A DIGITAL DIFFERENTIAL PROTON MAGNETOMETER BY JOHN D. MUDIE

Dept. of Geodesy and Geophysics, Cambridge University

N archaeomagnetic surveys the daily variation of the Earth’s magnetic field tends to obscure small magnetic features, i.e. anomalies less than four gamma. I To recover this detail, the observations may be corrected by subtracting inter-

polated values of the diurnal variation as observed by repeated readings made at a fixed reference point during the period of the survey. In August 1961 the author decided that it should be possible to construct a differential proton magneto- meter which could be used to remove the effect of the diurnal variation directly by either:

(i) having one ‘fixed’ bottle (detector) and subtracting its reading from that of the other used as a ‘search’ bottle, or

(ii) fixing the two bottles with respect to each other and using the differential magnetometer to measure the gradient as suggested by Aitken (1 962). Mechanical and muscular considerations have at present hindered ready application of this method.

The digital differential proton magnetometer described consists of a fully transistorized accumulator-powered instrument connected by 40 metre coaxial cables to two independent bottles. The instrument weighs 7.7 kilograms and the accumu- lators 8.1 kilograms and together they occupy a case 44 cm. by 12 cm. by 40 cm.; as this particular instrument was the prototype model, it should be possible to reduce these dimensions slightly. The magnetometer can be used to measure the magnetic field associated with either of the two bottles or the difference in magnetic field between the two bottles. All readings appear OH a set of five meters scaled 0-10 and operation is initiated by momentarily pressing a button.

Differential proton magnetometers have been described by Rikitake (1 960), Aitken (1962) and Scollar (1963). The present note contains a preliminary descrip- tion of a new method of obtaining the differential reading. The method is at present the subject of a British Patent application (No. 29018/62) by the National Research and Development Corporation on behalf of the co-inventors, Dr. J. C. Belshk, Dr. F. Gray, Mr. L. H. Flavill, Mr. J. D. Mudie and Dr. I. Scollar.

The differential reading is determined by simultaneously polarizing and depola- rizing the two bottles and by measuring the time taken for 1000 proton cycles to occur in one channel less the time taken for 996 proton cycles to occur in the other channel and the difference is displayed after addition of a constant (to avoid the need to consider positive and negative differences). Although the instrument is slightly more complex to understand electrically, there is a simplification of field operations from the use of a single magnetometer as the differential instrument always gives the same difference reading between two points even though the overall magnetic field may have changed due to the diurnal variation of the magnetic field. The present instrument has an accuracy of approximately 1.4 gamma when operated in Britain but there is no inherent difficulty in reducing this error to 0.7 gamma by increasing the counting time. The magnetometer measures the difference between the two bottles simulthneously in contrast with that of Scollar’s design in which the two measurements are made sequentially. The simultanaeity of reading has the advantage that the instrument is less sensitive

136 A R C H A E O M E T R Y

I O T

NG UIT

CRYSTAL ICLOCK I

I O T

NG UIT

CHANNEL B

AMPLIFIER i TUNED

I

POCARIZE

FIG. 1. Schematic diagram of the computing circuit when used to make a “single bottle” measurement of the magnetic field. The decade counters A,, A: and A, open the gate for the time taken for lo00 proton cycles to occur in the channei selected by the switch. The time for which the gate was held open is determined by allowing clock pulses from 100 Kilocycles crystal oscillator into the decade counters P, to Pa and displaying their counts on a set of

meters.

- - - BOTT

! CHANNELB : TUNED i

RESET j AMPLIFIER NB

POL’ARIZE TO 4

I

FIG. 2. Schematic diagram of the computing circuit when used to make I differential measure- ment. A pulse from As opens gates 1 and 2 and the next pulse from channel B closes gate 1. As B, is reset to 4, the next event which occurs is the output pulse from B, which will re-open gate 1 and close gate 2. Gate 1 is reclosed by the output pulse of A, and the counters C, to

C indicate the difference reading added to a constant.

A R C H A E O M E T R Y 137

to rapidly changing fields, caused for instance by electric locomotives starting, but the main advantage is that the two bottles do not interfere with each other as the polarizing fields are removed simultaneously and measurements can be made with the ‘search bottle‘ near to the ‘fixed’ bottle. The use of an electronic counting system gives this instrument a more quantitative measure 01 field diEerences than that obtained by the differential magnetometer described by Aitken and Tite (1 962).

The differential proton magnetometer consists of two separate bottles and amplifier chains which feed into the computing section; this can be operated in two modes. In the ‘single bottle’ mode illustrated in Fig. 1, the set of decade counters P, to P, display the number of clock pulses which occur in 1,000 proton cycles in the usual fashion. Proton pulses from either channel can be selected by the switch. Let this reading be HA for channel A and HII for channel B. In the differential mode as illustrated in Fig. 2, both bottles are polarized and released simultaneously; this operation can be carried out by separate relays. During polari- zation the decades A, to A,, B1 to B,. and C , to C, are all reset to zero; decade B,, however, is reset to 4. Four flip-flops, not shown in the diagrams, govern the following sequence of events by ‘or’ or ‘and’ gates. After d delay of 1,000 pulses to remove the initial transients the output pulse from decade A, opens gates 1 and 2. Gate 1 allows the clock pulses to feed into counter C, and gate 2 allows the next puls;from channel B to close gate 1 . After 996 pulses from channel B have accumulated in B, to B, the output pulse from B, closes gate 2 and opens gate 1 allowing the clock pulses from the 100 kilocycles crystal oscillator to pass into decade C,. Gate 1 is finally closed by the output pulse from A, which the 2,000th pulse in channel A causes. It can be shown that the count in C, to C,, which is displayed on the front panel of the magnetometer, is then equal to HA -H~+4/1,000 HA provided that - 3/1,000 < H, - HI% < HA. The detailed circuitry and theory will be published shortly by Gray, Flavill, Mudie and Scollar. In practice HA remains almost constant and the count in C is a direct indication of HA - HH. As the decades B,, B,, C,, C1, C, in the differential magnetometer are used as P, to P, in the single bottle magnetometer only a few extra components are required to convert a single bottle magnetometer into a differential magneto- meter, viz. two binary units, one decade counter, one gate and a bottle and amplifier chain.

This digital differential proton magnetometer ha5 been used in three archaeologi- cal surveys at present and has proved a useful instrument. The differential magneto meter was built before reversible decade counters were readily available; since the advent of reversible decade counters, Scollar (1962), a design change has been contemplated which is illustrated i l l Fig. 3. Both bottles would be polarized and depolarized simultaneously and, after the initial settling down period, a pulse from A, would open gate 1 and start the reversible counters C , to C, counting clock pulses in a positive direction. The 10th pulse in channel A would open gate 2 allowing the next pulse in channel B to close gate 1. Counters A and B continue to count the proton pulses in their respective channels until the output pulse from A, opens gate 1 and starts the reversible counter C counting in a negative direction. The output pulse from decade B, causes gate 1 to close. The resulting count, which would be displayed on a set of meters, would be H A - H B provided that Hk -H~<10/1,000 HA. The system is simpler in conception than the first but has few practical advantages over it.

138 A R C H A E O M E T R Y

~

- CRYSTAL CLOCK

CHANNEL A

FIG. 3. Proposed layout of a computin circuit for a differential proton magnetometer using reversifhe decade counters.

REFERENCES Aitken, M. J. and Tite, M. S. (Dec. 1% Rikitake, T., and Tanaoka, I. (1%0), Bb,. Earrhq. Res., Inst., Tokyo, 38, 317. Scollar, I. (1963)) Efec. Eng: 35, 421, 1’17. Scolar, I. (Sep. 1962), Elec. A 1 . 33, 403, 597.

1. Sci. Instrunt., 39, 625.