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 Southampton 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 Southampton Water, adjacent to ship hulls have been reported by Harrison [9]. the Solent. 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 Port of Southampton, 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).