| SATELLITE REMOTE SENSING: Satellite Imagery Maps Mineral Prospects in Peru
Progressive mining companies give their geologists an edge with the use of satellite imagery.
By Russ Cowart Imagine the challenges faced by prospectors working in the Andes. The topography is incredibly rough, limiting visibility and restricting mobility. The elevation tops out over 16,000 feet where simply walking can become a taxing aerobic exercise. There are few accurate maps and fewer roads. And to top it off, most mineral deposits are invisible to the naked eye. Yet hundreds of millions of dollars are spent annually to try and discover multi-billion dollar mineral deposits.
So how do the most progressive companies meet the challenges? What do they use to help rapidly identify the most promising targets? How do they give their geologists an edge? What do they do to beat the competition?
They use satellite imagery. A Powerful Tool
It's not a magic bullet or all of the world's deposits would already be discovered; but the effective integration of satellite imagery in a mineral exploration program results in faster acquisition of higher-quality prospects at a lower cost.
Some specific benefits include:
• improved geologic mapping
• the ability to delineate specific alteration zones
• enhanced structural identification (faults, folds, etc.)
• better logistical planning and execution ¥ and reduced manpower and support costs.
Although these benefits are available any time satellite imagery is used, they are especially valuable in remote terrains where existing information is scarce, access is expensive, and the competition is fierce.
The images also provide an excellent framework on which to compile other data such as occurrences, geology, geophysics, and geochemistry. As well, they provide a comprehensive overview of the situation which fosters improved communication between scientists and engineers from different disciplines, or between resource companies and local governments.
A key to gaining the most benefit from satellite data is to actively involve field staff during the processing and interpretation stages. Their first-hand knowledge of an area can transform the data from a pretty picture to a key exploration tool. Of course, there are some general techniques that can be applied if a field geologist is not available at the time. The Satellite Systems
Although this article focuses on Landsat TM, the RADARSAT, SPOT, ERS and JERS satellites are also commonly used for mineral exploration, especially structural mapping and logistics. Table 1 compares the critical characteristics of today's most popular commercial systems. The Toquepala Mine
A heavily mineralized area in Southern Peru will serve to illustrate the advantages of this imagery. The Toquepala mine is a major producer of copper and molybdenum concentrates. The mine is located in the dry coastal Andean mountains at an elevation of 3,400 m. According to reports1, the mine was put into production in the late 1950s at a cost of over $200 million. Several hundred million more dollars have been invested to update the mine and processing facilities to date. Toquepala is a porphyry copper deposit set in Mesozoic and Tertiary volcanic rocks which have been intruded by diorite stocks. The pit has a diameter of just over 1 kilometer. The Imagery: Landsat TM
Landsat Thematic Mapper (TM) is the traditional workhorse of the mineral exploration business (at least where clouds are not a problem). Its seven spectral bands can help differentiate rock types and identify alteration zones associated with mineral deposition. It is very cost effective, easy to interpret and imagery exists for most of the world.
Figures 1 - 4 illustrate some of the most useful maps which can be made from TM imagery. The first image is natural color created from a combination of TM bands 3, 2, and 1 (displayed as red, green and blue) which is very similar to what you would see if you were looking out the window of the satellite. This image is useful for logistical considerations, i.e. to answer the question,"where are the roads or access to my area of interest?" It can also be a valuable reference image while working with some of the more esoteric combinations.
The second image is a composite of TM bands 7,4, and 1. These bands do a good, first-cut job of differentiating rock types or lithologies. This image has also been converted into a different color space, enhanced and sharpened, and then converted back. These processing techniques begin to exploit the high information content of the TM data, allowing the user to do his or her job better.
The third image was enhanced to highlight structural features trending N45W, although many other trends are readily visible. Notice the zone of structural intersections in the mine area and the circular feature to the northwest. Structural features act as the plumbing system through which hot hydrothermal systems flow. These systems frequently change or "alter" the host rocks (changing the mineral make-up of the rocks) and occasionally deposit economic quantities of gold or other metals.
Finally, the fourth image is a principal components analysis. This type of processing emphasizes zones of hydrothermal alteration which is commonly associated with gold and copper systems. Notice especially the areas similar in color to the Toquepala pit. Does It Help?
Look again at the natural color image. Pretend for a minute that you are a geologist faced with exploring the area covered by the image. Where would you begin and how many months would it take?
Now look at the alteration image. Notice how the mine lies on a circular feature at the intersection with a WNW trending fault. Also notice that much of the area along both the circular feature and the fault have the same alteration signature as the mine. Now can you select the areas of higher potential?
Many other enhancements are available, depending on the goals of the users. An even more powerful resource is the integration of satellite imagery with the other data layers like geology, magnetics, gravity, radiometrics, geochemistry, digital elevation data, radar, etc. Costs
Archive TM imagery (more than 10 years old) can be acquired for about $500. The processing demonstrated here would typically cost about $1,000. Add in another $500 or so for prints and the whole package costs around $200 or about 6 cents per km2. Compare that with the savings from the increased efficiency and effectiveness of the geologist in the field and you can see what a value remote sensing can be. About the Author:
Russ Cowart is the president of i3 - information integration & imaging in Fort Collins, Colo., a producer of value-added satellite image products and orthophotography. He may be reached at: [email protected] (e-mail) or 970-482-4400 (telephone). 1 Review of Current and Potential SX/EW Copper Producers, Pincock, Allen & Holt 1995 Back |
