Remote sensing in geology

Remote sensing is used in the geological sciences as a data acquisition method complementary to field observation, because it allows mapping of geological characteristics of regions without physical contact with the areas being explored. About one-fourth of the Earth's total surface area is exposed land where information is ready to be extracted from detailed earth observation via remote sensing. Remote sensing is conducted via detection of electromagnetic radiation by sensors. The radiation can be naturally sourced (passive remote sensing), or produced by machines (active remote sensing) and reflected off of the Earth surface. The electromagnetic radiation acts as an information carrier for two main variables. First, the intensities of reflectance at different wavelengths are detected, and plotted on a spectral reflectance curve. This spectral fingerprint is governed by the physio-chemical properties of the surface of the target object and therefore helps mineral identification and hence geological mapping, for example by hyperspectral imaging. Second, the two-way travel time of radiation from and back to the sensor can calculate the distance in active remote sensing systems, for example, Interferometric synthetic-aperture radar. This helps geomorphological studies of ground motion, and thus can illuminate deformations associated with landslides, earthquakes, etc. Remote sensing data can help studies involving geological mapping, geological hazards and economic geology (i.e., exploration for minerals, petroleum, etc.). These geological studies commonly employ a multitude of tools classified according to short to long wavelengths of the electromagnetic radiation which various instruments are sensitive to. Shorter wavelengths are generally useful for site characterization up to mineralogical scale, while longer wavelengths reveal larger scale surface information, e.g. regional thermal anomalies, surface roughness, etc. Such techniques are particularly beneficial for exploration of inaccessible areas, and planets other than Earth. Remote sensing of proxies for geology, such as soils and vegetation that preferentially grows above different types of rocks, can also help infer the underlying geological patterns. Remote sensing data is often visualized using Geographical Information System (GIS) tools. Such tools permit a range of quantitative analyses, such as using different wavelengths of collected data sets in various Red-Green-Blue configurations to produce false color imagery to reveal key features. Thus, image processing is an important step to decipher parameters from the collected image and to extract information.

Background

In remote sensing, the electromagnetic radiation acts as the information carrier, with a distance of tens to thousands of kilometers distance between the sensor and the target. Proximal Sensing is a similar idea but often refer to laboratory and field measurements, instead of images showing a large spatial extent. Geophysical methods, for instance Sonar and acoustic methods, shares similar properties with remote sensing but electromagnetic wave is not the sole medium. Geotechnical instrumentations, for example piezometer, tiltmeter and Global Positioning System (GPS), on the other hand, often refer to instruments installed to measure discrete point data, compared to imagery in remote sensing. A suitable sensor sensitive to the particular wavelength region, according to the designated use, is selected and employed to collect the electromagnetic wave reflected or emitted from the target object.

Working principles

In remote sensing, two main variables are measured in a typical remote sensing system: the radiance (or intensity) and time of arrival for active systems. The radiance (i.e. returning signal intensity) versus wavelength is plotted to a spectral reflectance curve. As a point to note, the data collected is a blend of both reflection of solar radiation and emission (according to Planck's law) from the object for visible and near infrared (VNIR) region. The thermal infrared (TIR) region measures mainly emission while microwave region record backscattering portion of reflection. The radiance is determined by radiation-matter interactions, which is governed by the physio-chemical properties of the target object. Prominent absorptions in specific wavelength shown on the spectral reflectance curve are the fingerprints for identification using spectroradiometry. The two-way travel time of the radiation could infer the distance as the speed is approximately equal to the speed of light, roughly 3 x 10^8 m/s. This allows application in ranging in light detection and ranging (LiDAR) and Radio detection and ranging (Radar) etc. Since the sensors are looking through the atmosphere to reach the target, there are atmospheric absorption. Three main atmospheric windows, which allow penetration of radiation, can be identified. They are 0.4–3 micro-meters (Visible and Near-Infrared (VNIR)), 3–14 micro-meters (Thermal Infrared TIR) and few millimeters to meters (microwave). Camera in everyday life is a passive imaging system in VNIR wavelength region. A simple classification of prevailing remote sensing instruments in geology, modified from Rees (2013) in accordance with context of this page. Text in [ ] refers to the related instruments.

Carrying platform The sensor could be spaceborne (carried by satellite), airborne (carried by aircraft, or most recently Unmanned Aerial Vehicle (UAV)) or ground-based (sometimes called proximal sensing). Data acquired from higher elevation captures a larger field of view/ spatial coverage, but the resolutions are often lower. Prior mission planning regarding flight path, weight load, carrying sensor etc. have to be done before deployment. The resolution requirement is often high in geological studies, hence airborne and ground-based systems prevail in surveying.

Advantages and limitations

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