Offshore seismology today: more, better, faster, cheaper

Display and computer power simplifying imaging

Dennis W. Nesser
Pohlman and Associates
John C. Pohlman
Pohlman International

Geoquest's GeoViz software enables an interpreter to combine all available data to characterize and delineate the reservoir. [33,478 bytes]
Landmark Graphics' OpenVision software integrates geoscience and engineering applications, allowing a multidiscipline team to visually integrate diverse data types. [15,566 bytes]

Whether projected as a virtual mass, shown on a computer monitor, or printed on paper, the modern display of a 3D seismic cube is a product of integration and technology. Geoscientists have always known that they wanted to see the world in three dimensions. Within the last 20 years, desire has become a visual reality.

On one display, we see seismic horizon times, fault planes, amplitudes, well tracks, production volumes, and other characteristics of the subsurface. Bringing this information richness to the consumer requires many people and many machines. The capacity of these machines and the tools used by the people were hardly imaginable just two decades ago.

The display begins with data acquisition and processing. For marine seismic, that means huge vessels towing massive cable spreads. Positioning systems locate each receiver at each shot. Others steer the vessel in the proper direction and attitude to gather the data despite wind, waves and current. Noise suppression systems filter out specific and random noises.

Some onboard computers demultiplex, deconvolve, filter, demultiple and DMO stack the data. Some allow onshore processors to look at the data during onboard processing. Once onshore, the data goes to huge centers where multi-Gigaflop machines complete the pre- and post stack processing to order.

Stored in robotic tape libraries with multi-terabyte capacities, the data are fed over high-speed networks directly into an interpretation workstation. One of these workstations has more compute and storage capacity than an average oil company had ten years ago.

Within the workstation, the data is subject to analysis and evaluation well beyond any that could be done with paper methods. Helped by disciplines outside the geosciences (like artificial intelligence and perceptual psychology), the interpreter matches the evolving image of the data with the model of earth processes gained from experience. Joining well, lease, production and other data to the work in progress focuses the image and enhances the process. Joining the interpreter may be experts in several fields, all contributing to and working from a single image of a reservoir - real or potential.

Acquisition

Marine seismic acquisition occurs worldwide. Seismic vessels can travel in any waters of interest to explorers. Towing cable spreads exceeding half a mile wide and five miles long, recording six to 24 lines simultaneously, these vessels use brawn and brain. Even a brief look at the numbers tells of the size of the effort.

A single block in the Gulf of Mexico post-stack is approximately 90,000 traces, or 1.3 Gigabytes of data. Pre-stack data can be 40-60 times that volume, without including header and identification data.
Onboard computers demultiplex, resample, deconvolve, demultiple, filter and DMO stack the raw data. Some contractors offer quick looks at reduced data volumes, while others allow onshore processors in client offices to see work in progress onboard.
Noise suppression routines contend with ever-present boat, cable, wind and wave noises. They also attack signals from other crews shooting in the area.

Paul Huff, Senior Vice-President Data Processing Operations for PGS Tensor notes: "We can now keep lines that previously would have been discarded. That makes acquisition cheaper and faster for our clients." At Western Geophysical, Danny Stegall, Vice President for Marine Acquisition agreed, claiming, "Noise suppression begins with our new solid cable. We're equipping our vessels with this technology as quickly as we can take delivery."

Brawny machines deploy, tow and retrieve up to ten 8000-meter cables simultaneously. Others charge tuned airgun arrays for a shot every 75 meters(alternating between paired sources to give an effective 37.5-meter shot spacing). Huge propulsion systems move the vessel. Even so, the vessel and its tow remain as precisely located as synchronized swimmers.

Onboard computing compresses the time for delivery of product to the consumer. "Processing that took a year to complete in 1994 is done in three months now," notes Dennis O'Neill, Business Development Manager for Geco-Prakla. In some cases, clients can accept delivery of data volumes just weeks after the last shot in a marine survey.

VSAT communications allow some contractors to replace onboard processors with onshore processors in client's or their own offices. Using data compression and restricted data volumes, these processors can monitor and direct the onboard computing. Cooperation between clients and contractors can cut processing time by 50%.

Onshore processing

There will always be a need for onshore processing. No matter how capable, onboard computers cannot keep up with the volume of seismic data processing. Seismic data has always challenged the capacity of existing computing resources.

Today a 32 gigaflop partition of an MPP computer may spend a month doing a pre-stack depth migration for a single offshore survey. True, that survey may cover 60 offshore blocks (over 500 sq miles) and require a robotic tape library to manage the data. The result, however, is an image that can peer below salt.

Oil companies are finding modern seismic data so reliable that they are willing to use it to correct well deviation surveys. Onshore facilities, like the offshore vessels, ring the globe. Growth in these facilities continues at exponential rates.

In Houston, Geco-Prakla has trebled their capacity (and halved their floor space) over the last year. Today, contractors measure their compute power in teraflops across their centers. Tomorrow, they would like to have that power in a single machine. "Then we'd want as many of those machines as we could get our hands on," Paul Huff noted.

Interpretation systems

Data move from onshore facility to client site by network or by tapes. Large data volumes still require tape movement. However, networking can provide on-site look and feel while the data and processing remain remote. Interpretation begins during the processing step as velocity decisions and processing parameters shape the signal at the target horizons. Once loaded to a geoscience workstation, the data volume is subject to repeated analysis and display.

At this stage, other data begins to join the picture. Well data, land data, cultural data, and remote sensing data all contribute to the development of an economic reservoir image. Taking the best from each discipline to overcome the weaknesses in the other disciplines, this image is the best scientific prediction of what lies in the subsurface.

If the image shows a potential reservoir, it will become the working model. The model persists until new evidence from exploratory wells provides a reason to update the model. This same model moves seamlessly to the engineering disciplines for reservoir analysis and well planning.

It is hard to overstate the importance of the display. "Landmark's integrated information management solution includes OpenVision for merging 3D objects from small to massive volumes of disparate data sources to enable multidisciplinary teams to gain new understanding and perspectives. OpenVision appears to be the most advanced and easy to use solution for reviewing complex spatial relationships of multiple layers of data.

The resulting 3D views allow teams to visualize not only seismic rays, well horizons, well and pipeline locations, faults and other data, but also enhances the collaboration between geophysicists, geologists and engineers," said John Gibson, Executive Vice President, Integrated Products Groups, Landmark Graphics.

New tools

Two new tools stretch the value and the application of seismic data. The first of these, 4D seismic (repeating 3D surveys over time), is becoming a mature methodology. Using ocean bottom cable or towed arrays, these surveys can replicate data volumes to a high degree of accuracy.

Processing and bandwidth improvements over the last five years allow geoscientists to interpret changes in seismic responses as fluid-flows in the reservoir rock. Larry Denver, Manager of Product Planning for GeoQuest says that commercialization of this technology is a high priority for them.

"Companies will struggle with the decision when to shoot their next 3D survey," Denver noted. "We've done good work in our Cambridge office defining the parameters that make that decision economically sound." Good 4D work makes for better drainage plans. The result is more production and better economic results. Over the next two years, GeoQuest will introduce a series of tools to make 4D interpretation more manageable.

The second, marine multi-component shooting (4C), is in the early development stages. Land multi-component (3C) shooting has been around for a long time. The land system uses three geophones mounted to capture motion in each of the three dimensions (x,y,z).

Finding a way to get the same information offshore has been a goal of oil companies and contractors alike. Some solutions in use today include cables and sleds that anchor sensors to the seafloor. Another uses robots to mount seafloor sensors in a pattern. Then the marine data adds the traditional pressure sensor to these three motion detectors to get the fourth component. Data volumes, of course, increase by a factor of four.

Similarly, the interpreter must have a way to display and analyze each component of the data. This makes the data management and data handling complex. The payoff comes when interpreters use the additional information to image parts of the subsurface obscured in the traditional compression-wave only recordings.

Russ Sagart, in charge of GeoQuest Product Planning for Geophysical Products noted that interpreters would need new ways to view the various combined data images. This will require intelligent tools (for example, ones that can match P and S reflections even given the timing and signature differences) and improved data handling.

Full exploitation of the multi-component information gives access to physical measurements well beyond the ability of compression-wave seismology. With these measurements, geoscientists can now hope to map porosity, permeability, fluid fill and other significant measurements with great accuracy.

The contractors will continue to improve their data gathering skills. Researchers now are looking for new ways to control streamer groups. Today, the practical limit lies between 800 meters and 900 meters separation between outside cables. Companies want to double that width to increase the value of each shot.

Ashore, researchers are looking at ways to compress data to multiply the effective bandwidth of communications channels. While VSAT is rated at 2 million bits/second, the effective sustained rate is less than one-eighth that speed. Data compression is needed if onshore processors are to get the instant response they crave.

Research continues in the interpretive tools arena as well. Here, the vendors are continuing to integrate different data types and different applications into a seamless whole. Landmark, for example, is working on a 4D shared earth model to help in this integration.

Shared models allow geoscientists and engineers to use consistent tool sets to describe the subsurface. The payoff is reduced duplication and overlap, with less time spent moving data and converting formats. Data compression techniques will also help reduce the demand for physical storage. Here, intelligent compression systems that allow users to specify an acceptable degradation level will allow storage needs to be matched to geoscience requirements.

Art of the display

We cannot overlook the importance of art in this environment. While the science is demanding and critical to the successes in the industry over the last five years, art plays a very significant part as well. Geoscientists have long known that before they can commit to testing their theories with holes in the ground, they must sell their ideas to managers and executives. Successful explorationists viewed this presentation as the most critical part of the exploration process.

Presenters have always had to choose the data that best represents their arguments. Today, their range of choices is far wider than it was even five years ago.

Color selections may divide data into appropriate ranges. It may highlight a specific transition that is of interest. Color may be used to define the extent of a characteristic or to draw attention to a specific area.

Other choices have multiplied as well. Choices of texture and lighting can pack the final display with information. The key is to make that information easily digestible by the first-time observer.

Exploration today merges brains and brawn, science and art in ways quite different from those used just a few years ago. Geoscientists today routinely do tasks that no paper interpreter could do. Color, texture and lighting allow far more information in a single display than black and white ever could.

Computer systems show no sign of slowing their growth rates. We can only speculate about the wonders that these developments will provide us in the next few years.

Authors

Dennis Nesser is Vice President and COO of Pohlman and Associates. He has been in the industry for 26 years and previously was with Shell and Union Pacific Resources, where he was responsible for providing technology to processors and interpreters. He holds and MS in mathematics from Denver University.

John Pohlman is President of Pohlman International. He went into business in 1986 after a career as an explorationist and designer of computer systems. He was trained as a geologist and archeologist.