Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 438, 1998 Aug; :1-2
Problem: Locating subsurface conductive solutions is important for many applications, including in situ mining and environmental protection. Drilling boreholes for sampling is expensive and can increase contamination by mixing water from different aquifers. Various noninvasive geophysical methods are available, but previous comparisons were hampered by different methods being used at different sites or at different times. The U.S. Bureau of Mines (USBM) compared methods at one site using the same conductive solution to improve the reliability of the comparisons. Locating deep (> 100 m) solutions is most difficult, so the USBM selected methods applicable for such depths. Objective: Improve the ability to locate subsurface conductive solutions by evaluating and comparing applicable geophysical methods. Approach: The experiment was performed cooperatively with other agencies to increase the number of methods studied. The USBM, the University of Arizona, Sandia National Laboratories, and Zonge Engineering and Research, Inc., conducted cooperative field tests of six electromagnetic (EM) geophysical methods to compare their effectiveness in detecting salt water brine of conductivity 3.6 S / m collecting at the water table, 157 m subsurface. The brine was injected through two boreholes. The test site was the University's San Xavier experimental mine near Tucson, AZ. Geophysical surveys using surface and surface-borehole, time-domain electromagnetics (TEM); surface controlled-source audiofrequency magnetotellurics (CSAMT); surface magnetic field ellipticity; surface-borehole, frequency-domain electromagnetics (FEM); and crosshole FEM were conducted before and during brine injection. A total of 110,000 L of brine was injected over one week, but the effective target volume of brine was much smaller because of mixing with ground water and dispersion. Results: Surface TEM and magnetic ellipticity appeared most cost effective because of good results and ease of use. However, each system revealed advantages and disadvantages. The surface TEM results showed a broad decrease in resistivity with injection, along with an interfering narrow response from a surface metallic conductor that appeared as a deep feature in a resistivity pseudosection. Surface TEM allowed rapid data collection because it did not require grounded electric dipoles, but it appeared more susceptible than the frequency-domain systems to electrical noise and the effects of the surface metallic conductor. Surface-borehole TEM data indicated strong changes as a result of brine injection and located brine-filled fractures. CSAMT measurements with a commonly used receiver orientation did not detect the brine, but measurements with an alternative configuration indicated some decrease in resistivity. Data collection was slower with CSAMT than with TEM because it required grounded electric dipoles. CSAMT data were less affected by the surface metallic conductor, but static corrections for the effects of near-surface conductive regions on the electric dipole measurements were large. The surface magnetic field ellipticity surveys also indicated a decrease in resistivity at depth following brine injection. This method offers the advantage of a frequency-domain system's reduced susceptibility to electrical noise. Unlike CSAMT, however, it avoids grounded electric dipoles and the resulting static effects. This method is still experimental, and not generally available from geophysical consulting companies. Surface-borehole FEM showed high sensitivity to the brine. While measuring time-dependent changes due to injection, surface-borehole FEM detected 600 L of brine with the receiver at a depth of 91 m, over 35 m from the injection point and over 60 m above the water table. A novel approach using the two horizontal components of the magnetic field to compute ellipticity was very responsive to brine injection. The crosshole FEM method provided data consistent with a conductivity log. Conductive zones generally appeared as peaks at correct locations and changes in the data with brine injection were easily recognizable. Monitoring conductive solutions presents a different set of challenges than exploration does. Collecting data before and after injection is important because monitoring requires separating the effects of geology from the effects of the solution. Measuring all the components of electric and magnetic fields available with selected methods was beneficial. With CSAMT, only the less commonly measured fields showed a response to the brine. TEM horizontal magnetic components helped to identify a source of interference, and FEM surface-borehole horizontal magnetic components allowed surface-borehole ellipticity to be calculated. Detecting the small amount of brine at the depth of this experiment was a significant achievement for these systems. The results demonstrated considerable promise for expanding the role of EM methods in detecting and monitoring subsurface conductive solutions.
Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 438