Isometric sampling: The next milestone in integrated 3D marine seismic

November 7, 2013

After a decade of R&D spanning the globe, a new class of marine seismic technology has now been commercially deployed. Nina Rach spoke with Chris Cunnell of Schlumberger.

In an ideal world, a seismic survey would have a grid of sensors across the full area of interest, but the constraints of working at sea have never permitted this. The reality is that surveys have towed a fairly narrow spread of streamers. To approach the ideal, our industry has increased the number and width of streamers, but there are still limitations. In order to make seismic surveys efficient, previous common practice was to increase the size of the spread, with fairly widely spaced streamers, typically separated by 50-100m. In very specific circumstances, such as high-resolution site surveys, the spacing is narrower.

The problem is that although we’re able to measure the seismic wavefield anywhere from 12.5m down to 3.125m on a Q-Marine point-receiver marine seismic system along the streamers, the sensor separation between the streamers is still 50-100m.

This results in an asymmetric sampling of the subsurface and it affects data conditioning and migration, ultimately leading to “smeared” images which add uncertainty to subsequent interpretation.

Marine isometric seismic technology offers a new approach to seismic acquisition and processing, based on isometric sampling, which improves the density of measurements, both inline and crossline. The benefits are wide ranging and include increased exploration efficiency, broadband imaging in three dimensions, and the potential for fresh geological insights from fine-scale subsurface characterization.

The company presented examples of the new technology at the recent Society of Exploration Geophysicists (SEG) annual conference in Houston in September. Based on data collected in the North Sea, Dr. Malcolm Francis discussed advantages this high resolution imaging brings to shallow hazard analysis, well planning and the design of well bores, and enhanced reservoir characterization.

Theory

The cornerstone behind the technology is the premise that the gradient of the pressure wavefield (ΔP) in a medium is directly related to the acceleration of particle motion (the rate of change of velocity, ∂V/∂t) as follows: ΔP = -ρ ∂V/∂t, where ρ is the medium density. If we are able to measure particle acceleration in two directions, we can extract the gradient of the pressure wavefield and use this in the seismic processing workflow.

Measuring the pressure wavefield gradient enables two new developments:

IsoMetrix

IsoMetrix Technical Marketing Manager Chris Cunnell describes the 10-year development of isometric sampling techniques as one of the largest research and engineering projects in Schlumberger history. The company has developed a new type of multimeasurement point-receiver streamer which combines traditional hydrophone P-wave measurements with multi-sensor accelerometer measurements. These are based on micro-electromechanical systems (MEMS) technology, and are used to record both vertical and horizontal crossline measurements of pressure gradient. It is this combination of all three measurements that provide the information which allow 3D deghosting and proper wavefield reconstruction between cables.

OE: Why hasn’t this been done before?
CC: The basic premise has been known for some time, but until now we were unable to create a multimeasurement sensor that captured high-fidelity and robust accelerometer measurements with good signal-to-noise separation across the full frequency spectrum.

During the early stages of development, we made an effort to understand the very complex sources of streamerborne noise modes. Experimental and modeling studies helped us to design a system that would accurately measure noise and then filter it out.

OE: When was this system available to clients?
CC: We completed field testing in late 2011, close to our development site, in the Norwegian North Sea. The IsoMetrix system was officially launched in June 2012 at the EAGE conference in Copenhagen. Since then, we have completed seven projects in a wide range of settings; including the North Sea, offshore United Kingdom and Norway; South Africa; the Barents Sea; and a survey off Canada.

These surveys range in size and objective, from exploration to reservoir characterization for appraisal and development, including a 4D qualification in the UK sector of the North Sea, and cover a range of customers including independents, IOCs as well as Schlumberger multiclient projects. We have a strong backlog of activity into 2014, and will start a large program for Woodside off northwestern Australia this quarter.

All surveys to date have been acquired using the WG Vespucci, and we plan to upgrade more vessels in 2014.

OE: How do you handle this much data?
CC: The large amount of data is a major step-change for marine seismic acquisition. It’s not just the extra components we are measuring, which triples the amount collected. In order to characterize and remove the noise, we must sample data more densely along the cable. The legacy is 3.125m spacing, but we sample at shorter intervals. To handle this enormous quantity of seismic data, we’ve built an integrated acquisition and processing system that allows key elements of the processing to be done on board. The compute power on-board the WG Vespucci—a vessel—makes it the fourthlargest Schlumberger data processing center, worldwide.

There are two elements to onboard processing:

1. Take raw measurements through an initial online pre-conditioning step to remove noise associated with the streamer construction and behavior in the water. 2. Production of a dense, isometrically sampled up- and down going pressure wavefield on a 6.25-m x 6.25-m surface grid.

Additional fast-track processing runs in parallel, and allows the client a quick look at the data. Then, we send the data ashore for further processing. Here we are able to leverage the experience of our petro-technical experts from data conditioning, through imaging, inversion and interpretation to extract the maximum value from the data.

OE: How does the client benefit?
CC: IsoMetrix technology offers quality, efficiency and operational (including safety) benefits. We are not bound by limitations on streamer tow spacing; so for example, in an exploration setting we can tow the cables further apart to efficiently cover an area more quickly, reducing turnaround time, whilst reconstructing the wavefield to replicate closer-towed streamers. Wider spacing also mitigates the chance of streamer tangling. We can tow cables deeper in the water, where the ambient environment is quieter and they are less affected by weather, but of course the tow depth also depends on the survey geophysical objectives.

The quality uplift can be clearly seen in the standard seismic image volumes, but it is necessary to look beyond these to 3D interpretational attributes and inversion products to extract the full value. For example, IsoMetrix volumes are showing great promise when partnered with ant-tracking to map faults and fractures, and with eXchromaSG which uses coloration to isolate geologic information of other unconformities and lithology variations. Together, they allow for fresh geological and interpretational insights.

OE: Is the technology unique?
CC: Yes. Although there is a range of broadband solutions, only IsoMetrix provides fully 3D deghosted broadband data in all dimensions that is unique in the crossline gradient measurement, allowing accurate reconstruction of the pressure wavefield. The density of sensors along the cable is important in characterizing and removing the noise, to make best use of the accelerometer information.

What it really comes down to is the quality of images for explorationists and reservoir engineers. The fundamental benefits of the technology is that it provides fine isometric sampling to make data more suitable for a wide variety of applications. The system provides high-fidelity point-receiver seismic data, that allows us to capture the seismic wavefield on a 6.25-m x 6.25-m grid, and yields the first true 3D volume using towed streamers.

The earth is not simple, the subsurface is complex, and it is vital to get the full picture. OE

Image Caption (top): Chris Cunnell

Image Caption (center): The multisensor streamer system deployed offshore Canada to acquire an IsoMetrix marine isometric seismic survey.
Photo: WesternGeco.

Image Caption (bottom): Broadband imaging in three dimensions for complex sand injectites using IsoMetrix technology.
Photo: WesternGeco.