49 2. Refraction surveying a b c d Figure 2.9 Automatic process for first arrival picking (Coppens, 1985). To improve the picking in noisy traces, it is recommended that a spatial coherency filter is applied to seismic traces sorted in constant – offset gathers. Figure 2.9-c shows the efficiency of the coherency filtering on a noisy constant – offset section. The associated picked times are shown in Figure 2.9-d. The number of erroneous picks was considerably reduced. The remaining erroneous values can be edited manually or filtered by a median filter (Mari et al., 1999, 2015).
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