Kansas Geological Survey, Open-file Report 2001-17
by Jianghai Xia
Kansas Geological Survey
1930 Constant Avenue, Campus West
Lawrence, KS 66047
KGS Open File Report 2001-17
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 microGal). The sensitivity of the most advanced gravitymeter is at a one microGal 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 x 100 ft x 100 ft buried at a depth 400 ft, a typical depth of salt voids in Hutchinson area. The maximum anomaly from the cavern is approximately 25 microGal, assuming that the cavern is completely empty. In actuality, the maximum anomaly due to the cavern will be much less than 25 microGal because caverns are always filled with water, soil, and/or rocks, which makes a density contrast considerably smaller. To detect this 25-microGal anomaly, sensitivity of the gravitymeter and accuracy of elevation measurements are critical. The sensitivity of the most advanced gravitymeters available in the market is 1 to 10 microGal. 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-microGal sensitivity level.
Figure 1. Well 8C, an abandoned brine well, cost two people their lives.
Elevation measurements are the other main challenge in the microgravity survey. The error associated with elevation measurements is 5 microGal 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 amplitude less than 25 microGal, the sensitivity of gravitymeter should not be less than 4 microGal (in the range of 1-4 microGal) 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-microGal 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 maximum 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.
Figure 2. EM survey with a GEM-2.
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 investigation 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).
Figure 3. An EM survey grid is centered at Well 8C.
Figure 4. Arrows indicate the walking direction.
|Well 8C--Click on figures to view larger versions|
|Figure 5a. 2430 Hz in Phase.||Figure 5b. 2430 Hz Quadrature.|
|Figure 5c 7290 Hz in Phase.||Figure 5d 7290 Hz Quadrature.|
|Figure 5e 18270 Hz in Phase.||Figure 5f 18270 Hz Quadrature.|
|2,430 Hz (I)||2,430 Hz (Q)||7,290 Hz (I)||7,290 Hz (Q)||18,270 Hz (I)||18,270 Hz (Q)|
I expected to see a bulls-eye shape anomaly from the well. The anomaly shown in Figure 3 is in an ellipse shape with a longer axis in an east-west direction. I walked along an east-west direction with a GEM-2 oriented in the same direction. The sample point is about 2 ft away from the receiver coil. Data recorded along each line were linearly interpolated based on the starting point and the ending point when generating Figures 5a-5f. Therefore, I believe that the ellipse-like anomaly caused by well 8C instead of a bulls-eye anomaly is due to the way I acquired data and the 6-ft distance between a transmitter coil and a receiver coil of a GEM-2. The 6-ft distance between a transmitter coil and a receiver coil of a GEM-2 is also a criterion of a horizontal location of an anomaly object. Based on the results from well 8C, I would conclude that half the distance between a transmitter coil and receive coil (3 ft) of a GEM-2 might be an estimated accuracy of the horizontal location of an anomaly object. The elliptical effect due to the 6-ft coil distance and the way of walking along lines will be reduced when lines are much longer than the testing grid at well 8C. Thus, bulls-eye anomalies are my main objectives in locating abandoned brine wells.
When using the signals to compare data from an assigned area, two other characters may be changed. The depth of a well header affects the size of a bulls-eye and amplitude of signals. The deeper the well, the broader (in horizontal dimensions) the bulls-eye and the lower the amplitude will be. Abandoned wells were normally buried 3-4 ft under the ground. The signals from well 8C are from a well header on the ground. Thus, the bulls-eye anomalies from buried wells should be broader than the bulls-eye (Figure 5) from well 8C with a normally lower amplitude.
With a different GEM-2 instrument, different signals, polarity and amplitude, may be acquired on the same site due to calibration of the instrument. Thus, it is important to obtain signature readings from something that is known to be what the survey is looking for.
Figures 7a - 7f present the south part of the GEM-2 results. The anomaly located at (110, 40) was not identified during anomaly verifications in the March trip (Xia, 2001). This anomaly showed a negative bulls-eye on all components except for the 18,270 Hz quadrature component (Figures 7a - 7f). Due to a target depth, it is expected that anomaly might disappear in higher frequency components. The 2,430 Hz results showed the highest amplitude in three frequencies (Table 2). Comparing Table 2 with Table 1, Xia (2001) interpreted that this anomaly to be caused by a buried well.
Table 2. Amplitudes (in ppm) of EM signals from anomaly at point (110, 40).
|2,430 Hz (I)||2,430 Hz (Q)||7,290 Hz (I)||7,290 Hz (Q)||18,270 Hz (I)||18,270 Hz (Q)|
To review Table 2, strong in-phase anomalies are shown in all three frequencies. A relatively strong quadrature anomaly is shown in the 2,430 Hz frequency results. This indicates that, based on the skin depth concept (Won, 1980), the investigation depth of a GEM-2 for a 4-inch well in diameter could be as deep as 20-30 ft in the Hutchinson area if frequencies around 1,000 Hz are used.
Won, I.J., 1980, A wideband electromagnetic exploration method--Some theoretical and experimental results: Geophysics, 45, 928-940.
Won, I.J., Keiswetter, D., Hanson, D., Novikova, E., and Hall, T. 1997, GEM-3: A Monostatic Broadband Electromagnetic Induction Sensor: Journal of Environmental and Engineering Geophysics, 2(1), 53-64.
Won, I.J., Keiswetter, D.A., Fields, G.R.A., and Sutton, L.C., 1996, GEM-2: A new multifrequency electromagnetic sensor: Journal of Environmental & Engineering Geophysics, 1, 129-137.
Xia, J., 2001, Feasibility study on using the electromagnetic method to locate abandoned brine wells in Hutchinson, Kansas: Kansas Geological Survey Open-file Report 2001-10.