b'Environmental geophysics avoid any further use of large, dragged plastic sheets.The next stage in the development of the AgTEM system consisted of designing and building a large wooden platform for the transmitter loop. This proved to be too costly to maintain due to wood rot and was cumbersome to run. A receiver dragged on a separate mat far behind the wooden cart made the whole system challenging to drive, especially through farm gates and in traffic. Many designs followed with narrow towed mats and towed carts made from PVC pipe. The receiver antennae mounted on plastic mats never lasted long enough in the field, which led to their abandonment yet again, but those early plastic pipe carts showed considerable promise.As plastic pipes are not designed for Figure 3.A sample of AgTEM data collected in one day revealing potential groundwater sources in deepstructural flexure, I started working on rock fractures. Data are projected along survey lines, to depth of 40 m. an extruded fibreglass cart, but that was components failed due to the damageand Duncan Massie were doing). Usingultimately too costly to duplicate (and done by the high inductive load. the terraTEM solved my instrumentationoperate). With a total weight of about problems. After that, most of my700 kg its non-metallic wheels and axles Back in the early 2000s I got involvedproblems were practical, structuralfailed too quickly. I ran a number of with some work that Zonge Australiaproblemshow do you tow a sufficientlysurveys using that system, including one was doing with a University of Adelaidelarge loop to get signal into the ground,in the Burdekin Delta, where the system MSc student (Brian Barrettwith one ofwithout any structural metal, in fieldmapped aquifers and saline intrusion to his supervisors, Mike Hatch), they wereconditions that are often rough enoughas deep as 90 m.towing a Zonge NanoTEM equippedto damage a 4WD? I went back to the PVC pipe construction, wooden frame behind a boat, collectingthis time simplifying the physical TEM data, while I was working on towingI got to test some interesting towingdesign by eliminating the need for a a resistivity streamer, as part of my PhDideas by bootstrapping funding I gottroublesome rear towed mat receiver. research. We were looking for salt waterto run some TEM trials on a platinumIt took some effort and thought but I intrusion into the river bed and bothexploration programme. I tested andended up using a 4 m2 receiver loop techniques were successful. Inspired byran towed TEM surveys as an adjunctthat overlapped the transmitter loop in the success on the Murray River withto a standard 100 m x 100 m moving- such a way that the system minimized Zonge, I tried the same on land using aloop TEM survey. For this work I towedthe mutual induction between the crazy, low-cost 6 m x 6 m loop of PVCthe transmitter and receiver loops on areceiver and transmitter loops. Once pipe and electrical cable reels. Again, Isheet of plastic (super tough irrigationthis was stabilised I found that the data used the Zonge NanoTEM equipmentchannel liner) behind a quadbike. Thewere just as good as those collected and single turn transmitter and receiver2 mm thick sheet only wore out afterusing the towed mat receiver loop (and loops. It was good, but I thought that Isurveying several hundred kilometres,the system was far more compact and could do better. but it was very difficult to get the 7 m xuseable). The receiver and transmitter About this time I got the chance to visit7 m sheet through the many farm gatesloops were mounted on boom arms that the Hydrogeophysics Group (HGG) atin the field area. But the data sets werewere held in place with elastic straps Aarhus University in Denmark where theawesome, providing the client with highand could therefore fold up during the PATEM towed system (a transmitter sledquality subcrop mapping; the subcropsurvey and pass through most farm gates with a receiver towed behind) was beingcomplexity was surprising given the lackand gaps between trees. I patented the developed; this system was towed by aof evidence on the ground surface. structural concept and set to develop it tracked walk-beside vehicle. This projectcommercially. The original null mutual went by the wayside months later asI conducted similar towed TEM surveysinduction cart was rather large, so I also HGG focused on developing the SkyTEMusing plastic sheets at Menindee Lakesstarted working on making smaller carts. airborne TEM system, which has beenthe following year; again the resultsWith the help of an industrial engineer a huge success for that group. Seeingwere impressive revealing a number ofwe also developed a lighter-weight, the potential in the PATEM system, I waslocations that were potentially suitablethinner fibreglass pipe structure.inspired to continue to develop a nichefor managed aquifer recharge. It was towed TEM system. on this survey that a road train passedAt about this time drought started to grip us at 100 km/hr as we surveyed theeastern Australia, and demand for surveys I got my next transmitter/receiver systemroad verge and my 200 kg plastic sheettook away all of our time and resources from Monex Geoscope (a spinoff of somewas lifted into the air, demolishing thefor development, as we sited bore after research work that Professor Jim Culltransmitter. This led to the decision tobore for farmers and local councils. When 47 PREVIEW DECEMBER 2021'