Seismic tests at Southern Ute Nation coal fire site

Conclusions

We conducted a seismic test at the Southern Ute Nation coal fire site using minimal equipment. Data quality is at the high end of what can be expected for sledgehammer-source data. The recorded frequency content is strong up to $100$ Hz. The test shots were interpreted to contain several refracted events and a reflection event.

The refraction and reflection events are interpreted for a subsurface velocity profile. The velocities were higher than anticipated by a previous modeling effort (de Ridder and Haines, 2008). The fast layers that are interpreted to overlie the coal pose a difficulty to any seismic surveying because they are an impediment to deeper wave propagation. As we expected, the fissures present a major impediment to wave propagation and substantially degrade data quality. In addition, the test shots indicate highly dispersive ground roll and strong statics in the area.

Refraction and reflection analyses suggest a meter-thick layer of a little less than $500$ m/s on top, and a layer of about 9-10 meters of a velocity of about $1300-1400$ m/s overlying a lower layer with a velocity as high as $3000$ m/s. The reflection event is originating at a depth of approximately $11$ m, which is well above the coal layer. It might hide possible reflections from the coal layer.

There are no distinguishable events deep enough to adequately characterize the coal layer. The major difficulty is unexpected fast layers above the coal, as well as a relatively strong reflection event from a layer above the coal. It is conceivable that the depth estimates are inaccurate and the reflection event is from the top of the coal, but this is unlikely. Even if the reflection happens to be from the top of the coal, it is only clearly visible on only a few shot gathers. Thus we must conclude that further seismic work at the site is unlikely to be successful at imaging the targeted coal or ash layer.

Seismic tests at Southern Ute Nation coal fire site