Complex Resistivity (multi-frequency Induced Polarization)
Each equipment box in the diagram above is linked to the corresponding specification file.
Papers and Case Histories
50 years State of the Art in IP and Complex Resistivity
The use of resistivity and spontaneous potential by the Schlumberger brothers is documented at least as early as 1900 - over 100 years ago! Conrad Schlumberger received a patent on the IP technique in 1912. However, it was almost forty years before Newmont renewed interest in its use and application. From that time (the late 1940's) activity flourished for roughly forty years in both theory and practice, mainly in the search for disseminated sulfides; more specifically porphyries. However, with the crash of copper prices in 1983, interest in disseminated sulfide (porphyry copper) deposits declined dramatically with a concurrent drop in research concerning the source and nature of the induced polarization (IP) response. The precipitous decline in oil prices in 1985 further reduced interest in IP, which was being used as one of the non-seismic alternatives in hydrocarbon exploration. Only in the last few years has interest been renewed.
Despite this general lack of interest in the use of IP and IP research during the past 15 years, the development of instrumentation applicable to resistivity and IP surveys has continued at a fast pace, capitalizing on the development of powerful, high speed, low cost microprocessors. These new microprocessors also fueled the development of robust data processing routines and 2- and 3-D modeling and inversion programs.
Today research continues on the effects of hydrocarbons and other groundwater contaminants on the IP response. IP is used extensively in the search for precious metals by mapping areas hosting disseminated sulfides that may occur in conjunction with precious metals. Interest has been renewed in porphyry deposits in third-world countries, and complex resistivity (CR) or spectral IP is being used in attempts to discern the source of IP responses and to discriminate between valid metallic IP responses and electromagnetic (EM) coupling effects. Most recently IP has been found to be a cost-effective method in environmental surveys.
The Complex Resistivity Method
Induced Polarization methods have become increasingly important in geophysical exploration since their first major use in the 1950's. A reasonable number of successes have been attributed at least indirectly to these methods, and gross domestic IP expenditures have increased at a rapid rate-some 40% per year through the 1960's and 70's.
As useful as IP has been, there are several significant problems that have limited its success in certain areas. First, data are often contaminated by electromagnetic coupling due to array geometry and geologic layering, making it difficult to determine the economic validity of a measured anomaly. Second, economic and non-economic sources of induced polarization (e.g., chalcopyrite-versus pyrite, graphite, clays, etc.) are not distinguishable in conventional IP data, regardless of the coupling situation. Additional problems involve limitations in depth penetration due to array geometry and problems obtaining repeatable data with standard IP instrumentation in high-noise environments. As a result, IP is often a capable reconnaissance technique, but it is not well adapted for detailed exploration, especially outside the realm of hardrock mining interests.
In order to address the problems, a number of researchers began to investigate the possibilities of multi-frequency IP, know as "complex resistivity." The mineral discriminating capabilities of multi-frequency measurements became apparent through the work of Zonge (1972a,b) Van Voorhis, Nelson and Drake (1973), Katsube and Collette (1973), and others, but wide-scale field applications were not possible until a practical, general solution of EM coupling was found by Zonge in 1973. The proprietary decoupling methods developed by Zonge Engineering & Research Organization permitted the resolution of problems associated with IP and the application of electromagnetic methods to a wider range of exploration problems.
2-dimensional Inversion of Resistivity and IP data with Topography
Two-dimensional, smooth-model inversion of resistivity and induced polarization data produces image-like, electrical property sections which improve the data's interpretability. Recent software improvements enable routine smooth-model inversion of resistivity and induced polarization (IP) data. Nearly uniform starting models are generated by running broad moving-average filters over lines of dipole-dipole or pole-dipole data. Model resistivity and IP properties are then adjusted iteratively until calculated data values match observed values as closely as possible, given constraints which keep the model section smooth. Calculated values are generated with a finite element algorithm which can be adapted for accurate two-dimensional modeling of data collected in rough terrain. Smooth-model inversion of sample data show the method's utility as an interpretation aid and the importance of modeling topography in areas with significant relief.