NATURAL GAS EXPLOSIONS IN HUTCHINSON, KANSAS: DETECTING GAS WITH HIGH-RESOLUTION SEISMIC DATA

A series of natural gas explosions occurred in Hutchinson, Kansas, in January 2001. These explosions coincided with a leak at the Yaggy gas storage field, 7 miles northwest of the city. A shallow, high-resolution seismic reflection survey was conducted in order to detect seismic anomalies, which might indicate the presence of gas and identify potential gas pathways from Yaggy to Hutchinson. A 1/3 mile long, east-west seismic line was located in Rice Park, on the west side of Hutchinson, and a 3-1/2 mile long, north-south seismic line was located on Wilson Road, midway between Yaggy and Hutchinson. The Rice Park line was located near a well (DDV5) that had been drilled to vent gas from under the city. Seismic reflections on the two lines were tied to known rock formations using synthetic seismograms created from sonic and density logs from a well in Yaggy. At the level of the proposed gas-bearing interval, the Rice Park line appeared to show a dim out corresponding to the gas at DDV5. On the Wilson Road line, two amplitude anomalies, approximately 1/3 mile apart, were identified and drilled. One anomaly was a dim out and the other was a bright spot. Gas was found at both locations. Core and well log data from vent wells indicate that the gas-bearing interval corresponds to thin dolomite layers, which have higher P-wave velocities than the surrounding shales. Gas within fractures in these dolomites appears to reduce the velocity of the dolomite interval down to or below that of the shales. Seismic modeling indicates that, depending upon the magnitude of the gas effect, a dim out or bright spot is produced, and that the associated seismic anomaly is diminished as gas dissipates from a given location.