Collecting Seismic-Reflection Data From Depths Shallower Than Three Meters

Within the last three years, the ability to<br>describe the upper three meters of the earth’s<br>subsurface seismically has advanced significantly.<br>One reason is a better understanding of the<br>requirements for successfully collecting data at<br>these shallow depths. First, geophones placed in<br>close proximity to each other do not produce<br>significant mechanical or electrical crosstalk, and<br>high-quality seismic-reflection data have been<br>collected at several locations with geophone<br>intervals as small as 5 cm. This high-density<br>coverage of the near-source wavefield has allowed<br>the identification of ultrashallow (less than 3-m<br>deep) reflection events that otherwise would not<br>have been visible. Second, previous notions about<br>seismic sources, specifically when they concern<br>information contained in the near-source<br>wavefield, have been adjusted. For example, nearsource<br>waves are possibily distorted by nonlinear<br>displacements at the source. By greatly reducing<br>source energy, identifying reflection and<br>refraction events at frequencies of 500 to 1000<br>Hz within a few meters of the source becomes<br>possible. Third, an increased understanding of<br>the effects of near-surface moisture conditions on<br>data quality has been gained. Data collected over<br>time at a site with all things equal except for<br>variable surface-moisture conditions have yielded<br>results with widely varying quality.