Kansas Geological Survey, Open-file Report 2003-48
by Jianghai Xia, Kansas Geological Survey,
Chao Chen, China University of Geosciences,
Sihao Xia, The University of Kansas, and
Dave Laflen, Kansas Geological Survey
Report to Dennis Clennan, City of Hutchinson
KGS Open-file Report 2003-48
A natural gas explosion on January 17, 2001, destroyed two downtown Hutchinson businesses. Another explosion occurred a day after at a mobile home park 3 miles away. Two residents died of injuries from that explosion, which forced the evacuation of hundreds of people as gas geysers began erupting in the area. The geysers, following pathways to the land surface at both the explosion sites, were discovered to be abandoned brine wells once used for solution mining of salt. To find these abandoned brine wells is a part of the Hutchinson Response Project. After successfully locating one abandoned brine well by an electromagnetic method during a testing phase in 2001 and five abandoned brine wells by a high-resolution magnetic method during phase two in 2002, a high-resolution magnetic method was again proposed to search for wells in 2003 when a second sensor was employed to acquire data for calculating the pseudo-gradient of magnetic fields. The City of Hutchinson designated eight sites with a total area of 1,024,000 ft2 for investigation in 2003. These sites were divided into grids with an average size of 10,000 ft2. Survey-line spacing was 3 ft with a data density of 2.3 measurements/ft.
Magnetic anomalies and gradients from known brine wells were first recorded to help determine what buried brine wells might look like on magnetic data. Of forty-seven verified anomalies by excavation with a backhoe, twenty-nine anomalies were due to wells buried at depths from 0 to 8.5 ft: twenty-one 6- to 8-inch wells were abandoned brine wells, seven 1.5- to 3-inch wells were probably (water?) wells, and one 16-inch well was a dewatering well for a construction at a depth 3 ft. Two 4-inch wells were found without excavation because they were on the ground surface. Approximate monopole shape anomalies were observed from all these wells after data corrections. However, a wide range of amplitudes of magnetic anomalies—7,000 to 28,000 nT—from these abandoned brine wells was measured, which is mainly due to the thickness of wells and depths of buried wells. Anomaly amplitudes from the 1.5- to 3-inch wells are 4,000 to 8,000 nT that are linearly correlated with the buried depth. One 3-inch well that caused an anomaly with amplitude of 13,000 nT could be an inner pipe of a brine well. Gradient anomalies are roughly in a range of 100 to 200 nT/in for 1.5- to 3-inch wells and 200 to 300 nT/in for brine wells.
As indicated by the potential-field theory, gradient data possess higher horizontal resolution than magnetic field itself. Gradient data provide valuable assistance in deter-mining horizontal locations for excavation of anomaly sources. In practice, however, improvement in horizontal resolution is limited by survey line spacing. If only one sensor is used in a survey, a rapid decrease in the horizontal resolution results when the sensor height increases from 14 to 44 inches, indicating that it is critical to maintain sensor height as close to the ground as possible in hunting buried wells that are close to each other. It also suggests that downward continuation is useful to increase the horizontal resolution in well hunting.
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