Nick Moldoveanu

Over/under towed-streamer acquisition A method to extend seismic bandwidth to both higher and lower frequencies

Seismic resolution of towed marine-streamer data is affected by free-surface reflections. They strongly modulate the spectrum reducing energy at the so-called notch frequencies, fn = iv/2z {i = 0,1,…,∞}, in which v is the water velocity and z is the source or receiver depth. As a result there is a very strong loss of useful low-frequency energy and usually a similar loss at higher frequencies. The ghost effect occurs both at the source and at the receiver. At the source, the upgoing part of the source wavefield is reflected off the free surface with inverted polarity before its delayed journey into the Earth. At the receiver, the upgoing part of the receiver wavefield is reflected off the free surface with inverted polarity before it travels down to the receiver again. The over/under acquisition method allows separation of the up- and downgoing wavefields at the source (or receiver) using a vertical pair of sources (or receivers) to determine wave direction. In this article, we present the results of two ...

What is the next step after WAZ for exploration in the Gulf of Mexico

The concept of circular geometry for towed-streamer marine acquisition was introduced in the early 1980s by French (French, 1984). Due to the limitations in marine technology and processing methods at the time, circular geometry had a short time span. Recently, it was proved that is feasible to use this type of geometry to acquire fullazimuth (FAZ) data using a single vessel, if the vessel sails along a pattern of overlapping circles (Moldoveanu, 2008). This technique is called Coil Shooting* single-vessel full azimuth acquisition. In this paper we demonstrate the benefits of implementing coil shooting acquisition with multiple vessel configurations.

Full-azimuth imaging using circular geometry acquisition

Wide-azimuth (WAZ) towed-streamer acquisition has improv-ed the quality and reliability of subsalt imaging in the Gulf of Mexico, where WAZ surveys have typically been ac-quired using 3–4 seismic vessels, each shooting in straight parallel lines. However, acquisition of WAZ data using a single vessel and a circular geometry offers several potential ad-vantages, both for operational efficiency and geophysical analysis of subsurface rock properties. Modeling exercises indicate that circular acquisition geo-metry can deliver a better range of azimuths and offsets than parallel WAZ geometry. A feasibility test in the Gulf of Mexico shows that it is possible to sail along circles while main-taining constant streamer separation and achieving very accurate receiver positioning. Single-sensor recording enables effective attenuation of the additional coherent noise introduced by shooting in a curve. Prestack depth-migrated data from the circular geometry test compare favorably with a parallel geometry WAZ data set...

Multicomponent technology: the players, problems, applications, and trends: Summary of the workshop sessions

The 2000 SEG/EAGE Summer Research Workshop drew a large proportion of participants from contracting firms (Figure 1a). The affiliation of first authors was also largely from contractors (Figure 1b), with the remainder of participants and first authors being evenly distributed between oil companies and academia. If these statistics can be taken as a genuine cross-section of the geophysical community, they indicate that contractors have the most interest in multicomponent technology. It would also appear that they perform most of the research work. More than 40% of the papers presented in the workshop were related to processing, model building, and imaging with PS waves, thus indicating where the current emphasis on research and applications lies. Less emphasis was observed in other important areas of application for multicomponent technology, such as lithology, fluids, fracture characterization and time-lapse studies.

Full Deghosting of OBC Data With Over/under Source Acquisition

The resolution of marine seismic data is affected by ghost and reverberations occurring in the water layer. A benefit of acquiring ocean-bottom cable data or node data is that the receiver-side ghosts and reverberations can be attenuated by combining pressure data, recorded with hydrophones, with velocity data, recorded with vertical geophones. Resolution of seismic data is also affected by the source ghosts. Acquisition of seismic data with over/under sources allows attenuating the source ghost and in this way to completely deghosting the ocean-bottom recorded data. In this paper, we present the results of an experiment where a 2D oceanbottom cable line was recorded with over/under sources.

Random Sampling: A New Strategy for Marine Acquisition

Seismic data acquired in marine acquisition is typically undersampled for both receivers and sources, and regularly distributed along parallel lines. The theory of compressive sampling considers that if data is undersampled, the seismic wavefield is better reconstructed if the measurements are randomly distributed. In this paper we investigate how random sampling can be implemented in different types of marine acquisition geometries. 3D finite-difference modeling data were generated to simulate acquisition geometries with regular and random sampling.

A wide-azimuth streamer acquisition pilot project in the Gulf of Mexico

A 3D wide-azimuth (WAZ) marine streamer survey was designed to image deep objectives below a complex salt structure in a deepwater area of the Gulf of Mexico. Numerous technical and logistical challenges were considered in developing the design for this survey. The final candidate designs were tested with synthetic data modeling and imaging. The final selected wide-azimuth design was used for a survey acquired over the Friesian prospect in the Gulf of Mexico. We present some of the modeling studies supporting the wide-azimuth survey design, as well as acquisition and processing results from the Friesian WAZ pilot project.

Some aspects of survey design for wide-azimuth towed-streamer acquisition

Summary Wide-azimuth towed-streamer acquisition emerged in the last two years as a step change in marine acquisition technology in the Gulf of Mexico. This came about because the risky exploration and development of deepwater subsalt reservoirs required seismic data to have better illumination, higher signalto-noise ratio, and improved resolution. Early results from the initial programs confirm that the expected benefits can be obtained. However, although the survey design principles for land and OBC wideazimuth programs are well understood, the fact that the towed-streamer implementation is still in its infancy means that survey design principles in that case are still evolving. The objective of this paper is to discuss some of those principles.

Attenuation of High Energy Marine Towed-streamer Noise

Marine seismic data have been traditionally contaminated by bulge waves propagating along the streamers that were generated by tugging and strumming from the vessel, paravanes, tail buoys, and lead-in cables. With the progress of streamer technology bulge-wave interference has been significantly reduced. However, weather and flow noise still affects marine seismic data. The level of cross-flowinduced noise is increased when the data are acquired during turns or along circles, like in Coil shooting, and when marine currents are strong. In this paper, we present a new technique to attenuate towed-streamer noise acquired in these conditions. The method has been used successfully to process coil and wide-azimuth data in the Gulf of Mexico and offshore Brazil.

A single-vessel method for wide-azimuth towed-streamer acquisition

Summary Wide-azimuth (WAZ) towed-streamer acquisition has been established as a successful method for exploration and development of the complex subsalt structures in the Gulf of Mexico. Wide-azimuth data acquired so far have proved to have better illumination, higher signal-to-noise ratio, and improved seismic resolution. The acquisition geometry used for WAZ surveys is a parallel geometry that employs multiple vessels. In this paper, we propose to acquire WAZ surveys with a single streamer vessel using a circular geometry and we describe the features of this new WAZ method. We also present the results of a feasibility test performed with circular geometry acquisition.