Brian Barley

Dispersion patterns of the ground roll in eastern Saudi Arabia

Seismic surveys on land must be designed so that the source‐generated noise, such as ground roll, is preferentially attenuated before P‐wave signal amplification and recording. The correct specification of spatial and frequency filters requires prior knowledge of the noise properties in the area. We show that the strong Rayleigh wave component of source‐generated noise has a wavelength range which is predictable on a regional scale, using widespread P‐wave velocity measurements in shallow upholes. This predictive capability decreases the number of noise analyses required to map the boundaries between areas with different Rayleigh wave properties. The case history presented is for northeastern Saudi Arabia, an area of roughly 150,000km2. The data comprise 80 noise analyses and a data base of over 10,000 up‐hole measurements of P‐wave velocities, supplemented by maps of topography and geologic outcrops. Examples show that the frequency‐wavenumber transforms of time‐offset records can be interpreted in detai...

Deepwater problems around the world

Deepwater is whatever depth our industry is struggling to make money in today. Ten years ago, deepwater meant beyond conventional diving depth, say 600 ft. Today in areas where the metocean conditions are kind, development and production in 1000–2000 ft of water is really not news. The shallow end of “deepwater” in most people’s minds is now around 2500 ft, and the battlefront where our struggle to make money is going on is at around 6000 ft (Figure 1). It’s here that engineers and scientists are working beyond their previous limits, making it work for the first time. And it’s entirely consistent with our definition of “deep” that a new word, ultradeep, is beginning to be used for 6000–10 000 ft, where we stop trying to convince the boss that he can make money and instead distract him with visions!

Multi-azimuth and wide-azimuth seismic: Shallow to deep water, exploration to production

Ten years is a long time in geophysics. In the early days of 3D seismic surveys, in the 1980s say, it might have taken two years to acquire and process a 3D survey, and many business decisions simply could not wait that long. By the mid-1990s, our seismic contractors had done a wonderful job of making 3D seismic faster and cheaper while making it better—truly a “class act.” Thanks to them, the overwhelming technical advantages of 3D are in routine use in almost every possible application today.

Quantifying and increasing the value of multi-azimuth seismic

Conventional marine exploration offshore Nile Delta is challenged by shallow heterogeneities and a deep, complex anhydrite layer called the Messinian. These subsurface complexities cause variable illumination and strong diffracted multiples resulting in imaging challenges below the anhydrite on conventional narrow-azimuth towed streamer acquisition because the recording antenna is too small (Figure 1a). Additionally, it has proved difficult to build the detailed depth migration velocity model required to correctly image below the Messinian layer. Multi-azimuth (MAZ) acquisition using time-domain processing however, has been very successful in addressing both the noise and illumination problems in the Nile Delta. The increased azimuthal and crossline offset coverage—bigger antenna—is achieved by acquiring six conventional marine surveys over the same area at 30° sail-line increments (Figure 1b).

Multi-Azimuth towed streamer 3D Seismic in the Nile Delta, Egypt

A thin but complex layer of partially eroded anhydrite and other facies lie at a depth of around 3km across large areas of the Nile Delta in the Mediterranean. Wavefield distortion, attenuation and the generation of complex multiple diffraction noise cause the quality of the underlying seismic image to be highly variable. (Figure 1) In this paper we describe the problem and then demonstrate how multi-azimuth seismic is able to improve the PreMessinian image.

Multiazimuth versus wide-azimuth acquisition designs for sub-Messinian imaging: A finite-difference modeling study in West Nile Delta, Egypt

In this paper, we present a recently completed, extensive finite-difference (FD) modeling study over the West Nile Delta area in Egypt. The primary objective was to analyze the impact of various acquisition geometries on the pre-Messinian image quality. We used an innovative methodology for building the velocity and density models representative of the complexity of the offshore Nile Delta. Synthetic data showed many of the challenging features observed on field data. This allowed us to meaningfully quantify the impact of various acquisition geometries on image quality. We found that a surprisingly determinant factor influencing the final image quality is the offset distribution of the contributing traces. As offset distributions can be adjusted after the fact by applying a suitable offset-dependent weighting to recorded data, we find that by making careful use of offset weighting we can reap some of the same benefits of more complex acquisition schemes with simpler acquisition design.

How many azimuths do you need

Examples abound of marine seismic quality uplifts due to multi- and wide-azimuth acquisition geometries (Figure 1), reflecting the fact that these techniques are rapidly becoming routine and widespread.

Building representative velocity and density models for a finite-difference modeling study in offshore Nile-Delta, Egypt

Summary Full scale 3D finite-difference modeling studies improved our understanding of the influence of acquisition geometry in complex geological situations. In order to develop an efficient acquisition geometry using finite-difference modeling studies we need to use velocity and density models representative of the subsurface. In this paper, we present an innovative methodology for building the velocity and density model for the complex geological setting of offshore Nile-Delta in Egypt. Numerical data generated with these models represent the various features observed in field data.

Noise maps for shallow water OBC - a case study from SE Asia

Summary OBC wi de azimuth surveys record a large percentage of data before first direct arrivals (FBs) from the shot. Maps of RMS amplitude calculated from these data can be used to set realistic noise specifications for shallow water OBC. Based on this approach, the survey discussed here set thresholds for pressure and velocity at 20Bar and 2m/s respectively. These values are considerably higher than typical values for deep water OBC: 5Bar and 0.5 m/s respectively. Noise maps enabled QC personnel to identify numerous types of noise quickly and reliably enabling them to suspend recording when the noise exceeded the specified level. The noise maps display pressure in Bar or velocity in m/s in coordinates of shot index number and channel number which can be interpreted as an ordinal time scale and a spatial coordinate across receiver lines respectively. The most serious type of noise originated from the previous shot and suggested an increase in time between shots for future surveys. Scattering from the shoreline of the bay at distances of 15–20km from the receiver lines could produce pressure signals of 35150Bar when the travel path was unimpeded by seafloor ridges. Similarly, ground roll recorded on the vertical velocity component and travelling directly from the previous shot could exceed 2.5 to 5m/s. The characteristics and amplitudes of all types of noise are readily identified on these noise maps.