Agrogeophysical methods for identifying soil pipes

Authors: WODAJO, LETI - University Of Mississippi; RAD, PURSA - University Of Mississippi; SHARIF, SHARIFUL - University Of Mississippi; ABAS, MD - University Of Mississippi; MAMUD, MD - University Of Mississippi; HICKEY, CRAIG - University Of Mississippi; Wilson, Glenn

Submitted to: Journal of Applied Geophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2021
Publication Date: 6/1/2021
Citation: Wodajo, L.T., Rad, P.B., Sharif, S.I., Abas, M.A., Mamud, M.L., Hickey, C.J., Wilson, G.V. 2021. Agrogeophysical methods for identifying soil pipes. Journal of Applied Geophysics. 192:104383. https://doi.org/10.1016/j.jappgeo.2021.104383.
DOI: https://doi.org/10.1016/j.jappgeo.2021.104383

Interpretive Summary: The contribution of erosion below the soil surface along the inside of large pores called soil pipes, to total soil losses from a watershed and the collapse of these soil pipes to form sinkholes and gullies are important to measuring soil losses from agricultural lands. The formation and evolution of soil pipes occurs below the surface where measurements based upon surveying their openings at the surface, such as sinkholes and gullies, often fail to provide a complete image of their location or importance because of lack of knowledge of where they occur below the surface or how their pathways change with time. The various locations of soil pipe collapses do not always correspond with the surface topography which controls the paths of runoff. This lack of connection between the surface path and the subsurface path makes it difficult to predict where soil pipes occur based upon their surface features such as open and closed sinkholes and gullies. Geophysical methods that scan the subsurface from the surface without disturbing the subsurface provide a better alternative for developing detailed images of the soil pipe networks. In addition to being non-invasive, geophysical methods require less labor and are lower cost than methods that require excavation and surveying of soil pipes. In this paper, the feasibility and efficiency of two geophysical methods, ground penetrating radar (GPR) and electromagnetic induction (EMI), for identifying soil pipes is presented. The evaluations are based on field measurements at a research site known to have soil pipes based upon the existence of open and closed sinkholes and ephemeral gullies. Geophysical signals from the two methods were evaluated using a combined cross-section plot that includes measurements by cone penetrometer logger (CPL). These signals were then used to construct maps of the area. The EMI results allow for identification of zones in the fields that have soil pipe networks. The GPR data provides much more detailed images of the pipe locations. The GPR data can be used to construct maps with images that appear as slices of the earth that represent different depths. The results demonstrate the great potential of EMI and GPR as tools for finding and mapping soil pipes.

Technical Abstract: The direct contribution of internal erosion of soil pipes to soil losses or their collapse to form ephemeral gullies is a vital component in understanding total soil loss from agricultural lands. The generation and evolution of soil pipes occurs below the surface where survey-based measurements and analysis often fail to provide a comprehensive image of these important phenomenon or their erosional evolution. The distribution of soil pipe collapses does not always correlate with surface flow paths as indicated by surface topography. This lack of correlation makes inferences from traditional remote sensing or manual surveying of surface features subject to large uncertainty and lack of correspondence to the actual subsurface flow path. Non-invasive geophysical methods provide a better alternative for high spatial resolution imaging to delineate, characterize, and map the distribution of soil pipe networks. In addition to being non-invasive, geophysical methods are more expedient and lower cost than invasive methods. In this paper, the feasibility and efficiency of two geophysical methods, ground penetrating radar (GPR) and electromagnetic induction (EMI), for delineating soil pipes in cross-sectional and planview maps is presented. The evaluations are based on field measurements at a research site with established soil pipes and ephemeral gullies. Geophysical signatures from the two methods is evaluated using a composite (combined) cross-section plot that includes results of an invasive method known as cone penetrologger testing (CPL). These signatures are then used to construct plan view maps of the area. The EMI results allow for delineation of larger zones of fields having soil pipe networks. The GPR data provides much better resolution. The GPR data can be used to construct maps (depth slices) representing different depths. The results demonstrate the high potential of EMI and GPR as useful tools for studying internal soil pipes.