f1.cY' /SSN 0/4/ OIi/4. Journu! 01 thr SociUl'f;lr Undmratcr Techno!ogl', Vo/. 24. No. 2. pp. 51-{)() , 1999!2000 ltndc ec no ogy

GI•• caD. Electric Field Signatures 01 Ships in A. 'i Water u .c'e u GI M. VARNEY, S. BAILEY and M. SIDDALL I- Sclzool of' Ocean and Eartlz Seiences, Univers/tl' 0/ Soutlzampton. Soutlzampton Oceanograplzy Centre, European Wal'. Soutlzampton SOI43ZH. UK

Abstract reasons, with an emphasis on acoustic (and recently, non-acoustic) methods far detecting Continuous measurements of electric poten- (and hiding) ships, submarines and mines. tials were taken from a fixed point at the Simulations of underwater electric fields near entrance to , adjacent to ship hulls have been reported by Harrison [9]. . This provided a regular and wide spectrum of electric field sources for study. Southampton Water has a considerable move- 2. Introduction ment of vessel traffic entering and leaving the , and a significant tidal All sea-going vessels emit an electric current into movement each day. Underwater electric the surrounding seawater. Steel hulls may be con- fields were measured adjacent to a shipping sidered as large floating corrosion cells-complex channel of approximately 7 m depth. Analysis in nature because of the presence of many dissim- of signals allowed various signatures and ilar metals. To minimise corrosion, it is universal sources of electrical noise to be identified. practice to install cathodic protection systems on Regular ferry traffic allows the reprod ucibi Iity ships. The 'impressed cathodic current system' of the sensor response to be examined. This applies a current from a sacrificial electrode in permits background sources of noise to be the opposite direction to the natural anodic cur- better identified and therefore increases the rent, thus suppressing the electrochemical reac- ability to discriminate the various signals of tion and thereby preventing corrosion. The interest. resulting electric field (Figure I) propagates con- siderable distances through seawater and is easily detected using a sensitive electric field sensor. 1. Historical Other environmental sourccs of clcctric fields include the movement of the tides, geostrophic Ever since Faraday [I] tried to measure the flow flow of water through the Earth's magnetic of the Thames by measurement of the electrical field, and atmospheric electrical activity potential between electrodes, scientists have (including a variety of meteorological events measured electric fields in water, including such as thunderstorms). Tidal movement of induced and natural fields. Electromagnetic meas- water induces an electromagnetic field (emf) and uremcnt of ocean currents began approximatcly periodic noise at the frequency of the tidal move- 100 years ago; it event'ually became possible to ment. There are many complex velocity fields in observe from moving vessels [2] in the mid twen- estuarine systems; motionally induced fields are tieth century. Filloux [3] researched ocean electric widespread and complex. Those due to wave currents and their interaction with the sea floar action are generally random noise but are of and atmosphere. lesser importance at greater depth, as wave The relationship between the Earth's magnetic action is gene rally restricted to the upper 10 m field and clectric fields is very dose. The magnetic of the water column. 'Schumann resonances', field of the Earth has been measured in consider- genera ted from thunderstorms, cause large elec- able detail, and in theory, by inverting this data, it trical impulses to pro pagate through seawater. should be possible to determine electric current Sunspot activity is also a source of random (ar field) information accurately. Unfortunately, noise ('telluric noise'). However, sunspot activity this has not been possible yet [4]. In the last ten and geostrophic f10w cannot be resolved within years there has been much research on water the short (seconds to hours) time-scale of obser- f10ws [5, 6] and electric fields in sea currents [7], vation described herein. The most conspicuous but there has been little subsequent research into electric fields are those associated with (i) tidally changes in the field caused by marine vessels. induced water motion, (ii) interna I circulation Research by Hirota el al. [8] looked at the effects processes (including turbulent motion), and (iii) of (induced) dipole moments from shipping. impressed cathodic protection systems (the Defence research is being undertaken for obvious highest intensity signals of all).

51 M. Varney et al. Eleclric Field Signalures o{ Ships in S'oulhamplon Waler

y x p

( y

x

Figure 2 A schematic diagram of the vector dipole moment as seen at a d'lstance, r, away from the signal source, using polar co-ordinates. In the derivation of equa- tions that express the X- and Y-direction electric fields, a dipole (ship) is assumed to travel parallel to the Y axis. In reality, the electric field sensor is located in 3D space. and spherical co-ordinates are used.

deeper waters, Although the propagation of the electric field is attenuated more with increasing frequeney, alternating field sensors are more sen- sitive and the environmental hackground noise is lower [10].

3. Theory

Eleetrie fields in seawater, a semi-infinite eon- dueting medium, are expressed simply as Figure 1 An idealised view of an electric field around avessei possessing an electric dipole. Potential lines are at right angles to the current lines. , i Although the current (and hence the potential) continues out into seawater, E=--v·B the electric field strength becomes less than the background noise quite a quickly and determines the sensitivity of any such sensor to respond to Electric fields are measured in units of ~V/m electr'lc fields. (substituted for the less convenient MKS unit of V rn-I), where i = measured eurrent. 1 Environmental factors which affect the a = seawater eonductivity (units R- rn-I), I response of an eleetric field sensor ean be divided v = water current velocity (units ms- ), and B into periodie, transient and other time-related is the Earth's geomagnetic field (units nweber effeets. Frequeney analysis of the signals ean m 2). For the purposes of this study, the electrie therefore differentiate between the various field due to the movement of water through the sourees: measurements from passing ships are Earth's magnetic field (vB) is assumed to be neg- transient but natural sources are cyclie, usually ligible eomparcd to the responses measured from diurnal. loeal eurrent sources. Modern electric field sensors can detect two For an eleetrie dipole moving in three-dimen- kinds of electric fields: the underwater electric sional spaee perpendieular to the Y axis (Figure potential (EP) and the extra low freq ucncy elee- 2), the souree strength of an eleetrie dipole is trie field (ELFE), known as static and alternating usually expresscd as a eurrent moment with fields, respeetively, Typieally, such sensors have units of ampere-metre (Kraishman, 1976). The responses in the 5 mHz to 1kHz frequeney potential measured at any point in spaee a dis- range, and sensitivities in the nVIm/ JHz range, tanee, ,., away from the souree is given by At present, not enough is known about the mag- V __ l_p . cos e _ p y nitude (or significanee) ofthese background fields - 41TrJ" ,.2 - 47ra(x2 + y2)3/2 in the low frequeney range (1-100 mHz), The statie eleetrie field is the most useful for deteetion The eleetrie potential deereases as 1/,.2. The purposes, The alternating field (usually from potential gradient, oV /0,., will therefore dcerease rotating maehinery and generators on board as 1/,.3. The expression for the eleetromagnetie ships) does not pro pagate as far, espeeially in field of an eleetric dipole in an infinite homo-

S2 Underwater Techn%gy. Val. 24 No. 2, 1999/2000

.s geneous conducting medium can be derived by x10 considering the field along eaeh individual X, Y 2 and Z axis, The Z axis is vertical in the water eolumn, and will be ignored for the moment. For the purposes ofthis approach, the orthogonal X axis is treated as eonstant, i,e. o E = av = _ p (x2+l)3/2_~Y(X2+l)I/2(2y)

r ay 41T(T (x2 + y2)3 .1

.2 2 ,2 E = _L x - J ·2 .1' 41T(T (x2 + y2)5/2 150

100 120 When x = 0,

and when y = 0, (a) P I oS E =--- x10 )' 41T(T x3 4 Similarly, along thc X axis, the potential gradient is given by 3

- 8V py 3.2 2 5 Ex = -,- = --4 (-,,)(x + y) -:;:(2x) ox 1T(T ~ 2 3p xv E -- ' x - 4 0 ) S 17 1T(T(x" + y)/~ The motion of a dipole past the observing point is o therefore a trigonometric progression of the 150 above equations in either of the X or Y direetions 120 100 (Figure 3) and forms a method for modelling thc response of the sensor to a passing ship. Ey 50 Low frequency signals are attenuated least (with inereasing distanee), but environmental (b) noise also becomes more significant. Mid- frequency bands, which can be detectcd at Figure 3 Ex and Ey modelied plots 01 the electric lield responses as seen on the higher amplifier gains, are perhaps a more X and Y axes of the clectric lield sensor (the sensor is assumed to be below the exact centre 01 each plot). useful indicator of the signal source.

the ship's steel hull as the anode. This dissolves in 4. Cathodic Current Protection Systems preference to the steel, thereby protecting it. Conventionally, cathodic protection can be The basic 'corrosion cell' consists of an anode, applied in one of two ways: saerificial anodes eathode and electrolyte. The anode will dissolve, (zinc blocks or rods are common), or impressed by virtue of a conventional galvanie eurrent currcnt (using non-dissolving anodes and a cur- leaving the hull surfaee into thc seawater, causing rent sourcc). Because of thc direction of thc cur- metal ions to be oxidised to metal ions. At the rent, this clectrode is called the impressed current cathode, oxygen is reduced, and the solution im- anode and is gcnerally of a material which will not mediately adjaeent to the 'eleetrode' beeomes dissolve at any great rate. The anodes are gener- more alkaline (which on steel structures passi- ally plaeed at a number of strategie points along vates and helps prevent furthcr rcaction or eorro- the hull. A single power unit is controlled by mon- sion). itoring a referenee electrode (midships, for Anodc: M ;;=: M2+ + 2c- instanee) to inerease the impressed current suffi- eiently to depress the hull's natural galvanie reae- tion, but not so high as to eause damage to any To prevent or minimise eorrosion, a number of paintcd eoatings or ehemically damage the steel. techniques are traditionally applied. These Beeause the electrodes are always powered on and include cxclusion of the seawater (paint or active, there is no direct maintenance associated grease), or application of Impressed eurrent with their routine operation. (eathodic) protection. A cathodic system usually The eurrent rating and number of dipoles fitted has a material whieh is more eleetronegative than to a ship's hull must be adequate to prevent free

53 M. Varney et al. Electric Field Signatures vI Ships in Southampton Water

corrosion, and the dipole is given as the product contained within an electrolyte of similar compo- of the current rating of the anodes (in amps) and sition to thai of seawater, exposed to the outside the distance between the anodes (in metres). It is seawater via a semi-permeable membrane. The important to note that current flows in the sea- differential voltages are amplified to measurable water from the sacrificial anode to both anode levels by ultra-Iow-noise pre-amplifiers and and cathode sites. In general, this gives rise to a heavily filtered into a narrow low-pass frequency significant electric field signal since a current band, from I mHz to 2 Hz. These signals have a flowing in seawater will continue to infinity, and dynamic voltage range of ±2 mV m -[ , which are the range at whieh this signal can be detected will converted from analogue to digital and synchro- depend on the resolution of the measuring nised by means of a dock generator. The high system. For alternating electric fields, the decrease precision volta ge differential amplifiel' is an in the current density with distance is dependent underwater unit with an in-built modem that on the frequency of the signal. communicates the 10 Hz readings along a 2 km co axial cable to a top unit on shore where they 5. Operation are stored on disk prior to further processing. This cable also supplies the constant OC power A simple pair of electrodes placed a short distance from the top unit to the sensor. apart in the sea are able to detect electric fields of the order of I mV m -I. This would be sufficient to 7. Sampling Site detect a small vessel in shallow water. However, this simple system would be limited to a range of a The test site seleeted was Spit, located at few met res and, at its crudest, only detect changes the mouth of Southampton Water and is: (i) to static fields. High resolution, high sensitivity located in seawater away from sources of fresh measurements of eleetric fields in the sea ean or brackish waters, (ii) free from direct sources only be made through the elimination of extra- of pollution, (iii) a stable flat sea floor free from neous noise resulting from eleetrode self-noise, fishing and trawling activities, (iv) away from electronic noise, and background noise. The other extraneous sources of electric fields (pipes, (normal) self-noise of silverlsilver chloride elec- cables, structures, etc.), (v) in water depths of trodes is normally in the nV region-which is in 10m, and (vi) approximately 300 m from the practice much smaller than that due to vessels. shore station. The estuary is the entrance to one Sinee there is a simple chloride ion equilibrium of the busiest ports in the south of , with existing between the electrode and seawater, many large fleet cruise liners, cargo, cmde car- they are ideally suited for use in electric field sen- riers, gas, oil and fuel tankers. There are several sors as they present a low impedance to the meas- regular ferry routes to the , con- urement of electric potential. The performance of sisting of both car ferries and fast catamarans. the sensor improves by use of massive surface There are also many yachting mariners in the areas of silver chloride. Electronic noise can be area, giving rise to high numbers of sailing and reduced to a minimum through careful design pleasure vessels under power and sail. (especially of the pre-amplifier circuitry), avoid- The three-axis electric field sensor was ance of aerial or transmitted noise, and the suit- deployed on the sea floor within 20 m of the able selection of good quality electronic main Southampton Water shipping channel, in components. For a modern electric field instru- approximately 7 m of water (at mean HW) dose ment, the separation between the electrodes to the mouth of the Solent (Figure 4). The sensor need not be more than I m. housing on its concrete base was deployed by boat directly on to the seabed in August, without 6. The Electric Field Sensor special site preparation. The seabed was sand! gravel and moderately stable. This work was Electric field sensors measure potentials in water completed within the summer months, and gen- using pairs of ultra-Iow noise Ag!AgCI electrodes erally encompassed a wide range of meteorolo- along the X, Y and Z axes. The electrodes are gical conditions for several months thereafter. mounted flush with the surface of alm diameter Approximately 1500 m of coaxial cablc con- glass fibre hemisphere: the X and Y electrode nected the underwater sensor housing to a topside pairs on the horizontal plane I m apart, and one modem link and computer station. On inspection Z electrode at 0.5m in the vertical axis. The by divers, the orientation of the X-axis was found second Z electrode is a 'virtual' summation of to be at 225°N, perpendicular to the channel. The the X and Y potentials. The Ag/ AgCI electrodes sensor was in an arca of extreme hydrodynamic introduce variable (electrochemical) signals hard activity there are high tidal currents at mid-ebb, to maintain below approximately 100 n V. These and sandbanks are evident throughout the region mask extremely low frequency signals unless very at low tide, indicating extensive and continuous sophisticated techniques (such as multiple elec- sediment transport processes in operation. trodes, internal calibration, or multiplexing The range and bearing of shipping were esti- inputs) are used. The electrodes themselves are mated by comparison to the Admiralty charts of

54 Underwater TechnoloKY, Val. 24 No. 2, ]999/2000

, ,, , ,, , , , ,, ,, , ,, \ ,, ., ,, , , ' \ ::::.•. -", . :""0 '.•.•0 ~ ., ~ , " ,\ ,,- " ,\ ,,-

x+~

y+

o 3km ~

Figure 4 A sehematie map of Southampton Water showing the loeation and orientation of the eleetric field sensor relative to that of the main shipping ehannel, and magnetic north. Also shown is the approxlmate position of the sandbanks (dotted lines) around Calshot Spit and their relationship to the main shipping ehannel. The slower, flood tide generally follows the coastal profile, but the faster ebb water flows straight out of the channel. On the ebb tide there is a eritieal point where adjaeent sandbanks become exposed and water ean no Ionger flow direetly over them This ehanges the water direetion, and presumably causes eddies, increasing the turbulenee in the water. the area. The channel at Calshot Spit is approxi- The Z axis is an order ofmagnitude smaller signal mately 300 m wide. and für the purposcs of this because of the oblique angles involved. Despite work was arbitrarily classificd as near-, mid- or being a smaller signal, it is visually as clcar as far-ehanneL The majority of the data analysed the X and Y axes, but the Z-axis response is not involvcd signatures of vesse\s from the near reliablc in idcntifying vesse\s at distanee. channel (outbound traffic). The shape of thc signal on the X and Y axes follows a pattern whieh is found by considering the motion of the dipole parallel to the X and Y 8. Results axes of observation. Any differenee between the The field electrodes respond to a potential gra- orientation of the sensor X axis relative to the dient and most significantly to the passage of heading of the ship, results in a more eomplieated shipping. Larger vessels generally emit stronger pattern as the overall eleetrie field response will be electric fields. duc to thc larger surface area in in geometrie proportion to the individual x, y and 2 the water and the more powerful cathodic protec- z signatures, i.e. E = E~+ E~+ E;. The strength tion systems used. The 60000 tonne cruise ship of the signatures clearly diminishes rapidly with Oriana (260 m length. 33 m width and 7.9 m distance. This (potentially) forms the basis of draft) shows a typical response (Figure 5). dctcrmining a ship's eourse and distance. E, and Signals are seen in both the X and Y axes, directly Ey ean be ealculated (from the equations above) in proportion to the strength of thc dipole current when the electric dipole (x, )" z coordinate) moment and the distance away from the source. moment sails on a course parallel to the X axis,

55 M. Varncy ct al. Electric Field Signatures o{ Ships in Southampton Water

5 x10 3 2 1 o -1 246.809 246.81 246.811 246.812 246.813 246.814

5 x10 2 o

-2

246.809 246.81 246.811 246.812 246.813 246.814

4 x10 2

o -1 246.809 246.81 246.811 246.812 246.813 246.814

Figure 5 The electric lield sensor responds most significantly to the passage 01 shipping. The largest magnitude signals are seen on the X and Y axes. The response is due to the cruise ship, Oriana, out-bound lram Southampton Water. All voltages (Y axis) are measured in nV. The time axis is plotted as Julian day (where 0.005 day is approximately 7.2 minutes). Each 10-minute segment contains 6000 data points

and is an orthogonal distance (x + y) away part of the channel. The electric field response (Figure 6). from in-bound ferries using the far channel A car carrier, Titus (280 m), shows a larger varied in clarity, dependent on the background signal (Figure 7) despite its near-identical size to noise. The best signals were seen in the early Oriana, indicating that the electric field strength is ho urs of the morning, becoming increasingly a function of induced cathodic protection swamped by noise during the day. Many signals currents. It is also possible that the ship's physical can be obscured (or confused) by larger carriers, size physically constrains the field within a sm aller cargo ships and other vessels further up (or down) sectional area adjacent to the estuary floor. The the channel. The largest responses from the ferries 5 magnitude of signals from large vessels is gener- were up to 1 X 10 nanovolts between the elec- ally of order I x 105 nV; the strength of the trodes on the X and Y axes. Thc Z axis had an (En E\., EJ signals is so high that they dominate order of magnitude sm aller response, typically the response. A 200 m container vessel had a 1 x 104 nanovolts. response approximately half that from the larger Through extendcd periods of observation, it ships (Figure 8). Although the transit speed of all was possible to observe many other artefacts. ships through this section of the channel was When avessei comes into harbour, for much the same (approx. 10 knots), the X and Y instance, it may have its propulsion system off signatures (especially the Z axis) are smaller in and be under tow. The IPCC system may magnitude and time, commensurate with a be struggling to maintain stable protection cur- smaller electric field surrounding the vessel. The rents. Other ships were seen to show variations Red Osprey (82.4 m long, 17.5 m wide and in the dipole signature though the use of bow draught 2.75 m) providcs a signal directly in thrusters which appears as areversal in the direc- proportion to its smaller size (Figure 9). Red tion of flow of the cathodic protection current. Osprey is one of three car ferries travelling When comparing a ship under normal and between Southampton and the Isle of Wight. reversed dipole conditions, the signals are very The ferries generally follow the same pattern on similar. each pass: leaving Southampton Water on the The Red Funnel Islc of Wight Red Jet pas- west side of the channel (nearer to the sensor), senger ferries travel at higher speeds (often in and re-entering the channel on the far side. On excess of 40 knots). The electric field response is some occasions the vessels navigate in the mid- correspondingly less (Figure 10). When these

56 Underwuter Technology, Val. 24 No. 2, 1999/2000

-6 Ey x10 4

3

2

o

-1 -100 -80 -60 -40 -20 o 20 40 60 80 100

-7 Ex x10 5

o

-5 -100 -80 -60 -40 -20 o 20 40 60 80 100

Figure 6 The electric field sensor provides a description of local electric field behaviour. The signatures of the X and Y axes can be easily modelied using the Ex and Ey expressions lor the electric field due to a dipole. A ship's heading (and speed, given certain constraints) can be easily obtained Irom a simple theoretical analysis along the X and Y axes and comparison to the measured signatures. However, as the ships are confined to the shipping channel, headings were easily categorised (into north-west or south-east) by the overall shape 01 signals along either X or Y directions.

5 x10 6 4 2 o -2 246.813 246.814 246.815 246.816 246.817 246.818

5 x10 2 o -2 -4 -6 246.813 246.814 246.815 246.816 246.817 246.818

4 x10 4

2 o

-2 246.813 246.814 246.815 246.816 246.817 246.818

Figure 7 The car carrier, Titus, has a response which is approximatley twice that of the Oriana, despite their near-identical size. Although signals are seen on both the X and Y axes, the Z signal is approximately an order of magnitude lower because 01 the oblique angles involved.

57 M. Varney et al. Elec/ric Field Signa/ures ol Ships in SOUlhamp/on Water

4 x10

5 0 -5 -10 245.863 245.864 245.865 245.866 245.867 245.868 245.869

4 x10 5 0 -5 -10 -15 245.863 245.864 245.865 245.866 245.867 245.868 245.869

245.863 245.864 245.865 245.866 245.867 245.868 245.869 Figure 8 The response decreases in size as smaller ships are examined (this is a 180 m cargo ship, Geest) and the size 01 the background signal becomes more apparent, especially on the Z axis.

4 x10 10 5

-5 246.604 246.605 246.606 246.607 246.608 246.609 246.61

4 x10 5 o -5 -10 246.604 246.605 246.606 246.607 246.608 246.609 246.61

-5000 246.604 246.605 246.606 246.607 246.608 246.609 246.61

Figure 9 A car passenger lerry, Red Osprey, provides an opportunity to collect regular signals that can be examined to assess background nOlse statistically.

30 m catamaran vessels travel at high speeds, they pressed in time. However, their speed through 5 rise up on hydroplanes, reducing the overall SUf- the water still induccs a signal of 2 x 10 nV. face area in the water. Because of their speed past However, the ambient noise levels need to be the sensor, the response is considerably com- low in order to distinguish c1early these types of

58 Underwater Techn%gy, VoL 24 No. 2, 1999/2000

4 x10 2

245.569 245.57 245.571 245.572 245.573 245.574 245.575

4 x10 2

245.569 245.57 245.571 245.572 245.573 245.574 245.575

5 x10

1 o

-1 245.569 245.57 245.571 245.572 245.573 245.574 245.575 Figure 10 The Red Jet series of catamaran passenger ferries travel so fast past the sensor that the signal is much smaller in magnitude and shorter. Of note is that very short transient peaks (Z axis, in this case) are olten seen in the data-which cannot be ascribed to any surface shipping activity, bow waves, or to environmental effects such as tidal motion. signals. In principle, although they possess tidal current velocity. The noise increases directly cathodic protection systems, the opportunity for in proportion to tidal height, reaching a the electrodes to establish a protection current 'crescendo' associated with the maximum ebb (and therefore a steady, stable field) is consider- currents. ably reduced. Commonly, the cathodes are The measurements obtained by an electric field located in or around the rear thrusters. sensor can also be influenced by secondary fac- Plastic or GRP hulls (yachts and motor boats, tors, including water temperature, water salinity, for instance) have !ittle impact on the signals, marine fou!ing and sediment accumulation. sometimes even when sailing direct1y overhead Although apparent, they remain significant effects of the sensor. Most vessels possess some form of which have yet to be thoroughly investigated. cathodic protection-sometimes only a few zinc- Self-induced electrode noise is not a consideration plated bolts somewhere in the rudder or keel. The under the conditions reported here. current magnitudes are so small that the electric A better understanding of the origins and field genera ted is not sufficiently higher than the causes of background noises is nceded to improve background noise to be detected. the accuracy and precision of underwater electric field measurements. Although some origins can be distinguished, there are many others that 9. Summary require more fundamental investigation and understanding. There are a wide variety of electric field signatures due to shipping. There are complex contributions to these signals from hydrological activity, gener- Acknowledgements ally referred to as background noise. Electric field instrumentation is extremely sen- Subspection Limited for the donation of the sitive to local electric field phenomena. There is a equipment, Captain Stephen Young wide spectrum of noise due to various hydrolo- (Southampton Harbour Master), William Heaps gical and meteorological events. Distant signa- (Associated British Ports), Vessc1 Traffic Services tures (dipole magnitude, direction, ships' Southampton, Mike O'Brien (HM Coastguard), tonnage, etc.) in the presence of background the Nuffield Foundation, Qraham Etheridge noise are difficult to identify, especially for targets (skipper of RV Conway), DERA, the at longer distances. Tidal movement is a clear Environment Agency (Romsey sec tion) and source of noise, which generally increases with Valshot Activities Centre.

59 M. Varney et al. Eleclric Field Signalures o( Ships in Soulhamplon Waler

Abbreviations Used in the Text 3. Filloux, l., 1973, Techniques and instrumentation for the study of natural electromagnetic induction magnetic field (weber m 2) at sea. Journal o{ the Earlh ami Planetary Interiors, X component of the dipole moment 7, pp. 323-328. Y component of the dipole moment 4. Stevenson, D., and Bryan, K., 1992, Large-scale Z component of the dipole moment electric and magnetic fields generated by the oceans. Journal or Geophysical research, 97 (C 10), AC dipole moment frequency (rad S-I) = ftff1(J 467-480. 5. Luther, D.S., Filloux, l.H., el al. 1991, Low-fre- measured current flowing through seawater quency, motionally induced electromagnetic fields r distance between the dipole moment and the in the ocean. 2. Electric field and Eulerian current 2 electric field sensor [r - J(x + l + z2)] comparison. Journal or Geophysical Research, 2 I = I nanoweber m- 96(C7), 12797-12814. C dielectric constant of seawater 6. Larsen, J .c., 1992, Transport and heat flux 01' the 7 fL permeability of seawater (4 x 10- henry Florida Current at 2TN derived from cross-stream rn-I) voltages and profiling data: theory and observa- water velocity (m S-I) tions. Phi!. Trans. R. Soe. Lond., A338, 169-236. electrical conductivity of the seawater 7. Lopantnikov, V.I., 1995, Electric field in sea CUf- vertical angle between the electric field rents, induced by the horizontal component 01' sensor and the current-generating target geomagne1ic ficld. Physical Oceanography, 6 (3). 219-228. References 8. Hirota, M., Tcranishi. Y., el al., 1996, Study 01' underwater electric fields 01' a dipole moment in 1. Faraday, M., 1832. Bakerian Lecture IV: shallow water. .Japanese .Journal or Applied Experimental research in electricity. Philos. Physics, 35 (5A), pp. 2870-2878. Trans. R. 50c., 175. 9. Harrison, S.H.l., 1994, Kent, Nexus Hause. 2. Von Arx, W.S., 1950, An eleclromagnetic melhod 10. Talbot, P.B., 1995. Modern Mine Sensors. NexLls for measuring the velocities of ocean currents from HOLlse, Kent. a ship under way. Papers or Physical Oceanography amI Meleorology, 11 (3), 1-62.

MAN-MADE OBJECTS ON THE SEAFLOOR 2000 2 Day Conference 2-3 Mey, 80 Colemen Street, London EC2R 5BJ

The seafloar is no longer a remote and pristine environment. From the shallow continental shelves to the deep ocean floar it is visited regularly by sampling devices, autonomous instruments, and human beings; cables are laid, structures placed, waste dumped, shipwrecks and lost equipment inevitably finish up - on the seafloor. Man's activities on the seafloor have become so extensive in the past half century that marine operators are increasingly concerned about potential interference with other users of the sea. The enabling technologies which have made these activities possible will be described. Man's intrusion into the marine environment can be both harmful and beneficial. Understanding of the marine environment is the essential prerequisite to its responsible use. Thismeeting has been organised on behalf of the Society for Underwater Technology by Geotek Limited.

Far further information contact: Jean Pritchard, SUT Aberdeen Office, Innovation Centre, Offshore Industrial Park, Bridge of Don, Aberdeen AB23 SGX,UK Tel: +44 (0)1224823637, fax: +44 (0)1224820236,e-mail: [email protected] or visit our website www.sut.org.uk

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