48 Seismic Imaging Figure 2.8 Up-hole survey. The calculation of static corrections is based on the measurement of first arrival times. The geophysicist’s task is made easier with the use of automatic picking algorithms (Hatherly, 1982; Gelchinsky and Shtivelman, 1983; Coppens, 1985). Figure 2.9-a shows a seismic record and the 255 m constant offset section, the arrow on the shot point indicating the selected trace. The seismic source is a weight drop. On the constant offset section, the refracted signal evolves very little from trace to trace, which helps with the identification of first arrivals. The Chronos method is based on the criterion of a sudden energy increase associated with the arrival of a refracted wave. Each seismic trace is transformed into an energy trace representing the ratio between the energy contained in a small sliding window W2, and the cumulative energy contained in a window W1, which starts at time zero and ends at the end of window W2 (Figure 2.9-b). The abscissa τ0 of the maximum of the energy function gives the approximate time of the refracted arrival. The exact time is then accurately determined by picking the extreme (peak or trough) closest to τ0 (Figure 2.9-b). If it is an impulsive source, the measured time is reduced by the rise time.
RkJQdWJsaXNoZXIy NjA3NzQ=