The Leading Edge; 2005; v. 24; no. Supplement;
p. S36-S41; DOI: 10.1190/1.2112390
© 2005 Society of Exploration Geophysicists
Broader spectrum, fewer folks—gravity and magnetics from 1980 to the present
Pat Millegan
Marathon Oil Company, Houston, USA
When considering how to summarize the changes in gravity and magnetics during the last 25 years, I felt I needed a theme to bind the discussion together. I also wanted to include input from my gravity and magnetics (G&M) colleagues. I decided to use my career as the vehicle for my initial, rather philosophical, discussion of changes that I have seen in the business end of G&M. I end with a list of important technical advances since 1980. I want to offer my thanks to the dozens of G&M experts, professors, gurus, and friends that gave me input; many of the most arresting quotes have been inserted verbatim in the text at points where I felt they were particularly on target. I also recommend that you read the 75th Anniversary technical summary with articles in the November/December 2005 issue of GEOPHYSICS.
I started my geophysical career in 1976, so the period 1980-present covers 25 of my 29 years as a "grav/mag guy" in petroleum applications, mostly exploration. During those 29 years with three employers (Aero Service, Mobil, and Marathon), I have seen substantial changes in G&M. The most extreme involve computing, business issues, and personnel.
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Computing
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In 1976, we rolled through miles of analog magnetic profiles just to edit noise. And when an edit was necessary, you punched it on cards, carefully placed them in a 3-ft card deck, and gave it to the operator of the mainframe computer. More paper profiles would be generated on a pen plotter, and you would go back to rolling through them. By 1980, we had evolved to run streams, the digital equivalent of the card deck, and electrostatic plotters. Today, you can get a laptop and off-the-shelf software to do nearly all that you need to do, from making contour maps to building simple 2D models to manipulating complex 3D inverse models, all with GIS integration.
Alan Reid (Reid Geophysics): "The PC and the Internet let an individual consultant accomplish things that could not be done by a whole department 25 years ago."
The first 2D gravity model I constructed for Marathon in 1983 was done on a programmable HP calculator. Bill Pearson (the current SEG second vice president) gave me the program on a magnetic strip. This one-layer model ran overnight, printing gravity values at a fixed interval on a paper tape, like a grocery store receipt. I then hand-plotted the values on graph paper to determine that we had a viable basin depth. (This is a bit misleading, because larger majors like Mobil already had the first proprietary versions of cumbersome but interactive 2D modeling software.)
In 2005, in 2D modeling, we interactively grab polygon vertices, yank them around and get a real-time update of our calculated field, with our model superimposed over our registered seismic image. 3D modeling has made great strides too, allowing inversion of gradient data and joint inversion of multiple data types such as gravity, gravity gradient data, and magnetics. It is still our dream, however, to have a "power glove" that allows the interpreter to mold the surface, again with a real-time calculation.
During that same 1983 exploration effort mentioned above, I had to hand-color a paper map of the contoured aeromagnetic data over an international area one-third the size of Texas. The US$1 million survey was flown in doublets, two lines 4 km apart and each set of doublets 20 km apart. This type of acquisition conserved budget, but it made interpretation a challenge. Our research lab furnished the results of a 2D idealized single-layer magnetic model of a simple rift basin at our particular magnetic inclination/declination, run on our Burroughs mainframe. I used that picture and my eyeball to determine if we had basins and, if so, locate the edges. The goal was to plan the even more expensive helicopter-supported inertial gravity survey which, in turn, would be used to help plan the even more expensive 2D seismic survey. I still have a 35-mm slide of that magnetic interpretation (Figure 1). In simple yellow, red, and white (I had more yellow pencils than any other color), several basins emerged. The interpretation still stands at roughly 95% correct, even with my low-tech equipment and my fairly immature (at the time) eye.
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Figure 1. Vintage 1983, hand-colored aeromagnetic interpretation. Scanned from 35 mm slide. The original map was  4 ft square (Courtesy Marathon Oil).
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Today, in between working on this article, I made a color-contoured map of a huge area in a matter of minutes and I sent it to an HP color inkjet plotter that plotted it in even less time. I like this better!
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Business
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In contrast to the massive expansion in computer capability during my career, there has been a substantial contraction in G&M personnel and contractors. In part due to the dramatic shift from 2D to 3D seismic surveying, contractors saw the market for marine G&M shrink. At the same time, airborne contractors saw a decrease in mining and governmental surveying. In response, full-service G&M contractors merged to the point of near monopoly and/or not being immediately recognizable as G&M contractors. However, history suggests that competition will eventually come to the marketplace and cause ever-more changes, generally lower prices, and squeezed margins. At the very least, the G&M contracting business has remained viable.
Overall, there are benefits to these changes, but a red warning flag needs to be raised about the future.
Over the last 25 years the "oil company G&M business model" has evolved from strong, in-house potential fields groups to single (or a few) specialists that have to outsource work to handle the load. In spite of numerous predictions in every decade of the demise of G&M, it is still a mainstream exploration tool in 2005. G&M workflows are an integral part of major oil company exploration programs, especially in support of subsalt imaging and pre/poststack depth migration.
When I joined Mobil in 1978, the G&M group at MEPSI in Dallas had acquisition, processing, data enhancement, and interpretation teams. As I recall, Mobil had at least 15 people in G&M. Jack Peters, our manager, had been with Mobil for 42 years. This was mind boggling to me (then 28 years old). The experience that people like Jack, Willy Frink, Aart Berkhout, Lee Zollars, Al Moss, and others passed along has proven invaluable. Those times are past, and that experience base is gone.
I was one of five young geophysicists with some experience or interest in G&M that Mobil hired for a special five-year program. The goal was to have a specialist in every business unit worldwide. After an intense 18-month on-the-job training program that included about one day per week of "classroom" training by seasoned professionals, I was transferred to the Houston office and worked onshore and offshore Texas and New Mexico. All of that detail just to make the point that it was a very different time.
Mobil built its own seismic vessel, the M/V Fred T. Nelson, while I was in training at MEPSI. Willy Frink and I manned a magnetometer base station on the beach on the west end of Galveston Island for 13 days because Mobil was testing its towed magnetic horizontal gradiometer just offshore. This was not a tough assignment; we lived in a four-bedroom beach house, I got a great tan, and I learned a lot.
As oil companies divested of their own geophysical vessels, contractors picked up the slack. When seismic contractors were not making much off the acquisition of G&M (indeed, G&M data were often thrown in for free), they sold off their equipment, and companies like Edcon, Austin Exploration, MGAL, and LCT (now Fugro Robertson) became the source of marine G&M data.
At one time oil companies put a lot of effort into R&D of their proprietary systems. Now the contractors have that responsibility. Some argue that this outsourcing helped the marine G&M business. And, in fact, it did allow its survival. Others bemoan the loss of some great oil company R&D.
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Personnel
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Oil company G&M groups were decimated dramatically in the 1980s. Majors created service organizations that charged their specialist's time to the business units. Often, the in-house specialist's "price" could not compete with contractors and consultants. In the meantime, our industry's well-known compression was under way and G&M did not seem important to the people who made personnel decisions. It is important to note, however, that we are still here, albeit well hidden.
You may be surprised to hear me say that G&M are technologies that are easily outsourced. Its specialized applications require good software and experienced handling. The risk that G&M outsourcing poses to oil companies is that success depends heavily upon integration. Integration becomes difficult unless the G&M consultant works in-house.
The rise of the importance of contractors and independent consultants is arguably the most significant change I have seen in the business side of G&M since 1980. Contractors and individual consultants have become more important in the front line of G&M acquisition, processing, and interpretation. I believe that the next generation of G&M in the industry will consist primarily of work outsourced to consultants, but with the consultants working at least part-time inside the oil company's facility. The burden will be on the consultants to stay visible and to provide noticeable value.
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Demographics
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I started my career as many of my G&M contemporaries did, at a large contractor. Aero Service (now extinct) was the largest aeromagnetic acquisition contractor in the business. Aero and Edcon were the training ground in 1980 for many G&M specialists around today.
In the mid-1980s there was still a pretty good pool of trained young talent that oil companies could hire away from contractors. The number of young people in G&M today is small. I will not dwell on this, as this concern has been expressed many times in recent years, but industry seems unaware that demographics are not in favor of skilled G&M specialists being readily available in the future.
In 2005 young talent is hard to find. Denver and Houston in the United States, several venues in Canada, and Leeds in the United Kingdom, stand out in my mind as the few places for mentoring the next generation of specialists.
Elizabeth A. (Betty) Johnson (Unocal): "Technical meetings in Europe are filled with young, enthusiastic geophysicists with diverse scientific interests. Why don't we see this at SEG?"
Australia has a strong reputation for G&M and has trained some promising young talent. When he was SEG president, Fred Hilterman told me that I should attend an Australian SEG meeting, because G&M attendees are the majority and seismic is the minority. So, the strong programs in Canada, Europe, and Australia bode well for the future. But if young people do not see opportunities for a full career with oil companies or contractors in the United States, then I doubt they will choose G&M, or the industry for that matter.
Will I have the ability to mentor a younger generation? About the only way is to be involved with groups like the Colorado School of Mines, the University of Houston, or similar institutions. I was fortunate to have a recent CSM grad as an intern—one of our industry's few talented young people with a G&M background. She is currently employed by Ultra Petroleum as a "generalist" geophysicist. Two other young geophysicists skilled in G&M have been hired away from a contractor, one as a generalist for Amerada Hess, the other as a specialist for Total. The generalists will be challenged to stay involved with the G&M community and to keep G&M integrated with their seismic interpretations.
It is very good news through all these industry changes that G&M are alive and providing value to most oil and gas companies, regardless of size. The current generation of in-house specialists has done a very good job of focusing effort on appropriate geologic problems, of building trust, and of forging strong team-based integrated workflows at their companies. The watchwords of this generation of practitioners have been "be careful," "do it right," "integrate," and "give honest assessments of interpretations and confidence."
Jerry Hensel (Chevron): "My job has been to know the data through the whole process. That includes well-planned and well-specified data acquisition; careful processing to preserve content; geologically constrained data enhancement (anomaly separation); and interpretations that integrate every available piece of information. In the end, our job is to communicate our most honest assessments to the exploration group and/or management. This includes a confidence level. In fact, the whole process is confidence building."
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The evolution of G&M technology
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Listed below, in no particular order, are highlights of the technical advancements in G&M over the last 25 years.
- The development of GPS means we now have the best survey positioning in history. This greatly improves acquisition, processing and interpretation, especially the dynamic corrections for marine and airborne gravity.
The importance of this technology in contemporary grav/mag work cannot be understated. For example:
Alan Herring (Edcon): "GPS has provided the means to measure boat and aircraft velocity changes very accurately. The accuracy of the corrections that we can apply to dynamic gravity has led to faster reading of the gravity meter, more accurate corrections, less filtering, and less distortion of signal because of filtering."
Reid: "GPS has had a revolutionary impact on G&M field practice. It's the most important single change. This is the real ‘plowshares’ effect."
- GIS tools appear to be taking the lead in our geophysical industry in terms of our quest for a common data model that allows easy sharing of information.
Chuck Campbell (Accel Services):
"Integration is more than having all your maps at the same scale."
Johnson: "Now with GIS and the ability to geo-rectify virtually anything, we can spend our time on the thinking process of integration. Logical extensions of GIS technology have been visualization and integrated earth modeling (e.g., GOCAD and Petrel). These geographically referenced integration tools have made it much easier for G&M specialists to contribute to integrated team work and decisions."
- Computers, software, and the Internet which everybody knows about, of course, but some nice perspective comes from ...
Bob Van Nieuwenhuise (Earth-Wave Geosciences): "Because of today's computer speed and RAM size, we can actually write programs to do inversions that work. We can record data at breathtaking rates (1026 Hz versus 0.1 Hz) and use the Internet for data quality control, rapid data transmission, and processing."
We now have access to workstations with integrated interpretation software which allows such sophisticated operations as overlaying your 2D G&M model on a seismic image or easily integrating 3D forward and inverse G&M modeling with seismic volumes.
John Bain (Bain Geophysical): "Gone are the days of ‘working in a closet’ to develop interpretations based strictly on G&M. Now we find ourselves in collaboration, often working side by side with seismic geophysicists to create integrated earth models, that must either satisfy all of the data (wells, seismic, G&M, EM, MT) or the unresolved portions of the model must be flagged for subsequent study as being geophysically unresolved. This often leads to the most interesting discussions, and can, at times, produce significantly different results."
Harold Yarger (Chevron): "The transition from mainframe to Unix workstations created a sea change in turnaround time for G&M processing and modeling. Since about 2000, 3D gravity modeling plays an integral role in interpreting and processing PSDM seismic volumes."
The Internet is now used for everything from software licenses to downloadable databases to interactive integration and "Net" meetings.
- High-resolution aeromagnetics (HRAM), a revolutionary development (vis-à-vis what was standard fare in 1980), is well defined by ...
Pearson: "HRAM involves a magnetically clean platform, high accuracy alkali vapor sensors (airborne and ground), low altitude (100–150 m above ground), high sample rate (10 Hz or faster), precisely located (1 m DGPS) and closely spaced flight lines (250–400 m). HRAM surveys sample magnetic effects of cultural features (well casing, tank, pipeline, building, tower, etc.) accurately enough to allow removal of the cultural noise features individually without the need to high-cut filter the entire survey."
We are capturing and using the broadest spectrum of wavelengths ever, which has changed the way magnetics are applied—e.g., we have broadened the application of magnetics from crystalline basement and volcanics to the sedimentary section.
Reid: "Widespread availability of Alkali Vapour and Overhauser Effect magnetometers has given us at least an order of magnitude improvement in sensitivity."
V.J.S. (Tien) Grauch (USGS): "When acquiring a modern HRAM survey, 80% of the information to be gained is apparent in the preliminary data that arrive from the contractor—it is quite exciting! The next 10% is obtained through hard work, but usually serves to answer most of the salient geologic questions. Tackling that last elusive 10% is where the real research and the new insights into the sedimentary section lie!"
I know of at least one 25 million barrel oil discovery in the mid-1990s based on a US$200 000 HRAM survey, integrated with a strong, ground-truthed geologic model. Seismic imaging was impossible. The discovery well and eight delineation wells were based on HRAM. Six of the eight wells were oil wells, one dry hole was predicted, and one dry hole was a disappointment...not bad!
- Satellite-derived gravity (SDG) is ideal for tectonic frameworks, basin analyses, survey planning (for all data types), reconnaissance, and "new venture" work. David Sandwell and Walter Smith deserve an award for their pioneering work, as they have provided these data, which cover most of the offshore, in the public domain, downloadable over the Internet (Figure 2).
Gravity data, derived from staggeringly precise satellite altimetry measurements of the ocean's surface, contain wavelengths possibly as short as 15–20 km. There are now various versions from "regular" to "high resolution." (Some say SDG contains wavelengths as short as 12 km, but there are differing opinions on this, so ask your local specialist).
- Very useful data compilations are available from government, academic, and commercial sources such as USGS, UTEP, the Geological Survey of Canada, Scripps, KMS, and Getech. (Derek Fairhead won a much deserved SEG Special Commendation for the University of Leeds, now Getech, worldwide continent-scale compilations.)
R.I. (Dick) Gibson (Gibson Consulting): "Large-scale data compilations provide a framework for understanding continental architecture as well as basin-scale tectonics. They provide a context for understanding local features, including definition of salt basins and generalized sediment distribution pathways, important fault and accommodation zones, and in some cases, structure and tectonics of particular oil and gas fields or groups of fields."
Bain: "Digital terrain models, most of which are public domain and/or readily available at low cost, while serving to dramatically improve the gravity terrain correction process, also have significantly improved our modeling and interpretation studies. The top surface in our modeling efforts is terrain data. It is used in the calculation."
Tiku Ravat (Southern Illinois University at Carbondale): "Paradigms are comforting, but they can also make one dangerously myopic. We need new lenses to break out of their web—very large magnetizations, for example—this one took a whole new planet, Mars, to appreciate them on the earth."
- Gravity gradiometry, full-tensor gradiometry (FTG) in particular, is a US Department of Defense spin-off technology that has both marine and airborne capability. It provides high-resolution information that can be integrated with full field gravity, magnetics, and seismic for detailed integrated modeling. This technology can also be used in 4D to monitor steam flood progression (Figure 3).
Ed Biegert (Shell): "These instruments are so sensitive that they can sense a fist at a foot or a man at a meter."
Johnson: "The high-frequency content and improved signal/noise ratio of FTG has allowed better constraints for density models; however, to properly model the complete salt picture, we must also incorporate regional gravity, magnetics, and a good geologic understanding of the area."
John Brett (Bell Geospace): "In 1998 mobile gravity-gradiometry became a reality. Today there are three companies engaged in providing survey services in airborne gravity-gradiometry and one in providing marine gravity-gradiometry surveys. There are two somewhat different approaches both using the Lockheed Martin (formerly Bell Aerospace) technology."
- Airborne gravity was just becoming available 25 years ago. Frank Carson and Bill Gumert developed the technology during the late '70s and early '80s, and should be congratulated for their persistence in getting the market off the ground! Today it provides fast, good-quality dynamic gravity data in remote, hostile areas. The first airborne gravity gradiometer was developed by BHP Billiton and Lockheed Martin in the 1990s.
Malcolm Argyle (Sander Geophysics): "The Airborne Inertially Referenced Gravimeter system was the first purpose-built airborne gravimeter, and it was designed specifically for the unique characteristics of the airborne environment. The system includes a gravimeter on a three-axis inertially stabilized platform, combined with high-resolution DGPS to correct for vertical accelerations. The gyro-stabilized inertial platform makes the gravimeter much less affected by horizontal accelerations. This allows it to achieve higher resolution. Systems are currently flying fixed-wing and helicopter surveys worldwide."
- Magnetic gradiometry is now commonly used to track a pipeline or find unexploded ordnance. Gradient-enhanced gridding improves total field mapping and leads to better interpretations.
- Multiple magnetic depth-estimation techniques now exist in a single interactive software package that allows rapid interpretation with improved accuracy and better integration. Many variations of software are easily available for grid-based magnetic depth estimates with extended Euler deconvolution leading the pack. In 1997 the Best Paper in GEOPHYSICS was "Automatic conversion of magnetic data to depth, dip, and susceptibility contrast using the SPI method" by Jeff Thurston and Richard Smith.
- Digital sampling is now standard for instrumentation in most realms from HRAM to land meters, and we have access to hyper-sensitive gravimeters.
Steve King (Austin Exploration): "The ZLS fluid-damped gravity meter has no cross-coupling (which is one of the reasons gravity data deteriorates in rough weather) and is not sensitive to vibration, which has always been a problem on a seismic vessel."
- Borehole gravity, in general, may be underutilized (it is expensive and results in rig downtime), but it has been quite successful in specific areas. BHGM can investigate features 1500 m from a well; most other devices have a range of 8–18 inches.
Andy Black (Edcon): "BHGM well log surveys provide ultradeep investigation density measurements from open or cased hole. The applications of the tool range in scale from salt structural mapping to monitoring fluid saturation changes in reservoirs. The large volume investigated by the tool gives it the ability to detect fracture and vugular porosity in carbonate sediments that is often undersampled and missed by the normal suite of porosity tools."
Van Nieuwenhuise: "Amoco made seven borehole gravimeters that worked. One was a slimhole BHGM (5.5-inch diameter). They increased oil and gas production in east and west Texas by about 300% by finding missed multiple pay zones. This great value was soon lost when three times as many reserves were ‘not enough’."
- Unmanned airborne vehicles have been developed that are preprogrammed to fly the survey on its own within specifications using real-time GPS positioning updates (Figure 4).
Terry McConnell (Fugro Airborne Surveys): "The demand from resource exploration companies to collect higher and higher resolution aeromagnetic data from an airborne platform, and the subsequent push to fly lower and lower, has been coming up against the airborne survey company's comfort zone in terms of personnel safety. One solution is the use of unmanned drones. Recent advances in UAV technology, automated flight control systems, equipment miniaturization, and GPS navigation have resulted in the use of drones as small as 10 ft in wingspan and 40 lbs in weight."
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Figure 2. Sandwell & Smith public domain satellite-derived gravity with interpretation. (Courtesy Marathon Oil, Fugro Robertson and Gibson Consulting)
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What is coming?
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The increased use of UAV, as described in the previous paragraph, would seem one obvious example.
McConnell: "As experience in the use of unmanned vehicles accumulates, larger drones with additional sensor packages will become a reality."
But everyone expects much more in addition.
Dale Bird (Bird Geophysical): "The future of G&M technology will include new methods such as the emergent use of gradiometry and electromagnetics. It will flourish because its exploration value has been demonstrated by new techniques that integrate G&M data with other geophysical data and especially geological information."
Some "things" that the G&M community expect in the coming years are more improvements in airborne gravity and airborne gradiometry; 4D gravity and gradiometer monitoring of reservoir properties; better integration of gravity and magnetics with seismic; potential fields data sets with more GIS information such as projections, geology, interpretations, etc; better methods of inverting G&M data with a priori information and geologically reasonable constraints from earth models (a move toward artificial intelligence); smaller borehole gravimeters that can work in deviated holes; recognizing the fractal (chaotic) nature of magnetic sources and exploiting it for depth estimation; tensor magnetic gradient surveying; and routine use of neural networks, wavelet transform and equivalent source methods for processing and interpretation.
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A final word
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From 1980 to the present, G&M have been vital to many exploration and production applications. The G&M business has been evolving as a result of improved technology, new technology, and in response to employment stresses on personnel and demographics. G&M are now being used in some plays that are not usually associated with G&M. Normally thought of as reconnaissance or large-scale regional tools, today G&M are being used down to the prospect level in highly sophisticated plays, not just in the Gulf of Mexico—fracture plays, for example, and even reservoir monitoring. Some big resources are being discovered in unconventional plays where G&M have a role. The past 25 years have been an exciting time for G&M and its practitioners. I am proud to have been part of a relatively small but highly talented community that made so much progress in that period despite a drastically altered business climate.
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Footnotes
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Pat Millegan's academic background is mathematics (bachelor's degree) and geology (three years graduate study and teaching assistant) at Baylor University. He worked for two years in aeromagnetic processing and interpretation with Aero Service Division of Litton Industries and was a Mobil G&M specialist for almost five years. He has been with Marathon for the past 22 years. His professional achievements include an SEG/AAPG joint publication and the discovery of Ras El Ush oilfield in Egypt's Gulf of Suez (drilled strictly on purpose-flown aeromagnetic data and his interpretation). Millegan was the first coordinator of TLE's Meter Reader column and his two-year tenure solidified it as a regular feature of the magazine.
Copyright © 2008 by Society of Exploration Geophysicists
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