THOUGH COMMONLY USED IN MINERALS EXPLORATION, TRANSIENT ELECTROMAGNETIC (TEM) METHODS ARE LESS COMMON IN ENVIRONMENTAL AND ENGINEERING APPLICATIONS. SEVERAL ASPECTS OF THE TECHNIQUE MAKE IT A VERY USEFUL TOOL, INCLUDING THE FLEXIBILITY OF LOOP SIZES AND GEOMETRIES, AND THE RECENT IMPROVEMENT IN ELECTRONICS THAT ALLOW FASTER TRANSMITTER TURN-OFF TIMES, AND THEREFORE SHALLOWER SOUNDINGS. FASTER ELECTRONICS ALSO ALLOW THE ACQUISITION OF ALL THREE MAGNETIC FIELD COMPONENTS SIMULTANEOUSLY, INCREASING THE AMOUNT OF INFORMATION AVAILABLE FOR INTERPRETATION AND MODELLING. OUR DISCUSSION INCLUDES ENVIRONMENTAL APPLICATIONS OF TEM DATA IN STANDARD GEOMETRIES (SUCH AS IN-LOOP, FIXED-LOOP, ETC.) BUT WITH HIGHER RESOLUTION THAN USUALLY OBTAINED. IN THIS CASE, "HIGH RESOLUTION" REFERS TO HIGHER RESOLUTION SPATIALLY (MUCH HIGHER DATA DENSITY THAN IS NORMALLY USED IN MINERALS EXPLORATION TEM) AND TEMPORALLY (MUCH FASTER TURN-OFF TIMES AND FASTER SAMPLE RATES). ALSO OF INTEREST IS THE ADDED BENEFIT OF UTILIZING THE LATE TIME TEM DATA, AFTER THE BACKGROUND EARTH RESPONSE HAS DECAYED, AS A DEEP-SOUNDING METAL DETECTION TOOL.
DATA EXAMPLES FROM A RECENT PROJECT SHOW VERY INTERESTING AND USEFUL CHARACTERISTICS OF THE EARLY-TIME DATA FROM THE HORIZONTAL COMPONENTS OF TEM SURVEYS USED IN DEEP METAL DETECTION FOR TARGETS SUCH AS USTS, UXOS, AND UTILITIES. NORMALLY, MOST DEEP METAL DETECTION SURVEYS UTILIZE A SYSTEM IN WHICH ONE OR TWO TEM MEASUREMENTS OF THE VERTICAL COMPONENT (HZ) ARE ACQUIRED. THESE SYSTEMS USUALLY ACQUIRE DATA AT RELATIVELY LATE TIMES (HUNDREDS OF MICROSECONDS AFTER TRANSMITTER TURN-OFF), TO ALLOW THE BACKGROUND EARTH RESPONSE TO DECAY TO ZERO. A GOOD EXAMPLE IS THE POPULAR GEONICS EM-61 SYSTEM. BY RECORDING DATA AT NUMEROUS TIME WINDOWS FOR ALL THREE COMPONENTS (HX, HY, AND HZ), HOWEVER, FROM EARLY TIMES (A FEW MICROSECONDS) THROUGH LATE TIMES, ADDITIONAL SIGNIFICANT INFORMATION IS ACQUIRED. IN ONE RECENT PROJECT, A 55-ACRE SITE WAS SURVEYED (BY ANOTHER CONTRACTOR) WITH AN EM-61, AND SEVERAL SUBSURFACE TARGETS WERE IDENTIFIED FOR EXCAVATION. SMALL AREAS AROUND THESE TARGETS, TOTALING ONLY 2.25 ACRES, WERE RE-SURVEYED USING A MULTI-COMPONENT, EARLY-TIME SYSTEM. IN ADDITION TO VERIFYING THE TARGETS, FOUR ADDITIONAL ANOMALIES THAT WERE NOT EVIDENT IN THE HZ DATA WERE DETECTED, INCLUDING TWO BURIED POWERLINES. EXAMINATION OF THE HORIZONTAL COMPONENT DATA OF THE FIELD ALSO APPEARS TO BE PARTICULARLY USEFUL IN DISCRIMINATING TARGETS. FOR EXAMPLE, LINEAR FEATURES SUCH AS PIPELINES AND POWERLINES ARE EASILY DISTINGUISHED FROM 3-DIMENSIONAL TARGETS WITH ONLY A SINGLE LINE OF DATA, INSTEAD OF REQUIRING AN ARRAY OF LINES TO INTERPRET THE TARGETS BASED ON THE GEOMETRY OF ANOMALIES ON ADJACENT LINES.
THERE ARE A LIMITED NUMBER OF METHODS CURRENTLY IN USE TO GATHER INFORMATION ON THE VARIOUS HYDROGEOLOGICAL/ENVIRONMENTAL PROBLEMS THAT ARE PART OF "EVERYDAY" LIFE. TRADITIONALLY, GROUNDWATER PROBLEMS HAVE BEEN EVALUATED AND THEN MONITORED USING A CAREFULLY DESIGNED NETWORK OF WELLS WHERE WATER DEPTH AND QUALITY ARE MEASURED ON A REGULAR BASIS. IN RECENT YEARS, SOME OF THE VARIOUS MINING AND PETROLEUM ORIENTED GEOPHYSICAL TECHNIQUES HAVE BEEN MODIFIED FROM THEIR DEEPER APPLICATIONS TO SAMPLE AT SHALLOWER DEPTHS. OFTEN THE GOAL OF THESE SURVEYS IS TO HELP GEOLOGISTS AND ENGINEERS DETERMINE WHETHER THEIR ASSUMPTIONS ON WELL LOCATION AND WATER FLOW ARE CORRECT. TECHNIQUES THAT HAVE BEEN USED INCLUDE SHALLOW SEISMICS, DC RESISTIVITY, GROUND PENETRATING RADAR, FREQUENCY DOMAIN ELECTROMAGNETICS (FDEM), AND TIME DOMAIN ELECTROMAGNETICS (TEM).
RECENT ADVANCES IN SAMPLING SPEED, CIRCUITRY SPEED, AND DATA RECORDING HAVE ALLOWED THE DEVELOPMENT OF TEM TECHNIQUES WHERE DATA CAN BE TAKEN FASTER (AND THEREFORE START CLOSER TO THE SURFACE), AND WITH BETTER RESOLUTION OF THE TOP 15-50 M. THESE TECHNIQUES INCLUDE THE ZONGE ENGINEERING NANOTEM SYSTEM AND THE FAST SAMPLING MODIFICATIONS TO THE SIROTEM-3 SYSTEM.
THIS PAPER BRIEFLY SUMMARISES THE TEM RESULTS FROM THREE SEPARATE STUDY AREAS ENCOMPASSING A RANGE OF HYDROGEOLOGICAL AND ENVIRONMENTAL PROBLEMS, EACH OF SOME IMMEDIATE IMPORTANCE IN AUSTRALIA AT THIS TIME. THE FIRST STUDY, AT THE STOCKYARD PLAIN DISPOSAL BASIN (SPDB) NEAR WAIKERIE, SOUTH AUSTRALIA, EXAMINES THE CHANGING HYDROLOGICAL ENVIRONMENT AROUND A GROUNDWATER DISPOSAL BASIN IN THE MURRAY-DARLING SYSTEM. THE SECOND STUDY, IN THE WILLAURA CATCHMENT IN VICTORIA, EXAMINES WATER MOBILITY IN AN EVOLVING DRYLAND SALINITY SYSTEM. THE THIRD STUDY, AT AN ABANDONED MINE SITE IN NEW SOUTH WALES, ATTEMPTS TO DELINEATE THE EXTENT OF ACID-MINE DRAINAGE IN THE AREA AROUND THE MINE.
AT THE SAN MANUEL COPPER MINE IN SOUTHEASTERN ARIZONA, RECOVERY OF COPPER FROM THE OXIDIZED PORTION OF THIS PORPHYRY MINERAL RESOURCE IS BEING ACHIEVED THROUGH A LARGE IN SITU LEACHING OPERATION USING WEAK SULFURIC ACID SOLUTION. IN THE PAST, THIS ACTIVITY WAS COORDINATED WITH OPEN PIT AND UNDERGROUND MINING, BUT IN TODAY'S ECONOMIC CLIMATE, ONLY THE IN SITU OPERATION CONTINUES. THE ACID SOLUTION (20 GRAMS PER LITER) IS INJECTED IN WELLS UN-PRESSURIZED AT VARYING DEPTHS UP TO SEVERAL HUNDRED METERS, USUALLY AT RATES OF ONLY A FEW TENS-OF-GALLONS PER MINUTE. THE COPPER-BEARING PREGNANT LEACH SOLUTION (PLS) IS RECOVERED EITHER IN NEARBY RECOVERY WELLS OR IN COLLECTION AREAS IN THE UNDERGROUND WORKINGS 350 M TO 500 M (1200 TO 1600 FEET) BELOW THE SURFACE. A THOROUGH DESCRIPTION OF IN SITU MINING IN GENERAL AS WELL AS AT SAN MANUEL SPECIFICALLY CAN BE FOUND IN SWAN AND COYNE (1992). DUE TO THE ECONOMIC EFFICIENCY OF THIS MINING METHOD, THE IN SITU OPERATION AT SAN MANUEL HAS EXPANDED FROM TWO TEST WELLS IN THE MID-1980S TO MORE THAN 900 WELLS COVERING OVER 650,000 SQUARE METERS OF THE OPEN PIT MINE. OVER THE PAST TWELVE YEARS, GEOPHYSICAL SURVEYS HAVE BEEN USEFUL IN BOTH PLANNING AND MONITORING THE EXPANSION OF THE IN SITU FIELD.
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.