Laserfiche WebLink
Svmposimn on Geotechnical Methods for Mine Mapping Verificatiom Charleston, West Virginia October 29 2002 <br />Case History No. 6 — Locating Deep Mine Workings for Methane Extraction with the <br />S -Wave Seismic Reflection Technique in Illinois <br />A short (235 -foot long) S -wave profile was obtained by Bay Geophysical Associates over <br />an abandoned coal mine to detect mine openings that would be suitable targets for the in- <br />stallation of methane extraction wells. Two boreholes had been drilled on the basis of an <br />old mine map, without encountering the mine. An S -wave survey was selected to detect <br />the mine voids because S -waves will not transmit through a void whereas P-waves, the <br />type of seismic wave normally associated with seismic exploration, can be transmitted <br />through a void space. Mine voids should therefore appear as a "bright spot' on the S - <br />Wave record. <br />The S -wave survey was conducted with the following equipment setup and field parame- <br />ters: <br />• Bison 9048 seismograph, 48 channels <br />• Oyo Geospace 40 Hz horizontal geophones <br />• Source: hammer -struck railroad tie <br />• Trace interval: 2.5 feet <br />• Coverage: 48 fold <br />The data were processed by a commercial processing firm following steps routinely fol- <br />lowed to process oil and gas exploration S -wave data. The coal seam could be related to <br />the seismic reflections and an area where the floor of the mine had a relatively low ampli- <br />tude reflection compared to the roof was targeted for an exploratory boring. Mine workings <br />were encountered by that boring at a depth of close to 700 feet, as predicted. <br />Case History No. 7 — S -Wave Seismic Survey over Abandoned Coal Mine Workings, <br />Des Moines, Iowa <br />Control borings at a proposed construction site in Des Moines, Iowa encountered a col- <br />lapsed mine in one borehole and coal was found to be missing in several other borings. <br />The attached figure shows a high-resolution S -wave seismic reflection seismic profile that <br />crosses a control boring that encountered a void with wood cuttings (from timbers used to <br />support the mine). This profile was obtained by a high -frequency S -wave vibrator devel- <br />oped by Bay Geophysical Associates specifically to be able to measure high-resolution <br />profiles for engineering application to a maximum depth of approximately 300 feet. The <br />coal seam of interest in this case is present at a depth of between about 50 — 60 feet. The <br />profile shows a break in the reflection from the coal where the control boring encountered <br />mine workings. <br />The seismic profile also shows a discontinuity of the coal seam between stations 60 and <br />110. When several seismic profiles were correlated, it was found that this break in the coal <br />seam was actually a natural channel cut through the coal. The geotechnical borings that <br />had not encountered the coal were found to have been drilled through this natural channel <br />fill. Abandoned mine workings, such as penetrated by the single geotechnical boring and <br />also visible on the seismic reflection profile, were not found to be a problem over most of <br />the construction site. <br />17 <br />