As hinted at in Section 2, in the early days of Landsat and other space observing systems, geologic applications were near the top of a general list of uses to which remote sensing is put. Now agriculture and land use have moved up in importance. But, the market remains strong for Geology because companies involved in mineral and petroleum exploration continue to acquire and interpret a variety of space products in their hunt for new resources. This first page reviews some of the basic concepts and strategies pertinent to exploration for mineral/oil resources. Of special note is the attention given to finding metallic deposits using telltale surface alteration, often evident in space imagery.
We can synopsize the role of aerial photography and space imagery in searching for natural mineral, metal, and hydrocarbon resources in this diagram of an exploration model.
5-1: Before reading the next paragraph, try to deduce from the above diagram what is the single most critical step in determining whether the explorer has found, and proved, a mineral or petroleum deposit worthy of commercial development. ANSWER
Starting with the first stage (reconnaissance mapping of a broad region), the flow of activities leads us to select specific targets that reduce the total area to a few locations that we can effectively and economically examine in detail. Landsat, SPOT, and other space systems provide the regional inputs to visual databases that we must correlate with existing information from maps and publications. Computer image processing converts raw data into specialized thematic maps that emphasize certain characteristics of surface materials (telltale guides) and structures. We know from experience that these characteristics associate with concentrations of ore deposits and hydrocarbons . Once we designate candidate target areas , we begin more precise mapping, using imagery as a base but support it with ground checks and even detailed (local) mapping to pinpoint final targets. These targets can also be examined by appropriate subsurface geophysical methods, such as seismic refraction and gravity surveys. These methods may be applied early as a means of focusing on these ultimate targets. While information to this point may strongly indicate the presence of economically-viable deposits, the decisive test is nearly always to drill into the promising site(s). By drilling, we recover samples to evaluate the economic worth or, in the case of oil/gas, we encounter commercially-viable hydrocarbons.
Space-acquired data (principally images) supercede previous approaches, which relied on extensive ground and aerial surveys, simply by showing regional tracts of land. A full Landsat scene covers more than 34,000 square kilometers (13,000 square miles). The images can be examined as units by photointerpreters and by computer-aided image analysts, including the approach known as Geographic Information Systems (GIS) described in Section 15. After appropriate processing, these full scenes often reveal much smaller locales that display guides to mineralization or oil/gas deposits. This process potentially can save major exploration costs, because large unsuitable areas can be eliminated and field efforts confined to favorable areas. We illustrate this concept by demonstrating Landsat's capability to bring out surface signs of mineralization and then by evaluating a case study, which considers surface alteration and structural control that may point to underlying oil and gas traps.
Here is just one example of how remote sensing can aid in the search for extractable (commercially viable) mineral (ore) deposits: Prospectors and miners for centuries, and exploration geologists for the last one hundred years or so, have sought certain telltale signs of valuable mineralization, especially gold, silver, copper, nickel and other metals. These metals often appear as abnormalities or anomalies at the Earth's surface. Most widespread among these guides is "gossan", a miner's term for rust. It is, in fact, a mix of several forms of hydrated iron oxides, including the mineraloid group limonite (FeO(OH) nH2O), that develop most commonly when iron-bearing minerals react with water and other chemicals during the natural weathering or alteration processes. The brownish to yellow-brown soil color and yellows and oranges of many sandstones (especially in the U.S. West) owe those distinctive tints to these secondary iron compounds. The reds in rocks usually result from the presence of hematite, a non-hydrated iron oxide (Fe2O3).
Gossan, as seen in the field, is conspicuous. And in hand specimen it can be a dominant constituent. Here are two examples (see captions for details):
The chief reason that gossan forms in areas where useful metals are concentrated is that pyrite, (FeS2), a common mineral also known as "fools gold" because of its brassy-yellow metallic color, is deposited with these other substances. Pyrite and other mineralized substances come from hydrothermal (hot water) and other kinds of solutions that usually originate out of deep, hot magmas. The solutions migrate upward into the Earth’s crust until they encounter conditions favoring precipitation as ore minerals. The most common ores of silver, copper, lead, zinc, nickel and cobalt are compounds of these metals with sulphur (as sulphides). While several of these compounds also have discrete alteration products, the most conspicuous (whether seen from a pack mule or in a color aerial photo) is normally gossan and related iron products, that derive from the chemical breakdown of pyrite. This decomposition usually is brought about either by groundwater or by near surface weathering from rainwater that percolates into soil and bedrock. Secondary minerals (including sulphates, carbonates, and clay minerals) that also may form by alteration, afford additional clues to the possible location of prized metals. Of course, the presence of gossan does not automatically guarantee the presence of economically extractable metals, because iron sulphides (usually worthless) may be the only or dominant mineralization.
As will unfold in the next four pages, remote sensing can play a pivotal role in searching for new mineral deposits. We illlustrate this with this synopsis of how iron deposits (consult this Wikipedia web site for a synopsis of this subject) can be found with sensors on air and space platforms:
IRON ORE DEPOSITS CAN BE VARIED IN THEIR NATURE AND APPEARANCE. IF THE IRON IS PRESENT AS THE MINERAL "MAGNETITE", IT CAN BE FOUND USING A MAGNETOMETER, OR A DIP NEEDLE. THIS IS BEST DONE FROM AN AIRCRAFT. MOST MAGNETITE OCCURS SCATTERED IN ITS HOST ROCK. BUT MAGNETITE CAN OCCUR IN LAYERS IN A ROCK TYPE CALLED "TACONITE".
HEMATITE HAS ITS OWN DISTINCTIVE MINERAL SPECTRAL SIGNATURE. HYPERSPECTRAL REMOTE SENSORS CAN DETECT THIS SIGNATURE. BUT THE HEMATITE MUST BE EXPOSED AT THE SURFACE AND THIS IS RARE (USUALLY, SOIL OR VEGETATION COVERS THE HEMATITE) UNLESS THE HEMATITE IS IN A DESERT OR ARID CLIMATE. ALSO, RED SEDIMENTARY ROCKS GET THEIR COLOR FROM HEMATITE, BUT THE AMOUNT OF HEMATITE IS TOO LITTLE TO BE AN ORE DEPOSIT.
IF THE HEMATITE OR ANY IRON-RICH MINERAL HAS BEEN WEATHERED OR AFFECTED BY SOME OTHER ALTERATION PROCESS, THEN LIMONITE IS THE RESULT. THIS PRODUCES "GOSSAN", WHICH CAN BE DETECTED RATHER EASILY BY REMOTE SENSING. BUT LIMONITE DEPOSITS ARE MARGINAL AS ORE BODIES (THE IRON CONTENT MAY BE TOO LOW). SOME LIMONITE DEPOSITS IN FRANCE HAVE BEEN MINED SUCCESSFULLY.
RATHER RARE IS "SIDERITE", THE IRON CARBONATE, WHICH HAS BEEN SUCCESSFULLY MINED FOR ITS IRON CONTENT. IT PROBABLY HAS A DISTINCT SPECTRAL SIGNATURE, BUT AGAIN ONE HAS TO USE HYPERSPECTRAL REMOTE SENSING TO GET THIS SIGNATURE. IN GENERAL, IRON ORE PROSPECTING USING HYPERSPECTRAL R.S. IS THE BEST WAY TO CONDUCT AN EXPLORATION EFFORT.
The search for iron deposits continues to be a major effort in mineral exploration. New ore bodies can be found using remote sensing as well as more conventional methodology. Or, they can be found by plain "dumb luck". The Carajas deposit in the jungle of the Amazon in Brazil was discovered accidentally in 1967 when a team of exploration geologists noticed red earth at the site where their helicopter set down to refuel. The size of the deposit, which contains not only iron but recoverable gold, manganese, copper, and nickel, is estimated to be about 18 billion tons of iron. In 2007, 296 million tons were extracted. The mine is an open pit, which contrasts strongly with the surrounding jungle. Here is a detailed image, made by the Advanced Land Imager on NASA's EO-1. Below it is an image of a larger area (the mine is at the upper left), which shows that the land to the east has since 1967 been systematically deforested to convert to agriculture. This deposit would not have likely been found by remote sensing that sought visual signs of the ore body, largely because of the tree cover. There is an indication of red earth in clearings in the forest (one is an airstrip) and in the farmland north of the mine. But, an airborne magnetometer survey might have picked up an anomaly. Lets further illustrate the subject of iron prospecting through use of remote sensing by extracting several images from this USGS Spectral Lab web site. Multispectral (using Landsat-7 ETM) and hyperspectral (using AVIRIS; see page 13-5) sensors were trained on various mineral districts in Utah; see the site for details. Several different kinds of iron-rich minerals and clay minerals were detected in the resulting imagery. The iron minerals have distinctive spectral signatures:
Some of the iron-rich minerals are alteration products, such as Jarosite. The tourist attraction "Big Rock Candy Mountain" is made up entirely of this mineral: An AVIRIS map of the Dragon Mine area, Main Tintic, shows red = Jarosite; Green = Goethite; Yellow = Jarosite + Goethite; Light and Dark Blues = Hematite.
Landsat-7 ETM+ data were processed to produce this mineral map of altered rocks near Marysvale, Utah
The key to the deposits: Red = Hematite and similar Fe+3minerals; Yellow = Fe3 plus certain alteration products; Green = clays, sulphates, micas; Blue = dry vegetation; Cyan = moist vegetation; white border = argillic alteration; CI = central monzonite intrusion.
5-2: Would you expect there to be some kind(s) of surface manifestations of petroleum reservoirs; if so, what? ANSWER As we proceed to more case studies, if after these you are interested, then try this website: Remote Sensing for Exploration Geology, or google to some other titles on the Internet.
