b'Environmental geophysics and he certainly did not know me, I justwas of primary importance. In addition, knew it was one of the hottest places towe assumed that the addition of an learn. So there he was, Frank Morrison,inductive contribution to the response suave, with his wavy hair, a touch of graywould permit the AC method to look at the temples, like a movie star fromdeeper into the ground, at least if good the forties with a big smile and a sparkconductors were present, such as the in his eye. He took me under his wingwell conducting Tertiary clay which often and showed me around: And here is theformed the bottom of the aquifers in office and our secretary (I just caught aEastern Jutland, Denmark. Initially, there glimpse of a small personal computer inwere of course a lot of difficulties to be the corner with a student in front of it,overcome with this new instrument, but and I was very impressed), and here is Kiby and large, it fulfilled our expectations. Ha Lee (as taken out of a samurai movieThe instrument was developed by Kurt by Kurosawa) and here is Carlos TorresSrensen (surprise!) together with our Verdin. You will share an office with him.electronics engineer, Niels Breiner, How lucky I was with that I found outand the rest of the electronics lab later. And here are some of the students:team, and it was the first computer-David Alumbaugh, Mike Wilt, Guimin Liu,controlled instrument developed at our Mike Hoversten. A big mouthful on mydepartment, programmed by Frederik first day and an even bigger one laterHusted Andersen. I wrote the software when I found out that I had just skipped afor inversion of the sounding data and few grades and moved up into a differentdeveloped an optimal field procedure.league. However, after some time, it became My intention with going to Berkeleyclear that although the AC method was to learn more about the Audio- produced data with better information Magnetotelluric Method with athan the traditional method and controlled source (CSAMT). Until theimproved the obtainable subsurface beginning of the 1980s, DC geoelectricalresolution, the productivity was even soundings were by far the mostlower than for the traditional DC method frequently used electric/electromagnetic-the daily production was only about four method in Denmark. However, thesoundingsand eventually that became productivity of the method was limited:the final nail in the coffin for the AC-three people could do 6-8 soundingsgeoelectrical method.per day, and there were seriousBut then what? At that time, the local limitations in the depth of investigationAarhus County was responsible for and considerable uncertainties in themapping raw material and ground water resolution of the subsurface resistivity,resources, and over a number of years shortcomings in the form of equivalencesit had become quite clear that dense characteristic for the method. measurements over extended areas In the early 1980s, we developed themade not only a quantitative, but also so-called AC-geoelectrical methoda qualitative difference: it permitted a at our department. The idea was tomuch more reliable interpretation of the send an alternating current throughnear-surface geology and a better insight the current electrodes at a series ofinto the processes of its formation. It frequencies instead of a direct currentbecame a credo for our activities, first thereby achieving not only galvanic (asformulated by Verner Sndergaard: for the traditional DC method) but alsoMeasure densely over large areas! This inductive information. The combinationwas the background for my wishing to of the two would make it possiblelearn more about the CSAMT method, to resolve some of the equivalenceswhich might have the potential to which plagued the DC geoelectricalimprove productivity, in particular with methods, and we also had the intentionrespect to mapping of deep aquifers.to increase the depth of investigation.In Berkeley, I was, however, presented Due to high resistivity equivalence, thewith the transient electromagnetic traditional DC geoelectrical methodmethod. I knew very little about had severe limitations in resolving thethe method, but soon found it very thickness of layers of dry sand abovefascinating. It is an inductive method the groundwater table. This was a veryand thereby good at resolving the important parameter for the mapping ofpresence of good conductors like clay raw materials, which played a prominentand saltwater-carrying sediments, and a role in the 1980s, and also in connectiondepth of investigation of 100-150 mwith hydrogeophysical investigationswas achievable with a central loop where the depth to the groundwaterconfiguration on the surface of only 33 PREVIEW FEBRUARY 2022'