Kansas Geological Survey, Open-file Report 2004-20
KGS Open-file Report 2004-20
O'Brien and Gere, Syracuse
A natural gas explosion on January 17, 2001, destroyed two downtown Hutchinson businesses. A day later another explosion at the Big Chief Mobile Home Park three miles away took the lives of two residents and forced the evacuation of hundreds of people as gas geysers began erupting in the area. The pathways to the land surface at both the explosion sites and the geysers were abandoned brine wells used for solution mining of salt. A high-resolution magnetic and gradient survey was performed at the Big Chief Mobile Home Park in November 2003 in hopes of finding any hidden brine wells. The total survey area was 670,000 ft2. High-resolution magnetic data and gradient data were acquired at 526,000 locations along lines three feet apart. Monopole-shape magnetic anomalies with high amplitude (> 20,000 nT), a high vertical gradient (> 150 nT/in), and a wide half-width (> 6 ft) were detected at locations of all five known brine wells in the survey area. Survey results did not suggest existence of any other brine wells. Forty-two anomalies were verified. One resident water well was found.
On January 17, 2001, a natural gas explosion and fire destroyed two downtown Hutchinson businesses. The next day another explosion occurred at the Big Chief Mobile Home Park three miles away. Two residents died of injuries from the explosion, which forced the evacuation of hundreds of people as gas geysers began erupting in the area. The geysers spewed a mixture of natural gas and saltwater. The pathways to the land surface at both the explosion sites and the geysers were abandoned brine wells used for solution mining of salt (http://www.kgs.ku.edu/Hydro/Hutch/Background/index.html, Allison, 2001).
The known brine wells in the mobile home park had steel cased pipes. Xia (2001a, 2001b, 2002a) successfully located one buried abandoned brine well by an electromagnetic method during a testing in 2001. In a noisy urban environment, however, electromagnetic signature recognition and the investigation depth by the electromagnetic method still remain a challenge (Xia, 2002a).
The Kansas Geological Survey has a long history of using gravity and magnetic methods to solve regional geological problems (Yarger, 1983, 1989; Lam and Yarger, 1989; Xia et al., 1992, 1995a, 1995b, 1995c, 1996). Algorithms related to data processing and interpretation were developed (Yarger et al., 1978; Xia and Sprowl, 1991 and 1992, Xia et al., 1993). Although this past research focused on deep (> 700 ft) geology, the fundamentals of anomalies induced by the geomagnetic fields in the near-surface (< 20 ft) materials remain the same.
The length of vertical steel pipes in brine wells in the Hutchinson area normally is 400-700 ft. The maximum magnetic signal caused by this amount of pipe can be higher than 15,000 nT on top of the normal geomagnetic field in Hutchinson, Kansas (Appendix A). This huge anomaly shows promise in locating brine wells in the city noise environment. Five abandoned brine wells, five water wells, and one probable gas pipe were located by a high-resolution magnetic method in the summer of 2002 (Xia, 2002c; Xia and Williams, 2003, 2004). We also found that wells could be in the vicinity of only a few feet, which requires that a geophysical method should possess a certain horizontal resolution. The horizontal resolution of the high-resolution magnetic method employed in that phase may not be high enough in some situations (Xia et al., 2003).
To increase the horizontal resolution, a vertical gradient method was proposed to search for wells in the summer of 2003 when a second sensor was employed to acquire data for calculating pseudo-gradient of magnetic fields. The City of Hutchinson designed eight sites with a total area of 1,024,000 ft2. Magnetic anomalies and gradients from known brine wells were first recorded as signatures to use in identifying anomalies caused by possible buried brine wells. Of forty-seven anomalies verified 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 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 from these abandoned brine wells was measured. It was from 7,000 to 28,000 nT, mainly due to the thickness of wells and depths wells were buried. Anomaly amplitudes from 1.5- to 3-inch wells were 4,000 to 8,000 nT and linearly correlated with the buried depth. One 3-in well that caused an anomaly with amplitude of 13,000 nT could be the inner pipe of a brine well. Gradient anomalies were 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 (Xia et al., 2003, 2004).
Based on the results of the magnetic survey around well C8, where two residents died of injuries from the explosion, the other brine well that was 14 ft north of well C8 was found (Xia et al., 2004). A high-resolution magnetic and gradient survey was performed over the entire Big Chief Mobile Home Park in November of 2003, hoping to find any hidden brine wells.
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Kansas Geological Survey, Geophysics
Placed online Feb. 10, 2006
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