AIRBORNE GEOPHYSICAL STUDIES ON THE AMERICAN SIDE OF THE SAN PEDRO VALLEY OF ARIZONA AND MEXICO HAVE ALLOWED US TO MAP DEPTH TO CRYSTALLINE BASEMENT IN THIS AREA WHERE GROUNDWATER IS CRITICALLY IMPORTANT. THIS BASIN, WHOSE HEAD LIES IN NORTHERN MEXICO, HOSTS A MAJOR US-MEXICO MIGRATORY BIRD FLY-WAY. A DESIRE TO PRESERVE THE SURFACE WATER IN THE SAN PEDRO RIVER LED TO THE CREATION OF THE SAN PEDRO NATIONAL RIPARIAN CONSERVATION AREA IN 1988. TO PRESERVE THE SURFACE WATER, ONE MUST KNOW SOMETHING ABOUT THE AQUIFER UNDERLYING IT. ON THE AMERICAN SIDE OF THE BASIN, TIME-DOMAIN AIRBORNE GEOPHYSICAL METHODS WERE USED TO MAP THE RELATIVELY CONDUCTIVE GROUNDWATER TYPICAL OF AN ARID REGION TO DEPTHS OF 150 - 400 METERS IN THE ABSENCE OF HUMAN CULTURAL INTERFERENCE. IN ORDER TO BETTER UNDERSTAND THE HYDROLOGY OF THE BASIN AS A WHOLE, GEOPHYSICAL SURVEYING HAS BEEN EXTENDED SOUTHWARD INTO THE SONORAN SAN PEDRO VALLEY OF NORTHERN MEXICO. AN AIRBORNE MAGNETIC SURVEY IN NORTHERN MEXICO HAS BEEN PROCESSED TO DEPTH-TO-MAGNETIC-SOURCE, AND CONCATENATED TO A MAGNETIC DATA SET FROM SOUTHERN ARIZONA TO SHOW DEPTH TO BASEMENT FOR THE SAN PEDRO VALLEY DRAINAGE. WE THEN CONDUCTED A SCALAR AUDIO- MAGNETOTELLURIC (AMT) SURVEY OVER FOUR DIFFERENT LINES IN THE SONORAN SAN PEDRO BASIN, AND PROCESSED THESE DATA USING A SMOOTH-MODEL INVERSION TO CONDUCTIVITY-VS-DEPTH PROFILES. AS WE VIEW THE CONDUCTIVITY INVERSION RESULTS, WE ARE IN FACT VISUALIZING THE HIGHLY CONDUCTIVE WATER TYPICAL OF AN ARID CLIMATE - IN EFFECT, WE BROADLY IMAGE THE SATURATED SEDIMENTS. WE THEN USED AN ANALYTIC SIGNAL DEPTH-TO-SOURCE ALGORITHM ON MAGNETIC DATA ALONG THE SAME PROFILES TO CONSTRAIN THE AMT INVERSION. THE RESULT IS A UNIQUE SET OF GEOPHYSICAL PROFILES THAT CLEARLY SHOW BASEMENT STRUCTURE BENEATH THE SONORAN SAN PEDRO BASIN TO DEPTHS OF UP TO 800 METERS. THESE CONSTRAINED PROFILES HELP RESOLVE BASEMENT CONTROLS ON GROUNDWATER FLOW IN NORTHERN MEXICO LEADING TO THE US FRONTIER. IT IS IMPOSSIBLE TO UNDERSTAND THE GROUNDWATER REGIME EXCEPT IN THE CONTEXT OF THE VOLCANIC AND SEDIMENTARY HISTORY OF THE REGION, AND NEITHER THE GEOLOGY NOR THE GEOPHYSICS CAN BE CARRIED OUT INDEPENDENTLY OF THE OTHER, BUT THE WHOLE TOGETHER CONTRIBUTE SUBSTANTIALLY MORE THAN THE PARTS.
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.