Translations:Aliasing/2/en

Let us now apply the sampling concept to simple harmonic motion. We will see that the most important consequence of sampling a function at equally spaced time points is the phenomenon called aliasing. For example, in seismic recording, the sampling interval of 4 ms often is used. This causes frequencies greater than 1/(2)(0.004), namely 125 Hz (called the Nyquist frequency), to be aliased (see below). Figure 3a shows the amplitude spectrum of a wavelet with a 4-ms sampling interval. If the wavelet is decimated by resampling it at 8 ms (so that one-half of the values are thrown away), then the new Nyquist frequency becomes 1/(2)(0.008), which is 62.5 Hz. In Figure 3b, the portion of the spectrum beyond 62.5 Hz is folded back, as we see in Figure 3a. For this reason, the Nyquist frequency also is known as the folding frequency. The amplitude spectrum of the decimated wavelet is the sum of the parts shown in Figure 3b. This result is shown in Figure 3c (Robinson and Clark, 1991).