Deep-Sea Research, 1967, Vol. 14, pp. 275 to 276. Pergamon Press Ltd. Printed in Great Britain.

A new detector for the marine proton magnetometer

F. GRAY*

(Received 3 October 1966)

THE PRINCIPLE of operation of the proton magnetometer, and a marine version designed at Cambridge, have been described elsewhere (WATERS and PHILLIPS, 1956 : HILL, 1958).

The new detector to be described was designed so that the magnetometer could be used either in small motor launches or as a large ship installation. The mechanical requirements were that the towed " f i s h " and cable should be lightweight and easy to handle. It was decided to adopt the system of having a preamplifier in the " f i sh , " so that a special non-microphonic cable would be unnecessary, and also to minimise electrical noise from the ship. When a preamplifier is used several switching operations are required to be operated in the " f i s h " remotely, from the ship, and. it has been found possible to provide all the functions necessary, using a single co-axiai towing cable. Figure 1 shows how this has been done.

X

Y + ~,

R I

DI I A ~..~ D2I L~iA 2

D3

Clo \-

Fig. 1. Preamplifier and switching circuit.

(1) Polarising sequence

When the polarising supply is applied at the ship end of the cable, 12 V appear at the input XY with polarity shown. The diode D~ is forward biased and so relay RL2 operates and the bottle coil L is connected to the supply through contact A~. At the same time, the preamplifier input is discon- nected from the bottle. Also, contact B~ connects the supply to a rechargeable cell S via the stabilising circuit Rt and Da and during the polarising interval (5 sec) the cell is recharged. This means that the voltage supply for the preamplifier is maintained while the instrument is being used. Contact B~ also disconnects the preamplifier output from the line while polarising takes place. When the polarising current is switched off, the bottle is reconnected to the preamplifier, and also to a tuning condenser Cx, and the co-axial cable is reconnected to the output of the preamplifier. Precession takes place and the signal is fed to the preamplifier, which has a gain of 20,000. This means that the signal up the cable has an amplitude of about 100 mV.

(2) Tuning sequence

The capacitor C1 and the inductance L form a parallel tuned circuit, and the value of C1 will have to be changed from time to time depending on the change in precession frequency, and the Q of the resonant circuit. In the present design the Q has been reduced from 50 to 4 by using a low impedance amplifier to load the tuned circuit, and a useable signal is obtained with one value of C1 over 500 cycles of the precession signal. This means that the magnetometer will operate over a range of over

*Dept. of Geology, University of Durham, South Road, Durham.

275

276 Instruments and Methods

12,000 7 without retuning. If remote retuning is required, the additional components shown in Fig. 1, connected at point E are needed. A switch in the magnetometer reverses the current to the towing cable so that when the polarising current is applied, RL1 operates instead of RL2. Contacts A1 operate a uniselector U1 which selects a new value of tuning. In addition RL1 is held at the operate position by Ct~ for 1 sec after the polarising current is switched off. During this time resistor R2 provides positive feedback in the amplifier circuit and the amplifier oscillates at a frequency determined by the tuned circuit LC at the input. This signal is fed to the magnetometer and its frequency measured in the same way that the precession signal is measured, so that the tuning value is immediately known. It is in general, not worth the additional complication of remote tuning as a range of 12,000 ~, is usually sufficient for one survey, and it requires only a few minutes to open the " f i s h " and readjust

i Towing coble

A \\

the tuning should this be necessary.

A

If-]Vh , , , , , , , , ~/////I/I/I/I/I/I///I/1///,I//////////I I /

B. , JULY , Tufnol/~--..~l / \ \\ \\ I

/ \ Brass Preamp. / '\ chossis

Fibreglass Neoprene fube rubber

Fig. 2. Sealing arrangement at front end of "f ish."

Figure 2 shows the mechanical design of the sealing arrangement in the " f i sh ." The tufnol head is held in the end of the fibreglass tube by three screws (A). When the three recessed nuts (B) are tightened, the neoprene rubber disc is squeezed against the inside of the tube and also against the towing cable, providing an effective seal. The fibreglass " f i s h " which houses the preamplifier and switching, at the front end, and the bottle at the rear end, has to be at least 6-ft long to get the bottle far enough away from the magnetic effects of relays and towing cable. The diameter of the " f i s h " is 2½ in. Stabilising fins were fitted to the rear end of the " f i s h " but were later abandoned as they were found to be unnecessary. In latitudes where the earth's field is dipping at more than 45°, the horizontal bottle is satisfactory, but near the magnetic equator, a vertical bottle is necessary. This can be made by dividing the 18-in. long bottle into sections small enough to fit across the tube, and then connecting the sections in series.

The amplifier will not be described in detail as it is a conventional three-stage common emitter type with an emitter follower stage in the output. Various types of transistor were tried for low noise in the first stage, and it was found that a micro-alloy transistor (MAT 101) was the most satisfactory. The response of the amplifier was limited to the range of frequencies expected ( ~ 1000-3000 c/s) and it was found that a signal to noise ratio of over 10 to 1 couldbe obtainedwith the tuning bandwidth used (500 c/s) and this meant that additional tuning in the magnetometer was unnecessary. This instrument has been used over the last three years during the summer months, to carry out extensive magnetometer surveys in the Irish Sea.

R E F E R E N C E S

HILL M. N. (1958) A ship-borne nuclear-spin magnetometer. Deep-Sea Res., 5, 309-311. WATERS G. S. and G. Pm~LLrPs (1956) A new method of measuring the Earth 's magnetic field. Geophys.

Prospect., 4, 1-9.