Using High-Resolution Magnetic Method And Gradient Method To Locate Abandoned Brine Wells In Hutchinson, Kansas

After successfully locating one abandoned brine well by an electromagnetic method<br>during testing in 2001 and five abandoned brine wells by a high-resolution magnetic method<br>during 2002, a high-resolution magnetic method was again proposed to search for wells in 2003<br>when a second sensor was employed to acquire data for calculating the pseudo-vertical gradient<br>of magnetic fields. Total area surveyed in 2003 was 1,024,000 ft2, which was divided into grids<br>with an average size of 10,000 ft2 and distributed across eight different sites. Magnetic anomalies<br>and their vertical gradients from known brine wells were first recorded as signatures to identify<br>anomalies caused by possible buried brine wells. Of fifty-one verified anomalies, thirty-one<br>anomalies were due to wells buried at depths from 0 to 8.5 ft: twenty-one 6- to 9-inch abandoned<br>brine wells, seven 1.5- to 3-inch probable water wells, one 16-inch dewatering well for a<br>construction site at a depth of 3 ft, and two 4-inch wells on the ground surface. Approximate<br>monopole shaped anomalies were observed from all these wells after data corrections. However,<br>a range of amplitudes of magnetic anomalies from 7,000 to 28,000 nT from these abandoned<br>brine wells was measured. This range of anomalies is mainly due to the thickness and depth of<br>buried wells. Anomaly amplitudes from 1.5- to 3-inch wells are 4,000 to 8,000 nT and linearly<br>correlate with the buried depth. One 3-inch well that caused an anomaly of 13,000 nT could be<br>the inner pipe of a brine well. Gradient anomalies are roughly in a range of 100 to 200 nT/inch<br>for 1.5- to 3-inch wells and 200 to 300 nT/inch for brine wells.<br>As indicated by the potential-field theory, gradient data possess higher horizontal<br>resolution than the magnetic field itself. Gradient data provide valuable assistance in determining<br>horizontal locations of anomaly sources for excavation. In practice, however, improvement in the<br>horizontal resolution is limited by survey-line spacing. If only one sensor is used in a survey,<br>there is rapid decrease of the horizontal resolution when sensor height increases from 14 to 44<br>inches. This indicates that it is critical to keep the sensor as close to the ground as possible when<br>hunting buried wells that are close to each other. It also suggests that the downward continuation<br>is useful to increase the horizontal resolution in well hunting.