Using the Electromagnetic Method to Locate Abandoned Brine Wells in Hutchinson, Kansas
Jianghai Xia

Summary
An electromagnetic (EM) survey with a GEM-2, an electromagnetic instrument, was conducted in Hutchinson, Kansas, as a part of effort to locate abandoned brine wells. EM results successfully located one uncapped abandoned brine well, 4 inches in diameter and buried at a depth of 5 ft. This survey result indicates the potential investigation depth with a GEM-2 would be as deep as 20 ft in locating abandoned wells in the Hutchinson area. The survey also demonstrated the importance of acquiring target signals in inter-preting anomalies. The results of EM survey in Hutchinson demonstrated successfulness and effectiveness in locating the abandoned brine wells. Introduction On January 17, 2001, a natural gas explosion and fire destroyed two downtown Hutchinson businesses. The next day at a mobile home park 3 miles away, another explosion occurred. 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.ukans.edu/Hydro/Hutch/Background/index.html). To find these abandoned brine wells is a part of the Hutchinson Response Project. Hutchinson City and state officials estimate that there may be more than 160 abandoned brine wells in and around Hutchinson. It costs about $60,000 to plug an abandoned well (Hutchinson News, May 9, 2001). Some known wells in the mobile home park had steel cased pipes (Figure 1). A microgravity survey was not proposed to locate abandoned brine wells because anomalies due to brine wells or salt voids are too weak to be detected by this method. The length of vertical steel pipe normally is 400 - 700 ft. The predicted maximum gravity signal caused by this pipe is only 4 – 6 µGal (microgal). The sensitivity of the most advanced gravity-meter is at a one µGal level so this anomaly is too weak to find using microgravity survey. I also calculated the gravity anomaly caused by a salt cavern with a volume of 100 ft × 100 ft 100 ft buried at a depth 400 ft, a typical depth of salt voids in Hutchin-son area. The maximum anomaly from the cavern is approximately 25 µGal, assuming that the cavern is completely empty. In actuality, the maximum anomaly due to the cavern will be much less than 25 µGal because caverns are always filled with water, soil, and/or rocks, which makes a density contrast considerably smaller. To detect this 25-µGal anomaly, sensitivity of the gravitymeter and accuracy of elevation measure-ments are critical. The sensitivity of the most advanced gravitymeters available in the market is 1 to 10 µGal. It takes much longer (normally more than 15 minutes/station) to × acquire a microgravity data than a normal exploration gravity survey in order to achieve the 1-µGal sensitivity level. Elevation measurements are the other main challenge in the microgravity survey. The error associated with elevation measurements is 5 µGal per inch. In practice, one inch accuracy could be achieved by the most advanced Trimble GPS system. To confidently identify a gravity anomaly, the maximum anomaly should be at least three times higher than possible errors. Therefore, to see an anomaly with an ampli-tude less than 25 µGal, the sensitivity of gravitymeter should not be less than 4 µGal (in the range of 1-4 µGal) and the accuracy of elevation measurements should be within one inch. It is very difficult to achieve an accuracy of elevation survey with one-inch range. In addition, to detect this 25-µGal anomaly in an urban area, culture noise will become a serious problem. A 3-D ground penetrating radar (GPR) survey may be useful to locate these wells. The ground is dirt fill, however, and there could be a lot of reflected/diffracted events caused by objects other than the brine wells. Furthermore, time spent on 3-D GPR data acquisition and processing could be much longer than might be expected. I proposed to use the eletromagnetic (EM) method to search for wells. A GEM-2 (Figure 2) is an EM instrument that can survey an area quickly and with great detail (Won, 1980). Data can transferred into a notebook computer and maps generated within a few minutes after the survey is done. The GEM-2 is a portable, digital, broadband electromagnetic sensor. Multi-frequency data are acquired simultaneously with a maxi-mum sampling rate of 30 Hz when an instrument operator walks along a survey line. For each frequency, both in-phase and quadrature components of the induced EM field in ppm (parts per million relative to the primary field) were recorded. The measured in-phase and/or quadrature responses can be used to calculate apparent conductivity and apparent magnetic susceptibility based on a homogeneous half-space assumption by Won et al. (1996 and 1997). Apparent conductivity and apparent magnetic susceptibility are parameters that in general are related to targeted electrical and magnetic properties. Calculation of apparent conductivity and apparent magnetic susceptibility is a method of normalization of the EM data; it makes data analysis and interpretation easier for both geophysicists and other scientists. If the earth were truly homogeneous, the apparent conductivity would be the same at all frequencies and equal the true earth conductivity data (Huang and Won, 2001). In the real world, conductivity measurements are “bulk” or apparent conductivity. We will omit a word “apparent” from now on. Quadrature data are proportional to the ground conductivity in the low to middle induction numbers, but are inversely proportional to the conductivity at middle to high induction numbers. Thus, a moderate conductor may produce a strong quadrature anomaly, whereas a good conductor may produce a weak anomaly or no anomaly. In either case, in-phase data have to be used for further analysis (Huang and Won, 2001). An anomaly shown on conductivity maps should also show on in-phase and/or quadrature data. The investigation depth is dependent on the frequency of the instrument used in the survey, conductivity and magnetic susceptibility of a target and surrounding materials. There is no exact relation between instrument frequencies and the investigation depth. A skin depth concept (Won, 1980) may be used to obtain rough estimates of the investiga-tion depth in a specific survey area. An EM survey was conducted in the open field on the southwest corner of 11th and Chemical Streets (Xia, 2001). Four anomalies were identified and reported. Anomaly four was caused by an abandoned brine well, 4 inches in diameter and buried in 5 ft deep. The first three anomalies were discussed in Xia (2001).