The Earth is oscillating persistently even in seismically quiet periods. For example, ocean swells excite seismic surface waves at periods of about 10 s, known as microseisms. Because microseisms are excited persistently by random ocean swells, the wavefield seems to be at random. The wavefield is so complicated that seismic exploration using micorseisms seems to be difficult. However, a simple statistical processing of seismographs, cross-correlation analysis enables us to explore the Earth's interior. The method is known as seismic interferometry. Here we show a simple demonstration of the cross-correlation analysis.
Cross-correlation function (CCF): one excitation source
Please click the left button of your mouse. You select the location of an excitation source. You can start this demo by press of the "s" key. We can identify the wave propagation from the source. The waveforms show the velocity response by the impulsive force, which correspond to time derivative of the Green's function. The right upper panel shows cross-correlation functions (CCFs) between pairs of stations, where number in the lower panel shows station number. The peaks of CCFs show travel time differences between the pairs of stations.
Cross-correlation function: many random sources
In this demo, 100 excitation sources are located along a circle. Each source hits the ground at random with each other. You can start this demo by press the "r" key.After about ten minutes, you can find a ballistic wave propagation. For example, we focus on the CCF between station 1 and 2. You can find the propagation from 1 to 2 in positive time lag (called as the causal part), and the propagation from 2 to 1 in negative time lag (called as the acusal part).
- Open circles show stations from 1 to 5. The lower right panel shows the seismograms.
- The waveforms and the color shows the particle velocity. Blue color shows positive, and the red one shows negative velocity
- The upper right panel shows CCFs between pairs of station. For example 12 shows a CCF between the 1st station and the 2nd station.
- Source location for one excitation source: left button
- Radiation pattern for one excitation source: 0: isotropic, 1: dipole
- Start and stop: press the "s" key
- Switch from one excitation source to 100 random excitation sources : press the "r" key.