Multiscale geophysical imaging of the critical zone

Abstract

Details of Earth's shallow subsurface-a key component of the critical zone (CZ)-are largely obscured because making direct observations with sufficient density to capture natural characteristic spatial variability in physical properties is difficult. Yet this inaccessible region of the CZ is fundamental to processes that support ecosystems, society, and the environment. Geophysical methods provide a means for remotely examining CZ form and function over length scales that span centimeters to kilometers. Here we present a review highlighting the application of geophysical methods to CZ science research questions. In particular, we consider the application of geophysical methods to map the geometry of structural features such as regolith thickness, lithological boundaries, permafrost extent, snow thickness, or shallow root zones. Combined with knowledge of structure, we discuss how geophysical observations are used to understand CZ processes. Fluxes between snow, surface water, and groundwater affect weathering, groundwater resources, and chemical and nutrient exports to rivers. The exchange of gas between soil and the atmosphere have been studied using geophysical methods in wetland areas. Indirect geophysical methods are a natural and necessary complement to direct observations obtained by drilling or field mapping. Direct measurements should be used to calibrate geophysical estimates, which can then be used to extrapolate interpretations over larger areas or to monitor changing processes over time. Advances in geophysical instrumentation and computational approaches for integrating different types of data have great potential to fill gaps in our understanding of the shallow subsurface portion of the CZ and should be integrated where possible in future CZ research.

A gridded global data set of soil, intact regolith, and sedimentary deposit thicknesses for regional and global land surface modeling

“…Part of this uncertainty stems from the lack of global data sets to leverage for model calibration and validation. There is, however, a growing consensus among critical zone scientists (driven largely by recent shallow seismic refraction and scientific drilling studies) that regolith thicknesses are typically in the range of 10–40 m, with thicker regolith values usually occurring beneath topographic divides and thinner values beneath valley bottoms [ Holbrook et al ., , Parsekian et al ., ; St. Clair et al ., ]. Given the importance of the intact regolith layer as a reservoir for water that plants can tap during droughts, we believe it is more beneficial than not to place what constraints we can on the thickness of regolith (with caveats).…”

“…[] to zero near valley bottoms. After application of this factor, the range of predicted regolith thickness in uplands is consistent with typical values (10–40 m) inferred from seismic refraction surveys [e.g., Holbrook et al ., , Parsekian et al ., ; St. Clair et al ., ], except in arid and semiarid upland regions, where values as high as 50 m are common.…”

A review of CO2 and associated carbon dynamics in headwater streams: A global perspective

“…For instance, Mwakanyamale et al [] were able to reliably quantify exchange versus non‐exchange zones from a portion of the Columbia River with fiber‐optic distributed temperature sensing data. Such geophysical methods provide useful tools to improve our understanding of p CO 2 origins in headwater streams by quantifying changing CO 2 influxes over time at much higher spatial resolution than direct measurements [ Parsekian et al , ].…”

Understanding fracture distribution and its relation to knickpoint evolution in the Rio Icacos watershed (Luquillo Critical Zone Observatory, Puerto Rico) using landscape‐scale hydrogeophysics

“…While the effectiveness of near-surface geophysical methods for the characterization of landforms and imaging of the subsurface has been proven for many decades (see review from Van Dam (2012)), studies that fully integrate such methodology with studies of the critical zone are still limited (Holbrook et al, 2014;Parsekian et al, 2015;St. Clair et al, 2015;Orlando et al, 2016).…”