Geophysics Is Not a Silver Bullet, but Worth a Shot.

Hydrogeologists have long been interested in using geophysical measurements to answer questions about the Earth. Originally used to find ore bodies or oil and gas, geophysics’ sensitivity to water was originally considered a nuisance. While looking for metallic ore using electricalinduced polarization, for instance, early geophysicists found that groundwater controlled most of the measured signal. Others later capitalized on this “nuisance” by using geophysical methods to explore for water directly, or to attempt to estimate hydro(geo)logic properties. Unfortunately, no universally reliable relations between measured geophysical properties, such as seismic velocity, and hydrologic properties of interest, like a concentration flux, exist in the field. For instance, both positive and negative correlations were found between hydraulic conductivity and electrical conductivity. So, early hydrogeophysics research capitalized on imaging spatial differences in geophysical properties to map potential hydrogeologic facies as much as the more difficult problem of attempting to estimate hydrologic properties directly. While Doll et al. (2012) note that the field of near-surface geophysics was booming in the 1980s, environmental applications of geophysics were still only about 0.1% of reported geophysical activity in 1990. From early years, some thought geophysics could be made to “see” whatever the user wanted (e.g., Blau 1936), such that the methods were in “ill repute” within hydrogeology. Fortunately, opinions are changing. The field of hydrogeophysics really came into its own in the early to mid 2000s, in part by addressing problems of interest to the hydrogeology community. To me, this evolution was about asking the right questions with geophysics, and using geophysics to complement traditional methods and fill in gaps, rather than using it to substitute for hydrologic measurements. I’d always prefer a single borehole with a pressure transducer to trying to find the water table with a geophysical method. That said, there is no better way to see between the sparse “hard” data than with geophysics. Geophysics can also map time-lapse changes over broad spatial and temporal extents, and