Spectral Analysis of Surface Waves. Spectral analysis of surface waves (SASW) is cur- rently a research tool but may find use as an evaluation tool in the near future. As shown in Fig. 3.53, two or more accelerometers are attached to the pavement in line with the test point. An instrumented hammer is used to generate surface waves. The accelerometers measure the travel time of the surface waves. By varying the distance between two accelerometers or by using multiple accelerometers, data can be gathered for the deeper pavement layers. Analysis of the surface waves can be used to determine the modulus and thickness of each layer in the pavement surface.