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Landscapes on the Edge: New Horizons for Research on Earth’s Surface APPENDIX C Observing and Measuring Earth Surface Processes Scientific research is based on the collection of reliable and accurate data and observations from which interpretations, models, and concepts can be generated and tested. Some of the conceptual advances, opportunities, and challenges in Earth surface processes research highlighted in this report have been made possible by the development, particularly within the last 10 to 15 years, of a number of analytical tools to observe and measure features or activities at the Earth’s surface. This appendix reviews prominent tools, instruments, and techniques used in Earth surface processes research as a basic reference; many of these tools were highlighted by community experts during the committee’s open meetings and by respondents to the committee’s community questionnaire. The tools here are grouped very generally into those that address (1) measurement and visualization of Earth’s surface including topography; (2) timing or age of landforms or Earth surface processes; (3) composition (chemical or biological) of Earth’s surface features; and (4) mass flux or physical properties of the landscape. C.1 REMOTE SENSING Widespread availability of digital data of Earth’s topography and other surface attributes, collected predominantly by satellites and aircraft, allows study of vast regions, comparison of different parts of Earth’s surface, monitoring of Earth surface hazards in real time, and quantification of properties such as terrain dissection, vegetation type and height, ground moisture content, and soil mineralogy. These data can be viewed in the form of imagery that has become familiar to many through various Internet resources. The combination of Earth imagery and accurate topographic data is particularly powerful. Widely used tools include the following: Light detection and ranging (lidar) uses a laser beam mounted on an aircraft, satellite, truck, ship, or hand-held device to measure accurate distances to a target
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface point on land or under-water that is accurately located in a geographic coordinate system. The data can be used to produce a digital surface model (DSM), which can then be processed to yield digital elevation models (DEMs), digital terrain models, contours, and three-dimensional feature data. Interferometric synthetic aperture radar (InSAR) uses radar mounted on an airplane or satellite to measure the strength and round-trip travel time of a microwave signal to gather elevation data; these data can be used to produce DSMs and DEMs, among other products, as with lidar. High-resolution swath bathymetry uses a transducer mounted on a ship to measure the round-trip travel time of sonar waves emitted in the water below the ship. The time elapsed between emission and reception of the signals allows the depth to the seafloor and other features to be determined. Sensors mounted on satellite platforms measure various bands of the electromagnetic spectrum to produce images of land and water surfaces at wavelengths appropriate to determine vegetation type, general chemical composition, temperature, water content, and other aspects of a given land or water surface. Data from federally, commercially, or internationally owned and operated satellites are available either freely online or for purchase. The reader is referred to NRC (2008) for information on a variety of such satellite missions. Global positioning system (GPS) technology measures a highly accurate three-dimensional position on Earth’s surface at a precise time relative to known positions of several satellites. Information can be used to determine rates of tectonic motion, including uplift rates. Differential GPS measurements have yielded detailed observations of glacier surface velocities and temporal variations in those velocities that are important in understanding the glacier dynamics. C.2 DATING TECHNIQUES AND CHRONOLOGY The issue of “timing” or “age” of landforms and surface attributes is of great interest to Earth surface processes research. Accurate age data enable the history and rates of processes to be determined—for example, dating techniques are being used to study the rates of floodplain sedimentation and soil processes, faulting histories and paleoseismic recurrence intervals, and the topographic evolution of mountain ranges. A wide range of dating techniques exists; some of the most frequently used types are listed below: Fallout-derived and short-lived isotopes are used for quantifying sediment transport, deposition, and chronology on time scales of days (for example, beryllium, lead, and cesium isotopes).
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Thermoluminescence and cosmogenic radionuclide (CRN) techniques can be used for dating terrestrial deposits that range in age from several thousand to millions of years. These techniques may include, for example, the use of beryllium, aluminum, or chlorine isotopes or optically stimulated luminescence (OSL). Low-temperature thermochronometer techniques are used to provide information on hundred thousand- to million-year rates of erosion and topographic evolution (for example, U-Th-Sm/He, fission track, and 40Ar/39Ar thermochronology techniques). Sidereal methods count annual events, such as dendrochronology (tree ring dating) and varve chronology (sediment layer dating). C.3 BIOGEOCHEMISTRY Although some biogeochemical data can be gleaned from remote-sensing applications, most biogeochemical measurements require physically sampling rock, water, or vegetation material and subsequently conducting analysis in a laboratory. Such data are therefore slow to accumulate compared to remotely sensed data and are sparser with respect to distribution over time and space. However, this sparseness is balanced by the rich nature of biogeochemical data that can provide information about more than 3,000 minerals, more than 100 chemical elements, and many chemical species and isotopes. Biogeochemical tools are used to study weathering rates and mechanisms, issues of soil fertility, interpretation of biological fluxes, contaminant geochemistry, bioavailability and mobility of elements, soil organic matter, fractions of carbon, evapotranspiration, elemental cycling, exposure age dating, interpretation of hydrology, and biotic characteristics, among others. Broadly speaking, these tools can be categorized by the type of analysis they support. Primary types of analyses are listed below, along with examples of strategies for measurement. Elemental analysis—emission and absorption spectroscopies and X-ray and photo-electron analysis can produce bulk elemental compositions, as well as spot analysis as a function of depth or position. Chemical speciation analysis—chromatography, electrochemistry, and X-ray absorption analysis can be used to identify the chemical species present in solids, liquids, gases, and biological material. Mineralogical analysis—X-ray diffraction, thermogravimetric analysis, electron and neutron diffraction, and electron microscopy can identify the mineralogical structures and particle sizes of solid phases. Textural analysis—electron and optical microscopes, surface topographic analysis, image analysis, particle size analysis, and neutron diffraction can obtain particle
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface size and grain size and shape, as well as support topographic analysis and pore size analysis. Isotopic analysis—mass spectrometry coupled with various inlet strategies can identify the isotopic composition of phases, spots, layers, individual molecules, or biota in solids, liquids, or gases. Organic carbon analysis—mass spectrometry and coulometry can identify soil organic matter, fractions of carbon, fertility of soil, and other information. Molecular biological analysis—polymerase chain reaction, metagenomics, and some mass spectrometric tools can yield data to identify genomes, DNA, RNA, enzymes, and metabolisms. Surface area analysis—Brunauer-Emmet-Teller (BET) measurements, porosimetry, and neutron diffraction can determine surface areas, particularly as they relate to transfer of elements into water. Dissolved organic matter (DOM) analysis—fluorescence spectroscopy provides information on the chemical composition (DOM) of marine water, freshwater, and wastewater. C.4 PHYSICAL PROPERTIES Obtaining quantitative physical information about Earth’s surface is possible with tools that allow measurement of the dynamics and kinematics of mass flux and storage and of material properties such as density, temperature, magnetism, porosity, and permeability. Methods used to obtain this type of information include the following: Advanced seismic methods that use two- and three-dimensional imaging with acoustic waves to resolve fossilized surface features to resolutions of 1 meter or better in the field and, up to several kilometers below the surface; Electromagnetic resistivity, tomography, and magnetometers measure electrical and/or magnetic fields to yield data on groundwater flow and subsurface geometry and structures; electrodes or magnetometers are placed at the surface or in boreholes; Rock core-logging tools are attached to a drillstem to measure downhole neutron density, natural gamma radiation, resistivity, and sonic properties of rock during drilling to yield data that indicate density, porosity, permeability, and to some degree, composition of the rock column; Ground-penetrating radar yields two- and three-dimensional subsurface images at imaging depths between about 1 and 50 meters; radar pulses are emitted and collected from units that can be hand-held or mounted on vehicles;
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Stream gauges are part of a national network coordinated by the U.S. Geological Survey and partners in 50 states to measure flow and/or water level in streams and rivers; and Miniaturized instruments and dataloggers including pressure transducers monitor, for example, flooding occurrence in floodplains or storm surges along coastlines, surface heave, and electrical conductivity. C.5 REFERENCE National Research Council (NRC). 2008. Earth Observations from Space: The First 50 Years of Scientific Achievements. Washington, D.C.: The National Academies Press.
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