SEISMIC INVESTIGATION OF UNDERGROUND COAL FIRES; A FEASIBILITY STUDY AT THE SOUTHERN UTE NATION COAL FIRE SITE, DURANGO, COLORADO.

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1 SEISMIC INVESTIGATION OF UNDERGROUND COAL FIRES; A FEASIBILITY STUDY AT THE SOUTHERN UTE NATION COAL FIRE SITE, DURANGO, COLORADO Sjoerd de Ridder, Department of Geophysics, Stanford University. Nigel Crook, Department of Geophysics, Stanford University. Seth S. Haines, U.S. Geological Survey. S. Taku Ide, Department of Energy Resources Engineering, Stanford University. Abstract We conducted a near surface seismic test at a coal fire in the Southern Ute Nation near Durango, Colorado. The goal was to characterize and image the unburned coal and adjacent burned zone in order to determine the feasibility of any future seismic surveys. The field survey was preceded by a numerical study to optimize the survey design for the field test. The numerical study suggested that field experiments would rely on creating energy with sufficiently high frequencies, ideally greater than 125 Hz. Reflections or refractions from the top of the coal layer might indicate its presence or absence. Separately imaging of both the top and bottom of the coal layer or burned zone likely would be beyond the resolution of a reflection survey. Data from line 1, which overlies unburned coal at approximately 16 m depth, show useful frequency content above 100 Hz and a reflection that we interpret to originate at approximately 11 m depth. Data from line 2, which crosses the burn front and many fissures, are of lower quality with predominantly jumbled arrivals and some evidence of reflected energy at one or two shot points. It seems that neither the refraction nor reflection method is capable of imaging down to the coal layer; due in part to the presence of unexpected high-velocity layers overlying the coal. We conclude that information about the coal is obscured by a reflection from shallower layers and by chaotic arrivals generated by fissures. Based on our data, we suggest that further seismic work at the site is unlikely to successfully characterize the coal fire zone of interest. Introduction In March 2009 personnel from Stanford University, the U.S. Geological Survey and the Southern Ute Department of Energy collected compression (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 image and characterize the coal and ash layers of interest at the site, and to determine the feasibility of any future seismic surveys. The field survey was preceded by a numerical model in a simple geometry based on an estimate of the subsurface. The field site, located near Durango in southwestern Colorado, is generally open terrain with a gently dipping (10 ) ground surface. The shallowest geology consists of sandstone (highly fractured and fissured in many places). Throughout most of the site, the sandstone is overlain by a thin layer of soil, about.5 m. Well data shows the coal layer to be about 8 m thick and to be dipping in the same direction as the ground surface, at a slightly higher angle (20 ). Figure 1 contains a log recorded in a well near the seismic survey locations. The top of the (partially burned) coal layer 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 630

2 site. Open fissures with visible red-hot rock.5 m below the ground surface and noxious gasses from vents, indicate the presence of a shallow fire. Numerical seismic simulations, performed prior to the site visit, indicate that imaging the unburned coal would be possible using a high-frequency source, provided interference from fissures and shallow stratigraphy are minimal. A short site visit enabled the collection of a simple data set. We performed basic data processing to assess data quality and identify arrivals. For line 1, located above unburned coal, where no fissures where identified at the surface, data quality is good and reflections interpreted to be from the sandstone layer that overlies the coal are visible. For line 2, positioned across the burn front where numerous fissures were identified at the surface, data quality is lower. Simple velocity analysis indicates that neither refractions nor reflections image provide useful information from the depth of the coal layer. Previously unknown high-velocity layers and a strong reflector above the coal hinder characterization of the coal fire zone of interest. Figure 1: Well log and lithology at well #3. A 8-m-thick coal layer is situated at approximately 16 m depth. 631

3 Pre-Fieldwork Numerical Model Synthetic Model A model was designed prior to the site visit, based on a geological description of the site. The model consists of a homogeneous background with a gentle vertical velocity gradient overlain by a 10 dipping structure that represents the coal layer (Figure 2). The burn front was estimated to be about 20 m deep. Sandstone velocity was estimated as 2000 m/s with a gentle vertical gradient of s-1 (Table 1). Coal and ash velocities are rough estimates, as 1200 m/s and 300 m/s respectively. Ash velocity was set very low, but greater than zero to avoid numerical instability. Complicated geology such as fissures and voids in the ash layer were neglected; this is a major simplification of the real-life complexity expected in coal fire areas (e.g., Wolf, 2006) and that needs to be considered when interpreting the modeling results. Figure 2: Velocity model for numerical simulation; a dipping, partially burned (grey) coal (black) layer in a sandstone (white) background. P and S wave velocities are given in Table 1. Table 1: Seismic P and S wave velocities and velocity gradients. Unit V p (m/s) z V p (s -1 ) V s (m/s) z V s (s -1 ) Sandstone Coal Ash We synthesized a seismic survey across the model with shots and receivers at 1-m spacing, using the code e3d that is described by Larsen and Grieger (1998) and Martin (2006). The source was a vertical force with the waveform of a 125-Hz Ricker wavelet that is intended to simulate a better-thanaverage sledgehammer impact (e.g., Miller et al., 1992 and 1994). The receivers record both vertical, Vz, and horizontal, Vx, (in-plane) components of particle velocity. Model Results For interpretation we focused on the vertical (PP) component because we anticipated that compression (P) wave methods would perform better in a complex overburden than shear (S) waves. Figure 3 shows a pair of shot gathers for a shot at position 150 m, directly over the end of the burn front. The reflection arriving at 0.03 s at zero offset is the reflection from the top of the coal seam. There is no clear refraction observed in the vertical component; we know that even for our idealized velocity model it would be weak. From these models, we expected that in a field experiment we may be able to discern a reflection from the top of the coal bed. In addition, we expected changes across the burn front, such as diffractions, that could allow for delineating the burned zone. However any strong un-modeled heterogeneity would make imaging much more difficult. Separately imaging the top and bottom of the coal would require frequencies at least as high as those used in the model, preferably higher. 632

4 Figure 3: Shot gathers for a shot at 150 m. At t=0.03 s we see the reflection from the top of the coal layer. Seismic Tests at the Southern Ute Nation Coal Fire Site Data Collection 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 spacing. 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 m spacing, and then interleaving the resulting four 24-channel shot gathers, a 96-channel array was simulated, illustrated schematically in Figure 4 (left). This variation on normal walk-away testing is commonly used for acquiring data at a new field site; the interleaving technique assumes that geology does not vary too strongly along the transect. Two survey lines were selected, one parallel with the geological dip and surface fissures, and one transverse to the geological dip and surface fissures, see Figure 4 (right). Only five to six shot locations were selected along each line, to reduce the acquisition time. The shot locations are evenly distributed along the line, with one shot point off the end of each line to observe longer-offset arrivals. Figures 5-7 show three gathers along lines 1 and 2. For each location a raw recording is shown together with a version that has been processed with automatic gain control (AGC, with a 37.5 ms window) and a band pass filter (80 Hz to 500 Hz). Each shown shot gather is the result of stacking approximately 5 hammer impacts, after manual data checking for trigger errors and other problems. For most data in this survey, the noise reduction benefits of stacking are minimal; repeatability for each individual hammer impact is very high. 633

5 Figure 4: (left) Sketch of the interleaving approach. With Y geophones, at a spacing of X, and using N shots at a spacing of X/N, the interleave technique results in shot gathers with N x Y traces and a spacing of X/N. (right) Site map, showing mapped fissures as thin blue (cold fissures) and red (warm fissures) lines. The thick red lines show the seismic transects. Green lines denote the location of up-dip exposure of coal strata (edge of hill). Black triangles and squares denote well and probe locations, respectively. Thick black lines show road/path locations. Logs for well #3 are shown in Figure 1. Spectral Analysis Averaged, normalized frequency spectra of data recorded at lines 1 and 2 are shown in Figure 8 (left). The frequency content of our sledgehammer source is good, with energy above 100 Hz. The frequency wave-number spectrum in Figure 8 (right) shows significant high wave number noise. This is in part due to the interleaving technique. The energy associated with reflections, refractions and surface waves is concentrated below wave-numbers of 0.15 m-1 and frequencies below 25 Hz, leaving little opportunity to filter the refractions and surface waves from the reflection. Velocity Analysis In two of the better quality shot gathers (Figures 5 and 6) four different events can be distinguished: 2 refracted wave events, a direct P wave arrival, and a reflected wave arrival. These are annotated on the plot in Figure 9. At small offsets we see very slow and dispersive ground roll, annotated G. A weaker event (R 2 ) is barely visible in Figure 6 and Figure 9; it may be a second, deeper, reflection partially hidden behind the interpreted reflection R 1. The slopes of the first arrivals indicate three velocities: v1 = 480 m/s, v2 = 1400 m/s and v3 = 3000 m/s. These are relatively consistent for both gathers. The intersection times in Figure 9 (left) indicate two layer thicknesses of h0 = 1.2 m and h1 = 5.8 m. The intersection times in Figure 9 (right) indicate two layer thicknesses of h0 = 0.9 m and h1 = 9.1 m. 634

6 SAGEEP 2010 Keystone, Colorado Figure 5: At left, a raw record for the shot at position 18 m along line 1, and at right the same data after AGC and band pass. Figure 6: At left, a raw record for the shot at position 72 m along line 1 (southwest end of the line), and at right the same record after AGC and band pass filter. Figure 7: At left, a raw record for the shot at position 24 m along line 2 and at right the same data after AGC and band pass. 635

7 These differences are due both to estimation error and lateral variations of the geology and topography. However, they suggest two positive velocity discontinuities at approximately 1 m and 8 to 10 m. Comparing these depths to the log shown in Figure 1, suggests that these interfaces are not as deep as the coal and could be associated with the top of the thick sandstone bed. It is possible that the thin sandstone bed in the shale, or a sharp positive velocity gradient in the shale, is the reflector. The intersection time of the reflection is approximately 0.02 s. Using an estimated velocity of 1400 m/s, this would indicate a reflector at approximately 14 m depth. Normal Move Out Analysis In order to better constrain the velocities at the site and to gain certainty regarding the depth of the reflector, we analyzed the shot gather in Figure 6 to flatten the flank of the reflected event using normal move out (NMO), in Figure 10. Technically, NMO analysis is inappropriate for shot gathers; however we suggest that the approach is adequate because the seismic transect is a strike line and the reflector is not expected to dip strongly. Our goal is only to get an estimation of the velocity structure. Seismic line 1 runs along the geologic strike, so the reflecting layer should appear flat in these seismic data. 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 analysis. Various stacking velocities and normal incidence travel times were tested for their equivalent interval velocity and layer thicknesses. 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. It appears that the reflection originates at a depth of approximately 11 m. We did not perform NMO analysis on the possible second reflection (R2) because it is very weak and it is only seen on one of our shot gathers. Table 2: Various combinations of NMO stacking velocities and normal incidence travel times and their equivalent interval velocity and layer thickness. NMO stacking velocity (m/s) Normal incidence travel times (s) Interval velocity (m/s) Layer thickness (h) (m) 480, , , , , , , , , , , , , , , , Figure 8: (left) Average normalized frequency spectra of surveys at lines 1 and 2, (right) Frequency wave number spectrum of the shot recorded at line 1, near position 72 m. 636

8 Figure 9: At left is the shot located at 18 m along line 1 (same data as Figure 5), with a direct P-wave event annotated 1, and two refracted events annotated 2 and 3. At right is the shot located at the southwestern end of line 1, with possible reflected events annotated R 1 and R 2, a direct P-wave event annotated 1, and two refracted events are annotated 2 and 3. Dispersive ground roll is annotated G. Figure 10: The shot at southwestern end of line 1 (same as Figure 6). a) Plotted with no NMO, b) with NMO stacking velocity 1300m/s, c) with NMO stacking velocity 1150m/s, d) with NMO velocity 1000 m/s. Conclusions We conducted a simple seismic test at the Southern Ute Nation coal fire site, after conducting synthetic modeling of the experiment. Field data quality is at the high end of what can be expected for sledgehammer-source data. The recorded frequency content is strong up to at least 100 Hz. The test shots were interpreted to contain several refracted events and a reflection event. Refraction and reflection analyses suggest a one-meter-thick layer with velocity of a little less than 500 m/s on top, and a layer of about 9-10 m and a velocity of about m/s overlying a lowermost layer with a 637

9 velocity as high as 3000 m/s. The one prominent reflection event originates at a depth of approximately 11 m, which is well above the coal layer. It might hide possible reflections from the coal layer. 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 above the burned coal 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. There are no distinguishable events deep enough to adequately characterize the coal layer. Thus we must conclude that further surface seismic work at the site is unlikely to be successful at imaging the targeted coal or ash layer. Cross-well methods could find better success, though diffractions and other heterogeneity might also hinder these approaches. Acknowledgements We would like to acknowledge Lynn Orr from the Department of Energy Resources Engineering, Stanford University, for financial and aviation support of this project; Bill Flint, Kyle Siesser and Jonathan Begay from the Southern Ute Department of Energy for their financial support and expertise on the field site; Michael Krause from the Department of Energy Resources Engineering, Stanford University, for field support; the Southern Ute Indian Tribe Growth Fund for financial support and finally Jet West Geophysical Services for the log of Figure 1. We thank the sponsors of the Stanford Exploration Project for financial support of Sjoerd de Ridder. Bob Clapp and Biondo Biondi for helpful discussions on designing the numerical model. We are grateful to Shawn Larsen for providing and assisting with the e3d code. References to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. References Larsen, S.C. and J.C. Grieger, Elastic modeling initiative, Part III: 3-D computational modeling: 68th Ann. Internat. Mtg., , Soc. of Expl. Geophys. Martin, G.S., Marmoussi2: And upgrade for marmousi: The Leading Edge, 25, Miller, R.D., S.E. Pullan, D.W. Steeples, and J.A. Hunter, Field comparison of shallow P-wave seismic sources near Houston, Texas: Geophysics, 59, Miller, R.D., S.E. Pullan, D.W. Steeples, and J.A. Hunter, Field comparison of shallow seismic sources near Chino, California: Geophysics, 57, Wolf, K. H. A. A., The interaction between underground coal fires and their roof rocks: PhD thesis, Delft University of Technology. 638