of two to at least four seconds TWT are now both coherent and

strongly-represented at most locations along the line when the dual-sensor data is used to remove the effects of the receiver ghost and the multi-level source array is used to reduce the effects of the source ghost. New plays are apparent in both the shallow and deeper data. The data quality is in between the two results shown in Figure 12, when a conventional source array is used with the dual-sensor streamer. Velocity picking was greatly facilitated by the improved data quality arising from the combination of the dual-sensor streamer and the multi-level source. The output of this prototype source array was biased towards low frequencies. Future implementations will be designed to yield richer high frequency output too.

Fig. 11. When a conventional source array (upper) has all source elements fired simultaneously the (down-going) primary and ghost source wavefronts are parallel to the sea-surface, and effectively coupled by a constant time delay. In contrast, the staged firing of successively deeper source elements in the multi-level source array (lower) decouples the primary and ghost source wavefronts. The ghost source wavefront is no longer parallel to the sea-surface, but the primary source wavefront still is.

Fig. 12. Raw PSTM stack comparison of the test line in the target time window of 2 – 4 s TWT. When the effects of both the receiver and source ghost are present on conventional data (left) the target events are very weak, incoherent and impossible to interpret. In contrast, the dual-sensor streamer + multi-level source result (right) demonstrates a profound improvement in event strength, spatial coherency and interpretability.



Conclusions

Quantitative analysis of dual-sensor data acquired in a spatially-coincident manner with conventional streamer data on the North West Shelf of Australia demonstrate a four to five-fold boost in the low frequency signal content, about three times the high frequency signal content (before the application of any spectral enhancement processing), and a higher signal-to-noise content for all frequencies and for all depths. Furthermore, the frequency range of signal is boosted by 55–65% through the main target interval. The low frequency boost arises from deeper streamer towing and the removal of the effects of the receiver ghost in processing. The high frequency boost (up to at least 200 Hz) arises from the removal of the effects of the receiver ghost. Less noise is also recorded for deeper streamer towing, stabilising the wavefield separation processing. The high frequencies benefit the interpretation of subtle stratigraphic features and allow clearer imaging of features affected by transpressional fault reactivation. The pronounced low frequency boost has many benefits, including better velocity semblances (particularly in poor data areas affected by carbonates), more robust Q estimation, and significant signal penetration improvements for the seismic imaging of Triassic fault block plays and deeper dolerite intrusive features. Overall, the dual-sensor dataset has better resolution and is more interpretable at all depths.

Pre-stack relative impedance inversion analysis demonstrates a significant improvement in low frequency content and character, and high frequency resolution. Dual-sensor data thus provides a bridge through the historical gap between the seismic bandwidth available for inversion and the low frequency model derived from well log data.

Finally, a test using the dual-sensor streamer combined with a ‘multi-level’ source array also demonstrates the additional improvements derived from attenuating the source ghost, in addition to removing the receiver ghost. Data in one of the most problematic seismic areas of the North West Shelf of Australia is improved. Collectively, the pioneering lessons learned on the NW Shelf Australia demonstrate the diverse opportunities possible when the receiver and source ghost effects can be robustly and accurately removed.

Acknowledgements

Thanks to ESSO Australia and BHP Petroleum for access to the well data used for wavelet extraction in the seismic inversion exercise. DownUnder GeoSolutions produced the inversion results.

References

Carlson, D., Long, A., Söllner, W., Tabti, H., Tenghamn,R., and Lunde, N., 2007, Increased resolution and penetration from a towed dual-sensor streamer. First Break, 25, 12, 71-77.

Monk, D.J., 1990, Wavefield separation of twin streamer data. First Break, 8, 3, 96-104.

Özdemir, A.K., Caprioli, P., Özbek, A., Kragh, E., and Robertsson, J.O.A., 2008, Optimized deghosting of over/under towed-streamer data in the presence of noise. The Leading Edge, 2, 190-199.

Posthumus, B.J., 1993, Deghosting using a twin streamer configuration. Geophysical Prospecting, 41, 267-286.

Tenghamn, R., S. Vaage, and C. Borresen, 2007, A dual-sensor, towed marine streamer; its viable implementation and initial results: 77th Annual International Meeting, SEG, Expanded Abstracts, 989-993.

White, R.E., 1973, The estimation of signal spectra and related quantities by means of the multiple coherence function. Geophysical Prospecting, 21, 4, 660-703. ■

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GeoStreamer PESA News 042009 Andrew Long

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Transcript of GeoStreamer PESA News 042009 Andrew Long