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Gold Prospecting Methods -Photogeology
Photogeology:The use of aerial photographs to obtain both qualitative and quantitative geologic information is referred to as Photogeology. Aerial photographs are widely used today for identifying and mapping landforms, drainage patterns, structural features such as faults and folds, and rock or lithologic Units. Knowledge of the surface attributes of a landscape also enables a geologist to infer or predict subsurface characteristics and relationships. Air photos are routinely scanned for the following types of geologic studies:
- compiling topographic and geologic maps,
- exploring for mineral, hydrocarbon, and groundwater deposits,
- identifying hazardous features or sites such as active earthquake faults and areas prone to landslides
- identifying and mapping landscape changes caused by a natural hazard event such as a hurricane or earthquake,
- Selecting potential construction sites for critically engineered facilities such as dams or harbours.
The geologic interpretation of aerial photographs is based on the fundamental recognition elements that include shape, size, pattern, shadow, tone or colour, texture, association, and site. The quantity and quality of geologic information that can be interpreted from aerial photographs is dependent upon the training and experience of the Interpreter in geology and remote sensing.
The three-dimensional view of the terrain reveals important topographic information that cannot be obtained by viewing single photographs.
Although oblique air photos are often of value to the photogeologist, most studies make use of vertical air photos.
Most of the photographs used in Photogeology are acquired under relatively high solar illumination angles to ensure that ground detail is not hidden by cast shadows. When there is a need to enhance surface irregularities, special purpose photographs are acquired with low sun angle illumination. Whenever feasible, both high and low sun angle photographs are o btained for a given study area.
Medium to large scale air photos are best suited to the detailed study of a localized area, whereas small scale air photos, such as those obtained from earth orbit; find their greatest utility for regional surveys.
In Photogeology, the convergence of information principle is often employed, whereby an interpreter starts with small-scale photographs for a synoptic view and gradually focuses upon a local or target area by interpreting successively larger scale photographs
Regardless of scale, however, it is important to remember that ground observations (i.e., field geology) can't be replaced by Photogeology
Fig.4.1 Air photo interpretation of regolith-covered greenstone belt in central Brazil, original scale 1:60,000.
Insight into landscape evolution leads to more effective geochemical exploration, e.g.:
- what to sample and how to sample
- significance of anomalies
- location of the anomaly source
Accurate mapping of regolith units in the necessary detail requires the stereoscopic capability and fine resolution of air photos. Features mappable from air photos:
- exposed and concealed duricrusts - residual, transported, composite - relative ages
- duricrust debris and talus deposits
- outcrop, subcrop, bedrock or saprolite - usually subdivided as to general lithology
- cover rocks, young volcanics, veins and dykes
- active and inactive alluvium, terrace gravels, fan systems
- colluvium - proximal, distal, sheetwash, cemented
- wind-blown sand, fossil dunes
- detailed drainage, drainage changes, palaeodrainage interpretation
- weathering anomalies indicative of structure, lithology or alteration
Interpretation of structure and lithology is normally carried out at the same time.
For larger blocks, the synthesis of geomorphic and structural observations can reveal controls on erosion history, drainage changes and other factors affecting the concentration, secondary enrichment or dispersal of potential mineral deposits.
