The Critical Zone (CZ) is the region of Earth from that reaches from the top of the vegetation down through the soil, weathered rock, and fracture bedrock to the base of the water table. The physical, geochemical, hydrological, and biological processes that occur within the critical zone sustain life but are strongly affected by anthropogenic activity. It is for these reasons that this highly heterogeneous region is a topic of increasingly interdisciplinary scientific interest, as highlighted by the creation of the National Science Foundation (NSF) network of Critical Zone Observatories.

Although much of the top CZ, including the vegetation, surface water, and soil, can be easily accessed for direct measurements, it is much more difficult to access the deeper CZ. Understanding and imaging the structure of the deep CZ, which can range from millimeters to tens of meters below Earth’s surface, is essential for addressing important questions such as: What are the rates of bedrock weathering and what controls this rate? How thick is the regolith? Where is water stored and how does water storage change in response to natural and anthropogenic factors? Common methods used to quantify the structure of the CZ include digging soil pits, auguring, and drilling boreholes. However, these methods are limited in that they only provide point source information, do not capture the spatial heterogeneity present within the CZ, are invasive, and cannot provide temporal information. Furthermore, drilling, which can be used to provide the deepest information, is not possible in regions that cannot be accessed by large vehicles. Hydrogeophysics, which involves the use of near surface geophysical methods to image subsurface hydrologic properties and processes, has the potential to provide the critical information required to understand the deep CZ; however, hydrogeophysics is under-utilized in CZ science.

The funding this project was used to provide a forum in which to bring CZ scientists together with geophysicists and encourage collaborations. This was accomplished by holding a workshop, hosted by two major geoscience societies, the American Geophysical Union (AGU) and the Society of Exploration Geophysicists (SEG), called 2017 AGU-SEG Hydrogeophysics Workshop: Imaging the Critical Zone at Stanford University, California from July 24^th to 27^th, 2017. There were 75 attendees at the workshop from academic, government and industry sectors; 20 of the attendees were students.

The workshop was organized around four themes: (1) Interfaces in the Critical Zone, (2) Critical Zone Physical Properties and Rock Physics, (3) Hydro-bio-geo-chemical Processes in the Critical Zone, and (4) Scaling up of Geophysical Data to Address Critical Zone Science Questions. Associated with each theme was one invited CZ-science speaker, and one invited geophysicist speaker. In total there were 8 invited presentations and 57 poster presentations. In addition to the themes, one afternoon was dedicated to a group activity designed to have participants think broadly about how geophysical methods can be used to help address questions in CZ science. During the group activity, demonstrations of multiple different geophysical techniques (including ground penetrating radar, nuclear magnetic resonance, and electrical resistivity tomography) were provided so that participants unfamiliar with the methods could see first-hand how they could be implemented.  

Based on a survey of the participants following the workshop, the workshop was well received. Of the 25 respondents, 84% rated the AGU-SEG Hydrogeophysics workshop as above average or excellent compared to other specialty conferences attended. 80% said that they were somewhat to very satisfied with the interdisciplinary character of the meeting. 72% felt somewhat to very satisfied that they had learned new methods for use in their personal research. A database of the abstracts for all presentations from this workshop is available on the workshop website https://workshops.agu.org/hydrogeophysics/. 

Last Modified: 05/01/2018
Modified by: Kristina Keating