Geology involves the study of landforms, structures, and the subsurface, to understand physical processes creating and modifying the earth's crust. It is most commonly understood as the exploration and exploitation of mineral and hydrocarbon resources, generally to improve the conditions and standard of living in society. Petroleum provides gas and oil for vehicle transportation, aggregate and limestone quarrying (sand and gravel) provides ingredients for concrete for paving and construction, potash mines contribute to fertilizer, coal to energy production, precious metals and gems for jewelry, diamonds for drill bits, and copper, zinc and assorted minerals for a variety of uses. Geology also includes the study of potential hazards such as volcanoes, landslides, and earth quakes, and is thus a critical factor for geotechnical studies relating to construction and engineering. Geological studies are not limited to Earth - remote sensing has been used to examine the composition and structure of other planets and moons.
Remote sensing is used as a tool to extract information about the land surface structure, composition or subsurface, but is often combined with other data sources providing complementary measurements. Multispectral data can provide information on lithology or rock composition based on spectral reflectance. Radar provides an expression of surface topography and roughness, and thus is extremely valuable, especially when integrated with another data source to provide detailed relief.
Remote sensing is not limited to direct geology applications - it is also used to support logistics, such as route planning for access into a mining area, reclamation monitoring, and generating basemaps upon which geological data can be referenced or superimposed.
Geological applications of remote sensing include the following:
Syncline structures (in Pennsylvania) on SAR imagery
Structural geology plays an important role in mineral and hydrocarbon exploration, and potential hazard identification and monitoring.
Structural mapping is the identification and characterization of structural expression. Structures include faults, folds, synclines and anticlines and lineaments. Understanding structures is the key to interpreting crustal movements that have shaped the present terrain. Structures can indicate potential locations of oil and gas reserves by characterizing both the underlying subsurface geometry of rock units and the amount of crustal deformation and stress experienced in a certain locale. Detailed examination of structure can be obtained by geophysical techniques such as seismic surveying.
Structures are also examined for clues to crustal movement and potential hazards, such as earthquakes, landslides, and volcanic activity. Identification of fault lines can facilitate land use planning by limiting construction over potentially dangerous zones of seismic activity.
Why remote sensing?
A synoptic view of regional scale is a much different perspective than point ground observations when trying to map structural elements. Remote sensing offers this perspective and allows a geologist to examine other reference ancillary data simultaneously and synergistically, such as geo-magnetic information.
Certain remote sensing devices offer unique information regarding structures, such as in the relief expression offered by radar sensors. Comparing surface expression to other geological information may also allow patterns of association to be recognized. For instance, a rock unit may be characterized by a particular radar texture which may also correlate with a high magnetic intensity or geochemical anomaly. Remote sensing is most useful in combination, or in synergy, with complementary datasets.
A benefit of side looking radar is that the illumination conditions can be controlled, and the most appropriate geometry used for type of terrain being examined. Uniform illumination conditions provided by the sun, especially at equatorial latitudes, are usually not conducive to highlighting relief features. An extra benefit of airborne SAR sensors is that acquisition missions can be customized to orient the flightline parallel to the target orientation, to maximize the illumination and shadow effect.
In areas where vegetation cover is dense, it is very difficult to detect structural features. A heavy canopy will visually blanket the underlying formation, limiting the use of optical sensors for this application. Radar however, is sensitive enough to topographic variation that it is able to discern the structural expression reflected or mirrored in the tree top canopy, and therefore the structure may be clearly defined on the radar imagery.
Structural analyses are conducted on regional scales, to provide a comprehensive look at the extent of faults, lineaments and other structural features. Geologic features are typically large (kilometre scale) and applications therefore require small-scale imagery to cover the extent of the element of interest. Aerial photos can be used in temperate areas where large-scale imagery is required, particularly to map potential geohazards (e.g. landslides).
Structural mapping applications generally are not time sensitive (other than for project deadlines!) and so a fast turnaround is not required. Unless a time series analysis of crustal deformation is being conducted, frequency of imaging is not a critical issue either. The key factor for remotely sensed data are that they provide some information on the spatial distribution and surficial relief of the structural elements. Radar is well suited to these requirements with its side-looking configuration. Imaging with shallow incidence angles enhances surficial relief and structure. Shadows can be used to help define the structure height and shape, and thus increasing the shadow effect, while shallow incidence angles may benefit structural analysis.
Canadian vs. International requirements
Requirements for remote sensing parameters of structural features are fairly constant throughout the world. Those areas of persistent cloud cover will benefit from radar imaging, while areas at very high or low latitudes can benefit from low sun angles to highlight subtle relief for optical imaging.
Case study (example): Port Coldwell, Ontario: A case for SAR integration
The structural information provided by radar complements other spatial datasets. When integrated together, SAR and spatial geological datasets provide a valuable source of geological information. In this example, radioactivity information of the area of Port Coldwell, Ontario, was provided by an airborne gamma-ray spectrometry survey, which collected potassium, thorium, and uranium readings. This data is informative, but it is difficult to put the information into perspective without the layout and recognizable characteristics of the landscape. Airborne SAR image data was also acquired of the same region. The SAR image is quite interesting in terms of micro-topography and structure, but does not provide any other geo-technical information about the terrain. These two datasets were integrated, using an IHS approach (intensity-hue- saturation to replace the conventional red-green-blue colour display). The airborne gamma-ray spectrometry data are coded as the hue and saturation information, while the SAR terrain information is coded as the intensity information. The resulting integrated image is an excellent display of structural, relief, and natural radioactivity information, allowing a geologist to have a comprehensive view of the data with only one image.
Integrated image (natural radioactivity and SAR) of Port Coldwell
Mapping geologic units consists primarily of identifying physiographic units and determining the rock lithology or coarse stratigraphy of exposed units. These units or formations are generally described by their age, lithology and thickness. Remote sensing can be used to describe lithology by the colour, weathering and erosion characteristics (whether the rock is resistant or recessive), drainage patterns, and thickness of bedding.
Unit mapping is useful in oil and mineral exploration, since these resources are often associated with specific lithologies. Structures below the ground, which may be conducive to trapping oil or hosting specific minerals, often manifest themselves on the Earth's surface. By delineating the structures and identifying the associated lithologies, geologists can identify locations that would most feasibly contain these resources, and target them for exploration. Bedrock mapping is critical to engineering, construction, and mining operations, and can play a role in land use and urban planning. Understanding the distribution and spatial relationships of the units also facilitates interpretation of the geologic history of the Earth's surface.
In terms of remote sensing, these "lithostratigraphic" units can be delineated by their spectral reflectance signatures, by the structure of the bedding planes, and by surface morphology.
Why remote sensing?
Remote sensing gives the overview required to 1) construct regional unit maps, useful for small scale analyses, and planning field traverses to sample and verify various units for detailed mapping; and 2) understand the spatial distribution and surface relationships between the units. VIR remote sensing provides the multispectral information relating to the composition of the unit, while radar can contribute textural information. Multiple data sources can also be integrated to provide a comprehensive view of the lithostratigraphy.
Stereo imagery can also facilitate delineation and identification of units by providing a three dimensional view of the local relief. Some rocks are resistant to erosion, whereas others erode easily. Identification elements such as weathering manifestations may be apparent on high or medium resolution imagery and airphotos.
Images or airphotos can be taken into the field and used as basemaps for field analysis.
Two different scales of mapping require slightly different imaging sources and parameters.
In either case, frequency of imaging is not an issue since in many cases the geological features of interest remain relatively static. Immediate turnaround is also not critical.
Canada vs. International
Requirements for this application do not differ significantly around the world. One of the biggest problems faced by both temperate and tropical countries is that dense forest covers much of the landscape. In these areas, geologists can use remote sensing to infer underlying lithology by the condition of vegetation growing above it. This concept is called "geobotany". The underlying principle is that the mineral and sedimentary constituents of the bedrock may control or influence the condition of vegetation growing above.
In reality, the topography, structure, surficial materials, and vegetation combine to facilitate geologic unit interpretation and mapping. Optimal use of remote sensing data therefore, is one that integrates different sources of image data, such as optical and radar, at a scale appropriate to the study.
Even once geological unit maps are created, they can still be presented more informatively by encompassing the textural information provided by SAR data. A basic geological unit map can be made more informative by adding textural and structural information. In this example of the Sudbury, Ontario region, an integration transform was used to merge the map data (bedrock and structural geology information, 1992) with the SAR image data. The resulting image can be used on a local or regional scale to detect structural trends within and between units. The areas common to each image are outlined in black.