$S_H$ refraction

One $S_H$ refraction profile has been achieved in the centre of the site with two shot points at each extremity. The S-wave arrivals are picked with an estimation of the error, considering all directions for each shot. The observed traveltime-distance curve are shown in figures 6.26(c) and 6.26(d) by black dots and their associated error bars. From their shapes, it is obvious that a two-layer model is sufficient. The neighbourhood algorithm is then launched with three parameters ($V_s$ of each layer and thickness of first layer). The results are shown in figures 6.26(a) to 6.26(d). The generated ground models are presented in figures (a) and (b) and the comparison of the calculated traveltime-distance curves with the experimental curves is shown in figures (c) to (d). The lowest misfit found is around 0.2, similar for both shots. $V_s$ at the surface is between 100 and 400 m/s considering all models. For the best models, the velocity is between 260 and 290 m/s. Between 0 and 10 m, $V_s$ increases up to 500 m/s. The maximum possible velocity at 10 m is around 1000 m/s.

Figure 6.26: Refraction results obtained with travel time NA inversion for $S_H$ profile. (a) and (b) $V_s$ profiles obtained by inversion for the West and East shots, respectively. (c) and (d) corresponding calculated traveltime-distance curves (in the same order). The black dots and the error bars are the experimental times picked on recorded signals.