LANDSAT 6 IMAGES TO PROVIDE MOPE REMOTE SENSING MUSCLE

David G. KogerConsultant Fort Worth

With the launch planned for third quarter 1993 of Landsat 6, the newest civilian use imaging satellite, oil and gas explorationists will have been using satellite images for 20 years.

The economic benefits of remote sensing photogeology will get yet another boost as data from the "enhanced thematic mapper" become available almost immediately from a new ground station at Norman, Okla.

PHOTOGEOLOGY

Photogeology is the practice of examining photography of the earth to identify surface expressions of geologic features at depth.'

Scanners (fax machines and video cameras are common scanners) aboard satellites collect high-tech surface mapping data. Scanners capture more fight and do so with greater discrimination than film.

The abundance of information in these data make Landsat the tool of first choice for photogeology today. Other attributes of these data are:

• They cost far less than air photos. Unprocessed data cost less than one half cent/sq mile.
• They do not lose their signal strength or "fade" over time.
• They contain no inherent focus or shading distortion because there are no lenses through which light must pass onto recording or display media, and no (unstable) chemicals.
• They have greater dynamic range per light region recorded, can be enhanced through computer processing, and merge conveniently with other geologic and geographic data.
• Then, are safely used in secret to preview and analyze areas that are physically or politically remote (cover photo).
• They merit intensive analysis. They are best viewed on video screens (Fig. 1). The interpreter adjusts brightness, contrast, and color to reveal and scrutinize noteworthy patterns, shapes, and textures. 2

NEW SATELLITE'S PLUSES

The most exciting feature of the new satellite is the high resolution Panchromatic band. It contains a blend of visible and infrared light that is similar in spectral characteristics to Kodak infrared Aerographic 2424 film with Wratten No. 12 filter.

It samples the ground at 15 m resolution, twice the detail of the current satellite, and is automatically coregistered with the other bands.

Even sharper images and greater detail result, worthy of even larger scale analysis (in excess of 1:24,000, Fig. 2).

More detail to overseas exploration means even better logistical maps to augment the popular regional picture (Fig. 3). Domestic players will use this cost effective level of detail for more intensive analysis to provide better explanations for observed phenomena.

None of the multispectral characteristics of present satellites will be lost, and there will be better geometric fidelity. Landsat 6 also features switchable sensitivities to acquire data from both ocean shelf (dark) environments and over highly reflective desert sands.

Landsat 6 is compatible with even the oldest satellites that for 20 years have gathered earth data in all seasons, sun angles, weather conditions, and climates.

Before-and-after images make convincing testimony in environmental disputes. Romanticized global change discussions could be kept on a rational basis if this impartial, unbiased documentation were used to design effective policy. 3

When these archived data (80 m resolution)-and still commercially available-are merged with new Landsat 6 (15 m) data, information can be observed that exists in no other form.

Sensors in the older satellites uniquely capture certain photogeologic events, 4 too, but their coarse spatial resolution has made it especially difficult to accurately plot these phenomena in mature basins at larger scales. Land- sat 6, combined with older data, will provide new solutions for many applications.

USES IN EXPLORATION

Exploration managers have few resources and never enough information. Their commission is to evade nonproductive areas, uncover favorable targets, rank them, plan seismic programs, and exhibit cost efficiency.

Landsat delivers benefits at each of these stages.

Once a potential area is targeted, developing leads of good quality is critical. Remote sensing photogeology is an excellent choice at this juncture: time and dollars are rescued from bad (or no) leads-they are applied instead to areas that hold greatest promise.

Prospect generation moves quickly now with the site-specific application of other interpretive tools like well logs, subsurface mapping, and production data. Correlation with Landsat interpretation increases the confidence factor in gravity and magnetic data, too.

Top priority prospects are quickly readied for survey with nonconventional methods or seismic: the planning cost, and interpretation of which benefit from a model already in place. There is maximum return at minimum cost each step of the way.

Landsat is analyzed at whatever scale agrees with the targets and purpose at hand. Overseas, morphotectonic features, outcrop patterns, structural data, and the developmental history of an area are surveyed at small scale.

Regional trends and relationships are identified, located, and extended. The same data at larger scales (and different enhancement) are useful even in low-relief frontier areas where bedrock is poorly exposed.'

Potential favorable structures can be found; field parties "walk out" fewer areas, and seismic crews are better directed. Logistics benefit, too, and everybody at every phase finds his way around better with up to date satellite derived maps.

Very large scales are suitable for "focus" on domestic plays. Sometimes dip and strike can be calculated.

Landsat is often the only-and certainly most economic-tool to suggest mapping closure on a subsurface nosing 6 to indicate which way to drill out from a show (Fig. 2), to point out fracture trend orientation, or lead to a new field discovery amongst existing production. It would be difficult to use Landsat and not reduce finding costs.

HOW LEADS MAY APPEAR

Faults, fractures, fracture orientation, and joints can have surface expression as:

• right angle bends and other anomalous drainage courses;
• smaller-order streams enter main streams in direct opposition;
• moisture accumulation in linear patterns, alignment of tributaries, or springs and small ponds;
• subtle dip changes, varying lithologies or changes in rock texture; and

• variations in thermal signatures.

Positive structures at depth - at least buried, and often obscured by soft, un-deformed sediments, vegetation, and soils - can imprint the surface as variations in photo tone due to:

• differences in vegetation health, leaf water content, or population distribution;
• differential compaction, loading, and increased fracture density over and adjacent to buried structure;
• soil color and texture alterations (staining, bleaching, cobbling);

• local, slight topographic highs or lows;
• subtle variations in moisture accumulation on the flanks of buried tructure; phenomena too subtle to detect on air photos and not seen on topographic maps (Fig. 2);
• anomalous drainage patterns or more subtle ground and surface water effects due to permeability contrast, the upward movement of groundwater, fluctuations in water table depth which, crosscutting the surface effects of structure, can create alterations in vegetation, drainage, and soil type.'

Light hydrocarbons are known to migrate vertically from buried reservoirs and affect the surface in many ways. Landsat has captured many surface effects that are related to such seepage, including:

• secondary mineralization, "pseudo" structure, which can look like all the effects of positive structure above;
• soil chemistry alterations, which can affect color, type, and texture;

• vegetation anomalies; and
• anomalous drainage and moisture patterns.

OPTIMIZING DATA

Satellite data are best examined on an array of computer-driven video displays (Fig. 1), where an operator has interactive control of image parameters such as contrast, brightness, color assignment, and color purity.

This allows selective enhancement or suppression of data, more accurate comprehension, and points to methods for further processing. Here, the entire dynamic range and detail of digital satellite data can be exploited. Setups like these are expensive but pay for themselves quickly if exploration can keep up with the leads provided: more critical to a successful operation is operator training.

Before Landsat was commercialized in 1985, imagery was most often used in hard copy form. Eosat has emphasized sale of data in digital form because every hard copy image abbreviates the dynamic range and amplitude of Landsat data. A suite of images is a better idea, but the consulting industry acts as a "great equalizer" in delivering the same capabilities to independents as are available to larger companies that can afford and staff their own processing workstations. In no case will an interpreter produce better work than the photo allows.

AIR VS. SATELLITE PHOTOS

Aerial photos were the only medium for photogeology for 50 years. Study of them gave rise to reliable, tested methods that underpin the use of our modem observation platforms.

However, air photos record much less infrared light than do satellite photos and do poorly in areas that are highly reflective or somewhat dark. They don't mosaic well and contain focus and shading error.

Landsat images convey far more information. They are sharper, have more spectral (i.e.- color discrimination) range, and cover more physical area (as much as 12,000 sq miles).

Many combinations for imagery are possible from a single Landsat scene. This is an important reason why regional and local analysis (i.e., small and large scale work) can be performed on the same data set.

A comparison of the light regions sampled by the various photogeology platforms is shown in Fig. 4.

EXPLORATION LINK

Consultants that use satellite data for oil and gas exploration are called a value added industry.

Landsat's power to boost profits is made affordable by this industry segment, and a Landsat analysis can keep most explorationists busy for years. But despite the potential for profiting the oil industry and payoff to those whom it has benefitted, the market for photogeology services has remained small for many reasons.

Senior personnel who control exploration budgets are sometimes the least qualified to understand or judge an emerging technology (it happened with seismic too). Often remote sensing doesn't get the credit it is due (cynicism of a poorly understood tool is natural) when a fine subsurface workup is praised, and anybody has trouble explaining and defending a technique about which little can be found in trade journals.

Value added operations that process and analyze satellite data won't expand much, and starting such an operation will be difficult for the near future at least. Capital and sympathetic bankers are in short supply, established laboratories often compete for jobs with government (or other "nonprofit") entities.

Universities, market driven, won't train a workforce for Landsat photogeology until such jobs exist. Private enterprise has only lately taken over satellite data operations and sales, and the profit motive in place bodes well for promotion of this underused tool.

In addition to getting hard copy images and interpretations from the consulting industry, users may purchase raw data from Eosat in Lanham, Md., and air photos are archived at the EROS data center in Sioux Falls, S. D.

CONCLUSION

The need to constrain Landsat interpretations with all available geologic data is emphasized. Remote sensing photogeology replaces no common tools; rather it aids in the planning and layout of more expensive methods and augments the deciphering of these and direct subsurface data like well logs.

Most oil industry observers hold forth that "new technologies" will help today's explorationist survive in the face of increasing costs and environmental regulations. Landsat remote sensing photogeology - while not a new technology in the current sense - is a market that is approaching maturity.

REFERENCES

1. Koger, D. G., A close took at photogeology, remote sensing, and image analysis, OGJ, Dec. 5, 1988, p. 54.

2. Koger, D.G., Image creation for geologic analysis and photointerpretation, proceedings Southwest ASP-ACSM, San Antonio, Tex., 1984.

3. Koger, D.C., and Henderson, F.B. III, Observation, evaluation, and development of predictive models for global change: The need for establishing U.S. government-industry links, proceedings, first annual Earth Observations and Global Change Decision Making: A National Partnership, Environmental Research Institute of Michigan, 1989, Washington, D.C.

4. Koger, D.G., and Carter, J.C., Use of remotely sensed data in mature basins: considerations on creating useful imagery, proceedings Sixth Thematic Conference on Remote Sensing, 1988, Houston.

5. Berger, Z., Structural analysis of low relief basins using Landsat data, proceedings of the Third Thematic Conference on Remote Sensing, Colorado Springs, Colo., 1984.

6. Carter, J.S., and Koger, D.G., Successful applications of remotely sensed data for oil and gas Exploration, proceedings Sixth Thematic Conference on Remote Sensing, 1988, Houston.

7. Koger, D.G., and Ross, D.M. 111, Remote sensing method works in North Texas producing area, OGJ, Aug. 27, 1990, p. 51.

8. Berger, Z., and Aghassy, J., The use of Landsat data for detection of buried and obscured geologic structures in the East Texas basin, U.S.A., Proceedings of the Second Thematic Conference on Remote Sensing, Fort Worth, Tex., 1983.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.