Author
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LU, ZHIQU - University Of Mississippi |
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Wilson, Glenn |
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Submitted to: Journal of Applied Geophysics
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/21/2017 Publication Date: 8/1/2017 Publication URL: https://handle.nal.usda.gov/10113/5763091 Citation: Lu, Z., Wilson, G.V. 2017. Imaging a soil fragipan using a high-frequency multi-channel analysis of surface wave method. Journal of Applied Geophysics. 143:1-8. doi:10.1016/j.jappgeo.2017.05.011. Interpretive Summary: A fragipan is a naturally occurring soil layer that is characterized as being dense, brittle when moist, and restricts root and water from moving through it. Fragipan layers result in shallow perched water tables that can cause environmental problems. They typically do not cover a large continuous area of landscapes but instead they occur in smaller isolated areas which are very difficult to identify. The objective of this study was to develop a technique to identify fragipan layers without disturbing the soil using a high-frequency multi-channel analysis of surface sound waves called the HF-MASW method. The HF-MASW method is developed to measure the soil profile in terms of the shear wave velocity at depths down to a few meters. The present MASW method uses an accelerometer as a vibration sensor to detect the movement of Rayleigh sound waves that are generated by a shaker at the soil surface that is operated in a frequency-sweeping mode in three overlapped frequency bands. With the method, a subsurface picture is created that represents the soil properties in terms of shear wave velocity of a vertical cross-section of the test site. These shear wave pictures were measured, visualized, and evaluated. From the contrast of the image, the presence, depth, and extent of a fragipan were identified. The HF-MASW images compared favorably with field observations of the fragipan at a soil pit face and measurements obtained by soil penetration tests. The study demonstrated the capability of the HF-MASW technique for detection and imaging hard layers in soils such as fragipan. Given the good agreement between measurements and sound wave images, this innovative technology may enable improved site evaluations of soils if commercialized. Given that fragipans are subsurface horizons that restrict deep water percolation and affect the suitability of soils for some uses, this innovative application of sound waves may provide a valuable new tool for detecting and mapping their occurrence which can guide land management and land use decisions. Technical Abstract: A fragipan is a naturally occurring soil layer that is characterized as being dense, brittle when moist, and restrictive to root and water penetration. Fragipan layers result in shallow perched water tables that can cause environmental problems, are spatially variable in landscapes, and have proven very difficult to identify. The objective of this study was to noninvasively image fragipan layers using a high-frequency multi-channel analysis of surface wave (HF-MASW) method. The HF-MASW method is developed to measure the soil profile in terms of the shear wave velocity at depths up to a few meters in the vadose zone. The present MASW method uses an accelerometer as a vibration sensor to detect Rayleigh wave propagation generated by an electrodynamic shaker operating in a frequency-sweeping mode in three overlapped frequency bands. With the method, a subsurface image was obtained, i.e., the soil properties in terms of shear wave velocity of a vertical cross-section of the test site were measured, visualized, and evaluated. From the contrast of the image, the presence, depth, and extent of a fragipan were identified. The HF-MASW result was compared with field observations of a soil pit face and a 2- dimensional image obtained by penetration tests. They were in good agreement. The study demonstrated the capability of the HF-MASW technique for detection and imaging soil subsurface hard layers such as fragipan. |
