Hello All:
The recent discussion of Magnetic Anomaly Detection (MAD) of submarines in the General Discussion forum prompted me to dig out some old notes pertaining to the subject. In 1954 I was working as a research engineer in a group studying ASW problems. We had come up with the idea for a magnetic target which could be used to train MAD operators. We actually built a target and ran some tests on it. The target consisted of a steel tube about 15 feet long and 16 inches in diameter, wrapped in a coil of copper wire and powered by a set of lead acid batteries contained inside the tube. The tube could be ballasted for depth and was designed to be towed by a surface craft. We envisioned eventually a self-contained unit similar to a large torpedo which could be programmed to run a preset course and then surface for recovery. The Navy provided a Grumman S2F Tracker and we flew some at-sea tests. I rode with the MAD operator and we were able to detect and track the target. Eventually the Navy decided they weren’t interested in pursuing it further so the idea was never fully developed. However, it did lead to a lot of research on my part to dig up the data on MAD gear and magnetic anomaly detection. Much of what follows comes from my notes on this project.

As a point of interest, the development of the airborne passive sonobuoy and MAD occurred along parallel paths in approximately the same period in time. A Catalina flying boat operating from Quonset Point, Rhode Island successfully demonstrated a developmental MAD gear by detecting a submarine during the initial testing sometime prior to the attack on Pearl Harbor in 1941. The initial development of the sonobuoy occurred in 1941 and by March 1942 in tests off New London the propeller sounds of a submarine were detected at an underwater range of three miles. The MAD gear was first into operational use but so many false detections were occurring that the sonobuoy was brought along for use as a means of corroborating a MAD contact before weapon drop. However, the sonobuoy was also used extensively by the CVE carrier aircraft which were never equipped with MAD gear.

Magnetic Anomaly Detection (MAD) equipment operates by detecting the magnetic field of a submarine. The total magnetic field of a submarine is the vector sum of its “permanent” and “induced” magnetic fields. The “induced” field is that component produced by its presence in the earth’s magnetic field. The “permanent” field is the residual magnetization of the hull produced in large part by the working of the metal in the hull during the production process. The hull is subjected to impacts and heating during the riveting and welding operations. Both of these operations tend to align the magnetic domains in the metal with the earth’s field, in effect turning the hull into a “permanent magnet”. The “induced field” depends mainly upon the submarines geographical position and orientation to the earth’s field. As a result, the total magnetic moment of a submarine is highly variable and dependent upon both the state of its permanent component of magnetization and its current location and orientation on the earth’s surface.

To the MAD gear detector, the submarine appears approximately as a magnetic dipole. This means that the magnetic field strength of the submarine as seen by the detector is inversely proportional to the cube of the range, directly proportional to the magnetic moment, and varies as a sin/cos function depending on the angle to the dipole axis. It will be immediately apparent that due to the inverse cube relation to range, MAD gear is never considered a primary detection tool; the range is just too short, limited to a few hundreds of yards. However, because of its limited range, it was useful for determining a weapon release point and found its principal role in that mode.

The “flux gate” was the first detector developed for submarine detection. It is also the basis for the “flux gate” aircraft compass. Three bars of highly permeable metal (mu metal) are arranged orthogonly and when in use aligned with the components of the earth’s field. Each bar is wound with a primary coil and connected in series with the others. These primary coils are excited by an ac current source of approximately 5 kilohertz and amplitude sufficient to drive the mu metal bars into saturation. If no external magnetic field is present, the voltage induced in the secondary pick-up coils consists only of the odd harmonics of the drive frequency. However, if an external field is present, the core hysteresis loops are biased in one direction which causes the core to saturate sooner in one half of the drive cycle than in the other half. The induced secondary coil voltages then contain all the even harmonics as well as the odd. The amplitude and phase of the even harmonics are proportional to the magnitude of the external field and thus provide a measurement of the external field strength.

A disadvantage of this type of detector is the requirement for precise alignment with the external field. For an airborne detector this imposes the necessity for the detector to be mounted in a servoed gimbal to maintain alignment. The performance of the detector (S/N ratio) is degraded by all extraneous magnetic fields and care must be taken to isolate the detector from the “magnetic noise” produced by the gimbal system. In fact, the detector was flown in a towed “bird” to separate it as much as possible from all the magnetic noise produced in the aircraft. The handling of electrical wiring and the use of and placement of magnetic materials all had to be carefully considered.

A later interesting magnetometer development eliminated the precise alignment requirement. This is the Proton Precession Magnetometer (PPM) which had its roots in WW II research but to my knowledge was not used during the war. The PPM uses the magnetic and gyroscopic properties of protons in a fluid. The fluid could be as simple as a small container of kerosene which is a rich source of protons.

A proton has a dipole magnetic moment. A heavy current is passed through a coil wrapped around the container of fluid, producing a magnetic field far stronger than the ambient field to be measured. The free protons, acting like tiny spinning bar magnets, are jerked into alignment with this strong magnetizing field. The coil is then rapidly disconnected from the current source and switched into an amplifier. The formerly aligned protons, no longer aligned, begin to re-align to the ambient field being measured. As they re-align they precess and in doing so induce a voltage in the surrounding coil which is now connected to the amplifier. The rate at which they precess, and thus the frequency of the voltage induced in the coil is directly proportional to the magnitude of the magnetic field to which they are exposed. The coil is periodically switched between the magnetizing current source and the amplifier input and the measurement of the induced frequency in the coil is proportional to the external magnetic field strength. These magnetometers show a sensitivity between 0.1 and 1.0 nanotesla.

I hope this information is of interest.

Don B