Detailed characterization of the shallow subsurface is important not only in environmental, groundwater, and geotechnical engineering applications, but in neotectonics, mining geology, and the analysis of petroleum reservoirs (and analogs) as well. Extensive drilling or trenching to assist in characterization or monitoring, however, is at times imprudent or prohibitively expensive. In such cases, characterizing the upper layers of the Earth cost-effectively and noninvasively becomes important. Seismology in its various forms has been used to gather subsurface information for almost a century. There are numerous examples of using seismic reflection techniques to examine the upper 100 m of the Earth's subsurface. However, ultrashallow (less than 3 m) reflection seismology has only become possible over the last 20 years due to improvements in equipment, survey design, and processing procedures. This has allowed coincident seismic and GPR surveys (Figure 1) with vertical resolution ∼20 cm and detection of reflections as shallow as 60 cm.

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