THIS PROJECT HIGHLIGHTS TWO IMPORTANT POINTS: 1) THE VERY GOOD AGREEMENT OF RESULTS BETWEEN IP AND CSAMT SURVEY METHODS, AND 2) THE EXCELLENT COMPARISON BETWEEN THE INVERSION MODELS FROM THE TWO SURVEY METHODS. BOTH DATA SETS WERE ACQUIRED BY THE SAME FIELD CREW, USING THE SAME GDP-16 RECEIVER AND GGT-10 TRANSMITTER EQUIPMENT.
THE FOLLOWING EXAMPLE SHOWS THE DIPOLE-DIPOLE DATA AND THE CSAMT DATA THAT WERE ACQUIRED ALONG THE SAME LINE DURING A TRAINING SESSION AT WADI ALMARSAD IN THE KINGDOM OF JORDAN. THE LINE IS ON RELATIVELY FLAT GROUND, CROSSING A NARROW VALLEY. THE ALLUVIAL FILL MATERIAL IS OF AN UNKNOWN DEPTH, BUT EXPECTED TO BE LESS THAN 200 METERS, EXCEPT IN THE CENTER OF THE VALLEY.
FOR A NUMBER OF YEARS, ZONGE INTERNATIONAL HAS BEEN INVOLVED IN THE DEVELOPMENT OF INSTRUMENTS FOR ELECTRICAL RESISTANCE TOMOGRAPHY (ERT) IN CONNECTION WITH ONGOING RESEARCH AND DEVELOPMENT AT THE LAWRENCE LIVERMORE NATIONAL LABORATORY, A FACILITY FUNDED BY THE U.S. DEPARTMENT OF ENERGY. ERT IS BEING USED TO GENERATE RESISTIVITY IMAGES OF THE 'PLANE' DEFINED BY THE SPACE BETWEEN TWO BOREHOLES OR OTHER ELECTRODE STRINGS. ERT IS PRESENTLY BEING APPLIED TO MONITOR LEAKAGE AT HAZARDOUS WASTE SITES, TO MONITOR DYNAMIC FLUID INJECTION PROCESSES THAT ARE OFTEN INVOLVED IN WASTE SITE REMEDIATION. OTHER APPLICATIONS FOR ERT TECHNOLOGY EXIST AND ARE SIMPLY WAITING FOR THE AVAILABILITY OF EQUIPMENT AND SOFTWARE THAT CAN ECONOMICALLY ACQUIRE THE NECESSARY RESISTIVITY DATA REQUIRED AND THE SOFTWARE REQUIRED FOR ITS INTERPRETATION.
ERT INVOLVES THE ACQUISITION OF HUNDREDS, EVEN THOUSANDS OF 4-ELECTRODE RESISTIVITY MEASUREMENTS THAT ARE POSSIBLE BETWEEN MULTIPLE STRINGS OF ELECTRODES. FOR EXAMPLE, GIVEN TWO STRINGS OF 15 EQUALLY-SPACED ELECTRODES (30 ELECTRODES TOTAL), THERE ARE 632 DIFFERENT DIPOLE-DIPOLE MEASUREMENTS THAT CAN BE MADE (INCLUDING ALL RECIPROCAL MEASUREMENTS) INVOLVING TRANSMITTER AND RECEIVER DIPOLES WITH A FIXED LENGTH OF 2 ELECTRODE SPACINGS. THE DENSE DATA PROVIDE THE BASIS FOR SOLVING SOPHISTICATED INVERSE COMPUTER MODELS OF THE CONDUCTIVITY DISTRIBUTION IN THE GROUND. WHEN BOTH RESISTIVITY AND IP ARE MEASURED, THE RESULTING DATA SET RELATES TO THE COMPLEX IMPEDANCE OF THE EARTH AND THE TERM EIT (ELECTRICAL IMPEDANCE TOMOGRAPHY) IS SOMETIMES USED TO DESCRIBE THE TECHNIQUE.
OBVIOUSLY, THERE ARE TOO MANY MEASUREMENTS TO BE ACQUIRED MANUALLY. TO EFFICIENTLY MEASURE ALL THE DESIRED TRANSMITTER-RECEIVER ELECTRODE COMBINATIONS REQUIRES A COMPUTERIZED ACQUISITION SYSTEM THAT AUTOMATICALLY SWITCHES BOTH TRANSMITTER AND RECEIVER ELECTRODES AND HAS MULTI-CHANNEL MEASUREMENT CAPABILITIES. THE ZONGE ERT/EIT ACQUISITION SYSTEM HAS UNIQUE CAPABILITIES FOR THE EFFICIENT ACQUISITION OF ERT OR EIT DATA SETS. WITH THE ZONGE ERT/EIT SYSTEM, ACQUISITION OF THE DENSE DATA SETS IS NOW ECONOMICALLY FEASIBLE. SUCH DATA SETS ARE REQUIRED TO GENERATE CONDUCTIVITY IMAGES USING SOPHISTICATED INVERSION SOFTWARE. THESE CAPABILITIES GREATLY IMPROVE THE USEFULNESS OF THE VENERABLE RESISTIVITY MEASUREMENT FOR PROBLEMS WHEREIN THE RESISTIVITY METHOD IS TRADITIONALLY APPLIED. MOREOVER, THE CAPABILITY OF ERT/EIT TO GENERATE 'IMAGES' OF CONDUCTIVITY DISTRIBUTION GREATLY EXPANDS THE APPLICABILITY OF THE ELECTRICAL RESISTIVITY METHOD TO MANY PROBLEMS IN ENGINEERING AND HAZARDOUS WASTE SITE CHARACTERIZATION AND MONITORING, AND IN GEOPHYSICAL AND GROUNDWATER EXPLORATION AS WELL.
DIAGRAM OF THE SETUP OF THE ZETA SYSTEM.
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 MODELLING 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 MODELLING TOPOGRAPHY IN AREAS WITH SIGNIFICANT RELIEF.
DURING THE PAST TWO DECADES THE COSTS OF MOST ELECTRICAL GEOPHYSICAL SURVEYS HAVE DECREASED SUBSTANTIALLY, DUE TO THE IMPLEMENTATION OF DIGITAL RECEIVERS WITH MULTIPLE INPUT CHANNELS. THE COSTS OF DATA PROCESSING, MODELLING AND THE GENERATION OF COLOR SECTIONS AND MAPS HAVE ALSO DECREASED CONSIDERABLY WITH DRAMATIC IMPROVEMENTS IN LOW COST PERSONAL COMPUTERS ALONG WITH THE LATEST RELATIVELY HIGH-SPEED COLOR PLOTTERS. THE NET RESULT OF IMPROVEMENTS IN DIGITAL DATA COLLECTION AND PROCESSING HAS BEEN A REDUCTION IN THE COST-PER-STATION BY A FACTOR OF 10 OR MORE FOR SOME METHODS IN THE FACE OF INCREASED DAILY COSTS FOR PROVIDING GEOPHYSICAL SERVICES.
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 MODELLING 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 INDUCED POLARIZATION (IP) METHOD OF GEOPHYSICAL EXPLORATION IS CAPABLE OF DETECTING EVEN SMALL AMOUNTS OF METALLIC LUSTER MINERALS IN A ROCK MASS. CONSEQUENTLY, IN THE YEARS SINCE DISCOVERY, IP SURVEYING HAS BECOME THE MOST POPULAR GROUND GEOPHYSICAL SURVEY METHOD.
IT IS NOT TOO DIFFICULT TO UNDERSTAND MANY APPLICATIONS OF IP SURVEYING AS USED IN THE SEARCH FOR MINERAL DEPOSITS. HOWEVER, THE BASIC THEORY OF THE IP PHENOMENON IS NOT WELL DEVELOPED OR UNDERSTOOD, AND THERE HAS BEEN SOME DISAGREEMENT ON FUNDAMENTAL CONCEPTS.
IN ORDER TO MOST EFFECTIVELY APPLY THE IP METHOD IN THE FIELD IT IS NECESSARY TO KNOW THE PHYSICAL CHARACTERISTICS OF THE SOUGHT-FOR DEPOSIT INCLUDING ITS SIZE, SHAPE, DEPTH, AND ELECTRICAL PROPERTIES. WITH THIS INFORMATION AN OPTIMUM IP SEARCH ARRANGEMENT CAN BE DEVISED AND ONE COULD USE THE BEST POSSIBLE ELECTRODE INTERVAL, TYPE OF ARRAY, AND LINE SPACING. WITH UNCERTAIN TARGET CHARACTERISTICS AS ENCOUNTERED IN THE REAL WORLD, GEOLOGICAL GUIDANCE MUST BE USED TO HELP DIRECT AN OPTIMIZED IP SURVEY.
GEOPHYSICAL TECHNIQUES HAVE BEEN USED IN MINERAL PROSPECTING FOR THE PAST 300 YEARS, BEGINNING IN SWEDEN AROUND 1640 WITH THE USE OF MAGNETIC COMPASSES IN EXPLORING FOR IRON ORE. RESISTIVITY MEASUREMENTS FOLLOWED IN THE 1800'S IN THE SEARCH FOR BASE METALS, AND BY THE EARLY 1900'S THE SCHLUMBERGER BROTHERS WERE SUCCESSFULLY USING SELF POTENTIAL (SP) AND RESISTIVITY FOR THIS PURPOSE. BY 1912 CONRAD SCHLUMBERGER HAD PATENTED THE INDUCED POLARIZATION (IP) METHOD, AND HAD USED THE TECHNIQUE FOR FINDING ECONOMIC SULFIDE DEPOSITS.
THE USE OF APPLIED GEOPHYSICS FOR MINERAL AND HYDROCARBON EXPLORATION AS WE KNOW IT TODAY, PROBABLY BEGAN IN THE 1950'S, WITH THE ADVENT OF SENSITIVE MAGNETOMETERS, GRAVITY METERS, BATTERY-POWERED ELECTRONIC EQUIPMENT, AND THE APPLICATION OF INFORMATION THEORY AND COMPUTER PROCESSING TO SEISMIC DATA ACQUISITION.
SINCE THAT TIME, SEVERAL DIFFERENT FREQUENCY AND TIME-DOMAIN ELECTROMAGNETIC (FEM AND TEM) SYSTEMS WERE DEVELOPED TO MAP OUT LOW RESISTIVITY ANOMALIES FOR MASSIVE SULFIDE EXPLORATION. MANY OF THESE SYSTEMS CAME FROM CANADA, ALTHOUGH THE FIRST TEM SYSTEM WAS IMPORTED FROM RUSSIA IN THE 1950'S.
DURING THE PORPHYRY COPPER HEYDAYS OF THE 60'S AND 70'S, A NUMBER OF DIFFERENT GEOPHYSICAL EXPLORATION METHODS WERE USED WITH VARYING DEGREES OF SUCCESS: GRAVITY, MAGNETICS, INDUCED POLARIZATION AND SELF POTENTIAL. GRAVITY WAS USED TO MAP BASEMENT TOPOGRAPHY AND TO SEARCH FOR ALTERED INTRUSIVE BODIES; MAGNETICS WERE USED TO SEARCH FOR ALTERED ROCKS; AND IP AND SP WERE USED TO LOCATE DISSEMINATED SULFIDES, MAINLY PYRITE AND CHALCOPYRITE.
TODAY, THESE SAME METHODS ARE APPLIED, BUT CAN BE USED WITH GREATER ACCURACY AND SENSITIVITY DUE TO TECHNOLOGICAL ADVANCES OVER THE PAST 20 YEARS, ESPECIALLY IN THE FIELD OF ELECTRICAL GEOPHYSICS AND SEISMICS. FOR EXAMPLE, IP HAS EVOLVED FROM THE TRADITIONAL TIME-DOMAIN APPROACH TO MULTI-FREQUENCY IP, NOW CALLED COMPLEX RESISTIVITY (CR) OR SPECTRAL IP, WHICH CAN BE USED TO DIFFERENTIATE BETWEEN ANOMALOUS RESPONSES FROM ALTERATION, SULFIDE TYPE, AND ELECTROMAGNETIC COUPLING (AN UNWANTED ARTIFACT OF THE MEASURING PROCESS). VERTICAL SOUNDING METHODS SUCH AS CONTROLLED SOURCE AUDIOFREQUENCY MAGNETOTELLURICS (CSAMT) AND TIME DOMAIN OR TRANSIENT ELECTROMAGNETICS (TDEM OR TEM) CAN BE USED FOR MAPPING STRUCTURE AND MASSIVE SULFIDE BODIES. CR AND TEM ARE USED IN BOTH SURFACE AND DOWNHOLE SURVEY CONFIGURATIONS. DOWNHOLE TECHNIQUES ARE BEING DEVELOPED FOR IN-HOLE ASSAYING. CROSS-HOLE TOMOGRAPHY IS BEING DEVELOPED WHICH CAN BE USED TO ASSESS MINERALIZATION AND ALTERATION FEATURES BETWEEN DRILL-HOLES. AIRBORNE RADIOMETRIC TECHNIQUES HAVE BEEN DEVELOPED WHICH WILL AID IN LARGE-SCALE ALTERATION MAPPING. AND SEISMIC EQUIPMENT DEVELOPMENT AND DATA PROCESSING HAVE GREATLY INCREASED RESOLUTION AND INTERPRETATION CAPABILITIES FOR BOTH DEEP AND SHALLOW APPLICATIONS.