{"thcode":2,"term":{"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."},"uf":[],"bt":[{"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."},{"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."},{"code":734,"name":"methods","parent":1,"scope":"Techniques, methods, procedures, or strategies for research, management, collection, or analysis of scientific information in USGS."},{"code":1,"name":"Categories","parent":null,"scope":"Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."}],"nt":[{"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."},{"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)."},{"code":2084,"name":"ground penetrating radar","parent":326,"scope":"Electromagnetic imaging instrument using radar to investigate shallow subsurface structure."},{"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."}],"rt":[{"code":18,"name":"aeromagnetic surveying","parent":981,"scope":"Mapping of the earth's magnetic field with the use of airborne electronic magnetometers."},{"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."},{"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."}]}