b'Environmental geophysics Environmental geophysicsWelcome readers to this issueselectrical geophysics starting in the column on geophysics applied to the1970s through to the 1990s and beyond. environment. We are now on Scene 4 ofThere are no geophysical war stories in the reminiscences by Niels Christensenthis one, just an interesting recounting on his career at Aarhus University andof the challenges of the times. Over to that groups progress into and throughNielsMike Hatch Associate Editorfor Environmental geophysics michael.hatch@adelaide.edu.auPivotal moments: Seven scenes from a geophysics adventurein terms of field work, data inversionJim Macnae. Sometimes they just get it and geological/hydrogeologicalspot on, those guardian spirits.interpretation of the results. Even though the Selma inversion program that BoWe talked several times at the Holm Jacobsen and I had put togetherconference, and Jim directed my was more than an order of magnitudeattention to a thesis by Benjamin Polzer, faster than other programs that we hadwhich I requested to be sent to me when access to, an inversion with a 1D 3- orI was back in Aarhus. That was a truly 4-layer model took 45 minutes. Fieldoriginal piece of research, and I did my data were coming in a steady streambest to program up the approximate every day, and the computation time wasinversion that was laid out in the thesis, getting intolerably slow. but I could not make it work and I had to Niels B. Christensen put it aside, very frustrated that I could Professor Emeritus At the time I knew that certain optionsnot find the error.Department of Earth Sciencesexisted in terms of approximate inversion Aarhus Universityof TEM soundings data, and I hadFrustration often spurs you to think for nbc@geo.au.dk collected a few open source programsyourself, and I started to have some that existed at the time. However, I didclues about constructing approximate not find them to be particularly good.inversions. However, it was not easy Scene 4 My intention was to use the results offor me to find the time, as an associate an approximate inversion as the initialprofessor with teaching obligationsin which our protagonist has unusualmodel for the usual inversion to speedand many students to supervise, to luck, goes to Australia, and finally succeedsup convergence. I knew that Jim Macnaeconcentrate on the task, so it was not in finding the key to fast approximatefrom Australia had worked with fastuntil 1995 that I got some time to computing. approximate inversion, and I had readdig into it. I had asked Jim if I could some of his papers on the subject whichcome visit, and I spent two months EAGE conference, Paris 1992 as far as I could see were some of theat Macquarie University, just north of best on the subject, so wanted to have aSydney, and thats where it all came Im on the roof of the conference centretalk with him. together. I can still remember the where the EAGE conference is held inmoment when I knew that I had nailed Paris in 1992, enjoying a beautiful sunsetThese thoughts went through my mindit! The fruit of years of work! When I over the roofs of Paris with a glass ofthat evening on the roof, and I wonderedfinally relaxed, I looked at the code that cool white wine in my hand. Life couldif Jim Macnae would actually be at theI had written to implement Polzers certainly be worse. Besides making aconference. I just had to keep a lookoutapproach, andthere was the error! I presentation, I have another reason forover the next days among the ~4000corrected my program based on Polzers attending. Im searching for a person whoconference participants. I turned towardswork. And then came the next Eureka might, or might not, be here. the person standing to the right of me onmoment: The codes suggested by Polzer the rooftop who also had a glass of whiteproduced exactly the same results as By 1992 we had gathered a lot ofwine in his hand - you just had to startmy new code even though the concepts experience with the TEM method, bothsomewhereand read his badge. It read:behind them were quite different! When 31 PREVIEW AUGUST 2022'