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The Potential of Radar in Geological Exploration
At the present time side-looking air-borne radars (SLAR) are most useful to the geologist for regional studies of remote, previously-unmapped areas. Imaging radar systems may ultimately be most valuable for conducting regional investigations, but areas requiring detailed investigation can benefit from SLAR's unique ability to present certain types of data. Regardless of the nature of the study, dual-polarization imagery has proven the most effective in geologic investigation.
In regional studies, radar should be the primary, and in some instances may be the sole sensor. The value is best demonstrated in areas where photographic coverage is not possible (or feasible) and where ground investigations are difficult or impossible. Gross structural patterns are well-displayed on SLAR imagery because of the radar's capability to continuously image wide swaths of terrain. Such a use takes maximum advantage of the relative low resolution, which suppresses distracting and redundant detail. SLAR is especially valuable in the detection of regional-scale lineaments, and has demonstrated itself superior in most instances to aerial photography for such studies.
In detailed studies, the significant contribution of radar lies primarily in the tection of subtle changes of lithology. In some instances lithologic changes appear to be directly responsible for isolating anomalous areas. In other cases, the delineation of rock type may be achieved indirectly from radar imagery through examination of fracture textures, patterns of weathering, topography and vegetation. The full geologic significance of SLAR systems has not yet been demonstrated; however, it is anticipated that the value of radar to the geologist will be greatly enhanced when improved multi-band and dual-polarizing systems can be utiliezd to obtain geologic data. The value of long-wavelength imagery will be realized only when it is produced simultaneously with X- or K-band imagery.
Unfortunately, the geological community (along with others, we fear) has been subjected to a variety of baseless or unqualified claims regarding the value of radar with the result that among many geologists a degree of skepticism regarding the true value of radar has developed. Bold statements without a frame of reference, claiming that "radar is good for . . .," making unrealistic comparisons with displays of other sensors, and presenting results based on incomplete, unscientific or naive analysis of imagery—all have contributed to the aura of skepticism which surrounds radar in the minds of many in the scientific community.
In fairness to all investigators it should be noted that very little radar imagery has been available for geologic analysis, and that most has gone to a limited number of researchers. In the past three years, dual-polarization, K-band imagery has become available for approximately 320,000 square kilometers of the United States, and considerably less coverage is available from X-band radars. The availability for geologic evaluation of imagery from other than Kb-and and X-band systems is even more limited; however, our preliminary analysis indicates that we might expect dual-polarization, multi-band radars to be of great significance to the geologist. In spite of the limited amount and types of imagery available, it is apparent that radar can make a unique data contribution in geologic synthesis. Although it is becoming increasingly evident that imaging radar systems will ultimately be most valuable in conducting regional geologic investigations, radar's value has also been documented in areas requiring detailed investigation. In general, the unique contribution of radar is most likely to be at a maximum (1) in remote and poorly-mapped areas of the world, (2) where a regional rather than a detailed picture is desired, and (3) where climatic conditions are adverse to ground and aerial photographic investigation.
Our experience has indicated that in regional geologic studies and geological reconnaissance surveys, radar should be one of the primary sensors of any multi-sensor combination. In such investigations one can expect truly unique contributions of geologic data from radar imagery. Radar presents the following advantages: (1) Side-scanning systems (developed for military reconnaissance) can obtain imagery totally independent of daylight conditions or most weather conditions, and as a result, in some surveys radar may be the sole sensor which can successfully gather terrain data. (2) From radar imagery of one currently operational system, a topographic presentation can be prepared when it is not feasible or possible to collect aerial photography for mapping purposes. (3) Because of the broad areal coverage of side-scan systems, gross structural and topographic relationships are presented in such a fashion that individual and seemingly independent features can be integrated into a regional format. (4) In most terrain environments, radar is the best sensor for detection, integration, and analysis of lineaments. Realistically, one must remember that these generalizations are based on the results of analysis of primarily K-band imagery, a minimum amount of X-band imagery and a very limited amount of imagery from longer wave length systems.
In detailed studies, the most significant contribution of radar lies primarily in the detection of subtle changes of lithology. This capability has been demonstrated in a number of areas in which both ground investigations and aerial photographic studies have been conducted. In some instances contrasts in lithologic composition appear to be directly responsible for isolating anomalous areas on the radar imagery. In other cases, lithologic composition has determined the patterns of weathering and soil formation which are reflected as expressions of topography or vegetation. Thus lithologic compositions may have an indirect influence on the character of the return signal. Similarly, sytematic analysis of fracture-texture patterns which are so well defined on radar imagery should facilitate separation of rock types and the identification of anomalous areas. Side-scan radar has also proven valuable in several geologic investigations of previously mapped areas where lineaments were detected for the first time on radar imagery.
Two parameters (polarization and wavelength) of airborne radar systems appear to have greater significance in detailed geologic studies than in regional studies. In the traditional side-scan configuration, the electric vector is radiated and received horizontally, and thus receives the designation like-polarized (HH). Independent of the transmitted polarization, the return signal can be completely depolarized, and with one K-band system this signal (cross-polarized, VH) is displayed simultaneously, on adjacent film strips, with the like-polarized imagery. Differences between simultaneous like-and cross-polarized images are obviously the resultant of terrain features which are capable of effectively depolarizing incident signals. In studies which have been conducted where both the polarized and depolarized components are available for analysis, the depolarized component has made a significant contribution in lithologic differentiation. In at least one regional structural study, major lineaments have been detected only in the depolarized image.
The evaluation of multiband imagery on an extremely limited scale indicates that longer-wavelength systems should add a new dimension to side-scan radar as a tool for geologic investigation. Some of our preliminary studies indicate that when utilizing long-wavelength radar systems (such as L-and P-bands), some penetration of at least dry mantle can be expected, some vegetation penetration will occur, and indirect evidence of soil moisture content will be revealed. The full value of long-wavelength imaging systems will be realized, however, only when they produce imagery which is collected simultaneously with X-or K-band imagery.
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