Eivind Fromyr

Flam - A Simultaneous Source Wide Azimuth Test

Summary A wide azimuth acquisition test was performed in the Gulf of Mexico where two sources out of four in total were fired simultaneously. All firing times were randomized within a small window to minimize the interference from the near simultaneous source as well as minimizing the effect of remnant coherent noise from the preceding shot. Results show direct comparisons between a conventional wide azimuth acquisition and the simultaneous source method as well as decimated results which represent apple to apple comparison in terms of fold and shot positions. The simultaneous source method has the potential to significantly improve wide azimuth acquisition as well as other acquisition geometries.

An exploration‐scale wide azimuth towed streamer case study

Wide azimuth towed streamer surveys (WATS) have recently proven quite successful at improving illumination and attenuating multiple energy in difficult subsalt imaging environments. However, they present new challenges in terms of survey design, acquisition and processing methodology and operations. In particular, the sheer volume of data acquired requires pragmatic approaches to data processing. A 2007 deepwater Gulf of Mexico WATS survey of exploration scale (10,000 km2) illustrates these challenges and demonstrates the need for increased fold and cross-line offset.

Leveraging the Value of Multi-azimuth (MAZ) Seismic through MAZ-stack

The growing demand for energy requires that the search for hydrocarbons must extend into more challenging settings and become more reliant on technology, exposing the limitations of conventional 3D marine seismic. This decade has seen a major response to these challenges in the marine setting, in the form of Wide Azimuth seismic acquisition, like Multi-Azimuth, Wide Azimuth Towed Streamer, Nodes and OBC. These methods illuminate the sub-surface more completely, and sample problematic 3D noise across azimuth as well as offset for better attenuation during stack. These surveys are much more expensive to acquire and process, so it is thus important to lever all the value from the data. This paper discusses two seismic processing options to lever additional value from MAZ data. The first, MAZ-Stack, has been developed to weight up signal in areas of poor illumination, in other words, to favour strong signal over weak/absent signal. This approach has the effect of reducing fold and so decreases random noise suppression. The second technique, 3D warping, extends an existing method to align sub-surface seismic volumes, and so minimize registration errors between the azimuth stacks which result from imperfect knowledge of the sub-surface velocity field.

Simultaneous Sources: A Controlled Experiment On Different Source Configurations

The simultaneous firing of marine sources can provide a significant uplift in terms of acquisition efficiency. However, the seismic interference resulting from one or more “other” sources needs to be well understood and the appropriate processing techniques need to be investigated. This paper uses the well known BP Wide-Azimuth Towed Streamer (WATS) synthetic dataset to carry out a controlled investigation into the effect of additional noise in 3D surface-related multiple elimination (SRME), depth migration and source separation. Two different simultaneous source configurations are considered: a front and tail and a side by side survey design. Results show that the front/tail configuration gives naturally cleaner results after 3D SRME and migration, but some source separation should always be applied; 3D SRME or migration alone isn’t enough to suppress the additional noise.

Experience with towed streamer multi-azimuth processing and acquisition

Multi-azimuth towed streamer acquisition is a technique whereby conventional marine 3D seismic surveys are acquired in several distinct acquisition directions and then combined in some way to produce an improved image. The method as described here has produced some impressive comparisons in the literature, e.g., Keggin et al. (2006), Page et al. (2006), and is drawing increasing interest especially for areas of poor signalto-noise ratio. Over the years that this method has been pioneered, some standard acquisition and processing procedures have been established which will be discussed here with examples from BP’s six azimuth Raven survey from the Nile Delta, courtesy BP Egypt.

An Inversion Approach to Separating Sources in Marine Simultaneous Shooting Acquisition

In seismic exploration, there is continuous drive towards more dense data sampling to better image complex geological structures. Recent advances in acquisition such as Wide-Azimuth, Multi-Azimuth or Rich-Azimuth acquisition can deliver a more diverse range of source, azimuth and offset sampling. To collect such data, multiple source and receiver vessels are deployed, thereby increasing the costs of the survey significantly. In conventional acquisition, there is zero time overlap between shot records, and data are recorded discontinuously. The source domain is often poorly sampled, leading to aliasing. In simultaneous acquisition, data can be recorded continuously, and temporal overlap between shots is allowed. Consequently, more sources are fired during the same period of acquisition, which greatly enhances the flexibility in survey geometries. As a result, a more densely sampled data set in terms of source spacing, but also azimuth and offset distributions can be obtained. In terms of efficiency, simultaneous acquisition can contribute by reducing survey times, which is of particular value in critical situations where small acquisition timewindows dominate due to severe safety, environmental or economic restrictions. As such, from an acquisition point of view, simultaneous acquisition holds the promise of both efficiency and quality improvements. However, unless source separation can be achieved to a sufficiently high degree, the enormous potential benefits of simultaneous sources remain unrealized. Two approaches are currently utilized by the industry to achieve source separation: methods based on random noise attenuation and inversion-based methods. By regarding the energy from secondary sources as incoherent noise, after sorting the acquired data into an appropriate domain, the interference noise appears as random spikes which could be attenuated using well-known random noise removal procedures. Alternatively, inversion-driven methods aim to construct the separated sources through the minimization of a cost function that describes the “data misfit”. In such methods, incoherent energy is no longer regarded as noise that is to be removed. Instead, this energy is recognized as a representation of coherent events belonging to (one of) the interfering shots. An inversion approach aims to distribute all energy in the blended shot records by reconstructing the individual unblended shot records at their respective locations. In this abstract, an inversion-driven method is utilized that uses coherency measures to reconstruct the individual shot gathers from the blended data. The method is demonstrated to be capable of separating sources, even in the presence of strong diffraction energy. The method can be used in conjunction with other methods, resulting in so-called hybrid solutions. A simultaneous source wide-azimuth 3D data set from the Gulf of Mexico is presented.

Comparison of deep tow conventional hydrophone and dual sensor streamer seismic offshore Egypt

Summary Dual sensor streamer effectively removes the receiver ghost from seismic data through dual sensor summation. By doing so, one achieves preservation of a broader seismic bandwidth. In particular, low frequencies are significantly enhanced which contributes to better penetration without the compromising loss of higher frequencies. In this particular study it is shown that a deep tow of a conventional streamer improves deep imaging but resolution is lost and cannot be recovered in the shallower data. This is contrasted by dual sensor data which matches signal to noise for deep data, but at the same time preserves resolution in shallow data. These conclusions from theoretical considerations are confirmed by real data.

Simultaneous Source – Preprocessing and Imaging Challenges

This paper discusses a simultaneous source wide azimuth acquisition experiment which was performed in the Gulf of Mexico. Out of a total of 4 sources, two of them were fired simultaneously, with all firing times being randomized within a small window. Sta

Imaging and Characterization with PP and PS A feasibility study from the North Sea

The first step in successful application 4C technology is a feasibility study. The current study was done on the Sleipner field in the North Sea. The objective of the data processing and analysis was to investigate how 4C data can be used to improve imaging of structural geometry and aid in fluid and lithology discrimination. Then demonstrate that it can be achieved in a limited sense from existing 2D4C data. We emphasized in particular how 4C data can help delineate the Ty formation. The field has a gas saturated sandstone reservoir which is difficult to map with the 3D streamer data. The study was split into two main components; One modeling and analysis part based on well data and a one processing and interpretation part based on the 2D4C data available. The Ty formation was better imaged by converted wave data as suggested by the modelling. It was also concluded from the modelling that there is good separation between the sand and the surrounding shales in terms of Vp/Vs values suggesting that combined PP and PS data analysis should enable lithology discrimination. Well logs also suggest that Vp/Vs can be used to discriminate between water and gas saturated Ty sand. Well logs and modeling - Methodology In this study we had access to a few typical wells in the area. Well 15/9-26A coincided with the 2D4C line but did not have shear information whereas 15/9-D-1H had shear logs. The well log data available was used to cross-plot observations and also as a basis for forward modeling of PP and PS response. Since shear wave data was not available from all wells it was estimated based on the Krief approach (See Krief et.al 1990, Hilterman 1987 and Biot 1941). The mineral properties and the estimated shear wave response was calibrated in a well with shear observations. The synthetics were generated by doing Zoeppritz modeling based on calculated impedances and density maintaining well log resolution. The PP wavelet was derived from the seismic data and the PS wavelet was derived from the same with a stretch equivalent to a Vp/Vs ratio of 2. Well logs and modeling – Results Figure 1, shows the Vp/Vs vs. depth as a Vp/Vs semblance log plot, for the Well 15/9-D-1H, sampled over a 15 m interval. The plot highlights the separation between hydrocarbon sands and shales/wet sands along the Vp/Vs axis. Note the good separation between the gas-saturated sands (Ty) and the shales (Lista). Figure 2 shows the Vp/Vs versus porosity cross-plot for both wells (D-1H, 26-A) only for the Ty formation interval with the Z-axis as volumes of shale. The Vertical axis is indicating in colour the range for the three formations (Lista, Ty brine and Ty Gas). The synthetic PP and PS (Figures 3) show a stronger PS stack response than that of PP for the Ty formation.