<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="326" name="electromagnetic surveying" parent="375" scope="Method of determining the shape and constituency of near-surface earth structures by variations of responses of the materials to electrical and magnetic fields. Use for ground-based methods of imaging."/><bt><term thcode="2" code="375" name="field inventory and monitoring" parent="376" scope="Repeated observation or sampling at a site, on a scheduled or event basis, for study and analysis.  In general, this category excludes sampling programs in which materials are obtained in the field and brought back to a laboratory for study and analysis."/><term thcode="2" code="376" name="field methods" parent="734" scope="Research procedures and instrumental means to measure, collect data and samples, and observe in the natural areas where the materials, phenomena, structures, or species being studied occur."/><term thcode="2" code="734" name="methods" parent="1" scope="Techniques, methods, procedures, or strategies for research, management, collection, or analysis of scientific information in USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="2086" name="continuous resistivity profiling" parent="326" scope="Ship-based method of analyzing apparent resistivity of the sub-bottom (for example, spatial variation in pore-water salinity) by measuring the induced electromagnetic response to a current generated by the instruments."/><term thcode="2" code="2087" name="electrical resistivity imaging" parent="326" scope="Land-based resistivity survey where current is applied to the ground using an array of electrodes and apparent resistivity of the subsurface is measured, giving estimates of the spatial variability of resistivity (for example, variations in pore water salinity)."/><term thcode="2" code="2084" name="ground penetrating radar" parent="326" scope="Electromagnetic imaging instrument using radar to investigate shallow subsurface structure."/><term thcode="2" code="2085" name="magnetotelluric surveying" parent="326" scope="Passive geophysical technique that uses the earth's natural electromagnetic field to investigate the spatial variation of electrical resistivity in the subsurface."/></nt><rt><term thcode="2" code="18" name="aeromagnetic surveying" parent="981" scope="Mapping of the earth's magnetic field with the use of airborne electronic magnetometers."/><term thcode="2" code="325" name="electrical resistivity logging" parent="125" scope="Recordings made to identify the composition of subterranean materials at different depths in a borehole by measuring their ability to resist the flow of an electric current."/><term thcode="2" code="2298" name="inverse modeling" parent="713" scope="An iterative approach to estimate the form and characteristics of a physical system explained by observations or measurements. Frequently used to determine a subsurface geological structure from measurements of physical properties obtained at or near the earth's surface."/></rt></descriptor>