Induced Polarization with Electromagnetic Coupling: 3D Spectral Imaging Theory EMSP Project No. 73836
This report summarizes the research work completed on the project between December 2001 and September 2002. (1) A model of all Spectral IP capacitive couplings revealed that potential bearing electrodes should be carefully chosen to obviate some of the capacitive coupling problems. This need becomes more important for borehole sampling. Thus, work had been done to design a porous pot electrode hat has all the desired characteristics (low input impedance = 100{Omega}, low noise = 1 {micro}V/{radical} z, low temperature sensitivity = 10{micro}V/{sup o}C) and that can be implanted in a borehole for up to two ears. Further constructional/fabrication details will be given in the final report. The attached pictures are rom a sample of the prototype electrode. Four strings, each consisting of 14 electrodes (7 potential electrodes alternated with 7 metallic-copper current electrodes, each electrode 6ft apart), were constructed and are to be employed into the four boreholes. (They were eventually deployed in Dec. 2002 and measurements acquired in March 2003). (2) The MIT's Earth Resources Laboratory (ERL) performed Spectral Induced Polarization SIP and Time Domain Induced Polarization (TDIP) measurements at the A-14 Outfall during the summer of Y01 as a participant in a DOE-sponsored exercise to assess the state-of-the-art in cross-borehole IP technology for delineating subsurface contaminants. To demonstrate the utility of SIP to map DNAPL contaminants, we inverted cross-borehole SIP data, taken within a very narrow frequency bandwidth of 1/32 o 9/32 Hz. The narrow bandwidth was selected after carefully studying when effects of emc, electrode polarization, etc. begin to set in. The upper frequency is limited by electromagnetic couplings (emc) and strong capacitive behavior observed for the electrodes and the low-frequency limit is set by the time to take measurements. Because below 9/32 Hz, the IP response seems to be greater than emc in all our measurements, the data was considered invertible by our existing 2D/3D complex resistivity codes. The results of this exercise were inconclusive because the ground-truthing phase of the operation failed to detect ny concentration of NAPL above a requisite threshold of 40-50{micro}g/g. It is our understanding that this threshold level is based upon analytic chemical partition analysis, which is dependent upon the physicochemical properties of the soil, its pore-fluid, and organic constituents [Cohen and Mercer, 1993], and thus represents a necessary and sufficient condition to confirm the presence of NAPL. Therefore, since the round-truthing phase never found PCE concentrations in excess of {approx}3mg/g, there is no irrefutable evidence of NAPL; hence, the objective of the FY01 exercise could not be completed. Nonetheless, ERL's conversion results agree well with the ground truth considering the sparseness of the CRS boreholes, low concentrations of ''PCE'' (< 3{micro}g/g), and despite poor electrodes. Note that the displacement of the enter of the SIP phase anomaly from that of the ground truth data might be due to inaccuracies in the SIP conversion (which we are still working on improving) or sampling depth errors during the ground-truthing phase which could cumulatively amount to a meter or more. (3) As a continuation of the FY01 efforts, another et of surface and borehole SIP measurements were planned at another SRS site during FY02, which unlike he FY01 site,had been verified to have substantial DNAPL presence by SRS engineers. The plan to use phoenix Geophysics SIP equipment could not materialize because Phoenix discontinued its SIP line and planned to introduce a new line in the Spring/Summer of 2003, which will be too late for us. So our planned second fieldwork of FY02 defaults again to Zonge Engineering and Research Organization equipment. (4) developments on the modeling and inversion of the new complex resistivity code, that will incorporate conductive coupling, stalled to give way to the electrode design, fabrication and testing(s), which were not anticipated earlier.
- Research Organization:
- Massachusetts Institute of Technology, Cambridge, MA (US)
- Sponsoring Organization:
- USDOE Office of Science (SC) (US)
- DOE Contract Number:
- FG07-96ER14714
- OSTI ID:
- 834657
- Report Number(s):
- EMSP-73836-2003; R&D Project: EMSP 73836; TRN: US200433%%279
- Resource Relation:
- Other Information: PBD: 1 Jun 2003
- Country of Publication:
- United States
- Language:
- English
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