APPLICATION OF HIGH-RESOLUTION MAGNETIC METHOD AND GRADIENT METHOD TO LOCATE ABANDONED BRINE-WELLS IN HUTCHINSON, KANSAS, U.S.A.

After successfully locating one abandoned brine well by an electromagnetic method during testing in 2001 and five abandoned brine wells by a high-resolution magnetic method during 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-vertical gradient of magnetic fields. Total area surveyed in 2003 was 1,024,000 ft2, which was divided into grids with an average size of 10,000 ft2 and distributed across eight different sites. Magnetic anomalies and their vertical gradients from known brine wells were first recorded as signatures to identify anomalies caused by possible buried brine wells. Of fifty-one verified anomalies, thirtyone anomalies were due to wells buried at depths from 0 to 8.5 ft: twenty-one 6- to 9-inch abandoned brine wells, seven 1.5- to 3-inch probable water wells, one 16-inch dewatering well for a construction site at a depth of 3 ft, and two 4-inch wells on the ground surface. Approximate monopole shaped anomalies were observed from all these wells after data corrections. However, the range of amplitudes of magnetic anomalies from 7,000 to 28,000 nT from these abandoned brine wells was measured. This range of anomalies is mainly due to the thickness and depth of buried wells. Anomaly amplitudes from 1.5- to 3-inch wells are 4,000 to 8,000 nT and linearly correlate with the buried depth. One 3-inch well that caused an anomaly of 13,000 nT could be the inner pipe of a brine well. Gradient anomalies are roughly in a range of 100 to 200 nT/inch for 1.5-to 3-inch wells and 200 to 300 nT/inch for brine wells.As indicated by the potential-field theory, gradient data possess higher horizontal resolution than the magnetic field itself. Gradient data provide valuable assistance in determining horizontal locations of anomaly sources for excavation. In practice, however, improvement in the horizontal resolution is limited by surveyline spacing. If only one sensor is used in a survey, there is rapid decrease in the horizontal resolution when sensor height increases from 14 to 44 inches. This indicates that it is critical to keep the sensor as close to the ground as possible when hunting buried wells that are close to each other. It also suggests that the downward continuation is useful to increase the horizontal resolution in well hunting.