Instruments \ Electrical \ 

Description:
This wooden device had its origins in an airborne electromagnetic project at Continental Oil Company from 1952 to 1954. At the conclusion of work at Conoco, the equipment was sold to Aero Service Corporation in Philadelphia. Airmag may have acquired this unit from Aero Service since both were in Philadelphia.

This drogue with an electromagnetic receiver coil was towed beneath an Avro Anson twin engine aircraft. The Avro Anson was a plywood aircraft, making it better adapted for airborne electromagnetic purposes than conductive aluminum aircraft. The source coil was wrapped around the aircraft fuselage and powered by a vacuum tube amplifier of about 300 watts at somewhere around 500 Hz. The original receiver coil was mounted on the wing tip, but wing motion during flight encouraged the use of a drogue device to increase the source-receiver spacing. The drogue was designed by an outside consultant and built in the carpentry shops at Conoco. Wiley Haggard of Spring, Texas, clearly recalls the construction phase of this device. Enclosed within the drogue is probably a vacuum tube preamplifier to drive the electrical cable. This project was directed by Dan Mitchell, now deceased. Other involved were Graydon L. Brown, Reginald Childers (mechanic) and James K. Buttram (pilot) who live in Ponca City, Oklahoma.

The electromagnetic project was one of the first involving the new Geophysical Development and Research Division formed at Conoco. Originally promoted by Hans Lundberg of Toronto, Ontario, the earliest experimental work as at Toronto with Irwin Lerner as the Conoco representative from 1948 to 1951. After Lerner left Conoco, and Don Mitchel was hired to supervise. Graydon Brown started with Conoco in February 1952 when they had acquired an Avro Anson and were ready to begin development of their own system. Construction of new instrumentation took place through 1952 and 1953. This included local flights to test the system on conductive earth at the salt plains and nearby conductive roof buildings. Flight altitude for reliable detection of conductors rarely exceeded 150 feet. Wiley Haggard joined the project, and test flights at Roundup, Montana and Wind Caves, South Dakota, further evaluated the system. At Wind Caves, the goal was to detect the transmitter coil with a receiver coil in the cave to test EM penetration in earth media. Graydon Brown believes all the Roundup tests were with the wing-tip-mounted receiver coil. The drogue was used in 1954 tests with a manually operated winch to lower it from the aircraft when in flight. The aluminum fittings at the top were used to capture the drogue in stable position under the Anson when it was retracted.

Early Vibroseis (TM) experiments had become the primary focus at Conoco by 1954. Electromagnetic prospecting for oil seemed highly unlikely, though EM prospecting for sulfides and other highly conductive minerals was successfully developed. Conoco and Aero Service reached an agreement for the sale of the equipment including instrumentation and aircraft. The equipment was probably transferred in late 1954 or early 1955. Brown met with Aero Service after they began evaluation of the system. Other parts of the instrumentation consisted of a power amplifier, detector amplifier with primary field nulling, and twin pen-inking recorder.

Graydon Brown recalls that most of their flights were conducted early in the morning to take advantage of stable air, but nontheless, he still has memories of violent bouts of airsickness from the project.

(adapted from Letter to TLE, April 1998, by Graydon L. Brown)