Earthquakes—a naturally occurring source of low-frequency data

Seismic reflection data contain information at two very different length scales. The long wavelength or lowfrequency information is derived primarily from move-out velocity analysis and provides information on the order of kilometers (i.e., ~1 to 2 km). The short wavelength (highfrequency) information comes from reflection amplitudes and/or changes in those amplitudes with offset (AVO) and provides information at scales of tens of meters (i.e., ~10 to 250m). Between these two length scales is a gap. This missing information (between 1 and 6 Hz) causes a general uncertainty in all seismic-reflection inversions and creates a challenge for full-waveform inversion methods to properly converge without getting stuck in local mathematical minima. This problem is further compounded in regions with complex structure (e.g., basalt flows, sills and dykes, salt bodies, fold and thrust belts, etc.) where it can be difficult to obtain the long-wavelength information reliably from the data, thereby leading to poor imaging. One possible solution for both of these problems is to use local, regional and teleseismic (far-away) earthquakes as a source of low-frequency energy, exploiting the Earth’s naturally occurring seismicity. This paper describes a field experiment that was designed to test this idea.