Using Ground Penetrating Radar to Observe Hidden Underground Water Processes


Researchers present applications of radar technology for exploring the properties and movement of water beneath our feet.



To meet the needs of a growing population and to provide it with a higher quality of life, increasing pressures are being placed on the environment through the development of agriculture, industry, and infrastructures.



Soil erosion, groundwater depletion, salinization, and pollution have been recognized for decades as major threats to ecosystems and human health. More recently, the progressive substitution of fossil fuels with biofuels for energy production have been recognized as potential threats to water resources and sustained agricultural productivity.



The top part of the earth between the surface and the water table is called the vadose zone. The vadose zone mediates many of the processes that govern water resources and quality, such as the partition of precipitation into infiltration and runoff, groundwater recharge, contaminant transport, plant growth, evaporation, and energy exchanges between the earth’s surface and its atmosphere. It also determines soil organic carbon sequestration and carbon-cycle feedbacks, which could substantially affect climate change.



The vadose zone’s inherent spatial variability and inaccessibility make direct observation of the important belowground (termed “subsurface”) processes difficult. Conventional soil sampling is destructive, laborious, expensive, and may not be representative of the actual variability over space and time. In a societal context where the development of sustainable and optimal environmental management strategies has become a priority, there is a strong prerequisite for the development of noninvasive characterization and monitoring techniques of the vadose zone.


In particular, approaches integrating water movement, geological, and physical principles (called hydrogeophysics) applied at relevant scales are required to appraise dynamic belowground phenomena and to develop optimal sustainability, exploitation, and remediation strategies.



Among existing geophysical techniques, ground-penetrating radar (GPR) technology is of particular interest for providing high-resolution subsurface images and specifically addressing water-related questions. GPR is based on the transmission and reception of electromagnetic waves into the ground, whose propagation velocity and signal strength is determined by the soil electromagnetic properties and spatial distribution. As the electric permittivity of water overwhelms the permittivity of other soil components, the presence of water in the soil principally governs GPR wave propagation. Therefore, GPR-derived dielectric permittivity is usually used as surrogate measure for soil water content.



In the areas of unsaturated zone hydrology and water resources, GPR has been used to identify soil layering, locate water tables, follow wetting front movement, estimate soil water content, assist in subsurface hydraulic parameter identification, assess soil salinity, and support the monitoring of contaminants.



The February 2008 issue of Vadose Zone Journal includes a special section that presents recent research advances and applications of GPR in hydrogeophysics. The studies presented deal with a wide range of surface and borehole GPR applications, including GPR sensitivity to contaminant plumes, new methods for soil water content determination, three-dimensional imaging of the subsurface, time-lapse monitoring of hydrodynamic events and processing techniques for soil hydraulic properties estimation, and joint interpretation of GPR data with other sources of information.



“GPR has known a rapid development over the last decade,” notes S├ębastien Lambot, who organized the special issue. “Yet, several challenges must still be overcome before we can benefit from the full potential of GPR. In particular, more exact GPR modeling procedures together with the integration of other sources of information, such as other sensors or process knowledge, are required to maximize quantitative and qualitative information retrieval capabilities of GPR. Once this is achieved, GPR will be established as a powerful tool to support the understanding of the vadose zone hydrological processes and the development of optimal environmental and agricultural management strategies for our soil and water resources.”



The full article is available for no charge for 30 days following the date of this summary. View the abstract at: vzj.scijournals.org

Geoscientists use radar to locate lost graves





Acquiring GPR data at Wyatt Chapel Cemetery
Acquiring GPR data at Wyatt Chapel Cemetery

Participants in a summer course for educators used ground-penetrating radar (GPR) to locate a pair of lost graves at an abandoned cemetery outside Houston. The site might become a historical monument.



Nineteen in-service K-12 teachers from urban Houston school districts where the majority of students are members of historically underrepresented minority groups enrolled in the class — ESCI 515: Geophysical Field Work for Educators.



Alison Henning, lecturer in the Department of Earth Science, taught the course. Dale Sawyer, professor of Earth science, and graduate student Priyank Jaiswal also took part in the endeavor.



Henning said some faculty members at Prairie View A&M University (PVAMU), aware of a previous Rice project in the summer of 2006 that investigated the Evergreen Negro Cemetery in Houston, suggested they focus attention on a site near the campus about 50 miles northwest of Houston. The PVAMU faculty members are hoping to turn the site, which is believed to have originated as a slave burial ground in the 1850s, into a historical monument to the early settlers of Prairie View.


Joined by PVAMU students and faculty, Henning and her crew began their work last July. Over two weeks, the group acquired and interpreted 59 GPR profiles in Wyatt Chapel Cemetery and surrounding areas to determine the local stratigraphy and try to locate unmarked graves.



“The soil at Prairie View is ideally suited for GPR investigations, and we obtained some spectacular results,” Henning said. The stratigraphy in the area consists of three to six feet of reddish-brown, medium-grained sand overlying a light gray, highly compacted clay, she explained. The sand-clay boundary appears as a strong reflector on the GPR profiles.



“The class identified numerous subsurface anomalies that might have indicated unmarked burials,” Henning said. Archeologists from Texas A&M later joined the project in the field and excavated two of the anomalies. The first consisted of a pair of bright hyperbolae on the GPR, suggesting two edges of a metal object. This excavation resulted in the discovery of a metal plank thought to be a burial cover. The second anomaly consisted of a break in the horizon representing the top of the clay layer, and subsequent excavation revealed a grave shaft.



ESCI 515 is aimed at educators who are currently teaching science without a science degree. Participants in the Wyatt Chapel Cemetery project included elementary, middle and high school teachers. This summer experience is followed by a content-intensive academic year course in physical geology.



“GPR is an excellent tool for archeological and cultural investigations,” Henning said, “because it is nondestructive (no digging or trenching required). It also provides the opportunity for service learning. As a geoscientist, I find it very gratifying to use geophysics for community service.”