Abstract

Detection of fractures in bedrock, which is essential for hazardous waste site evaluation and for modeling of local ground-water flow in fractured terranes, is difficult where exposure is poor, but may be accomplished using a newly developed method based on soil gas analysis. This method is based upon the accumulation of thermogenic gas in soil above open bedrock fractures. Fractures may provide high permeability pathways through bedrock; if the underlying units contain natural gas, gas will migrate upward through open fractures, producing anomalously high methane concentrations in the soil above. Over ten thousand soil gas analyses from western New York demonstrated that fractures of the Clarendon-Linden Fault System could be traced with soil gas analyses through an area largely covered with unconsolidated glacial deposits. Interpretation of soil gas anomalies as indicators of underlying fractures was confirmed by correlation with outcrop, well log, seismic, and remote sensing data. Samples were collected at a spacing of 10 m, from a depth of approximately 60 cm, using a stainless steel probe driven with a sledgehammer. Analysis of the 60 cc samples was done on-site with a portable gas chromatograph. Ethane/methane ratios were used to distinguish thermogenic gas from biogenic methane. The excellent correlation of linear soil gas anomalies with mapped fractures suggests that where gas-bearing units underlie an area, soil gas analysis may provide a method of mapping faults and fractures where exposure is too poor for conventional mapping.

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