GeoStreamer PESA News 042009 Andrew Long


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Dual-sensor streamer (GeoStreamer) technology and case examples. I did the quantitative analyses.

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GeoStreamer PESA News 042009 Andrew Long

  1. 1. technology focus: streamer Australian Insights into Streamer Technology Innovations Andrew Long, Petroleum Geo-Services, of conventional streamers with the loss of been a forced trade-off between towing Introduction mid and high frequency amplitudes, dual- shallow to record high frequency (but noisy) T he evolution of towed seismic sensor data processing perfectly recovers all data at the cost of reduced lower frequencies, streamers has progressed from the frequencies without penalty. or towing deep to record low frequencies at the development of fluid-filled to solid- cost of reduced higher frequencies. filled streamers, and from sensors that only A pressure sensor in a towed streamer always record the scalar pressure field to combined records two wavefields that interfere with each It has long been understood that by recording pressure and particle velocity sensors within other. The two wavefields are the up-going seismic data from collocated hydrophones and the streamer. For many years the primary pressure wavefield propagating directly to the velocity sensors, and by properly combining focus of streamer development was upon pressure sensor from the earth below, and the their signals, ghost reflections can be cancelled. reducing weather, swell and operational down-going pressure wavefield reflected with The technology described here uses an noise effects recorded within the seismic opposite polarity downwards from the free efficient architecture with densely sampled data. Unfortunately, fundamental physics (sea) surface immediately above the streamer. groups of collocated pressure and velocity related to “ghost” notches in the recorded Thus, every recorded reflection wavelet from sensors in a low-noise solid-fill streamer with amplitude spectra restricted typical towing conventional marine streamers is accompanied distributed electronics and Ethernet telemetry depths to 5–9 m. Recent developments by a ‘ghost’ reflection from the ocean’s surface. (Tenghamn et al., 2007). In contrast to pressure with dual-sensor streamer technology, The combination we traditionally measure is sensors, velocity sensors are directional, so the however, have allowed this restriction to thus the ‘total pressure’ wavefield. The reflection down-going velocity wavefield is measured as be removed. Dual-sensor streamers can wavelet is undesirably elongated, reducing having equal polarity to the up-going velocity be towed very deep (15–25 m), providing temporal resolution. The consequence, as wavefield. Consequently, as observed in the greater operational flexibility combined with illustrated in the left side of Figure 1, is that a right side of Figure 1, receiver ghost notches for a step-change in data quality and frequency series of receiver ghost notches are introduced the pressure and velocity sensors are separated content. In contrast to the deep towing into the frequency spectra. There has historically in the frequency domain by regular intervals, 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. 34 | PESA News | April/May 2009
  2. 2. technology focus: streamer 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. 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 Unfortunately, as demonstrated above, the effect the receiver ghost can be removed in processing. of the receiver ghost has prevented conventional streamers from being towed any deeper than about 8 meters. At conventional towing depths streamer spread being towed shallow. By grossly project flexibility. Both low and high frequencies the low frequencies required for robust seismic relaxing the spatial sampling assumptions, the are boosted without compromise in fidelity impedance inversion are absent, operations are methodology is less sensitive to streamer depth or accuracy all the way up to the Nyquist exposed to surface-related weather, sea swell errors, and the deeper streamers are only used frequency. Thus, the only constraints upon the and other hazards that will create unwanted to contribute very low frequencies to the final recoverable high frequencies are the effects noise, and high frequencies required for thin bed processed result (e.g. 0–20 Hz). Thus, the trade- of the source ghost and the inescapable resolution are also often absent. off is that mid and higher frequencies retain the attenuation effects of the earth. The source down-going pressure wavefield effects (receiver ghost is briefly addressed below. Several historical efforts have been made to ghosting), and have a signal-to-noise content combine the frequency spectra of streamers appropriate to streamers towed closer to the Spectral enhancement in processing of high towed simultaneously at different depths; surface. frequencies lost to attenuation benefits from variously referred to as ‘twin streamers’ or dual-sensor data acquisition for two main ‘over-under’ towing. In the simplest approaches Many attempts have been made through the reasons: taken (Monk, 1990), time shifts and/or phase years to incorporate vertical particle velocity corrections are used in an effort to align and detectors into the towed streamer, but their 1. The high frequencies have signal-to-noise combine data acquired at different depths. response to vibration noise propagating content appropriate to streamers towed very More recent publications from Posthumus along the streamer always overwhelmed their deep; and (1993) and Özdemir et al. (2008) describe response to the desired seismic reflection 2. The removal of the receiver ghost effects extensions of these ideas to address residual signals. PGS eventually made the required have already boosted the signal content of noise from the down-going pressure wavefield, engineering breakthroughs, and introduced a high frequencies relative to conventional and reduce noise and errors introduced commercial solution (the ‘GeoStreamer’) at the streamer data anyway. or enhanced via the methodology. On the 2007 EAGE international technical conference. operational side it is obviously expensive Thus, high frequency amplitudes can be and challenging to tow half of the available By having collocated pressure and particle robustly recovered before noise levels become streamers at one depth and the other half of velocity sensors, there are no issues related unacceptable. the available streamers at another depth. Each to streamer depth control and the removal streamer spread will be exposed to different of the receiver ghost. In fact, the dual-sensor Field tests and data analyses in local forces and noise effects. In addition configuration enables a methodology to Australia: The receiver ghost to the well-known critical requirements to accurately identify the streamer depth to laterally align the shallow and deep streamers, within 0.2 m on an offset-dependent and The test line location a lesser-known problem is the sensitivity of shot-dependent basis. Any errors in receiver the methodology to small errors in streamer ghost removal are thus identified and removed A regional 2D streamer seismic survey was depth. As a consequence of vertical streamer early in processing. By towing the collocated acquired in two regionally overlapping phases separation errors, the up-going pressure data dual-sensors very deep (typically 25 m) the over the North West Shelf of Australia during derived from processing becomes less robust signal-to-noise content across all frequencies 2007 and 2008 (Figure 3). Water depths vary at higher frequencies; typically beyond 70 Hz after removal of the down-going wavefield between 50 and 3,500 m. Phase I was acquired in normal operating conditions. One pragmatic is high. This increased “insulation” from the with a 7,950 m fluid-fill streamer at a depth of solution is to tow a smaller number of coarsely- effects of bad weather also increases the 7 m, and Phase II was acquired with a 8,100 m spaced streamers at larger depths than the operational weather window and enhances solid-fill dual-sensor streamer at a depth of April/May 2009 | PESA News | 35
  3. 3. technology focus: streamer 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) Fig. 3. 2D survey location on the North West Shelf of Australia. The survey area is about 1000 km long. Two test and Base Tertiary. Campanian inversion of lines were acquired in multiple passes to quantify the removal of the receiver and source ghost effects from transpressional structures was accentuated streamer data. The line used to quantify receiver ghost removal described here lies in the northern Carnarvon by regional tectonism in the Neogene, and Basin; in the southern part of the survey area. The line used to address source ghost attenuation described here is associated with trap breach in the area. lies in the central Browse Basin; in the northern part of the survey area. Complex swarm faulting affects the Tertiary section, and requires rich high frequency 15 m. Receiver interval in both phases of the Typical of the area, processing follows a content for high resolution seismic imaging. survey was 12.5 m. A symmetric 2,980 in3 cascaded flow, incorporating velocity mute in Analysis of the dual-sensor data (refer to source array of Sodera G-guns was towed at the tau-p domain, SRME and high-resolution Figure 5) shows that brute stack frequencies a depth of 6 m for both surveys, with a shot radon demultiple. Pre-stack Kirchhoff time beyond the source ghost in the up-going interval of 37.5 m. Record length was 12 s. Phase migration used an optimised high-order velocity pressure wavefield have much stronger I acquisition and processing was completed function to accurately incorporate longer offsets amplitudes; up to at least 200 Hz. This in 2007. Phase II (dual-sensor) acquisition and into imaging. Specialised pre-processing and particularly benefits thin bed resolution and processing was completed in 2008. As observed wavefield separation used for the Phase II dual- subtle stratigraphic interpretation in the first in Figure 4, the recorded pressure and particle sensor data followed the methods described in second two-way time (TWT) below the water velocity data yield complementary information Tenghamn et al. (2007) and Carlson et al. (2007). bottom. Although no spectral enhancement about the subsurface, and together in The obliquity term is automatically included processing has been applied to any of the processing, result in significant data resolution, during wavefield separation, and amplitudes figures here, estimation of the quality factor quality and deep target imaging improvements. are equally correct for all incidence angles (Q) is more robust using up-going pressure 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. 36 | PESA News | April/May 2009
  4. 4. technology focus: streamer 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 Fig. 5. Superimposed amplitude spectra from brute stacks. Red is the up-going pressure data from the is evident during analysis, and associated dual-sensor streamer used in 2008, and black is the total pressure data from the conventional streamer improvements in later stage velocity analysis 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 subsequently derive from improved multiple demonstrates the combined benefits of higher resolution and high signal-to-noise content. removal. 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. April/May 2009 | PESA News | 37
  5. 5. technology focus: streamer 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. Although untested as potential hydrocarbon underlying Triassic sediments during the Early considerably easier to pick in poor data areas plays, the two strongest seismic markers Cretaceous, and again later by the regional by examination of the up-going pressure throughout the survey area correspond to Campanian inversion. Post-rift fault reactivation data. Additional low frequency imaging Top Triassic and Callovian events. These two is more clearly interpreted to affect internal benefits were proven in the Browse basin events can be easily confused with each other reservoir and trap geometry. using a dual-sensor streamer complemented wherever the Lower Jurassic section thins by a source array configured to attenuate the over Triassic structural highs – typically the Finally, the Triassic Mungaroo Formation source ghost (see below). top of rotated Triassic fault blocks. Various is a thick pre-rift section that contains investigations were made to address potential dolerite intrusions. Mapping the intrusives Quantification of signal and noise hydrocarbon-related amplitude anomalies at is a key to constraining basin evolution; the top of these Triassic fault blocks. The results notably areas of basin weakness during the It is self-evident that a seismic dataset rich from a relative impedance inversion study are main Callovian extensional event. Figures in very low frequency content and with high discussed below. 7 and 8 demonstrate the overwhelming signal-to-noise ratio across all frequencies and improvements in deep target image clarity for all two-way time (TWT) provides the best The largest gas field in the survey area is hosted and character on up-going pressure data. platform for the recovery of elastic rock and below the Valanginian Unconformity, and was The deeper target velocity field is surprisingly fluid properties in a hydrocarbon-saturated probably affected by fault block rotation of the variable throughout the region, and was reservoir. High frequencies required for thin bed 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. 38 | PESA News | April/May 2009
  6. 6. technology focus: streamer 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 Fig. 10. Comparison of velocity picking panels for a discrete analysis location with conventional (upper) quality. 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 Furthermore, it is our observation that dual- frequency content and higher signal-to-noise ratio on the raw dual-sensor streamer field data. sensor streamer data acquired in all global locations to date, demonstrate significant resolution are inevitably and universally lost ■ At maximum frequency, the signal is improvements in low frequency content and faster than low frequencies during transmission consistently an additional 10 dB (a factor signal-to-noise ratio. Thus, velocity model through the earth. It is only possible to recover of 3 to 4) stronger than noise for the first building and multiple removal are greatly some of these high frequency amplitudes at couple of seconds TWT below water bottom, facilitated from day one of any data processing large target depths during data processing tapering to lower values as attenuation in project when dual-sensor data are available. if the signal at these depths is demonstrably the earth affects deeper targets. stronger than the noise component of the ■ The frequency range of signal is consistently It is worth noting that four unique wavefields seismic wavefield. an additional 55–65% greater for the first are created during the wavefield separation in couple of seconds TWT below water bottom, dual-sensor processing: up-going and down- After data processing to recover the up-going tapering to lower values as attenuation in going pressure, and up-going and down-going pressure wavefield, completely free of any the earth affects deeper targets. particle velocity. Whilst the up-going pressure receiver ghost effects, a cross-spectral analysis ■ The signal-to-noise ratio is greater for all wavefield is the primary product for velocity was pursued using methodology influenced by frequencies and for all TWT. picking, interpretation and inversion, the other White (1973). The signal and noise components wavefields can be used for various high-end of the up-going pressure wavefield were used Seismic impedance inversion comparison multiple removal and seismic imaging pursuits. to derive frequency spectra for various TWT windows. A systematic set of comparative A pre-stack relative impedance inversion Field tests and data analyses in analyses were then possible. was done for the 2007 and 2008 surveys Australia: The source ghost using a simultaneous sparse spike algorithm In comparison to conventional streamer data, (refer to Figure 9). The wavelet through the A new approach to seismic acquisition has the dual-sensor streamer data show significant target interval was extracted using a Bayesian also been tested in the southern Browse Basin improvements in low and high frequency signal inversion scheme, but the well log data were during 2008, in an effort to remove both content, less low and high frequency noise not used to derive any low frequency model. source ghost and receiver ghost effects from content, greater frequency range for signal at all Thus, only the seismic data contribute to towed streamer data. Of particular relevance in TWT, and greater signal-to-noise ratio of signal each impedance inversion. It is demonstrated the test area is the almost complete absence across all frequencies and for all TWT: that the dual-sensor data inversion result on conventional streamer data of coherent has contributed at least half an octave of seismic events at depths greater than about ■ At 10 Hz, the signal is consistently an low frequency impedance information in two seconds TWT. A 100 km 2D line was additional 12–15 dB (a factor of 4 to 5) comparison to the conventional streamer result, acquired three times in a spatially-coincident stronger than noise for all TWT. and has also yielded more high frequency manner during 2008: With a conventional April/May 2009 | PESA News | 39
  7. 7. technology focus: streamer 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. source array and solid streamer in 2007, with of air gun elements at different depths. The strongly-represented at most locations along the a conventional source array and the dual- shallowest sub-array is fired first, sequentially line when the dual-sensor data is used to remove sensor streamer in 2008, and then finally with followed by the firing of successively deeper the effects of the receiver ghost and the multi- a prototype “multi-level” source array and source arrays. Thus, the source ghost wavefront level source array is used to reduce the effects of the dual-sensor streamer. The objectives in is decoupled from the primary source the source ghost. New plays are apparent in both this case were to address signal penetration wavefront. In the test described here, two the shallow and deeper data. The data quality problems by a combination of receiver sub-arrays were towed in vertical alignment at is in between the two results shown in Figure ghost removal and source ghost attenuation. depths of 12 and 18 m. The deeper sub-array 12, when a conventional source array is used Receiver ghost removal arises from the dual- was fired 4 ms after the shallow sub-array, with the dual-sensor streamer. Velocity picking sensor streamer processing, and source coincident in time with the arrival at 18 m was greatly facilitated by the improved data ghost attenuation arises from the multi-level depth of the primary source wavefront from the quality arising from the combination of the dual- source array. Again, true apples-for-apples 12 m sub-array. sensor streamer and the multi-level source. The comparisons were possible. output of this prototype source array was biased Reference to Figure 12 shows that data in towards low frequencies. Future implementations As illustrated schematically in Figure 11, the the target depth range of two to at least four will be designed to yield richer high frequency multi-level source array deploys sub-arrays seconds TWT are now both coherent and output too. 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. 40 | PESA News | April/May 2009
  8. 8. technology focus: streamer Conclusions References Quantitative analysis of dual-sensor data Carlson, D., Long, A., Söllner, W., Tabti, H., Posthumus, B.J., 1993, Deghosting using a acquired in a spatially-coincident manner with Tenghamn,R., and Lunde, N., 2007, Increased twin streamer configuration. Geophysical conventional streamer data on the North West resolution and penetration from a towed dual- Prospecting, 41, 267-286. Shelf of Australia demonstrate a four to five- sensor streamer. First Break, 25, 12, 71-77. fold boost in the low frequency signal content, Tenghamn, R., S. Vaage, and C. Borresen, 2007, A about three times the high frequency signal Monk, D.J., 1990, Wavefield separation of twin dual-sensor, towed marine streamer; its viable content (before the application of any spectral streamer data. First Break, 8, 3, 96-104. implementation and initial results: 77th Annual enhancement processing), and a higher signal- International Meeting, SEG, Expanded Abstracts, to-noise content for all frequencies and for all Özdemir, A.K., Caprioli, P., Özbek, A., Kragh, 989-993. depths. Furthermore, the frequency range of E., and Robertsson, J.O.A., 2008, Optimized signal is boosted by 55–65% through the main deghosting of over/under towed-streamer data White, R.E., 1973, The estimation of signal target interval. The low frequency boost arises in the presence of noise. The Leading Edge, 2, spectra and related quantities by means of from deeper streamer towing and the removal 190-199. the multiple coherence function. Geophysical of the effects of the receiver ghost in processing. Prospecting, 21, 4, 660-703. ■ 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. Advertiser | Geokinetics 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 Instruction | Open 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. April/May 2009 | PESA News | 41