Historically, travel times were measured from seismograms recorded on smoked or photographic paper. An example is shown in Figure 6.1. This is a seismogram, dated July 26, 1963, from the World Wide Standardized Seismograph Network (WWSSN), the state-of-the-art at the time. Arrival times were picked from such recordings by measuring the distance to the nearest minute mark, visible as small deflections at regular intervals. One major shortcoming of such photographic recordings is that the trace becomes hard to read when the amplitude is large and the light source moves quickly, giving only a short exposure of the photographic film. Short period recordings, an example of which is shown in Figure 6.2 are to be preferred for the picking of the P-wave arrival time, but may be less suitable to correctly identify the later arrivals. Though the network is now obsolete, scanned images of seismograms for a growing number of historical earthquakes are available in the public domain. These images can be digitized with suitable vectorising software, such as Teseo (Pintore et al.,).

Modern, digital instrumentation has greatly changed the practice in seismographic stations around the world. Figure 6.3 shows an example from a modern digital seismographic station. Digitized seismograms are much easier to manage, archive and analyse.

In this chapter we consider only digitized signals. Formally, a seismogram s(t) is digitized by convolving it with a Dirac comb ∑i δ(t − i Δ t), resulting in N-tuple of values, e.g. (s1, s2, … sN).