|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
BP, Houston, USA
Imagine a typical workday in the life of a seismic interpreter, and then make that day quite atypical and extraordinary by allowing it to span 75 years of interpretation history. Imagine that the interpreter begins that day at 8 a.m. with the technology of 1930, the year in which SEG was founded, and ends at 5 p.m. with the technology of 2005, the year of SEG's 75th Anniversary. Imagine that he's on the leading edge of the wave of innovation, able to actively employ interpretation technologies within the first few minutes of their development.
Proficiency in any art or science is not attained until its history is known.—HENRY G. TYRELL, History Bridege Engeneering, 1911
 |
Morning |
|---|
 TopAbstract Morning AfternoonAcknowledgments: |
|---|
Shortly after 8:30 a.m. he reads in the journal GEOPHYSICS about the development of the Rieber sonograph, a method by which seismic data are for the first time recorded in reproducible form so that they can be played back with different equipment settings and studied in greater detail. A few minutes before 9 a.m. he begins to regularly use a technique that comes to be known as "loop tying," a "loop" being a number of individual paper records arranged to form a closed traverse. In this technique correlation begins at a given point on the loop and proceeds around it, returning to the original starting point by way of consistent and geologically reasonable correlation from record to record. Although this approach helps to ensure that correlations are at least spatially reasonable, it doesn't solve the problems and uncertainties caused by variations in acquisition of individual records. At about 10 a.m. he begins to overcome these difficulties by using record sections for interpretation, on which reproducible reflection records are plotted as a continuous "section" with uniform display parameters. Even though the interpreter can see his correlations on these sections in a view akin to a geological cross-section, he is keenly aware that without velocity control and compensation for structural dip ("migration") his record sections do not accurately represent the true subsurface geology.
With the morning's developments fresh in his mind, the interpreter takes his customary coffee break at 10:15 a.m. But the pace of his day does not slacken—just after 10:30 a.m. he learns about a major advance in data acquisition, the capability to record analog reflection data on magnetic tape. This new technology broadens and deepens the need for specialization in data processing, and the interpreter quickly learns that while he is necessarily becoming more specialized as well, he must also be increasingly vigilant in keeping up with the progress in acquisition and processing that will affect his data—and not five minutes later he hears news about the vibroseis source, an important development in land seismic acquisition. At 10:45 a.m., he discovers that he can now choose among variable density, variable area and the conventional wiggle trace formats for display of his data, and often finds himself in heated discussions about which format contains more useful and meaningful information and, ultimately, which provides more "interpretable" sections. This, like many other aspects of his work, is a very personal decision, and he continues to cultivate his own personal interpretation style. A few minutes after 11 a.m., he sees that in response to technical advances and the accelerating pace of data acquisition, both on land and in the marine environment, data processing is being centralized in specific groups or departments within exploration companies and contractor organizations. This reflects what he believes is yet another step along the path of greater functional specialization, which offers clear business advantage but warns that communication among technical disciplines will become increasingly important. At the same time he moves into an office where he will continue his work in the company of other interpreters, geologists, technicians and support staff in the exploration department (at his new desk he takes note in GEOPHYSICS of Mayne's patent abstract for "a method of seismic shooting in which both the detectors and shot point are moved"—the original common depth point or CDP method—and then rereads Dix's paper on calculating seismic velocities from surface measurements). He'll still travel to the field, but only periodically and no longer as a member of a crew, much less as the boss. Some computational tasks that previously had been his responsibility are now delegated to technicians, and he's able to devote more of his time to correlation and mapping.
The interpreter looks up at the clock on the wall and sees that it's 11:30 a.m., time for lunch. He's managed to keep his wits about him through the morning's avalanche of technology, his move from the field to an office and the dizzying pace of exploration, and is very hungry now. What to do? He's heard about these new "fast food" restaurants, and decides to try one that's just opened down the street.
|
Afternoon |
|---|
|
TopAbstractMorningAfternoonAcknowledgments: |
|---|
|
At about 12:45 p.m. he hears about a new, unifying global context for all of his interpretations—plate tectonics. This theory, in its infancy of acceptance and commanding much attention in both academia and industry, helps him to more thoroughly understand the structural settings and depositional histories of the provinces in which he's working, and also gives him a new and powerful tool for establishing analogs for frontier exploration areas. The explosion in acquisition of marine reflection seismic data, together with the interpretive framework provided by plate tectonics, give him insights into plate margins and open a tremendous number of prospective unexplored basins, many of which are far removed from major population centers or established infrastructure. To the interpreter these basins represent the greatest of all challenges, and present opportunities to be perhaps the first person ever to see the geology of a previously unseen slice of the earth.
Up to this point in the day the interpreter has always worked with unmigrated records and sections, doing his mapping either by migrating sections by hand, using wavefront charts or actually drawing the wavefronts with a compass and then connecting points of tangency, or by migrating structure maps constructed from his unmigrated sections (also done by hand). At about 1 p.m., he hears that the geophysical contractors and the R&D community are beginning to produce their first computer-generated time-migrated sections. As the next few minutes pass he receives more and more migrated sections to interpret, and spends less and less time on the tedious manual migration processes that had consumed so much of his time before. But like most technical advances, migration of 2D lines has come with a price—although he can see a more accurate image of the subsurface on his migrated sections, he must now deal with the 2D mis-tie problem. At the same time he reads about observations that other interpreters have made in Cenozoic clastic basins such as the Gulf of Mexico, that strong or "bright" seismic amplitudes are often found to be correlative with hydrocarbon accumulations, particularly gas. This observation, which is fostered by the introduction of color in seismic displays, gives rise to what becomes known as "bright spot" technology, and the interpreter discovers not only that he now has at his disposal a technique he can use to directly detect hydrocarbons, but also that he no longer has an excuse not to learn about rock properties, and that he must become more familiar with petrophysics if he is to apply "bright spot" technology properly. It doesn't take long for him to find out that "all that's bright isn't right" in exploring with direct hydrocarbon indicators, that the physics of low-saturation gas in a reservoir will betray and frustrate him.
Just before 1:30 p.m., his world changes again with the advent of seismic stratigraphy, the concepts and practices of which provide a new approach for more thorough integration of geology into his interpretations. Much if not all of his previous effort, for right or wrong, probably was directed at interpreting structure and defining traps. With the many advances in data acquisition and processing resulting in ever increasing volumes of high quality data becoming available, and a global community of scientists sharing their results and integrating information from the burgeoning number of wells being drilled in more and more basins throughout the world, interpreters are able to describe stratigraphy to a degree of detail never before possible, with immediate and obvious consequences for exploration. At this time no interpreter wants to be without his own copy of the classic AAPG Memoir 26, Seismic Stratigraphy—Applications to Hydrocarbon Exploration, and he rapidly learns the terminology of this now essential dimension of his work. Using seismic stratigraphic techniques he interprets reservoir facies and infers presence and quality of those facies directly from his data, and integrated interpretation takes on a fuller meaning. At about this same time seismic inversion technology is developed, providing him with yet another tool for interpreting the subsurface in even greater detail, although he soon learns that inversion in the absence of calibration to well control and without careful pre-inversion processing is a very risky business.
At about 1:45 p.m. the interpreter hears that the first experimental 3D survey has been successfully acquired, and soon after he's invited by his data processing friends to review preliminary processing results of one of these early surveys. Up until now he's worked exclusively with 2D data, virtually all of them time-migrated, if migrated at all (there still were a few instances when only unmigrated data were available, and he had to dust off his older interpretation skills to get the job done). Throughout countless battles with his tireless mis-tie nemesis he had held out hope that some day 3D seismic data would become a practical reality, and that time seems to have finally arrived. A few minutes later the processors tell him that advances in computing technology and applied mathematics have made image-ray or depth migration possible, although they are quick to say that it will be some time yet before depth-migrated data become routinely available. He can envision how these two technologies, 3D seismic and depth migration, have the potential to revolutionize the industry, permitting companies to explore in structurally complex regions—but in the meantime he must carry on with his grids of 2D data and be content to wait for the day when 3D data are the rule rather than the exception.
|
A few minutes after 2 p.m., the interpreter sees that SEG has begun to publish a new general interest magazine called GEOPHYSICS: THE LEADING EDGE OF EXPLORATION. He notices also that 3D data are beginning to appear in front-line exploration, and interpretation technologies are evolving to enable him to take advantage of the power of 3D data. He is somewhat overwhelmed by the volume of data in his first 3D survey, and at the same time excited by the flexibility of his data "cube" and the different displays (time slices, horizon slices, "arbitrary" lines, to name a few) he can employ to see his data in entirely new ways, especially through the use of custom color schemes. He quickly finds that working at his desk or drafting table with maps, rolled or folded paper sections, pencils, dividers, rulers, tracing paper, map weights ("shot bags") and many of the other accoutrements of interpretation "by hand" gives way to using a computer workstation, a development necessitated by the wealth of data in a typical 3D survey and the desire to manipulate those data in real time. He also reads about the physical basis for an analytical technique called AVO, an acronym for amplitude variation with offset, which will enable him to investigate lithology and pore fluids using the amplitude information contained in his data. He finds out that this technique, like many others he has used before, depends critically on the quality of his data, and requires calibration to well control in order to be acceptably accurate and reliable. He confirms the lesson of his previous experience that he must work closely with data processors to gain a working understanding of the quality, and hence the limitations, of his data. It's at this time that he learns about a process called DMO (dip moveout), a sort of partial prestack migration that improves the quality of seismic velocity analysis and sharpens the final processed image, which among other things helps him pick faults more accurately.
As 3 p.m. approaches the interpreter finds that more and more of his exploration projects involve 3D data, and that large speculative or nonexclusive surveys are being acquired and energetically marketed by the major geophysical contractors—in some of the more mature hydrocarbon provinces, 2D data are on the wane. At the same time workstations are becoming more powerful and versatile, enabling him to perform basic interpretation tasks more quickly and efficiently. He is most impressed with automated functions such as horizon autotracking and amplitude extraction, and readily adapts to the new interpretation setting. With each succeeding interpretation he more fully appreciates how important motion, color, and the ability to rescale data can be in helping him correlate his data. But he grows wary of trusting "the machine" without question, after having had several automated tasks fail spectacularly, and so in the back of his mind he preserves his expertise in interpreting 2D data and doesn't discard his colored pencils or proportional dividers. He learns how to use computer-based gridding and contouring packages but doesn't allow his manual contouring skill to atrophy (he's always regarded this particular skill as a cultivated talent that reflects his ability to visualize the subsurface). He's somewhat amused when workstation technology grows to include functionality for interpretation of 2D data, because he's not sure when or if the workstation will ever be able to properly handle the 2D mis-tie problem. He's very pleased to see workstation hardware and software develop for dual screens, because this more closely approximates the conventional map-and-section setting—he has no particular preference for a mouse versus a joystick.
Shortly after 3 p.m., he realizes that 3D data and the workstation are the generally accepted standards for interpretation, although areas in which only 2D data are available still remain. The dominance of the workstation continues to grow, and its analytical power in particular expands. The interpreter is able to extract, manipulate and analyze more seismic attributes than he thought ever existed and is taxed to understand the physical meaning of many of them. Two attributes in particular that he learns to use frequently are dip and azimuth extractions; he finds these to be very useful for interpreting faults and fine structural details as well as stratigraphic features such as channels and channel-levee complexes (he doesn't know that in a few minutes these attributes will give way to coherence, which is destined to become one of his most useful 3D attributes). Together with refinements in seismic stratigraphy (or sequence stratigraphy, as it has become known to many), the attributes derived from 3D data enable him to generate very accurate depositional models for his areas of interest. At 3:15 p.m. he learns that time-lapse, or 4D, seismic surveys are beginning to be acquired. Carefully processed and compared to baseline surveys, these data will enable him to interpret the behavior of producing reservoirs, with obvious and profound implications for field development and management. He's happy to see this important development finally occur, because he had heard about it earlier in the afternoon and wondered what took so long for it to come about.
At about 3:45 p.m. the interpreter works with his first depth-migrated 3D survey. The data set is a poststack depth migration, and he learns how important the accuracy of the migration velocity model is to the validity of the results (the processors weren't quite ready to let the interpreter see the results of their early attempts at prestack depth migration, which of course consumed very much more computer time than the simple poststack depth migration). Soon he's actually interpreting the horizons needed for construction of the migration velocity model, and gains appreciation for the time he must invest in interpreting these horizons and coordinating his effort with data processors throughout the depth migration project. He's growing increasingly aware that the interpretation of 3D data on a workstation is breaking down the decades-old distinction between geologists and geophysicists, who earlier in the day had been intentionally separated in his formerly function-based organization. He believes this change has been long overdue, because he's seen first-hand that sound interpretation is the sole possession of neither the geophysical nor the geological community but of the geoscience community as a whole. But he thinks also that interpretation is definitely not a group activity, that it is an intensely personal and largely creative endeavor—that there can be only one hand on the mouse (or joystick) at a time.
By 4 p.m., he has become very comfortable working almost exclusively with 3D time-migrated data, although occasionally he works on a project for which only 2D data are available, or for which a carefully laid out and processed 2D line or two enables him to answer specific interpretation questions. As the last hour of the day passes he is pleased to be part of the rapid development of visualization technology, in terms of both software and hardware (including large-scale 3D and immersive visualization environments), because these enable him not only to more completely understand the subsurface but also to more clearly communicate the results of his interpretations to nongeoscientists. He finds himself interpreting more and more depth-migrated data, working closely with data processors on the critical task of building depth-migration velocity models because he understands the crucial dependence of depth migration results on the input velocity model. He's learned also that specific imaging problems are better solved by migration algorithms designed and parameterized for those problems, that no one type of migration is necessarily suitable for an entire project, and that he must keep up with advances in imaging technology in order to be effective as an interpreter. At about 4:30 p.m. he becomes aware of the value of using elastic impedance data derived from his reflectivity data to resolve fine details of producing reservoirs, and that these data can be tuned to highlight both lithology and pore fluid type. As the day closes he marvels at the great strides in computing capacity that enable him to generate multiple versions of data sets, all in a time frame he couldn't have dreamed of only minutes before. He senses that this staggering increase in capability has not come without its price—he knows all too well that no single data set can be used to answer all of the interpretive questions posed by his project, and that it's his responsibility to evaluate each available data set within the larger context of his project objectives. He knows that as geophysical technology advances and business demands inexorably increase, the unit length of time he'll have to evaluate multiple data sets will shrink, and managing his data, which always has been an inescapable element of his work, will consume more and more of his time.
It's 5 p.m. and time to call an end to the day. As he powers down his workstation the interpreter recalls all of the phenomenal developments he's witnessed on this remarkable day. Experience has taught him that he's an interpreter not because of the abundance of technology at his command, but by virtue of the skill, persistence and insight he brings to his craft. Only his imagination limits the interpretation he'll do tomorrow....
|
Acknowledgments: |
|---|
|
TopAbstractMorningAfternoonAcknowledgments: |
|---|
|
Footnotes |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
