Abstract:
A system for mapping and characterizing a hydrocarbon plume in seawater by measuring seawater capacitance. Multiple streamer cables are towed in the sea behind a ship, each at a different depth, simultaneously. Each streamer cable includes transmitters and receivers at the free end thereof. The free ends of the streamer cables pass through the plume and the transmitters transmit an electrical current into the plume. The receivers detect any secondary signals produced by capacitive effects of the hydrocarbon or hydrocarbon and dispersant surrounded by conductive seawater in response to the inducing electrical current. Pre-amplifiers connected to the receivers and a two-step calibration procedure and various grounding and shielding steps provide noise rejection. An electronics system on board the ship processes the secondary signals to provide immediate development of detailed maps of plume location, and to provide tracking and characterization of how the plume changes shape and character over time.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the U.S. Government for governmental purposes without payment of any royalties thereon. 
    
    
     BACKGROUND 
     The present invention relates to rapidly mapping and characterizing hydrocarbon and dispersant plumes in seawater. 
     In April 2010, the Deepwater Horizon oil drilling rig exploded, causing an oil spill at the Macondo Well in the Gulf of Mexico that was one of the largest accidental marine oil spills in the history of the petroleum industry. The well released over 200 million gallons of crude oil into the Gulf of Mexico, resulting in damage to marine and wildlife habitats and to fishing and tourism industries, as well as health concerns to inhabitants of the Gulf coastline. Although the well was eventually capped, the impact of this massive spill continues today, and will continue for years to come. 
     Masses of undersea oil, which have lengths spanning tens of miles, have been reported in the Gulf of Mexico. These oil or hydrocarbon plumes, plus the vast quantities of toxic chemicals (such as COREXIT oil spill dispersants by Nalco Company) intended to disperse them, move and spread in the Gulf seawater column, posing long-term threats to marine and coastal wildlife. The volatile components of the plumes floated to the surface early, where they could be burned off, but the vast majority of the oil either sank (smothering life on the seafloor) or drifted away with the Gulf Loop Current. 
     Such a huge and amorphous target is extremely expensive and difficult to accurately characterize. Determining the character of a hydrocarbon plume currently requires stopping a $100,000-per-day ocean-going vessel in the water for several hours, and dropping sampling bottles over the side to depths in excess of a mile. Only an extremely tiny sample of the water column is collected, which must then be analyzed later in a laboratory, adding days of additional time. The entire process operates under the implicit assumption that the oil plume does not move during the hours needed to do the sampling, which is not a valid assumption. 
     Other existing methods that can be used to map hydrocarbon plumes in seawater include sonic methods (e.g., side-scan sonar) and resistivity. However, sonic methods require a significant velocity contrast to function, and a dispersed hydrocarbon plume will have a sound velocity indistinguishable from unpolluted seawater, rendering this approach ineffective away from the erupting seafloor well-head. Resistivity methods will not work because the sampling current will short-circuit past the oil droplets following the path of least electrical resistance through the highly-conductive seawater, rendering this approach equally ineffective. 
     Besides the Macondo Well blowout, there have been a number of other major well-blowouts and oil leaks in the open ocean, including several larger than the Macondo Well event. Notable among these are the Ixtoc Well (Bay of Campeche, 1979) and the Persian Gulf (1991), which each released larger volumes of oil. At this time, there is no way to really know what remains from these huge pollution events, since divers can rarely descend below 100 meters, and people controlling remotely-operated underwater vehicles can see little more than divers in low-visibility, dark waters. 
     Also, there are over 6,600 active or removed oil platforms in the Gulf of Mexico alone, and each connects to a huge network of pipelines lying on or just below the seafloor. These pipelines convey oil from all the current and former offshore oil platforms and wells to collection points and refineries on land. Many of these pipelines are old, corroded, or damaged by hurricanes, and are known to be leaking. In addition, the Gulf of Mexico has many natural oil seeps. 
     To protect coastline and marine environments, new technologies are needed to detect, map, and characterize undersea hydrocarbon plumes, and to predict their movements. 
     SUMMARY 
     Induced polarization (IP) is a general term for a surface-sensitive physical phenomenon caused by several different electrochemical mechanisms, but all behave like a capacitance. On land, an induced voltage injected into the ground can cause ions in groundwater to adsorb onto sulfide mineral grains such as pyrite and chalcopyrite. When the inducing voltage is released, the accumulated charge bleeds back off, and this can be measured. This phenomenon is called “polarization” or “chargeability.” A large volume of rock filled with disseminated copper sulfides and pyrite (or clay) will behave as a large capacitive system to an IP transmitter-receiver array energizing the ground surface above it. 
     A simple capacitor can be characterized as two conductive plates separated by a resistive, dielectric material such as oil. It is used in electronic systems to delay a signal or to store charge in a power supply. A dispersed plume of oil droplets and blobs immersed in a highly conductive medium such as seawater is topologically equivalent to a simple capacitor. Therefore, a dispersed hydrocarbon plume presents a large surface area of oil-to-seawater, and is a strong polarizer, an anomaly in a sea that otherwise has no chargeability. The greater the dispersal of the hydrocarbons, the greater the polarizing surface area exposed to seawater for the same amount of oil, and the greater the subsequent chargeability. The size of a capacitor and strength of the dielectric between its plates also controls the rapidity of charge bleed-off, thus from basic physics, an oscillating induced voltage signal in polluted seawater causes a varying response with frequency depending on the size of the oil droplets (i.e., smaller droplets yield a higher frequency for peak response, and the more surface area exposed to seawater, the greater the volume capacitance). 
     In accordance with the invention, there is provided a seawater capacitance detection system and method that allows users to rapidly map hydrocarbon plumes in seawater in four dimensions: three spatial dimensions as well as how the plumes evolve and move over time. The measurement of plume movement over time allows future plume evolution to be reliably predicted. A towed electrical transmitter-sensor streamer array having three or more streamer cables is pulled through the water column at three or more depths to detect hydrocarbons in the seawater column by measuring seawater capacitance. This permits immediate development of detailed maps of where a pollutant plume is located, as well as the tracking and characterization of the plume over time. In a synoptic view, the streamer array can be towed in lawn-mower fashion as fast as the host ship can travel, rapidly sampling and characterizing the hydrocarbon plume volume at multiple depths at the same time. 
     In accordance with one embodiment of the invention, there is provided a seawater capacitance detection method for rapidly mapping and characterizing hydrocarbon plumes in seawater. The method includes towing a streamer array having three or more vertically stacked streamer cables at three or more depths, respectively, through a seawater column. Two or more transmitter electrodes and a plurality of receiver electrodes are placed on each streamer cable. Transmitter electrodes transmit an electrical current into the seawater. Transmission of the electrical current signal is episodically terminated, and the receiver electrodes measure the returned secondary signals produced in response to terminating transmission of the electrical current signal. The returned secondary signals indicate the presence of a hydrocarbon plume in the seawater if their capacitance is not zero. Based on the returned secondary signals, a capacitance value is calculated for discrete volume points within the hydrocarbon plume and seawater mixture sampled by the receiver electrodes. The hydrocarbon plume is then mapped and characterized using the capacitance values. 
     In accordance with another embodiment of the invention, there is provided a seawater capacitance detection method for rapidly mapping and characterizing hydrocarbon plumes in seawater. The method includes towing a streamer array having three or more vertically stacked streamer cables at three or more depths, respectively, through a seawater column containing a hydrocarbon plume. Two or more transmitter electrodes on each streamer cable are energized with an electrical current signal, the transmitter electrodes injecting electrical current into the seawater column. A plurality of receiver electrodes are arranged on each streamer cable into one or more receiver sets. Transmission of the electrical current signal is episodically turned off. Each receiver set samples a volume of seawater surrounding it and detects any returned secondary signals produced by a capacitive effect resulting from injecting the electrical current into the seawater column and turning off the transmission of the electrical current signal. Each receiver set successively samples a larger volume of water as the receiver sets are positioned further away from the transmitter electrodes. The returned secondary signals are processed, for each adjacent pair of receiver sets, by subtracting the smallest volume of seawater from the next smallest volume of seawater to yield a capacitance value of a donut-shaped sampling volume of seawater outside the smallest volume. The receiver sets positioned further away from the transmitter electrodes provide increasingly larger donut-shaped sampling volumes of sampling data of the capacitance of the seawater column through which the streamer cables are towed. The capacitance values of the donut-shaped sampling volumes of seawater are processed and geometrically corrected to yield a frequency-varying final seawater capacitance value for the volume of seawater sampled by each receiver set. The hydrocarbon plume is then mapped and characterized using the final seawater capacitance values of the receiver sets. 
     In accordance with another embodiment of the invention, there is provided a seawater capacitance detection method for rapidly mapping and characterizing hydrocarbon plumes in seawater. The method includes towing a master streamer cable through a seawater column, and attaching a sled to an end of the master streamer cable to maintain it substantially vertical in the seawater column. A plurality of parasitic streamer cables are attached to, and extend from, the master streamer cable. Each parasitic streamer cable samples different depths simultaneously. Two or more transmitter electrodes and a plurality of receiver electrodes are placed on each parasitic streamer cable. The transmitter electrodes transmit an electrical current into the seawater. Transmission of the electrical current signal is episodically terminated, and the receiver electrodes measure any returned secondary signals produced in response to terminating the transmission of the electrical current signal. A non-zero capacitance value of the returned secondary signals indicates the presence of a hydrocarbon plume in the seawater. Based on the returned secondary signals, a capacitance value is calculated for discrete volume points within the hydrocarbon plume and seawater mixture sampled by the receiver electrodes. The hydrocarbon plume is then mapped and characterized using the capacitance values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. The drawings are not necessarily drawn to scale. In the drawings: 
         FIG. 1  is a conceptual schematic diagram for explaining the topological equivalence of a classical capacitor and dispersed hydrocarbons in highly conductive seawater according to an exemplary embodiment of the invention; 
         FIG. 2  is a schematic perspective view of a ship towing an array of multiple, vertically-stacked streamer cables through an undersea hydrocarbon plume in accordance with an exemplary embodiment of the invention; 
         FIG. 3  is a schematic side view of the active portion of one of the streamer cables of  FIG. 2 , according to an exemplary embodiment of the invention; and 
         FIG. 4  is a conceptual block diagram of a control and signal processing system on board the ship of  FIG. 2 , according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The induced polarization (IP) method has been used for decades to detect certain minerals and clays that give rise to an IP effect in the ground or beneath the seafloor. A system and a method that use the IP method to detect minerals and clays in the seafloor are disclosed in U.S. Pat. No. 6,236,211 and in U.S. Pat. No. 6,236,212, both issued May 22, 2001, and both hereby incorporated by reference in their entirety. The typical IP method involves injecting an electrical current (through an induced voltage) into the subsurface. Electrochemical reactions occur between the minerals and surrounding water, causing the minerals to act as capacitors. While the current is on, electrical energy is stored. After the current is turned off, the stored energy is discharged, which causes current to flow from the minerals back to the water in which they are immersed. This method is used to measure the slow decay of voltage in the ground or subsurface—if a polarizer is present—after the current is turned off. Induced polarization is a phenomenon that can have several electrochemical causes, but all of them are capacitive in character and behavior, and all are sensitive to surfaces exposed to water rather than to volume. The invention described herein proposes a new physical phenomenon causing an IP-like effect, and proposes how to measure it. 
     One manifestation of the IP response of a subsurface polarizer is that the voltage on an array of detectors or receivers lags the primary or inducing voltage (produced by a transmitter dipole pair, described below) by a finite amount of time. This is always expressed as a phase-shift (i.e., a slight time-shift or lag of the wave-cycle between the transmitter and the receiver). The delay phenomenon is based on a complex interaction of ions in the electrolyte (the ground water) with the individual mineral surfaces. Because of this, IP is more sensitive to surface area than to volume, and is thus ideally suited for dispersed or disseminated targets. 
     An IP survey typically gathers both resistivity information, which is generally a measure of the porosity of the substrate, as well as polarization information, which is a measure of the reactivity or “chargeability” of certain minerals disseminated throughout the subsurface. Computer modeling is then used to arrive at models that best fit the observed data acquired on the land surface or at the seafloor, with the purpose of providing a true map of the three-dimensional nature of the underlying subsurface. 
     Evidence in the scientific literature (and found in public news media) after the Macondo Well erupted in the Gulf of Mexico in April 2010 indicated that the oil sampled in the seawater was not pure oil. It was always a mixture of hydrocarbons and saltwater, except where it accumulated on the coast and became subsequently dehydrated. Instead, the hydrocarbon pollution has been described as “dispersed” in the seawater. In other words, the oil was found in blobs and tiny droplets, each surrounded by conductive seawater, with the aggregate forming a vast cloud. In the invention disclosed herein, the strength and frequency response to such a mixture correlates closely with the surface area exposed to the seawater and the size-distribution of the droplets. 
     As shown in  FIG. 1 , a simple capacitor  102  can be characterized as two conductive metal plates  104  separated by a resistive dielectric medium  106 , commonly a chemically-doped oil. Oil dispersed as droplets and blobs in highly conductive seawater  108  is topologically equivalent to the simple capacitor  102  where the highly conductive seawater substitutes for the metal plates, and seafloor hydrocarbon seeps and hydrocarbon plumes in the water column, such as the Macondo Well output, are the oil/dielectric. In both cases, an inducing voltage can cause charge to be stored. When the transmitted inducing voltage is turned off, the charge will bleed back in a manner that can be measured at receiver electrodes. Therefore, a hydrocarbon plume in seawater is polarizable and behaves as a frequency-dependent capacitance to a chargeability-measuring array passing through the plume. 
     In essence, the dispersed oil plume is a giant, amorphous capacitor, with capacitance changing over spatial and time dimensions according to the movement, overall density, and droplet size distribution of the hydrocarbons present. There will be discrete different capacitance values according to droplet size—oil dispersed as smaller droplets will have greater volume capacitance. However, the capacitance values will be distributed because there will normally be a range of different droplet sizes in any hydrocarbon plume in the open ocean. The capacitance frequency-response will also vary according to droplet size—smaller droplets will have a higher frequency response (i.e., a faster bleed-back rate). A resistivity survey cannot detect hydrocarbon plumes and seeps in the seawater column because of their distributed nature. In other words, the measuring electrical current will short-circuit around the oil, following the path of least electrical resistance in the conductive seawater. Sonic methods will likely not detect a dispersed hydrocarbon plume for a similar reason—there is no significant velocity contrast. 
     Seawater has previously been considered a homogenous conductive medium. It almost always is homogenous because it is constantly mixing. Salinity and temperature may vary modestly in enclosed seas depending on depth and surface evaporation, but the sea has always been treated as no more than an electrically-conductive medium with biological content. Scientists would not normally consider measuring capacitance in something that was uniformly conductive—this would be like trying to classify colors in the dark. There is no capacitance in a metal wire or single metal plate. The system and method of the present invention, as described below, provides a way to measure seawater capacitance, such that hydrocarbon plumes in seawater can be mapped and characterized. The system and method described herein does this by measuring a multi-frequency phase-shift between transmitted and received signals (i.e., measuring a capacitance-caused time-lag between transmitter and receiver signals over a wide frequency range) to map and characterize hydrocarbons in seawater. In an electronic circuit, a capacitor causes a time-delay in the signal. The present invention uses this broadband signal time-lag to measure variations in, and characterize the frequency-dependent capacitance of, the seawater column, which is now described in greater detail. 
     Referring to  FIG. 2 , a ship  202  is used to tow a multi-component streamer array through seawater containing hydrocarbon plumes and plume stringers  204 . The streamer array includes a series of multi-wire-stranded streamer cables  206 . Preferably, between three and ten parallel streamer cables  206  are towed behind the ship  202  in a vertically stacked array. Three such streamer cables  206  are illustrated in the embodiment shown in  FIG. 2 . Each streamer cable  206  has multiple components, including depth-control cable depressors  208 , transmitter electrodes  210 , receiver electrodes  212 , pre-amplifiers  214 , a depth-sensing pressure transducer with an acoustic transponder (herein a “pressure transducer/acoustic transponder unit”)  216  located at the peripheral and distal ends of each streamer cable  206 , and a drogue  218 . For ease of illustration, the multiple components are shown only on the first or top streamer cable  206 . The streamer cables  206  are similar to one another, except the deeper-riding cables may be longer than the shallower-riding cables. 
     Each streamer cable  206  samples different depths and volumes of seawater at the same time. The depth-control cable depressors  208  maintain the depth of each streamer cable  206 . Each depth-control cable depressor  208  can be a small sled with adjustable fins or farings, for example, and may either be remotely adjustable or preset for a particular depth. The ship  202  includes a commercially-available three-dimensional (3-D) acoustic streamer element locator  220  deployed from the side of the ship  202  (described below), and an on-board control and signal processing system (described below and illustrated in  FIG. 4 ). Each streamer cable  206  carries a transmitted signal from the control and signal processing system down the cable and returns an amplified received signal back to the control and signal processing system aboard the ship  202 . Transmitted electrical energy does not travel very far in the highly conductive seawater of the ocean. For this reason, the streamer cables  206  are towed behind the ship  202  in a vertically-stacked array so more of the vertical water column may be sampled at the same time. 
     The pressure transducers/acoustic transponder units  216  are used to provide accurate real-time depth information for each streamer cable  206 . The drogue  218 , which may be something as simple as a knotted rope, is attached to the distal end of each towed streamer cable  206  for stabilization while underway. This minimizes cable whipping and undulation that would contribute an artificial electrical noise to the received signal (due to changing distances between transmitter and receiver dipoles, described below). The separation between streamer cables  206  is adjustable according to target depth-range to be mapped. In practical terms, the number of streamer cables  206  and target depth-range would be similar for most deep oceans such as the Gulf of Mexico, but would be modified (e.g., by using fewer streamer cables  206 ) for shallow coastal shelves. 
       FIG. 3  illustrates one of the streamer cables  206 . In the embodiment shown in  FIG. 3 , each streamer cable  206  is shielded and independently grounded  302  against outside electrical noise. A transmitter cable  304  supplies power to the transmitter electrodes  210  (i.e., to a transmitter dipole). Preferably, the transmitter cable  304  is a ground-shielded, twisted-pair of low gauge (e.g., eighteen or lower gauge) wire designed to provide a square-wave current signal to the transmitter electrodes  210 . A signal return cable  306  returns the received signal to the ship-board control and signal processing system. This data signal can be returned via either shielded coaxial wire cabling or optical fiber, depending on design considerations. A power supply cable  308  supplies power to the pre-amplifiers  214  and to the pressure transducer/acoustic transponder units  216 . The power supply cable  308  is typically a high-gauge twisted pair of wires. 
     In the exemplary embodiment shown in  FIG. 3 , the streamer cable  206  carries a pair of the transmitter electrodes  210  ( 210   a  and  210   b ). The transmitter electrodes  210  are spaced typically 20-100 meters apart, depending on the depth-range of interest and practical considerations such as how much current a given streamer cable  206  can support without overheating due to internal resistance. The transmitter electrodes  210  are preferably made of coiled titanium wire to prevent corrosion when used with 10 or more amperes of electrical current. Seawater is highly corrosive, especially in the presence of several amperes of current. The transmitter electrodes  210  are used to transmit current at relatively high amperages and at low voltages to increase the signal-to-noise ratio. The transmitter electrodes  210  take advantage of a non-linear surface-effect transition at about 15 volts, above which current will easily cascade from the transmitter electrodes  210  into the seawater. The maximum current is then effectively limited only by the capacity of a current transmitter  404  ( FIG. 4 ) on-board the ship  202  that sends the square wave to the transmitter electrodes  210  and the internal resistance of the transmitter cable  304 . The transmitter electrodes  210  are connected to each streamer cable  206  by using waterproof “take-outs” (not shown) to prevent seawater from penetrating into the streamer cables  206  under hydrostatic pressure and changing the electrical characteristics of the streamer cables  304  while in use. In this regard, a change in the cable phase characteristics caused by water entry would degrade the effectiveness of the calibration procedure, and reduce the reliability of the measured towed-mode phase-values (described below). 
     The transmitter cable  304  provides a square-wave current signal to the titanium transmitter electrodes  210 , while having minimum cross-talk with the received signal coming back up the streamer cable  206 . The waveform of the transmitter signal is optimized to include a wide range of frequencies in order to detect a varying-frequency capacitance between dispersed hydrocarbons and the highly conductive seawater medium. Typically, this optimization means using a square-wave transmitter waveform, which is made up of multiple odd-frequency harmonics. A 1 Hz square wave is composed of 1st, 3rd, 5th, 7th, 9th, 11th, etc. sine-wave harmonics, which means a Fast Fourier Transform can extract frequencies of 1 Hz, 3 Hz, 5 Hz, 7 Hz, etc. from this single square waveform. The typical range of frequencies of a square wave used on land in an IP search for sulfide minerals is 0.1-10 Hz, while a range of about 1 to about 100 Hz is used in a seawater capacitance application. 
     In the embodiment shown in  FIG. 3 , eight receiver electrodes  212  ( 212   a - 212   h ) are affixed to each streamer cable  206  in an equally spaced relationship to provide sampling at different distances away from the streamer cable  206 . As illustrated, the first or most proximal of the receiver electrodes  212   a  from the ship  202  is spaced at about one-half to about one times the transmitter dipole electrode separation away from the most proximal transmitter electrode  210   a , and subsequent receiver electrodes  212   b - 212   h  are equally-spaced farther along the streamer cable  206 . The actual numbers of transmitter and receiver electrodes and the distances separating the electrodes (called the dipole spacing) can be adjusted in conformance to the spacing between the vertically-separated streamer cables  206  so that detection-zones overlap. The receiver electrodes  212  are non-polarizable (i.e., they do not produce an arbitrary battery effect or voltage offset when in use, due to corrosion and electrolysis). Thus, in a preferred embodiment, the receiver electrodes  212  are composed of silver wire immersed in a stable-base silver-chloride gel that is, in turn, exposed to contact with the seawater, but will not itself corrode. The receiver electrodes  212  are typically encased in a plastic sheath (not shown) to provide protection against abrasion and damage during deployment, while having ports or cut-outs (not shown) to still afford electrical contact with the seawater. 
     The pre-amplifiers or preamps  214  are standard common-mode-rejection differential pre-amplifiers. In the embodiment shown in  FIG. 3 , six preamps  214  are used on each streamer cable  206  to remove common-mode noise by providing each preamp  214  a reference receiver electrode equidistant between two adjacent measurement receiver electrodes (described in more detail below). Each preamp  214  is connected through electrical links  310  to the receiver electrodes  212  on either side and also to its adjoining receiver electrode  212  to implement the common-mode rejection arrangement. The preamps  214  also strengthen and condition the received signal that is returned to the ship-board control and signal processing system through the signal return cable  306  to minimize parallel-wire cross-talk. Otherwise, the two-to-10 +  ampere transmitted signal would overwhelm and swamp the millivolt-level signals from the receiver electrodes  212 . The preamps  214  are located as close as possible to the receiver electrodes  212  to minimize the exposure to electrical streaming-potential noise, and should be compact in order to minimize the towed cross section presented by the streamer cable  206 . 
     The array of receiver electrodes  212  on each streamer cable  206  are grouped together to form a series of dipoles or “triplets”  312  ( 312   a - 312   f ), with the center receiver electrode  212  of each triplet serving as a reference for the corresponding pre-amplifier  214  that sits astride the middle receiver electrode  212  of each triplet. The center preamp  214  of each triplet is a noise filter, and also amplifies and sends back up the streamer cable  206  a single received signal for that triplet. The triplets  312  sample at greater and greater distances out from the streamer cable  206  as they themselves are increasingly distant from the pair of transmitter electrodes  210 . Adjacent triplets  312  can share receiver electrodes  212  at the same time because of the high input impedance of the preamps  214 . 
     Six receiver dipoles or triplets  312  are formed with the embodiment of the invention shown in  FIG. 3 . More particularly, a first dipole  312   a  is formed by the first  212   a , second  212   b , and third  212   c  receiver electrodes  212 . The second or central receiver electrode  212   b  in the first triplet  312   a  serves as the reference electrode. A second dipole  312   b  is formed by the second  212   b , third  212   c , and fourth  212   d  receiver electrodes  212 , so that two receiver electrodes  212   b ,  212   c  are shared with the first group  312   a . The third through sixth dipoles  312   c - 312   f  are formed in a similar way. This layout effectively allows sampling of the hydrocarbon plume  204  to an approximate distance up to about 2 to 3 dipole-lengths away from the streamer cable  206 . 
     The practical maximum size for an optimal electrode-spacing is limited by how much electric transmitter current the streamer cable  206  can bear. If the dipole-spacing is too large, or the transmitted current too low, the signal from the distal receiver dipoles near the end of the streamer cable  206  can be lost or fall below the noise threshold. Making the transmitter cable  304  thicker with lower-gauge internal wires allows for transmission of greater current, but the weight of the streamer cable  206  can become prohibitive for long lengths, and this must be optimized as part of the entire survey design. The greater the transmitted current, the larger the sampling volume size (i.e., “sampling donut,” described below) detected by the most distal receiver electrodes  212  and, therefore, the larger the spacing that can be permitted between the individual streamer cables  206  to still fully overlap each other and sample the entire seawater column. 
     The receiver triplet  312   a  closest to the transmitter electrodes  210  will sample a relatively small volume around it, while the receiver triplets  312   b - 312   f  further away from the transmitter electrodes  210  will sample proportionally larger volumes. When the smallest volume is subtracted from the next smallest volume, a resulting seawater capacitance number value represents the average capacitance of the ring or donut-shaped volume outside the smaller volume. This sampling-and-subtraction process can be done sequentially and automatically, providing increasingly larger “rings” or “donuts” of volume-sampling information on the capacitance of the seawater column that the streamer cable  206  is towed through. The vertical separation of the streamer cables  206  is optimized to provide overlap of the largest “volume sampling donuts” of each streamer cable, thus providing a continuous vertical volume sampling for the vertically-stacked array of streamer cables  206  as a whole. 
     Optimization of streamer cable  206  separations can be done experimentally or by numerical modeling, but the starting point is the type of cable that will be used for fabrication of the streamer cables  206 : the number of shielded conductor-pairs or fiber-optic lines composing the signal return cable  306 , their individual gauge(s), and the weight of the proposed cable per unit length. This latter consideration is important because of the limitations of the ship-board drums or spools (not shown) that are used to deploy the streamer cables  206 . 
     Ultimately, the choice of cable—which dictates the rest of the parameters including electrode-spacing and cable separation—is dictated by what cable-type is most readily available to the fabricator, who must balance cost of components as well as shipping in the decision process. The more receiver triplets  312  there are on each streamer cable  206 , the higher the vertical column sampling obtained, as long as there is sufficient transmitter current to be detected by the most-distal triplet  312   f . The number of instantaneous samples obtained as the array of streamer cables is towed through the water-column is approximately the number of streamer cables  206  multiplied by the number of preamps  214  on each streamer cable  206 , with some overlap. The preamps  214  are generic (including optical versus voltage output options), and there are many different commercial kinds available to design engineers, but a typical amplification factor would be at least 10. 
     In the exemplary embodiment shown in  FIG. 3 , two depth-sensing pressure transducer/acoustic transponder units  216  are located at the peripheral and distal ends of each streamer cable  206 . The depth-sensing pressure transducer/acoustic transponder units  216 , together with the acoustic streamer element locator  220 , add precision to the location of each sampling point on each streamer cable  206  in three dimensions (latitude, longitude, and depth). The same commercially-available power supply (not shown) that feeds the preamps  214  is used to power these additional small pressure transducer/acoustic transponder units  216  on each streamer cable  206 . Shipboard power supplies the 3-D acoustic streamer element locator  220  deployed from the side of the ship  202 . Depth information passes up each streamer cable  206  from the depth-sensing pressure transducer/acoustic transponder units  216 , and location information from the shipboard 3-D acoustic streamer element locator  220  passes independently to the control and signal processing system on the ship  202  for compilation into a 3-D image of the hydrocarbon plume  204 . 
     The final designed vertical resolution of the entire array of streamer cables  206  is decided by engineers according to the target of interest. In other words, the number of transmitter electrodes  210  and receiver electrodes  212 , the spacing between the transmitter electrodes  210  and the receiver electrodes  212 , the number of streamer cables  206  used, and the depth-settings of the cable depressors  208  will all vary according to the depth of the water to be tested. 
       FIG. 4  shows a block diagram of the on-board control and signal processing system. In this figure, three of the streamer cables  206  are shown, as well as the transmitter cables  304  that provide current to the transmitter electrodes  210 . The signal return cables  306  that carry the measured results (data) enter analog-to-digital (A/D) boards (not shown) within a multichannel data-acquisition, processing, and transmitter control unit  402 . The shield surrounding each twisted-pair transmitter cable  304  is grounded  302  (see  FIG. 3 ) and electrically isolated as much as possible to prevent cross-talk from the high-amperage transmitted signal into any received-signal links. The non-ship ground  302  is a sacrificial electrode, typically a copper plate, which is hung overboard so that the ground voltage of the on-board control and signal processing system is allowed to float independently of the electrical system of the ship  202 , thereby reducing or eliminating a potent source of electrical noise contamination. 
     The transmitter electrodes  210  are connected through the transmitter cable  304  to a current transmitter  404 . The current transmitter  404  can be a commercially-available device that is connected through a logic link  406  to the data-acquisition, processing, and transmitter control unit  402 . The linkage is designed to provide a precisely-controlled, stable square wave signal to the transmitter electrodes  210 . The signal return cables  306 , either shielded coaxial or fiber-optic, are connected from the streamer cables  206  to the data-acquisition, processing, and transmitter control unit  402 . The data-acquisition, processing, and transmitter control unit  402  is connected through an isolation amplifier  408  to transmit-signal-monitoring shunt resisters R  410  on each transmitter cable  304  twisted-pair, which are, in turn, connected to the transmitter electrodes  210  down each streamer cable  206 . 
     A control computer  412 , which in a preferred embodiment is a laptop computer or a tablet computer, is connected through a data download cable  414  to the data-acquisition, processing, and transmitter control unit  402 , and also to a Differential Global Positioning System (DGPS) unit  416  through an input connection or download cable  418 . Location data from the acoustic streamer locator  220  are fed to the control computer  412  through a link  420 . Also, the data-acquisition, processing, and transmitter control unit  402  sends depth data from the depth-sensing pressure transducer/acoustic transponder units  216  to the control computer  412 , which assembles the four-dimensional (spatial and time) seawater capacitance map. The DGPS data stream is added to the shipboard acoustic streamer locator  220  data to precisely position the seawater capacitance results, which are the individual sampling volume donuts for each streamer cable  206 , in a database in a 3-D (latitude, longitude, and depth) geographic framework with a date-time mark added for each point. Because the streamer array samples larger and larger volumes of seawater the further the receiver electrodes  212  are located from the ship  202 , a simple geometric correction algorithm, as described above, is used to convert the sampling volume donut values and their overlaps to the capacitance for each particular sampled point of the hydrocarbon plume  204 . The geometric correction algorithm is based upon geometric correction algorithms used in land geo-electrical surveys and can be created within the capabilities of one skilled in the art. An oscilloscope  422  is also connected by a line  424  to the data-acquisition, processing, and transmitter control unit  402  to monitor the received and transmitted signals to verify that there are no ground loops, and that no extraneous noise (e.g., 60 Hz or DC offsets from shipboard power systems) enters the data-stream. 
     The data acquisition, processing, and transmitter control unit  402  can be a commercially-available device, an exemplary embodiment being a 32-bit multi-channel system with individual A/D boards for each channel, along with an internal processing capability. It provides geophysical data acquisition and processing by completing Fast Fourier Transforms on the preamp waveform signals. These are deconvolved against a calibrate record (described below), which is acquired in a pollution-free area of empty sea before the data acquisition begins, to remove extraneous systemic capacitances in the return cables and the data-acquisition, processing, and transmitter control  402  hardware itself. The deconvolved waveforms of the transmitted signals measured at the shunt resistors R  410  are then subtracted to provide final signal phase-shifts for each electrode triplet  312 . This is the measure of the seawater capacitance for the instantaneous volume of water sampled by each individual receiver electrode  212  of each streamer cable  206 . 
     The isolation amplifier  408  is an isolation optical amplifier that enables the square wave signal from the current transmitter  404  to be precisely measured and instantaneously adjusted to keep it constant using feedback from the shunt resisters R  410  to the data-acquisition, processing, and transmitter control unit  402 . Because the current transmitter  404  signal is optically isolated from the received signal, no electrical noise, voltage offset, or crosstalk arrives at the data-acquisition, processing, and transmitter control unit  402  from the current transmitter  404 . 
     A system calibration is done in two stages or levels. In the first stage, after an on-board wire-harness (not shown) is set up, a series of phase and amplitude measurements are made for the on-board control and signal processing system for the full range of frequencies that will be used in the spectral mode described below, which is typically about 1 Hz to about 100 Hz. These results are Fourier transformed from the time domain to the frequency domain, and the transformed results for each frequency are stored in a data storage unit  426  connected through a link  428  to the control computer  412 , where they are later deconvolved against the received signal data during actual operation. An additional second level calibration operation is done by measuring data from the entire system (including the deployed array of streamer cables) in some location at sea where there are no hydrocarbon plumes, IP-reactive minerals, or metallic objects in close proximity. The final shipboard-acquired data, with the frequency-dependent system phase-shift deconvolved therefrom in real time, are then subtracted against the second level calibration to yield systemic noise-free data. 
     Turning now to the operation of the system described above, the transmitter electrodes  210  of each streamer cable  206  are energized with a precisely controlled square wave voltage generated by the current transmitter  404 , resulting in the injection of several amperes of electrical current into the seawater. The streamer cables  206  are pulled behind the ship  202  and are maintained at specific depths via the individual depth-control cable depressors  208  so that the hydrocarbon plume  204  is sampled simultaneously from top to bottom with precise depth information being returned for each point of data acquired. When the square wave is turned off episodically during the transmission square-wave duty-cycle, the electrochemical reaction (ion adsorption onto the oil droplet surface) reverses, and any subsequent secondary signals caused by this during transmitter off-time are detected using the non-polarizing receiver electrodes  212 . 
     If there are no hydrocarbons present, no secondary signal will be generated. There may be electromagnetic (inductive) coupling between the transmitter electrodes  210  and the seawater, but this is constant and is automatically removed by the calibration process described above. The array of streamer cables is designed using the depth-control cable depressors  208  so that the sampling field of each individual streamer cable  206  overlaps the sampling field of the nearest overlying and underlying streamer cables  206  by about 5% to about 10% to ensure that there are no unmeasured gaps in the vertical sampling plane. Also, the spacing of the sensing or receiving electrodes  212  on each streamer cable  206  is adjusted to permit detection-overlap between the streamer cables  206 . This overlap will, in turn, be dictated by the number of streamer cables  206  that are deployed simultaneously in a given vertical stack of streamer cables and by the amount of transmitter current the streamer cable  206  can tolerate without overheating and becoming damaged. 
     While underway, the streamer cables  206  cut through the seawater column and intersect any plumes or plume stringers  204 , providing a real-time reading on the location of the oil at multiple depths in a two-dimensional vertical plane. The data are acquired very rapidly at a single square-wave frequency (which by Fast Fourier Transform is converted to its individual harmonics), typically with a fundamental (1 st  harmonic) frequency of 1-4 Hz, in a continuous sampling mode at typical towing speeds of about 3 knots or higher. A very large area can thus be covered in this manner in a very short time, limited by how fast the ship  202  can move and still maintain the cable depressors  208  at a fixed depth. 
     The ship  202  makes a pass across the region of interest, and then makes a so-called “keyhole turn.” The tow-path is thus offset and folded over and parallel to itself, back and forth, to cover a large surface area on the sea and, thus, sample a large volume beneath the sea. Each towed-path sampling plane, which is a tall but narrow sampling volume, is parallel to previous sampling planes, the aggregate providing a rapid full volume of sampling over a wide area and the full depth range being searched for the hydrocarbon or hydrocarbon-and-dispersant plumes. This permits the assembly of a three-dimensional picture of both hydrocarbon density and droplet size, providing in aggregate the plume location and three-dimensional shape, as well as the droplet-size distribution. The survey can be repeated on subsequent days to determine how the plume is moving and evolving with time. This would then give scientists a predictive capability for future plume movement, as well as plume degradation rates (which are seen as a change in the frequency distribution of the measured seawater capacitance), which also contribute toward predicting its future location and state. 
     An alternative to the way in which the streamer cables  206  are towed behind the ship  202  is to have a robust sled (not shown) running at the top of the expected plume  204  depth. From this sled, multiple streamer cables  206  with depth-control cable-depressors  208  would be deployed. The sled would have much of the ship-board electronics for data-processing and current transmission. 
     There are also other geometrical alternatives to the seawater-sampling array. For example, a single, long master cable can reach down from the ship to great depth to a heavy, powered sled or remotely-operated vehicle that would track beneath the towing ship and turn with it. Multiple transmitter-receiver sets (i.e., parasitic cables) would be distributed down the main cable, each parasitic cable extending off and behind the master towed main cable, trailing in the sea in the direction opposite the direction of tow, each making its own measurements at different depths. These measurement results would all be collected simultaneously to assemble a near real-time, 3-dimensional picture of the location and state of the hydrocarbon plume. There would be only a single depth-sensing transducer required for each parasitic cable, and as few as a single pair of transmitter electrodes and a single triplet or non-polarizing receiver electrodes on each cable. A preamplifier would be used on each parasitic cable. 
     The present invention provides a near real-time approach to map and characterize hydrocarbon plumes  204  from major oil well blowouts and pipeline leaks, as well as from natural oil seeps. It does not require time-consuming drop sampling or chemical analyses of seawater pollutants, though lab-sampling of representative oil-saltwater mixes beforehand will increase its predictive accuracy. The related IP approach has previously been used to map minerals fixed in place beneath the land or seafloor. The streamer array of the present invention maps a moving target in the sea in four dimensions. In other words, the seawater capacitance measuring system and method described herein allows for the mapping and characterization (i.e., depth, thickness, density, droplet size, etc.) of the hydrocarbon plume, including its volume distribution in the open ocean, and how it evolves (i.e., moves and degrades) with time. 
     Although the present invention has been described relative to specific exemplary embodiments thereof, it will be understood by those skilled in the art that variations and modifications can be effected in these exemplary embodiments without departing from the scope and spirit of the invention.