Re Slingram etymology
In settling on loop frame as the translation of Slingram, I think the
discussion slipped off target. The online dictionaries are trying to
make a one-to-one match between words in the two languages, but losing
some meaning in what is bound to be an imperfect process. Sling in
swedish is from the same root as sling in english. As I understand it
(likely imperfectly), loop is a good english translation when one is
referring to a flexible loop for lifting things or simply a closed path.
Ram translates OK as frame, if one means a solid frame, framework, or
the enclosing fixed boundary of something, and it could appropriately
describe a coil winding on a fixed substrate.
In summer 1959 at Outokumpu, Finland, I worked with data from many 10's
of square kilometers of intensive Slingram (HLEM) surveying. I was told
that Slingram was best translated as "carried loop" or "slung loop";
describing how each of the two circular air -cored horizontal loops were
slung by straps from the operators' shoulders. This was a contrast with
earlier EM's where coils were usually tripod mounted. I was also shown a
famous picture of a mobile version of HLEM used in Lapland, where the
systems coils were mounted on sleds pulled by reindeer.
Certainly, Slingram and Turam were developed in Sweden well before 1950,
as described in several posts of this series.
Early (pre 1960s) EM systems were single frequency, and had minimal
electronics (limited to simple oscillators to generate the Tx current
and simple amplifiers to make the received signal audible in
headphones). Measurements were made with compensators, essentially two
dial AC bridges that had to be adjusted manually to annul the headphone
signal and then the data read from the potentiometer dials.
An alternative to Slingram at the time was Turam (two loop). It used a
large, usually rectangular, fixed horizontal loop as the transmitter,
with a moving receiver system traversing lines usually directed outside
and away from the Tx loop. The Rx consisted of two vertical axis loops
separated by one or more stations. This allowed one to measure the
complex ratio of the vertical field components at the two stations with
a compensator. Then, if desired, one could calculate back to the complex
field amplitude at individual stations. Bob Bosschart's PhD was the
bible for it.
By the late '50's, all kinds of alternatives were being marketed for
exploration to order of 30 m. Most were only single frequency, and used
~ 1kHz. This had the important S/N advantage of being within the
sferics amplitude minimum, but was mainly suitable for small scale
surveying in terrain of low conductivity, e.g. glaciated precambrian
shield). Stan Ward's PhD (~1955) was about 20 Hz to 20 kHz field tests
to find an optimum frequency. Jim Wait's PhD at U of T's Electrical
Engineering Dept provided the theoretical background for understanding
its role.
I especially want to second the remarks about Jack Betz' formidable
contributions to the very robust, multi-frequency version of Slingram
designed mid 70's by Apex Parametrics and called Max-Min (referring to
the fact that one could use it as a maximum or minimum coupled system).
Jack provided innumerable excellent example field responses with
detailed interpretations; all very well supported by scale modelling
studies to show how one could differentiate overburden and host rock
responses from those of localized conductors. It was pioneering work in
EM interpretation and an inspirations for our work on UTEM. Some of his
reports are still available at U. of T. I haven't seen or heard from
him in several years, but I believe he is still enjoying retirement in
rural Ontario.
Regards Gordon West
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