USGS Groundwater Information: Branch of Geophysics
Current trends in relation to hydrogeophysics and water science focus on three main areas: properties, processes, and scale.
This is an excerpt from the U.S. Geological Survey Water Science Strategy – Observing, Understanding, Predicting, and Delivering Water Science to the Nation, released April 2013.
In the past, geophysical characterization provided direct insight into geologic structure and properties but only indirect insight into hydrologic properties. Today (2012), there is a trend toward direct measurement of hydrologic parameters by use of geophysical tools. The prime example of this trend is the emerging interest in nuclear magnetic resonance (NMR) methods (for example, magnetic resonance sounding and borehole NMR; see fig. 10). At this time, the USGS is an "early adopter" of these methods, which involve use of a surface unit and a borehole system. These methods provide estimates of saturation, total porosity, and permeability. In the case of the borehole tool, one can also extract mobile/immobile pore fractions important for understanding and modeling hydrologic behavior (for example, rate-limited mass transfer).
Application of geophysical methods in time-lapse mode to monitor hydrologic processes is a productive area at the intersection of geophysics and water science. Permanent resistivity arrays are being used to monitor engineered remediation (for example, biostimulation) in 4-D, and resistivity/induced polarization (IP) and distributed temperature systems are being used to monitor tracer tests, natural processes (for example, hyporheic exchange, groundwater discharge to coastal bays and estuaries, unsaturated zone dynamics), and aquifer-management operations (for example, aquifer storage and recovery).
There is a trend toward multiscale characterization and monitoring, facilitated by the advent of mobile measurement platforms. Towed seismic land streamers are used for multichannel analysis of surface waves, refraction, and even some reflection surveys, and towed multifrequency electromagnetic and shallow electrical resistance tomography/ground penetrating radar systems provide broad-scale geoelectrical information at the study scale (for more than depths of about 20-30 meters). The transfer to civilian Federal agencies of military unmanned aviation technology developed since 9/11 has the potential to revolutionize our ability to acquire multisensor data in support of a wide range of science needs. It is expected that the application of unmanned aerial vehicle systems will soon have a transformative effect on USGS science.
Figure 10. Nuclear magnetic resonance (NMR), gamma, and electromagnetic conductivity (EM) logs from a borehole completed in sand and gravel on Cape Cod, Massachusetts (Johnson and others, 2011). The NMR data, which measure how hydrogen nuclei spin in response to a magnetic field and can determine how much and where water is stored in porous media, indicate decreased total water and increased proportion of bound water in silt zones identified by the gamma and electromagnetic induction conductivity (EMI) traces and drilling logs, as indicated by the shaded boxes.
Evenson, E.J., Orndorff, R.C., Blome, C.D., Böhlke, J.K., Hershberger, P.K., Langenheim, V.E., McCabe, G.J., Morlock, S.E., Reeves, H.W., Verdin, J.P., Weyers, H.S., and Wood, T.M., 2013, U.S. Geological Survey water science strategy—Observing, understanding, predicting, and delivering water science to the Nation: U.S. Geological Survey Circular 1383–G, 49 p.
Johnson, C.D., Lane, J.W., Jr., Walsh, David, and LeBlanc, D.R., 2011, Comparison of nuclear magnetic resonance (NMR) Logs in wells completed in glacial sediments in the northeastern United States, [abs.], in GSA Annual Meeting, 9-12 October 2011, Minneapolis, Minnesota, Proceedings: Geological Society of America, Boulder, Colorado.