Seismic tests at Southern Ute Nation coal fire site

Seismic data

We collected P-wave seismic data along two transects, using conventional hammer-plate-seismic techniques. The recording arrays consisted of a Geometrics Geode with $24$ live channels and $30$ Hz vertical geophones at $3$ m spacings. In order to create shot gathers with a greater number of traces at a narrower spacing, four shots were closely-spaced at each shot location. By placing these shots at a 0.75-m spacing, and then interleaving the resulting four $24$-channel shot gathers, a $96$ channel array was simulated, illustrated schematically in Figure 1. This approach is a variation on the ``walk-away'' testing that is commonly used for acquiring data at a new field site. The interleaving technique assumes that geology does not vary too rapidly along the transect. Any lateral geological variation would violate this assumption, and cause artifacts in the shot gathers.

reflection-interleave Figure 1. Sketch of interleaving approach. With Y geophones, at a spacing X, and using N shots at a spacing of X$/$N, the interleave technique results in a shot gathers with N$\times$Y traces and a spacing of X$/$N. Geology is assumed to be fairly constant in the lateral direction. $\mathbf{[NR]}$

Five or six shot locations were recorded along each line, rather than a shot between each pair of geophones. The shot locations are evenly distributed along the line, with one shot point off the end of each line to observe longer-offset arrivals. Each shot gather is the result of stacking approximately 5 hammer impacts, after manual data checking for trigger errors and other problems. It should be noted that for most data in these surveys, the noise reduction benefits of stacking are minimal; data quality for each individual hammer impact is good.

seismic-lines Figure 2. Site map with fissures (thin lines), wells (full triangles), thermocouples (open triangles) and seismic lines (thick lines). Courtesy of Taku Ide. $\mathbf{[NR]}$

Seismic tests at Southern Ute Nation coal fire site