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United States Department of Agriculture

Agricultural Research Service

Research Project: Technologies for Managing Water and Sediment Movement in Agricultural Watersheds

Location: Watershed Physical Processes Research Unit

Title: Imaging a soil fragipans using a high-frequency MASW method

Authors
item Lu, Zhiqu -
item WILSON, GLENN
item Hickey, C.J. -

Submitted to: Symposium on Application of Geophysics to Engineering and Environmental Problems Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 12, 2014
Publication Date: March 16, 2014
Citation: Lu, Z., Wilson, G.V., Hickey, C. 2014. Imaging a soil fragipans using a high-frequency MASW method. Symposium on Application of Geophysics to Engineering and Environmental Problems Proceedings. PP 1-8.

Interpretive Summary: 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 is to map fragipan layers without disturbing the soil using an acoustical instrument called a laser Doppler vibrometer (LDV) based on the high frequency (HF) multi-channel analysis of surface wave (MASW) method. The HF-MASW is developed to measure the soil profile in terms of the velocity that shear waves penetrate the soil to depths of several feet. While the conventional MASWs use surface vibration sensors called geophones, the present HF-MASW uses an accelerometer as a non-contact sensor to detect movement of Rayleigh waves generated by an electromechanical shaker operating in a chirp mode to achieve high frequency and high definition pictures. With this method, a two dimensional image of the subsurface soil properties in terms of shear wave velocity was measured, pictured, and evaluated. From the contrast of the image, the presence, depth, and extent of a fragipan was determined. The HF-MASW result was compared with an image obtained by a penetration test as well as standard soil description of the soil profile. They were in good agreement. The study demonstrated the capability of the HF-MASW technique for detection and imaging of dense soil layers such as a fragipan.

Technical Abstract: The objective of this study was to noninvasively image a fragipan layer, a naturally occurring dense soil layer, using a high-frequency (HF) multi-channel analysis of surface wave (MASW) method. The HF-MASW is developed to measure the soil profile in terms of the shear (S) wave velocity at depths up to a few meters. While the conventional MASWs use geophones as surface vibration sensors, the present MASW uses an accelerometer as a sensor to detect Rayleigh wave propagation generated by an electromechanical shaker operating in a chirp mode to achieve high frequency and high spatial resolution. With the method, the subsurface soil properties at a test site were measured, visualized, and evaluated. A 2-dimensional S-wave velocity image was obtained and from the contrast of the image, the presence, depth, and extent of a fragipan were identified. The HF-MASW result was compared with those of site characterization made by invasive methods and a 2-dimensional image obtained by a penetration test. The HF-MASW and soil charatcerization were in good agreement. The study demonstrates the capability of the HF-MASW technique for detection and imaging subsurface layers such as a fragipan.

Last Modified: 9/10/2014
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