
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.