Seismic tests at Southern Ute Nation coal fire site

Introduction

In March 2009 personnel from Stanford University, the U.S. Geological Survey and the Southern Ute Department of Energy collected compressional (P) wave near surface seismic data along two transects at the site of a coal fire on Southern Ute Nation lands. The objective of this effort was to determine the utility of seismic methods for imaging the coal and ash layers of interest at the site, and to assist with planning any future data acquisition efforts.

The field site is generally open terrain with a gently dipping ($\sim$10$^{\circ}$) ground surface. The shallowest geology consists of sandstone (highly fractured and fissured in many places); the sandstone is overlain by a thin layer of soil, of about $.5$ m, throughout much of the site. Abundant well data show the coal layer to be about $8$ m thick and to be dipping in the same direction, but a slightly higher angle ($\sim$20$^{\circ}$) than the ground surface. The top is approximately $5$ m deep at the up-slope (up-dip) edge of the site (to the northwest) and up to $16$ m deep in the down-slope part of the site. Open fissures with red-hot rock clearly visible $0.5$ m below the ground surface are one clear indication of the shallow fire. Noxious gases coming from the vents are another indication.

Numerical simulations of seismic experiments at the site indicated that imaging the unburned coal could be possible with sufficiently high-frequency source energy if the impacts of fissures and layering above the coal were minimal (de Ridder and Haines, 2008). A short site visit enabled the collection of a simple data set. A basic data processing analysis was performed to assess data quality and identify arrivals. For line $1$, located above unburned coal where no fissures have been mapped, data quality is good and reflections assumed to be from the sandstone layer that underlies the coal are visible. For line $2$, crossing the burn front and numerous mapped fissures, data quality is reduced. The reflection might be visible for one shot point. Velocity estimation indicates that neither refractions nor reflections image as deep as the coal layer. Two main reasons for the differences between simulations and field test is the presence of high-velocities layers above the coal, and a shallow reflector.

Seismic tests at Southern Ute Nation coal fire site