processing techniques were applied to the recorded data up to 200 repetitions of the experiment were summed at each placement of the source and receiver subsequent data processing techniques applied by computer to the digitized receiver signals were variable gain corrections as a function of source receiver separation to compensate for spherical spreading and bandpass filtering to improve the signal to noise ratio in particular low pass filtering suppressed the direct arrival relative to the refracted arrival and made timing the onset of the refracted event better the data shown in fig5 and 6 were summed from 200 individual repetitions of the experiments band pass filtered and gain adjusted verification that refracted signal has been recorded can be done by demonstrating that the extracted signals conform to theoretical travel times calculated from the minimum time paths defined by snell 39 s law this is possible because the material elastic wave velocities and source receiver geometry are known overlays of calculated travel times on the recorded data show the coincidence between recorded event and their theoretical travel times which establishes their mode of propagation this verification of the travel time calculations was done first on a model with only water overlying the cement slab the data in fig7 show the compression p w p c p w and shear p w s c p w refraction modes of propagation and the direct arrival note.