Anomalies, Conductors and Half-Widths

In 1969 and 1970 I was a data man and occasional navigator of an airborne geophysics crew, working for Siegel Associates (Scintrex) in northern Canada. The aircraft were either Bell Jet Ranger 206B helicopters or a de Havilland Otter float/wheeled planes. The aircraft were equipped with several geophysics instruments, air photos camera and a radio altimeter.

The air photo film, instrument charts and radio altimeter data were all related by an intervalometer which produced a series of marks on the charts and film, and numbers on dials that could be read by the navigator up front in the aircraft and the operator in the back seats. The navigator would periodically write the intervalometer numbers in wax pencil on his air photo mosaic at the beginning and the end of flight lines and at as many places as he could along the lines. The operator would write the same number on the charts. The film recorded the numbers serially frame by frame.

Once I had developed the film and plotted the actual flight lines, as flown, on my copy of the air photo mosaic, I marked regular intervalometer numbers along the lines. All marking was done with ink pens and errors were hard to erase, so I learned to work carefully, which was against my impatient, energetic nature. With the few intervalometer values scribed by the operator on the charts, I divided the charts with a regularly spaced reference lines using a Tacro ten-point proportional divider. Tacros were beautifully manufactured German machinist’s tools, and were very expensive. They are rarely seen any more, but I have my own pairs and still use them to plot line segments into regularly spaced sections.

The crew geophysicist (who often doubled as a navigator), looked over the charts and identified any geophysical anomalies. Electomagnetic (EM) anomalies on the charts showed buried conductive bodies, and were characterized by the half-widths of the phase shifts and amplitudes of the plots. For unusual high or low magnetic readings the number of gauss units were recorded. Magnetic anomalies showed local perturbations of the earth’s magnetic field due to a metalliiferous boy or a some structural feature like a fault. For radiometric data, anomalies were unusaully high radium, thorium, and potassium counts per unit time. The locations of the geophysical anomalies relative to the flight lines were identified by the intervalometer markings I had drawn on the charts. The geophysical data were then plotted using various symbols on the flight lines as located by the intervalometer numbers.

Once the mission was completed, conductors and other geophysical targets were evident as groups of inked symbols extending across several lines on the the air photo mosaic. The groups represented an area of ground underlain by a metalliferous body (“conductor”) or radiometric mass than was large enough to be investigated further by ground geophysics crews and geologists. Mistakes in plotting of flight paths could threw the data reduction into error: I had a lot of responsibility for someone who initially did not ken half-widths or gauss or potassium counts.I became good at being an alround dataman. I also learned to plot magnetometer data onto Mylar drawing plastic overlain on the marked up air photo mosaics. I then hand contoured the data, a skill that is now pretty well extinct given the fancy computer software available for performing this tiring but satisfying work.

The end result of our airborne reconnaissance-lvel work was the identification of areas to be inmvestigaterd more thoroughly using ground geophysics. So grew an interest and curiosity as to what the ground crews got up to. The men from Scitnrex ground crews seemed rougher and more rugged than we pampered airborne folk.But it was a long time before I found out what ground geophysics was. I became too good at what I was doing for Scintrex to release me work with their to ground crews. So, I eventaully left Scintrex to find a job with somebody else who would teach me ground geophysics. But that is another tale…

About Ed Medley

Ed Medley has been on a random walk for over 50 years. Many scribbles and snapshots at this site are from his vagabond transits; others are from his decades of international experience in geological and geotechnical engineering, academia, and mineral exploration prospecting.
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