ONE TRADITIONAL LIMITATION OF TRANSIENT ELECTROMAGNETIC SOUNDING METHODS IS THE LONG TURN-OFF TIMES OF THE TRANSMITTED SIGNALS. FOR SHALLOW SOUNDINGS, AND HIGHLY RESISTIVE AREAS, THIS HAS PREVENTED EFFECTIVE USE OF THE METHOD. THIS LIMITATION HAS BEEN OVERCOME IN THE NEWEST SERIES OF ZONGE TRANSMITTERS, THE NT-20. THESE TRANSMITTERS COMPLETE THE CURRENT FLOW TURN-OFF INTO A 20 METER LOOP IN ABOUT 1.5 MICROSECONDS. THIS RAPID TURN-OFF, AND THE HIGH SPEED ANALOG-TO-DIGITAL CONVERSION IN THE GDP-32 RECEIVER, ALLOWS DATA COLLECTION AT DEPTHS LESS THAN 2 METERS AND IN AREAS WITH ELECTRICAL RESISTIVITIES IN EXCESS OF 20,000 OHM-METERS.
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.
IT IS SOMETIMES DIFFICULT TO DETERMINE IN ADVANCE EXACTLY WHICH EQUIPMENT OR TECHNIQUE IS BEST SUITED TO A GIVEN PROJECT. ALTHOUGH THE RESPONSE OF A PARTICULAR TARGET MAY BE PREDICTABLE, THE LOCAL BACKGROUND RESPONSE MAY BE MORE DIFFICULT TO ASSESS UNTIL FIELD DATA HAVE ACTUALLY BEEN GATHERED. THIS PROBLEM BECOMES ECONOMICALLY SIGNIFICANT WHEN THE JOB SITE IS RELATIVELY REMOTE, REQUIRING DOWNTIME AND/OR ADDITIONAL AIRFREIGHT EXPENSES WHEN THE ACTUAL FIELD RESULTS DO NOT MATCH EXPECTATIONS AND A CHANGE IN EQUIPMENT SYSTEMS BECOMES NECESSARY. AN EXAMPLE OF THIS IS A RECENT SERIES OF SURVEYS IN INDIANA AND OHIO, IN WHICH THE GOAL WAS TO ENSURE THAT NO ABANDONED, BURIED OIL WELLS WERE PRESENT WITHIN A PRESCRIBED RADIUS OF PROPOSED INJECTION WELLS. AFTER LOCAL TESTS, BOTH THE PHYSICAL SURVEY LAYOUT AND THE DATA PROCESSING TECHNIQUES WERE VARIED IN ORDER TO DETECT THE VARIOUS POSSIBLE TARGETS IN THE DIFFERENT ENVIRONMENTS.
ONE TRADITIONAL LIMITATION OF TRANSIENT ELECTROMAGNETIC SOUNDING METHODS IS THE LONG TURN-OFF TIMES OF THE TRANSMITTED SIGNALS. FOR SHALLOW SOUNDINGS, AND HIGHLY RESISTIVE AREAS, THIS HAS PREVENTED EFFECTIVE USE OF THE METHOD. THE NEWEST SERIES OF ZONGE TRANSMITTERS, THE NT-20, HAS OVERCOME THIS LIMITATION.
IN ORDER TO OBTAIN VERY SHALLOW INFORMATION, THE TRANSMITTED SIGNAL MUST GO TO ZERO VERY RAPIDLY, WITHOUT "RINGING" OR OSCILLATIONS OF EITHER THE ELECTRONICS OR THE WIRE LOOPS THEMSELVES. DEPENDING ON LOOP CHARACTERISTICS, THE NT-20 TRANSMITTER TURNS OFF IN APPROXIMATELY 1.5 MICROSECONDS. THIS RAPID TURN-OFF, AND THE HIGH SPEED ANALOG-TO-DIGITAL CONVERSION IN THE GDP-16 AND 32 RECEIVER, ALLOWS DATA COLLECTION AT DEPTHS LESS THAN 2 METERS AND IN AREAS WITH ELECTRICAL RESISTIVITIES IN EXCESS OF 20,000 OHM-METERS. THE NT-20 CAN BE USED WITH EITHER THE GDP-16 OR -32 TO COLLECT NANOTEM DATA. THE RECEIVER RECORDS THE DECAY CURVE AS 31 WINDOWS (OR GATES) FROM APPROXIMATELY 1.5 MICROSECONDS AFTER TRANSMITTER TURN-OFF TO ABOUT 3 MILLISECONDS AFTER TURN-OFF.
BECAUSE OF THE FLEXIBILITY OF CHANGING TRANSMITTER AND RECEIVER LOOP SIZES, NANOTEM CAN BE USED FOR A VARIETY OF DIFFERENT TARGETS. SURVEYS HAVE BEEN PERFORMED USING FORTY METER (40 M BY 40 M) TRANSMITTER LOOPS WITH FIVE METER (5 M BY 5 M) RECEIVER LOOPS TO COLLECT RESISTIVITY SOUNDING IN HIGHLY RESISTIVE GROUND. THE SAME SYSTEM HAS ALSO BEEN USED WITH A TEN METER (10 M BY 10 M) TRANSMITTER LOOP WITH ONE METER (1 M BY 1 M) RECEIVER LOOPS TO LOCATE SMALL METALLIC OBJECTS.