Dual-sensor Streamer Data Research Articles

Getting it right the first time in the Ceduna Sub-basin: regional and target depth imaging in a frontier setting

The acquisition and depth imaging of almost 20400 km2 of broadband seismic data in the Great Australian Bight has created an excellent dataset fit for quantitative interpretation. This new dataset was derived from a merge of 12400 km2 of 2011 vintage conventional streamer data in an almost seamless manner with 8000 km2 of 2014 vintage dual-sensor streamer data. The Ceduna Sub-basin is the main depocentre of the Bight Basin. It lies adjacent to the continental shelf and slope and is covered by two broad bathymetric terraces in water depths ranging from <200 to >4000 m. A potentially prospective Late Jurassic syn-rift to Late Cretaceous post-rift sedimentary succession (fluvial to paralic sediments) >15 km thick is imaged with remarkable quality and resolution. Features of particular interest include large stacked fan and channel systems, as well as simple, structurally closed formations. Careful survey design and execution optimised efficiency, enabling each survey to be acquired in less than one season. Particular attention was given to amplitude versus offset and phase compliance, including customised flows to overcome a paucity of well control in this frontier area. Optimised preprocessing, velocity model building and survey merging were applied to ensure structural and depth integrity in the final images. Regional and targeted mapping and quantitative interpretation results testify to the value of the multifaceted geophysical and geological disciplines used in the overall project execution.

A robust gradient for long wavelength FWI updates

We introduce a robust method to produce long wavelength updates in gradient-based Full Waveform Inversion (FWI). The solution introduces dynamic weights in the velocity sensitivity kernel derived from impedance and velocity parameterization of the classical objective function. The new kernel implementation effectively eliminates the migration isochrones produced by the specular reflections, enhances the low wavenumber components in the gradient in heterogeneous media, and is able to deliver velocity updates beyond the penetration depth of diving waves. We use synthetic examples to illustrate how this dynamic weighted FWI gradient successfully recovers the velocity from pre-critical reflections. We also show with dual-sensor streamer data from deep-water Gulf of Mexico how the dynamic weighted FWI gradient can combine both transmitted and reflected energy in a global FWI scheme.

Wave-equation multiple imaging (WeMi) — Swimming between the flags

Deghosting dual-sensor streamer seismic data requires separation of the total recorded wavefield into upgoing and downgoing components during seismic preprocessing. Imaging of multiples exploits the wavefield separation process by using the downgoing wavefield at each receiver location as a virtual source. Multiple imaging has been shown to effectively remove shallow-acquisition-related artifacts commonly seen on wide-tow marine seismic surveys and improve imaging quality of the shallow overburden. Acquisition design factors, imaging condition, and cross-talk noise are the main factors that determine the effectiveness of separated wavefield imaging (SWIM) of dual-sensor streamer data. (SWIM is a common nontrademarked acronym used in the literature to describe multiple imaging of separated wavefields sampled from dual-sensor streamer data.) In a field trial, acquisition design did not allow for proper sampling of the total wavefield, with the result that multiple imaging did not prove to be as robust a technique as examples in the literature might imply. Understanding the effects of acquisition design parameters on image quality is essential to more successfully apply multiple-imaging techniques with greater relevance on the North West Shelf of Australia.

Complete wavefield seismic structural, stratigraphic, geomorphic and elastic interpretation in the offshore Browse Basin

The first quantitative interpretation project in the Browse Basin that used complete wavefields from dual-sensor streamer data to characterise a variety of overburden and reservoir seismic structural, sedimentary and geomorphic features is discussed in this extended abstract. Having established that complete wavefield pre-stack angle data are amplitude versus angle (AVA) compliant, the authors analyse the elastic response from thin gas sands in a location where well control predicts a weak gas response in terms of the Vp/Vs ratio and negligible response in terms of the acoustic impedance. Various seismic expressions in the near-surface are better resolved using complete wavefield imaging, including deltaic deposits, potentially prospective patch reefs and intersecting channel systems, and shallow hazards associated with historic drilling failures appear to be resolvable with greater confidence. On the larger scale, the overall data interpretability is substantially enhanced in texture and character by broadband imaging of primary reflections, and then by subsequent reservoir characterisation. Initial (deeper) interpretation focused around mapping of the prospective Early Jurassic Plover Formation and the Early Cretaceous Upper Vulcan Formation, in which evidence of channelisation can be demonstrated on conventional migration volumes and reinforced using complete wavefield migration. Ultra-low frequency signal penetration to the deeper Triassic level has been limited historically, but the first complete seismic wavefield data analysis presented demonstrates that a combination of volume attributes provide substantially improved geologic information from the seafloor to the deepest targets.

Migration of primaries and multiples using an imaging condition for amplitude-normalized separated wavefields

Migration of primary and multiple reflections leads to enhanced subsurface illumination and to increased image resolution. A joint migration approach using the complete wavefield requires properly imaged primaries and multiples of all orders. In recent works, primaries and multiples have therefore been imaged separately, using the upgoing and downgoing pressure wavefields obtained by decomposing dual-sensor streamer data. The matches between the corresponding depth images are still not found to be sufficiently accurate, so new and more appropriate imaging conditions need to be found. We reviewed the classical imaging approach used in one-way wave-equation migration, with the aim of extending it for simultaneous migration of primaries and all orders of multiples. Based on Rayleigh’s reciprocity theorem and well-known theoretical developments, we derived a new imaging condition described in terms of the upgoing pressure wavefield and a filtered version of the downgoing vertical-velocity wavefield. To evaluate the efficiency of this new imaging approach, primaries and multiples were separately migrated using synthetic and real data examples. These results were compared to those obtained using a conventional imaging condition. We found that the use of the new imaging condition led to a better match between the depth images and spectra of the migrated primaries and migrated multiples.

Dual sensor streamer technology used in Sleipner CO2 injection monitoring

ABSTRACTCO2 has been injected into the saline aquifer Utsira Fm at the Sleipner field since 1996. In order to monitor the movement of the CO2 in the sub‐surface, the seventh seismic monitor survey was acquired in 2010, with dual sensor streamers which enabled optimal towing depths compared to previous surveys. We here report both on the time‐lapse observations and on the improved resolution compared to the conventional streamer surveys. This study shows that the CO2 is still contained in the subsurface, with no indications of leakage. The time‐lapse repeatability of the dual sensor streamer data versus conventional data is sufficient for interpreting the time‐lapse effects of the CO2 at Sleipner, and the higher resolution of the 2010 survey has enabled a refinement of the interpretation of nine CO2 saturated layers with improved thickness estimates of the layers. In particular we have estimated the thickness of the uppermost CO2 layer based on an analysis of amplitude strength together with time‐separation of top and base of this layer and found the maximum thickness to be 11 m. This refined interpretation gives a good base line for future time‐lapse surveys at the Sleipner CO2 injection site.

Signal resolution and penetration from dual-sensor streamer data on the NW shelf of Australia

A regional 2D streamer seismic survey was acquired in two regionally overlapping phases over the North West Shelf of Australia during 2007 and 2008. 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 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, 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 2, 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.