A S E A F L O O R M A G N E T O M E T E R

F O R T H E C O N T I N E N T A L S H E L F

ANTONY WHITE

School of Earth Sciences, The Flinders University of South Australia, Bedford Park, South Australia 5042

(Accepted 20 October, 1978)

Abstract. Measurement of temporal magnetic variations on the sea floor is desirable in order to extend the technique of geomagnetic depth sounding into the oceans. This paper describes a recording three-component sea floor magnetometer and its use in continental shelf depths. The orientation and tilt of the instrument on the sea floor are recorded using gelatine solutions to 'freeze' a compass card and a ball-bearing, respectively. A backing-off procedure initially nulls the magnetic field components along each of the three mutually orthogonal fluxgate sensors. Magnetic variations along each sensor axis are then recorded within a range of -+ 300 nT of these nulled positions. The resolution is _+ 1 nT, and with a power drain of 800 mW the magnetometer can record continuously for 30 days. The instrument capsule is moored to surface buoys for recovery in continental shelf applications. The buoys may have marker flags, radar reflectors or radio beacons attached to them to aid in relocation.

Introduction

In the last two decades the measurement of geomagnetic variations (with periods

ranging from a minute to several days) has become popular in connection with a

technique known as geomagnetic depth sounding. In this technique, profiles or

arrays of recording magnetic variometers are left in position for periods of from

one to several months, during which time they record geomagnetic variations

such as storms, bays, pulsations and the normal daily variation. The variations

contain both external and internal parts, the latter being due to concentrations of

induced electric current flowing in the more conductive regions of the crust and

upper mantle. By mapping the patterns of induced currents and using suitable

analysis of the magnetic variations, theoretical models of the subterranean

electric conductivity structure may be inferred.

Land-based magnetometers have recorded many 'anomalous ' geomagnetic

variations associated with conductive bodies or regions, which include such

areas as inland seas and lakes, large sedimentary basins and the ocean waters.

These near-surface conductors can make it difficult to detect and interpret the

effects of deep conductivity structure and impose a potentially severe limitation

on land-based surveys directed towards a straightforward conductivity depth-

sounding experiment. Although the need to extend the technique to include sea

floor observations has long been recognised, relatively few attempts have been

made. Studies of the ocean--continent structure along the California coastline (Filloux, 1967; Cox, Filloux and Larsen, 1970; Greenhouse, 1972) included

Marine Geophysical Researches 4 (1979) 105-114. 0025-3235/79/0041~)105 $01.50. Copyright 0 1979 by D. Reidel Publishing Company.

106 A N T O N Y W H I T E

observations both on the continental shelf and on the deep ocean floor, while Poehls and Von Herzen (1976) chose to avoid structural and tectonic com-

plications by making observations on abyssal plains in the North Atlantic. This paper describes a sea floor magnetometer developed initially for use on the

continental shelf and which has been used in a study of the coast effect in geomagnetic variations in South Australia (White and Polatajko, 1978). In design it is similar to the Cambridge University sea floor magnetometer (Owen and Sik, 1972; Sik, 1973), with which, unfortunately, no sea floor observations of any significance were ever made.

T h e I n s t r u m e n t C a p s u l e

The design of a sea floor magnetometer falls naturally into two parts; the design of the magnetometer and the design of the instrument capsule which houses it on the sea floor. Although some design details will be given here as necessary, fully

detailed descriptions and specifications on all aspects of the sea floor mag- netometer are available in an unpublished report (White, 1977).

The physical layout of the instrument capsule may be seen in Figure 1 which shows the magnetometer being readied for launching by hand from a small

Fig. 1. The sea floor magnetometer being readied for launching from a fishing boat.

A S E A F L O O R M A G N E T O M E T E R F O R TIdE C O N T I N E N T A L S H E L F 107

fishing boat. Three pressure casings are attached to triangular framework of which each side measures approximately 1.6m. The magnetic sensors are housed in the smaller white pressure casing at the apex of the frame in order to

isolate them from possible distortion of the magnetic field caused by the rest of the magnetometer electronics which are housed in the long white pressure casing along the base of the frame. Electrical connection between these two casings is made by an underwater cable going from the left-hand end cap of the horizontal casing to the lower end cap of the vertical casing (this cable is not visible in Figure 1). The smallest, centrally mounted pressure casing is made of PVC and contains a doubly-gimballed compass which is filled shortly before launching with warm fluid gelatine solution. The gelatine takes several hours to set and the ensemble 'records' the orientation of the instrument capsule on the sea floor. Because the capsule usually lands in a position close to the horizontal the compass reading can be used to define the orientation of the two 'horizontal' sensors in the sensor unit with an accuracy of - 2 ~

Apart from the compass housing and four stainless steel ring bolts used for the

rope harness the material used throughout (including nuts and bolts) is alu- minium alloy which has been hard anodized to check corrosion. All surfaces except the mating surfaces of the tubes and end caps were coated with an epoxy based paint to protect the anodizing. The tubing is of 0.23 m outside diameter with a 25mm wall and the end caps are 50-mm thick. While these wall thickness are far greater than is required for continental shelf depths, a modification of the instrument for deep ocean use is envisaged. Collapse pressure of the tubing may be calculated from formulae given in mechanical engineering texts (for example, Roark, 1938). The critical parameter in this application is the compressive yield strength of the material, and, on the range of values quoted by the manufacturers, the aluminium casings would probably

withstand the hydrostatic pressure at 5-km sea water depth. The sealing of the end caps to the tubing is accomplished by an 'O' ring set in a groove in each end cap and they are held onto the tubing at low external pressure by four bolts extending the length of the tubing, as may be seen in Figure 1.

The Magnetometer

The magnetometer is a battery-powered three-component fluxgate instrument

with analogue recording on three individual strip chart recorders. The sensor unit, which is shown in Figure 2, contains three mutually orthogonal Vacquier- type fluxgate sensors (Smiths Industries, type LFG-A13) which each have backing-off (feedback) coils as well as primary and secondary coils. The unit fits uniquely onto the bottom end cap so that the orientations of the sensors have a known relationship with the capsule framework. The one-litre flask, also con- tained in the sensor unit, is for recording the tilt of the unit on the sea floor. It

108 ANTONY WHITE

Fig. 2. A close-up of the sensor unit. The three cigaret te-shaped sensors may be seen surrounding the ' tUtmeter ' flask.

contains a bronze ball-bearing which, in rolling to the lowest point inside the

flask, defines the vertical. Just prior to launching of the magnetometer the flask is filled with warm fluid gelatine solution so that the ball-bearing is ' f rozen' in position some hours after the magnetometer has settled on the sea floor. On recovery, its position is marked so that, subsequently, the two angles of tilt relative to the axes of the two sensors parallel to the bottom end cap may be measured. By this means it is possible to record tilts to an accuracy of --+ �89176 in each of these directions. The unusable area of the flask in the region of its rubber stopper is unlikely to be needed since this would mean that the capsule was standing on its 'nose' , a highly unstable position.

The rest of the magnetometer is housed in the large pressure casing and comprises the electronic circuit boards, recorders and battery pack as illustrated in Figure 3. A block diagram (one fluxgate sensor) is shown in Figure 4. The 1.5 kHz square wave current drive is supplied in common to each of the excitation (primary) coils of the fluxgate sensors. The demodulator circuit filters, rectifies and amplifies the second harmonic signal from the secondary coil, producing a d.c. voltage proportional to the magnetic field component along the sensor axis (the system is not a phase-sensitive one). This signal is recorded on a Rustrak chart recorder at a sensitivity of approximately 200nT per chart width. An electronic switch extends the recording range by one chart width each side of

A S E A F L O O R M A G N E T O M E T E R F O R T H E C O N T I N E N T A L S H E L F 109

Fig. 3. The magnetometer unit, showing the electronic circuit boards, the three recorders and a part of the battery pack at the right-hand side.

the recorder range so that a recording range of about 600 nT is achieved. A

crystal clock circuit provides 2-minute and l-hour time 'pips' on each trace.

The system uses a linear portion of the sensor response curve and, in order to

achieve the required sensitivity, the sensor must operate close to the null field

position. But there is no control over Lhe sensor orientations on the sea floor, so

there will be a magnetic field component of unkown magnitude and polarity along

each sensor axis bounded by +f, the total field magnitude. This magnetic field

component must be 'backed-off ' by feeding an appropriate stable current through

the backing-off coil of each sensor. This is achieved by the backing-off control

board which generates a stable backing-off voltage to be supplied to a 'latitude'

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Fig.4. A block diagram of the sea floor magnetometer (one set of sensor coils only is shown).

110 ANTONY WHITE

resistor in series with the sensor backing-off coil. This voltage is varied by means of a 22-turn potentiometer driven by a small electric motor which is powered by a comparator circuit. The backing-off voltage is initially set to give a backing- off field slightly greater than the total field magnitude in one direction along the sensor and is then continuously reduced through zero and out to the total field value in the opposite direction. Somewhere between these limits must lie the null field position and the comparator ceases to power the electric motor when the operating point at the centre of the recording range (-+20 nT) is reached. The power supply to the comparator circuit itself is only switched on by a clock- controlled relay about five hours after launching; it is switched off again ten minutes later by a second relay, during which time all sensors will have been nulled. The backing-off voltage set up by this nulling procedure is not thereafter altered during the experiment.

Subsequent to recovery, the backing-off voltage for each sensor may be measured and the magnitude and direction of the backing-off field along each sensor calculated. Together with the tilt angle information this allows the magnetic variation components to be oriented into any chosen reference axis set. This means that the (gelatine) compass information is superfluous, but it allows a useful cross check to be made. So far, the magnetic orientations deduced from the two methods have agreed more closely than the accuracy of estimation (-+2 ~ for the compass).

Performance of the Sea Floor Magnetometer

The important performance characteristics for this sea floor magnetometer are power consumption, baseline stability, calibration accuracy and signal resolu- tion. While no great pains have been taken to reduce power consumption in this design the total demand is less than 800 mW. A battery pack of 100 to ll0 'D' size alkaline dry cells (Ever Ready E95 or equivalent) is more than adequate to power the system for 30 days and is easily accommodated in the pressure casing. Although temperature variation presents no problem because of the stability of sea floor temperatures, the wide swing of the primary voltage from the battery pack as it discharges does cause a baseline drift in the magnetic variation traces. Up to 30 nT drift per battery pack discharge has been measured in the laboratory for this magnetometer; this is equivalent to 1 nT/day for a 30 day discharge rate.

The calibration of this magnetometer may be carried out either by using a calibration solenoid in which each sensor may be placed sequentially, or by an 'in situ' method in which the backing-off voltage to a sensor is varied and the equivalent field change calculated using the quoted constant for the backing-off coil. The methods have been found to tally to within 1%. However there is a temperature coefficient of the calibration factor. A series of calibrations made over a range of temperatures between 2.5~ and 30~ revealed a 1% increase in

A S EA F L O O R M A G N E T O M E T E R FOR T H E C O N T I N E N T A L S H E L F l l 1

sensitivity for every 3~ rise in temperature for all sensors. A knowledge of the

sea floor temperature at each site is therefore necessary. The recording sensitivity of approximately 200 nT per chart width was chosen

in conjunction with a signal resolution of - 1 nT. A test of the magnetometer was made in the Gulf of St. Vincent near Adelaide (see Figure5) where the water depth was only 13 m. A section of the original chart recordings may be seen in Figure 6. An example of chart stepping is visible in the record from sensor 1228. The timing marks appear as a series of small dots above the continuous trace; the higher dots, identified by numerals, are hour marks, while the lower dots forming a shadow trace are a succession of 2-minute marks. The function of these 2-minute marks is to indicate areas of non-linear transport of the chart paper which occurs when the sprocket drive slips a hole. Examples of this may be seen in the vicinity of the 0200 hour mark on sensor 1228 record and around the 2100 hour mark on sensor 1229 record. While the crystal-controlled clock maintains a timing accuracy of a few seconds a week, it is important when digitising the trace to preserve this accuracy of timing, particularly at times of

high f requency magnetic variations. After digitisation of the traces and orientation of the variations into the H, D,

Z reference axis set the records were replotted by a computer. Simultaneous variations from a land station at nearby Flinders University and from Toolangi Geophysical Observatory 800 km away were also included in the computer plot, which may be seen in Figure 7. It can be seen that there is excellent agreement

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Fig. 5. A map of the South Austral ian region showing the sea floor magne tomete r sites.

112 ANTONY WHITE

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betvceen the sea floor magnetometer data and the land magnetometer data at Flinders University. There is also excellent agreement between respective horizontal components of the sea floor magnetometer and those at Toolangi, with the Toolangi data showing somewhat larger amplitude events; this is expected in view of the latitude difference between the stations (Toolangi is close to Melbourne and is about 3 ~ South of Adelaide). The Toolangi Z

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Fig. 7. A computer plot of digitised data from the test (GULF), a land magnetometer at Hinders University and the Toolangi Geophysical Observatory near Melbourne. Comparison may be made with

the original chart recordings in Figure 6.

A SEA FLOOR MAGNETOMETER FOR THE CONTINENTAL SHELF 113

component shows the expected attenuation which is a result of its greater distance from the deep ocean. As may be seen by comparing Figures 6 and 7 the record from sensor 1228 looks similar to the D component, the record from sensor 1229 looks similar to the H component but with opposite polarity, and the record from sensor 1230 tallies closely with the Z component. In fact the orientation of the sensors on the floor of the Gulf was as follows; sensor 1228 virtually horizontal and pointing close to magnetic east, sensor 1229 virtually horizontal and pointing close

to magnetic south, sensor 1230 virtually vertical.

Continental Shelf Experiments with the Sea Floor Magnetometer

The map in Figure 5 indicates two continental shelf edge sites along the South Australian coastline where the magnetometer has been positioned. Two posi- tionings have been made off Robe and one off Port Lincoln. In all instances the water depths were approximately 150 m and successful recovery of the instru- ment was made after about three weeks in each case. Data acquisition was not so successful but an analysis of some data acquired off Robe in conjunction with a profile of land-based stations has already been reported (White and Polatajko, 1978).

In all of the three experiments the magnetometer has been moored to surface buoys with anchors preventing the motion of the buoys being transmitted to the capsule itself. A schematic diagram of the mooring system is shown in Figure 8.

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Fig. 8. A schemat ic diagram of the configuration of a continental shelf site.

114 ANTONY WHITE

Light cane marker buoys with 0.2-0.3-m-diameter polystryene floats were usually employed using black sighting flags about 3 m above the sea surface. In one instance, an inflatable PVC Polyform buoy was used to hold a galvanised steel mast on top of which was fixed a radar reflector. On one occasion also a radio beacon was fitted inside one of the cane marker buoys but this buoy was lost and

did not aid the subsequent recovery of the instrument. Polypropylene film rope is used throughout with a scope of at least twice the water depth because of the strong swells and tides experienced in the Southern Ocean waters. Additional polystyrene floats at intervals along the ropes are to serve as surface markers should the marker buoy be flattened or severed from the rope. In general, the

surface gear has been kept light but durable following the experience of the lobster fishermen in this area who mark their pots in this way.

Conclusion

The magnetometer has proved a viable instrument for use on the continental shelf. However the electronics is not at present very sophisticated and the power requirement could certainly be vastly improved. Modification of the circuits which is currently underway could cut the requirement down to 300 mW or less. This could result in either a less bulky instrument package or an extended recording capability. Adaptation of the instrument for deep ocean use is the ultimate goal in which case a ballast-release 'pop-up' capability will be neces- sary.

References

Cox, C. S., FiUoux, J. H., and Larsen, J. C.." 1970, 'Electromagnetic Studies of Ocean Currents and Electrical Conductivity below the Ocean Floor', The Sea, Vol. IV, p.637, A. E. Maxwell (Ed.), Interscience, New York.

Filloux, J. H.: 1967, 'Ocean Electric Currents, Geomagnetic Variations and the Deep Electrical Conductivity Structure of the Ocean-Continent Transition of Central California', Ph .D. Thesis, Scripps Institution of Oceanography, University of California, San Diego.

Greenhouse, J. P.: 1972, :Geomagnetic Time Variations on the Sea Floor off Southern California, Ph. D. Thesis, Scripps Institution of Oceanography, University of California, San Diego.

Oweu, T. R. E., and Sik, J. M.: 1972, 'A Three-Component Fluxgate Magnetometer for Sea-Bottom Use', Oceanology International, 72, 37--42.

Poehls, K. A., and Von Herzeu, R. P.: 1976,: Electrical Resistivity Structure beneath the North-West Atlantic Ocean', Geophys. J. R. Astr. Soc., 47, 331-346.

Roark, R. J.: 1938, Formulas/or Stress and Strain, McGraw Hill, New York. Sik, J. M.: 1973, 'Geomagnetic Variations on the Continental Shelf', Ph.D. Thesis Department of Geodesy and Geophysics, University of Cambridge.

White, A.: 1977, 'The Flinders University Sea Floor Magnetometer', unpublished internal report, School of Earth Sciences, The Flinders University of South Australia, Bedford Park, S. A. 5042.

White, A., and Polatajko, O. W.: 1978, 'The Coast Effect in Geomagnetic Variations in South Australia', J. Geomagn. and Geoelectr. 30, 109-120.