Mapping Techniques (World of Earth Science)
Geological maps portray the distribution of different rock types, the location of faults, shear zones and folds, and the orientation of primary and structural features. Mines, quarries, mineral occurrences, fossil localities, geochronological sampling sites, oil and water wells may also be shown. Geological maps illustrate rock relationships that enable the depositional, intrusive, and structural history of an area to be established, and the three-dimensional geometry to be visualized. They provide fundamental information for mineral and petroleum exploration and for hydrological and environmental investigations.
In small areas such as exploration tenements where detailed maps are required, it is common practice to undertake grid mapping. After a grid is surveyed and pegged, the geologist carries out detailed traverses along grid lines. Rock types, lithological contacts, and alteration are noted and structural measurements made using a compass-clinometer. Information may be recorded by hand on traverse maps or collected digitally. A complete map is compiled by interpolating between gridlines, collecting additional data where necessary. Aerial photographs, satellite or other remotely sensed images (as discussed below) serve as the base for recording regional map data. Digital data recorders integrated with GPS (global positioning system) location measurements enable lithological and structural data to also be digitally recorded in the field. Data can be directly input into a GIS (geographic information system) or custom computer package that enables different attributes to be displayed on a map and spatially analyzed.
A three dimensional, exaggerated view of the landscape is created when pairs of overlapping aerial photographs are viewed through a stereoscope. Stereographic views of aerial photographs assist in the identification and classification of landforms, the interpretation of rock types based on characteristic outcrop or weathering patterns, and the recognition of tectonic and intrusive structures. Areas where rock formations crop out can also be identified. Where aerial photographs record data in the visible (and sometimes infrared) parts of the electromagnetic spectrum, earth-sensing satellites collect data for several different wavelengths or bands. Some bands or ratios of bands highlight vegetation, whereas others respond to differences in water content of soils or different rock types. Individual bands, or ratio of bands are assigned to red, green or blue channels of image processing systems to produce false color images. There has been a marked increase in the resolution of commercially available satellite imagery from 262 ft (80 m) in early Landsat imagery to 2.3 ft (0.70 m) (panchromatic) and 9.2 ft (2.8 m) (multispectral) with the Quickbird-2 satellite. Several other satellites have resolutions of approximately 3.3 ft (1 m) (panchromatic) and between 6.6 and 16.4 ft (2 m) (multispectral). Whereas standard satellite imagery uses approximately seven bands, in hyper spectral remote sensing, data is collected simultaneously in over 200 narrow, contiguous spectral bands from sensors in high-flying aircraft or satellites. For example, NASA's AVIRIS (Airborne Visible/InfraRed Imaging Spectrometer) maps a strip 6.8 mi (11 km) wide with a ground resolution of approximately 21.8 yd (20 m). Hyperspectral data can be calibrated to distinguish different rock types. Bands that correspond to specific wavelengths at which minerals reflect or absorb energy are used to map the distribution of individual minerals, including clays and alteration minerals developed around mineral deposits. Side-looking airborne radar (SLAR) on aircraft or satellites transmits microwave energy and records the energy obliquely reflected from the ground. Radar imagery is useful in mapping areas covered by cloud that obscures normal satellite sensors and in structural mapping, especially in highly vegetated areas. Radar imagery currently has a resolution of 26.2 ft (8 m) or more, although 9.8 ft (3 m) resolution imagery will soon be available.
In contrast to the above techniques that record portions of the electromagnetic spectrum, magnetic and radiometric methods record differences in rock composition. The magnetic signature of a rock is due to the amount of magnetic minerals such as magnetite it contains. Some rock types (e.g., mafic volcanics and banded iron formations) have a high magnetite content and create magnetic highs. Other rocks low in magnetite (such as quartzite and shale) produce magnetic lows. Faults may be imaged due to magnetite destruction as a result of weathering or alteration during fluid flow along them. Local magnetic highs may indicate addition of magnetic minerals during mineralization and so provide exploration targets. Linear, crosscutting magnetic highs commonly represent mafic dikes. Aeromagnetic data is obtained by flying low altitude, closely spaced parallel paths with an aeroplane or helicopter mounted with (or trailing) a magnetic sensor. Ground magnetic data is recorded using sensors on a tall pole, either handheld or vehicle mounted. Data values are interpolated between recorded measurements. Raw magnetic data is generally first processed to appear as if the inducing magnetic field had a 90° inclination (a process called reduction to the pole). This simplifies magnetic anomalies and centers anomalies over the causative rock body. The distribution of different rock types, position of contacts, and form of folds and other structures can be interpreted from contoured or digitally processed magnetic images. Digital enhancements (such as artificial sun angles and vertical derivatives) are used to highlight faults, shear zones, and/or lithological contacts. Aeromagnetic data allows geological maps to be made in areas of no outcrop, even below lakes or superficial cover.
Airborne gamma ray spectrometry is a technique that measures variations in the potassium, thorium, and uranium content of rocks using sodium iodide crystal detectors mounted in aircraft (radiometric surveys are often carried out at the same time as aeromagnetic surveys). Radiometric data is presented as either individual images portraying the relative amount of each element or images of various element ratios. Radiometric images are useful in mapping compositional variations in granitic and high-grade metamorphic rocks (especially in areas with little or no transported sedimentary cover), rock types such as carbonatite that have unusual amounts or proportions of potassium, thorium and uranium, and alteration zones around mineralized areas. They can also be used to map the distribution of sediments derived by the weathering of granitic and other radiogenic source rocks.
While computer-enhanced images of remotely sensed data are increasingly used in geological interpretation, detailed fieldwork by geologists still provides the backbone for the creation of geological maps.
See also Bathymetric mapping; Cartography; Geologic map; Petroleum, detection; Physical geography; RADAR and SONAR; Remote sensing