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Chapter 3. Measurement of Geomagnetic Intensity at Sea
CHAPTER 3
MEASUREMENT OF GEOMAGNETIC INTENSITY AT SEA
THE TOTAL INTENSITY ANOMALY
In a moving body towed by a ship or an airplane it is impractical to measure components of the geomagnetic field with sufficient precision because the north and the vertical directions are difficult to establish. However, the magnetic anomalies seldom exceed 2% of the total earth's field, so that one can assume that the total magnetic vector stays undeflected over an area that is large in comparison to the wavelength of the anomalies, and that therefore its changes in magnitude can be treated as a vector quantity. Exactly the same kind of approximation is made in the case of gravity which is assumed to remain undeflected by the nonuniform lateral distribution of mass in the earth's crust. In Fig.24 let a geologic body produce an anomalous field AT which can be
8 T = A H c o s i t Z sin i Fig.24. Approximate treatment of the total magnetic anomaly as a vector quantity. The anomalous magnetic intensity AT has components AH and AZ. The sum of their projections on the direction 7' of the geomagnetic field, 6T is regarded as a vector in that direction, which is assumed to be the same over the area of the anomalies that are being interpreted.
split into a horizontal component AH and a vertical component AZ. What is actually measured is the difference between the planetary field T which in practice is the International Geomagnetic Reference Field (I.G.R.F.) and IT + A T [ .Most often geomagnetic model calculations deal with 6 T which consists of the sum of the projections of AH and AZ on the constant direction of T. The maximum errors that can be incurred by this approximation are given for different latitudes in Table 11.
28
MEASUREMENT OF GEOMAGNETIC INTENSITY AT SLA
TABLE I1 Maximum error in 6T from assumption that the anomaly does not deflect the direction of the total magnetic intensity (units are y) (From Kontis and Young, 1964) 500
30,000 35,000 40,000 45,000 50,000 55,000 60,000 65,000 70,000
1,000
2.000
11
61
14 13 11 10 9 8 8 7
51
50 44 40 36 33 31 29
3,000
150 129 113 100 90 82 I5 69 64
THE PROTON MAGNETOMETER
Any total intensity magnetometer of sufficient precision can be used. A sensitivity of 5 or 10 y is adequate for geomagnetic work at sea, since errors of the order of 30 y might come from rapid time variations of the geomagnetic field and uncertainty of the ship’s position. The modern proton precession magnetometers which are now universally used at sea record the field to one gamma at least once every minute. Except for the Russian nonmagnetic ship “Zaria” the hulls of oceanographic ships are made of steel, so that the magnetometer sensor has to be towed about three ship-lengths behind the ship to reduce errors on different headings to the 10-7 level (Bullard and Mason, 1961). The sensor consists of a coil immersed in a hydrogenous liquid such as 3 hydrocarbon oil or water. A d.c. current of several amperes, producing a field of over 100 r, is passed through the coil for several seconds to line up a small fraction of the protons, about one in l o 6 ,parallel to the axis of the coil. If this axis forms an angle to the direction of the geomagnetic field, when the d.c. field is.shut off, the protons will start precessing about this field with a frequency proportional to its magnitude. The precessing protons induce an e.m.f. in the coil the frequency of which is measured by comparison with a crystal oscillat6r. The effect depends on the magnetic moment and angular momentum of the protons. The product of the magnetic moment of the proton and the component of the geomagnetic field perpendicular to it furnishes the torque which acts on the angular momentum to produce the precession. It corresponds to the gravitational torque acting on a pendulous gyroscope which is governed by the same equations of motion. It is easily demonstrated that the precession frequency is independent of the angle the gyroscope makes with gravity (Page, 1935, p.137). To make the amplitude less dependent on coil orientation, systems of several coils or a toroidal coil is used instead of a simple cylindrical coil which
PROTON MAGNkTOMPTER
31 '
gives no signal when the coil axis is parallel to the geomagnetic field. The decrement of the amplitude of the precession signal differs for different liquids, being about 3 sec for water, and somewhat shorter for the oil mixtures. If the magnetic field T is measured in gammas, and the frequency in hertz:
T = 23.48682
* 8f
(N.B.S., 1971, chapter 2)
The constant is proportional t o the ratio of the angular momentum to the magnetic moment or the gyromagnetic ratio of the proton. One hertz corresponds to 23.5 y so that if one wishes greater sensitivity, one has t o measure fractions of a cycle. I n the past this has been done by counting cycles of a high-frequency oscillator between a given number of precession cycles. The number of high-frequency cycles counted is proportional to the period of the precession, requiring conversion to field values from a table. Modern magnetometers multiply the precession frequency and count it in time units of such size that the counter reads directly in gammas. Recording proton magnetometers are now built by several manufacturers for station, ship, and aircraft use. A block diagram of such an instrument is shown in Fig.25, and a photograph in Fig.26. The only part of the proton precession magnetometer that might need checking is the oscillator in the counter. The oscillators in such counters are usually controlled by thermostated quartz crystals the
Fig. 25. Block diagram of proton magnetometer Model V-4970. (From Varian Associates, Palo Alto, California, publication No. INS 1012A.)
30
MEASUREMENT OF GEOMAGNETIC INTENSITY AT S L A
Fig.26. Photograph of Varian Associates, Palo Alto, California, shipboard proton magnetometer Model V-4970.
frequency of w h c h remains constant to a few parts per million. It can be checked by a standard oscillator. The proton magnetometer has the great advantage over the oriented flux gate of not requiring calibration or measurement of base-line values. Its digital output is conveniently recorded on magnetic or punched paper tape, or directly fed into a shipboard computer system where the International Geomagnetic Reference Field can be subtracted from it to yield the anomalous field. Where the processing is not automatic, the magnetic record and the position on the track are correlated according to time after the ship’s track is worked out from navigational data.
MEASUREMENT OF GEOMAGNETIC INTENSITY AT SEA
THE TOTAL INTENSITY ANOMALY
In a moving body towed by a ship or an airplane it is impractical to measure components of the geomagnetic field with sufficient precision because the north and the vertical directions are difficult to establish. However, the magnetic anomalies seldom exceed 2% of the total earth's field, so that one can assume that the total magnetic vector stays undeflected over an area that is large in comparison to the wavelength of the anomalies, and that therefore its changes in magnitude can be treated as a vector quantity. Exactly the same kind of approximation is made in the case of gravity which is assumed to remain undeflected by the nonuniform lateral distribution of mass in the earth's crust. In Fig.24 let a geologic body produce an anomalous field AT which can be
8 T = A H c o s i t Z sin i Fig.24. Approximate treatment of the total magnetic anomaly as a vector quantity. The anomalous magnetic intensity AT has components AH and AZ. The sum of their projections on the direction 7' of the geomagnetic field, 6T is regarded as a vector in that direction, which is assumed to be the same over the area of the anomalies that are being interpreted.
split into a horizontal component AH and a vertical component AZ. What is actually measured is the difference between the planetary field T which in practice is the International Geomagnetic Reference Field (I.G.R.F.) and IT + A T [ .Most often geomagnetic model calculations deal with 6 T which consists of the sum of the projections of AH and AZ on the constant direction of T. The maximum errors that can be incurred by this approximation are given for different latitudes in Table 11.
28
MEASUREMENT OF GEOMAGNETIC INTENSITY AT SLA
TABLE I1 Maximum error in 6T from assumption that the anomaly does not deflect the direction of the total magnetic intensity (units are y) (From Kontis and Young, 1964) 500
30,000 35,000 40,000 45,000 50,000 55,000 60,000 65,000 70,000
1,000
2.000
11
61
14 13 11 10 9 8 8 7
51
50 44 40 36 33 31 29
3,000
150 129 113 100 90 82 I5 69 64
THE PROTON MAGNETOMETER
Any total intensity magnetometer of sufficient precision can be used. A sensitivity of 5 or 10 y is adequate for geomagnetic work at sea, since errors of the order of 30 y might come from rapid time variations of the geomagnetic field and uncertainty of the ship’s position. The modern proton precession magnetometers which are now universally used at sea record the field to one gamma at least once every minute. Except for the Russian nonmagnetic ship “Zaria” the hulls of oceanographic ships are made of steel, so that the magnetometer sensor has to be towed about three ship-lengths behind the ship to reduce errors on different headings to the 10-7 level (Bullard and Mason, 1961). The sensor consists of a coil immersed in a hydrogenous liquid such as 3 hydrocarbon oil or water. A d.c. current of several amperes, producing a field of over 100 r, is passed through the coil for several seconds to line up a small fraction of the protons, about one in l o 6 ,parallel to the axis of the coil. If this axis forms an angle to the direction of the geomagnetic field, when the d.c. field is.shut off, the protons will start precessing about this field with a frequency proportional to its magnitude. The precessing protons induce an e.m.f. in the coil the frequency of which is measured by comparison with a crystal oscillat6r. The effect depends on the magnetic moment and angular momentum of the protons. The product of the magnetic moment of the proton and the component of the geomagnetic field perpendicular to it furnishes the torque which acts on the angular momentum to produce the precession. It corresponds to the gravitational torque acting on a pendulous gyroscope which is governed by the same equations of motion. It is easily demonstrated that the precession frequency is independent of the angle the gyroscope makes with gravity (Page, 1935, p.137). To make the amplitude less dependent on coil orientation, systems of several coils or a toroidal coil is used instead of a simple cylindrical coil which
PROTON MAGNkTOMPTER
31 '
gives no signal when the coil axis is parallel to the geomagnetic field. The decrement of the amplitude of the precession signal differs for different liquids, being about 3 sec for water, and somewhat shorter for the oil mixtures. If the magnetic field T is measured in gammas, and the frequency in hertz:
T = 23.48682
* 8f
(N.B.S., 1971, chapter 2)
The constant is proportional t o the ratio of the angular momentum to the magnetic moment or the gyromagnetic ratio of the proton. One hertz corresponds to 23.5 y so that if one wishes greater sensitivity, one has t o measure fractions of a cycle. I n the past this has been done by counting cycles of a high-frequency oscillator between a given number of precession cycles. The number of high-frequency cycles counted is proportional to the period of the precession, requiring conversion to field values from a table. Modern magnetometers multiply the precession frequency and count it in time units of such size that the counter reads directly in gammas. Recording proton magnetometers are now built by several manufacturers for station, ship, and aircraft use. A block diagram of such an instrument is shown in Fig.25, and a photograph in Fig.26. The only part of the proton precession magnetometer that might need checking is the oscillator in the counter. The oscillators in such counters are usually controlled by thermostated quartz crystals the
Fig. 25. Block diagram of proton magnetometer Model V-4970. (From Varian Associates, Palo Alto, California, publication No. INS 1012A.)
30
MEASUREMENT OF GEOMAGNETIC INTENSITY AT S L A
Fig.26. Photograph of Varian Associates, Palo Alto, California, shipboard proton magnetometer Model V-4970.
frequency of w h c h remains constant to a few parts per million. It can be checked by a standard oscillator. The proton magnetometer has the great advantage over the oriented flux gate of not requiring calibration or measurement of base-line values. Its digital output is conveniently recorded on magnetic or punched paper tape, or directly fed into a shipboard computer system where the International Geomagnetic Reference Field can be subtracted from it to yield the anomalous field. Where the processing is not automatic, the magnetic record and the position on the track are correlated according to time after the ship’s track is worked out from navigational data.