by Lee Lawyer
Today, Lee Lawyer, retired Chief Geophysicist with the Chevron Corporation tells us how the Earth's Magnetic field is measured. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.
Measurements of the magnetic field of the earth are routinely made on land, on sea, in the air and even from satellites. The earliest compass was invented by the Chinese over two thousand years ago. Every Boy Scout knows that the compass needle points north. The needle doesn't lie flat; it tends to dip north. The amount it dips and the direction it points reflect two components of the Earth's magnetic field. Magnetometers record the strength of those components.
Gulf Oil Company developed a device to make the measurements from a moving aircraft in 1939. Electrical engineer Victor Vacquier led the research. The Navy flew the magnetometer over a submerged US submarine. It was easily detected. The National Defense Research Council hired 200 physicists and electrical engineers to develop the airborne magnetometer for military use. It was called Magnetic Anomaly Detector or MAD for short.
Vacquier installed it in a Blimp for testing at the Lighter-Than-Air Naval Headquarters in New Jersey. Often the Blimp couldn't make headway. Sometimes it even moved backwards relative to the ground below. After over 200 hours of testing in the blimp, the Navy decided that a conventional aircraft was more appropriate.
The airborne magnetometer uses a fluxgate principle. It's simple in concept. We wrap two coils around an iron core. We pass an alternating current through one of the coils. This drives the iron core through an alternating cycle of magnetization. This changing magnetic field in turn induces an electrical current in the second coil. The difference in the currents is the strength of an external magnetic field, in this case Earth's magnetic field.
Well over a million miles were flown using the fluxgate magnetometer. We use magnetic data to explore ore deposits and areas suitable for petroleum exploration. It was important in the discovery of Plate Tectonics and it confirmed ocean floor spreading in the North Atlantic. The magnetometer is used on every spacecraft to the solar system including earth satellites.
There're other applications. Package inspection, materials screening and the effectiveness of shielding are examples. It's used to detect buried subsea pipelines, fiber-optic cables and once was used to locate a missing US H-bomb buried 15 feet in a seabed off the California coast. All these applications clearly show that good technology, thoughtfully engineered and innovatively applied, stays with us well beyond its original purpose.
I'm Lee Lawyer. I've always been interested in the way inventive minds work.
L. C. (Lee) Lawyer's career with the Chevron Corporation spanned 38 years. He was Division Geophysicist for Chevron's Mid-continent and Alaskan Divisions, Chief Geophysicist of Chevron Overseas and Vice-President of Chevron Geosciences. When he retired in 1992, he was Chevron's Corporate Chief Geophysicist. He has been very active in the Society of Exploration Geophysicists. He was General Chairman of the SEG Convention in San Francisco in 1978, SEG 2nd Vice-President in 1987-1988, President-Elect in 1986-1987 and President in 1987-1988. He writes the monthly column, "From the Other Side" for The Leading Edge, published by the Society of Exploration Geophysicists.
Victor Vacquier was born in Russia in 1907, raised in France and received his education at the University of Wisconsin and is alive today. He joined Scripps in 1957. They had a 100th birthday party for him in October 2007. Interestingly, both Victor senior and his son Victor, Jr. (marine biology) ended up at the same institution. Victor Jr., will retire after 29 years with Scripps in January 2008.
The Fluxgate magnetometer first tested by Gulf measured only the vertical component of the earth's magnetic field. For submarine detection it was not necessary to actually measure the magnetic intensity. All that was needed was a magnetic profile of the variations caused by a magnetic object such as a submarine. The device to do this (called MAD -- magnetic airborne detector) was operational in December 1942. It detected and destroyed an enemy submarine in the Mediterranean Area.
The New Fluxgate Magnetometer (outside the "Bird" in which it flies below an airplane.)
The commercial version of the fluxgate magnetometer was used in mineral exploration. In ore body exploration, the applications are direct, survey an area and look for anomalous indications. In finding petroleum, the problem is indirect. The magnetic information is used to estimate the depth of magnetized rocks, usually the basement (the first layers below sedimentary rocks). Using those estimates, the location of thick sedimentary layers can be mapped. Those are the primary targets of petroleum exploration.
Much of the material associated with the early development of the fluxgate magnetometer comes from internal memoranda and reports from Gulf Oil Company. A simple search of the internet using "Fluxgate Magnetometer" results in sites that give directions for home construction of a magnetometer.
Images: Photos of the magnetometer in its aerial casing, called a "Bird," being installed and flown under a DC-3 are courtesy of Keystone Aerial Surveys.
I invited Paul Cloutier, Prof. of Physics and Astronomy at Rice Univ., to comment on the role of a magnetic field forming Earth's atmosphere. I include his following response as a matter of general interest:
In the case of a large planet like Jupiter, the planet forms in the outer solar system where temperatures are low enough to keep volatiles like water, carbon dioxide, methane and ammonia frozen as ices in the absence of local heat sources. The thermal speed of most gases in the atmosphere is less than the gravitational escape speed, so all gases are retained against thermal (Jeans) escape. However, other non-thermal mechanisms, such as solar wind pick-up, would cause erosion of the outer part of the atmosphere (exosphere) if the planet did not possess a magnetic field. Because these planets have the original atmospheres from their formation, the atmospheres are referred to as primary atmospheres.
Our small terrestrial planet was formed in the inner solar system where solar radiation makes the temperature too high for volatiles to exist as ices or liquids. The high surface temperatures of such newly-formed planets result in thermal speeds larger than gravitational escape speeds. The result is that such planets lose all of the gases present at their time of formation. Only after these planets' surfaces cool down enough to have thermal speeds lower than gravitational escape speed can these planets retain an atmosphere, and the gases in their atmospheres are the gases produced by outgassing from the planets' interiors by volcanism. These atmosphere are referred to as secondary atmospheres. Because the planets are small, the thermal speed is not much smaller than gravitational escape speed, so a small amount of gas continually escapes (the highest-speed fraction of the Maxwellian distribution of velocities). However, if the planets' magnetic fields are not strong enough to protect the exosphere from the solar wind, solar wind losses will greatly exceed the gravitational loss rates for the lightest atmospheric constituents like hydrogen and oxygen, causing high rates of water loss.
Magnetometer in its "Bird" casing, being installed beneath a DC-3.