Using Surface Waves for Shallow Soil Profile Exploration.

In a recent long-term field soil survey, it was found that sound speed correlates with water potential, expressed as a power law (Lu and Sabatier, 2009). Sound speed is also sensitive to pressure (Lu, et al, 2004). Therefore sound speed is a good parameter reflecting the variation of soil properties (water content, water potential, and porosity) and conditions (pressure and temperature).  In order to explore shallow soil properties in a non-invasive manner, a multi-channel analysis of surface waves (MASW) method was employed to obtain soil shear wave velocity profile up to a couple of meters. In the study, an electro-mechanical shaker operating in a frequency-sweeping mode was placed on the ground to excite Rayleigh waves propagating along the surface. A laser Doppler vibrometer (LDV) was used as a non-contact receiver to detect the surface vibration. The LDV was mounted on an aluminum frame and driven by a stepper motor to move along a straight line.  Multi-channel surface vibrations were obtained and analyzed to determine the dispersive curve, i.e. the Rayleigh wave velocity as a function of frequency. An inversion algorithm was used to convert the dispersion curve into the shear wave velocity profile. This technique can be developed for the applications of levee and dam evaluation as well as shallow soil properties imaging and characterization.