Comparison of soil excitation methods for surface wave speed measurements

<p>Rayleigh waves are surface seismic waves, which can be produced by a disturbance applied to the surface of a half-space. The dispersive behaviour of Rayleigh waves in real multi-layered media is widely utilized for near-surface acoustic characterization, e.g. in Spectral Analysis of Surface Waves (SASW), and more recently, in Multi-channel Analysis of Surface Waves (MASW). These methods, along with the associated field testing, result in the construction of a dispersion curve. However, it is often difficult to measure the speed of Rayleigh waves in a consistent manner. Other interference, such as direct, reflected and refracted body waves, backscatter from nearby objects, are unwanted and could contaminate the dispersion curve (i.e. the phase velocity) in a significant way. Thus, shear or compression wave energy is undesirable. Furthermore, additional surface waves coming from other sources and background noise can compound the problems.<br> A wide range of different active sources have been utilized for decades in surface wave-based methods, but the reliability and the differences among these have not been properly assessed. The main focus of this paper is to compare different types of soil excitation, in order to identify and quantify seismic waves generated by each different type and to find a feasible Rayleigh wave source for the survey of shallow depths. This will be in terms of type of energy generated, range of frequencies excited, and amount of Rayleigh wave energy with respect to the total. Particularly, in this paper, the excitations caused by inertial shakers, oscillating in both the horizontal and vertical direction, and by transient sources like sledgehammers and mallets, are examined. For each different type of excitation, two key aspects are varied: the magnitude of the impact and the coupling of the source to the soil. The transmission of the energy from the source to the soil will be investigated using rigid platforms of different size, shape and material. This paper investigates the limitations of each type of active source to be utilized. These goals are pursued with experimental tests, digital processing of data, and by comparison with analytical and numerical models.