ANSWERS¶
ANSWERS¶
` <>`__5-1: The answer is clearly “core drilling” whether it be to find that a sufficient quality (grade) of useful minerals exists below the surface or that oil or gas is present in recoverable quantities. Core is analyzed to assess the percentage of minerals in the rock. A series of holes are drilled to block out the three-dimensional extent of the ore deposit or the petroleum field. As we shall see, this comes long after the first indications of possible mineral or oil concentration that in many instances (but increasingly less frequently) have some kind of surface expression. Remote sensing is normally an early phase in the exploration sequence shown in the diagram. **BACK**
` <>`__5-2: Most oil and gas deposits are deeply buried, so evidence is likely only for those deposits within a few thousand feet or less of the present surface. While the petroleum fluids and the gas can migrate upward and escape to the surface, this is likely only when they are leaking from their reservoirs. Gases can be carried by groundwater and may be detected at springs, etc. In ascending through fractures, oil may lose volatile constituents, especially when it reaches the air, and be deposited as tar. In certain circumstances, escaping hydrocarbons - both oil and gas - can chemically modify near-surface rocks, creating gossan-like effects, that are detectable in air or space images. This is generally a rare occurrence; remote sensing is generally more valuable as a means of portraying regional structures, some of which may be favorable for oil/gas concentrations at depth. **BACK**
` <>`__5-3: If you follow the direction of elongated alteration east past White Mountain, there are several small buffy-orange patches that are part of the same gossan pattern. When visited on the ground, they did not show indications of potential ore deposits but they were never drilled. **BACK**
` <>`__5-4: There are three distinct alteration colors. A whitish patch of kaolinitic clay alteration lies near the center of the photo, and a second smaller one to its right. On the left center is a reddish-orange area of hematitic alteration. Near the upper right is a yellowish zone of alunitic alteration. In the background, near the photo top, is a dark gray representing unaltered volcanic rock. **BACK**
` <>`__5-5: The boundaries between alteration types, and between them and volcanic rocks, is not always sharp - is in fact somewhat gradational. When this area was mapped on the ground (aided with photos), the geologists choice of where to put the precise boundaries was somewhat arbitrary and indefinite. They probably relied on fresh rock samples, dug out with their hammers, to decide on which which mineral type was present. But, thin surface discolorations can blur or obscure the boundaries, as the same stains can be developed in more than one type, or loose float can be washed from one type onto another. Thus, what a map shows depends to some extent on subsurface sampling but the photo and image show only what is exactly at the surface. `BACK <Sect5_2.html#5-5>`__
` <>`__5-6: I’d select the ratio image 7/5 because it is such a distinctive pattern that can be followed well during reconnaissance on the ground. It tends to emphasize a dominant alteration pattern at White Mountain, namely, where clays and alunite have concentrated. That zone marks the most intense alteration but if one is looking for possible ore deposits then 3/1 may be more helpful since it brings out the hematitic alteration (in effect, a variant of gossan) that may more closely associate with some mineralization of value. **BACK**
` <>`__5-7: Take ‘em both. Each tells you something. But the one on the left seems to better highlight the primary alteration zone. However, it appears to underrepresent the hematitic alteration. **BACK**
` <>`__5-8: The first TM PCA does a fine job of encompassing the general alteration zone. The choice of colors within that zone has made separation of clay-alunite from hematite difficult. The second TM PCA is intriguing because of the black zone pattern. Keep in mind that the patterns in the higher numbered PCA (such as the fifth component) are more likely statistical artifacts than indicators of real features on the ground (this subject of correlation of patterns with reality has not been effectively examined, in the opinion of this writer). Probably the Bendix PCA is the best of the three in defining verifiable ground alteration and rock type discrimination and location; the bands used were narrower in spectral interval (width) and that may give these features better definition. All three PCAs tend to credibly separate the andesite from the basalt volcanics. **BACK**
` <>`__5-9: The differences between images are in part the result of different wave bands in the two cases and in part because of the way in which the classes were chosen. For example, in the Idrisi classification, the alluvium was subdivided into three categories. Likewise, the volcanics were subdivided differently; in the Bendix classification, a somewhat artificial class, vegetated volcanics, was selected at the time based largely on deciding that the sagebrush, cedars, and other vegetation varied notably in the aerial photo. The manner of setting up the alteration classes differs: in the Bendix classification, an attempt was made to try to establish and locate the same classes as in the map whereas the Idrisi classification consisted of a broader specification of alteration types. Also, training sites differed; experience has shown that the choice of precise site location does influence the classification outcome. **BACK**
` <>`__5-10: While the answer to this question reflects my personal opinion (or bias), I think this 4 person multi-linear map in which so few were consistently found by all is as much a consequence of the pyschology of picking linear features as it might have been the scientific application of geologic expertise. The human eye-mind approach to drawing lines wherever there seems to be some straight discontinuity has a large subjective element. Improper criteria (relying on choosing anything that appears linear without regard to its possible geologic significance), plus the subconscious desire to prove the value of space imagery by producing maps with myriads of lines proposed as true geologic structural features, led to “overkill” in the selection and reporting of linear features. In the first few years of Landsat, many papers were presented showing these features, accompanied by claims that they were valid geologic entities, but almost none included confirming evidence from field checks. The same features seldom turned up in aerial photos. This epidemic of fracture finds, especially in flatlands, subjected some Landsat users to ridicule by skeptics. In time, good science took over. The study of fractures in the Wyoming Wind River Mountains by Ed Decker, the Adirondacks by Yngvar Isachsen, and the Canadian Shield by Lowman, Short et al. proved that proper analysis could disclose genuine structural lineaments. One geologist in particular showed that Landsat lineaments analysis could pay off: by analyzing fracture patterns over several oil fields in Kansas, he noted that the oil-bearing structures were actually extended beyond the producing areas and his company then drilled 10 successful wells out of 10 tries. **BACK**
` <>`__5-11: Eason Oil (working with the Earth Satellite Corp.) did the right thing in their approach. They started with a known, well-documented oil and gas-producing region and tried to relate the observations they made using Landsat to specific fields within the Anadarko Basin. They found some correlations and also believed they had discovered some new criteria. Their hazy features, however, did not stand the test of field confirmation. These hazies more often than not did not coincide with known fields and no one drilled into the hazies to see if oil/gas was really below. Instead, most proved to be phenomena related to wind and other weather action. Over time, various oil companies tried Landsat, radar data, etc. and concluded that space imagery is valuable. What they sought was information usually at the regional level that suggested further ground mapping and geophysical surveying. This information included better definition of structures, geomorphic anomalies that were influenced by subsurface conditions favorable to petroleum, and at times linear features that were genuine fractures. Rarely has surface alteration been detected as a guide to escaping oil/gas. The prime value of space imagery remains as an early stage reconnaissance aid and, since that phase is quite costly, any such tool that offers improved means for deciding where to concentrate the exploration effort is a big plus. Ultimately, today oil and gas are found most frequently by a geophysical survey followed by exploratory drilling. **BACK**