FIG. N-3. Noise analysis or walkaway. (a) Source was vibroseis, geophones were spaced 1.5 m apart, offset to first phone was 425 m. The 1890 m/s arrival is a refraction from the base of the weathering; the 530 and 620 m/s arrivals are ground-roll modes; the 330 m/s event is an airwave. (Courtesy Chevron Oil Co.) (b) Frequency-wavenumber sketch for the data; see also Figure F-11.