Seismic tests at Southern Ute Nation coal fire site

Interpretation

In two of the better quality shot gathers, Figures 6(b) and 6(d), four different events can be distinguished. 2 refracted wave events are observed: a direct P wave event and a reflected wave event. These are annotated in Figures 9 and 10. The interpreted reflection is also visible at positions greater than about $55$ m in Figure 6(d). At small offsets we see very slow and dispersive ground roll, annotated $G$.

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Figure 9. Line 1, shot at southwestern end, 10 m SW of well #11. A possible reflection, annotated $R$, appears between positions 30 to 55 m. A direct P-wave is annotated $1$, and two refracted waves are annotated $2$ and $3$. A possibly hidden reflected event is annotated $R_2$. Dispersive ground roll $G$. $\mathbf{[NR]}$

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Figure 10. Line 1, shot on sandstone outcrop, in the road. A direct P-wave event annotated $1$, two refracted waves annotated $2$ and $3$. $\mathbf{[NR]}$

We performed simple refraction analyses to estimate a velocity profile (Stein and Wysession, 2003). The direct P-wave and two refractions' slopes indicate three velocities: $v_1=480$ m/s, $v_2=1400$ m/s and $v_3=3000$ m/s. These are relatively consistent for both figures, although errors in the range of 30% are possible. The intersection times in Figure 9 indicate two layer thicknesses of $h_0=1.2$ m and $h_1=5.8$ m. The intersection times in Figure 9 indicate two layer thicknesses of $h_0=0.9$ m and $h_1=9.1$ m. These differences are due both to estimation error and lateral variation of the geology and topography. However, they suggest two positive velocity discontinuities at approximately $1$ m and $8-10$ m. Comparing these depths to the log shown in Figure 11 (depth indicated in feet), suggests that they are not as deep as the coal and could be associated with the top of the thick sandstone bed. Perhaps the thin sandstone bed in the shale, or a positive velocity gradient in the shale, is the reflector. The intersection time of the reflection is approximately $0.02$ s, using a estimated velocity of $1400$ m/s. This would indicate a reflector at approximately $14$ m depth.

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Figure 11. Well log at well #3. Lithology is indicated below, with depth indicated in feet. A $9$ m thick coal layer is located at approximately $16$ m depth. $\mathbf{[NR]}$

The frequency wave-number spectrum in Figure 4 shows significant high wave-number noise. This is partly due to the interleaving technique. The energy associated with reflections, refractions and surface waves is all located and mixed together below wave-numbers of $.15$ $m^{-1}$ and frequencies below $25$ Hz. This offers little opportunity to filter the refractions and surface waves from the reflection. Applying Normal Move Out (NMO) to flatten the reflector on the shot gather of Figure 6(b) was consistent with the velocity estimates.

The shot gather after NMO with three different velocities is shown in Figure 12. A NMO velocity of $1150$ m/s seems optimal to flatten the flanks of the interpreted reflector. This NMO velocity was tested for consistency with the velocities coming from refraction. Various stacking velocities, $v_s$, and normal incidence travel times, $\tau$, were tested for their equivalent interval velocity, $v_i$, and layer thicknesses, $h$.

$$\begin{array}{llll} v_s=480, 1150 & \tau=0.0055, 0.02 & h=1... ...u=0.0075, 0.02 & h=1.8, 8.78955 & v=480, 1406.33 \end{array}$$ (1)

Most combinations are fairly sensible, but pushing the lower velocity to $1400$ m/s also pushes the slow top layer to $2$ m thick. This result is unrealistic considering our data and field observations. Regardless of exact numbers, it appears that the reflection originates at a depth of approximately $11$ m. Revisiting Figure 9, note a weaker event, $R_2$, that is possibly a reflection hidden behind the interpreted reflection $R$. These estimates are rough, but error is unlikely to exceed $30$ %.

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Figure 12. Line 1, shot at southwestern end, 10 m SW of well #11. a) No NMO, b) NMO with $v_s=1300$, c) $v_s=1150$, d) $v_s=1000$. Note that $1150$ m/s seems to be the correct NMO velocity. $\mathbf{[ER]}$

Seismic tests at Southern Ute Nation coal fire site