An electrode system for passive measurement of electric potential field

Abstract

The disclosure relates to an electrode system for passive measurement of electric potential field in seawater characterized by comprising: an underwater vehicle (100), four or more electrodes (20) arranged on the underwater vehicle (100), wherein the exact installation positions of the electrodes (20) are arranged to be paired i electrode pairs (21,22) wherein the pairs are set up to measure orthogonal electric field components in three dimensions, and a controller (150) provided with means to reconstruct the electric field potential in the underwater vehicle (100) coordinate system. The disclosure further relates to a method for passive measurement of an electric field in a fluid.

Description

[0001] An electrode system for passive measurement of electric potential field

[0002] Technical field

[0003] The present disclosure relates to an electrode system for passive measurement of electric potential field in seawater and a method for passive measurement of electric field in a fluid. More specifically, the disclosure relates to an electrode system for passive measurement of an electric potential field in seawater and a method for passive measurement of an electric field in a fluid as defined in the introductory parts of the independent claims.

[0004] A problem with the solutions of the prior art is that AUV’s used for measuring electric fields traditionally has been implemented by carefully aligning the measurement system electrodes to the AUV coordinate frame (aligning the axis of the electrodes to the axis of the AUV). However, such an approach is prone to error as true orthogonal placement of the electrodes are difficult. Further, such a method is also faced with challenges in that the desired electrode positions may be practically unfeasible due to either on-board instruments or features, or they may be error prone or unfeasible from a handling perspective.

[0005] Existing add-on electrode arrangements are prone to vibration and out of position measurements, which are likely to measure signals which are not “clean", in that arbitrary fractions of orthogonal fields may influence the measurement. Since the electrode arrangements introduce positional uncertainty and errors, the measured values mirror this uncertainty and errors.

[0006] Some past attempts comprise electrode rig providing a set reach, typical parallel, orthogonal and diagonal with the AUV. The orthogonal reach is then minimized to minimize the positional deviation from a standstill position, for example imposed by drag when the AUV is moving through the water. The longer a lateral rig is, the more vulnerable it is to drag and vibrations, and it is not possible to establish an absolute position of the electrodes.

[0007] Erroneous measurements of electric field made in sub-sea operations will lead to low quality surveys, and in some instances the resulting survey will not be usable, and the only remedy is to perform the survey a second time. Surveys are time consuming and, particularly in surveys where resources, time and man / vessel hours, are limited, it is a difficult task to decide if two or more passes is to be performed to remedy potential invalid surveys passes, or take the chance of making only one pass at the risk of the need to rerun the whole operation. The latter being even more resource demanding.

[0008] In WO 2021 / 107776 it is shown an electric field gradient sensor, having a sensor body having an outer surface; and a plurality of electrodes distributed over the outer surface, each electrode having an electrode surface facing outward from the surface.

[0009] A further problem with prior art is that once an AUV with a sensor-rig is deployed into the waters, any faults arising by one or more of faulty electrodes, damages to rig, faulty wiring, electronics failure, wear and tear, disturbances deviating from expected values, and other cannot be detected and mitigated, and may significantly reduce the quality of the measurements.

[0010] There is thus a need for an improved operational setup which comprise mitigating features and is more efficient, less error prone, and having a more flexible approach.

[0011] It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve some or all the above mentioned problem.

[0012] A system measurement setup according to the present invention provides an overdetermined system from which it is possible to assess the uncertainties within the measurement system itself.

[0013] According to a first aspect there is provided an electrode system for passive measurement of electric potential field in seawater characterized by comprising: an underwater vehicle, such as an AUV, having four or more electrodes arranged on the underwater vehicle, wherein the exact installation positions of the electrodes are arranged to be paired i electrode pairs wherein the electrode pairs are configured to measure orthogonal electric field components in three dimensions, and a controller provided with means to reconstruct the electric field potential in the underwater vehicle coordinate system.

[0014] It is thus provided a system that may compensate for variances in the electrode setup coordinate system relative the underwater vehicle, AUV, coordinate system.

[0015] According to some embodiments, the system comprises arranging the electrode pairs to form one or more loops with maximum physical electrode spread. A maximum spread of the electrodes in the electrode pair will maximize the volume encompassed by the loop, which again improves the SNR abilities of the electrode setup.

[0016] According to some embodiments, the electrodes are arranged in a first group in the front, and a second group in the back of the underwater vehicle, and the electrode pairs are configured in loops in a substantially symmetrical configuration, within the loop and / or between the loops.

[0017] Having one or more loops with electrodes being substantially symmetrical arranged within the loop, and the loops being substantially symmetrical to each other provides for a number of mitigating actions and possibilities when electrodes behavior / performance deviates from the expected.

[0018] Having the electrode pairs configured in loops in a substantially symmetrical or symmetrical configuration is advantageous, but not an absolute requirement in present disclosure. The symmetry simplifies the post processing of the measurements, but in embodiments where the symmetry is not achieved, the post processing of the data can take this into account.

[0019] According to some embodiments, the underwater vehicle is a non-conductive underwater vehicle.

[0020] According to some embodiments, the electrodes are arranged slightly recessed or in flush with the surface of the underwater vehicle.

[0021] It is advantageous to mount the electrodes slightly recessed with the outer surface of the underwater vehicle, AUV. Having a tight and non-flexible arrangement, secured by the hull of the underwater vehicle, AUV, reduce or eliminates disadvantages related to for example drag, vibration, vulnerability to debris collisions and other.

[0022] According to some embodiments, the numbers of electrodes are 8, and the electrodes are arranged in two loops of 4 electrodes each.

[0023] Two loops, preferably being symmetrical to each other, may provide an unprecedented redundancy feature, and the possibility to detect faulty loop or electrode pair is improved compared to a system having only one loop.

[0024] According to possible embodiments, the electrodes are separated at its furthest possible extent, longitudinal, horizontal and vertical. The longer the distance between electrodes of a pair or loop, the better the SNR ratios, and the better the volume embraced by the loop, the more complete will the measurements be, with good representation of the 3D electric field components.

[0025] According to possible embodiments, the electrodes are paired such that any first electrode of an electrode pair within the first or within the second group are chosen to be paired with a second electrode being arranged with the longest orbital length outside the underwater vehicle from the first electrode.

[0026] As this pair configurations within the first or within the second group provides for the shortest distance between paired electrodes, these pairs are most vulnerable to the selfinduced electric fields of the underwater vehicle, AUV. It has been found that these disturbances can be reduced by pairing the electrodes being furthest apart each other within the first or within the second group.

[0027] According to possible embodiments, the electrodes are paired such that any first electrode of an electrode pair in the longitudinal direction are chosen to be paired with a second electrode having one electrode in the first group, and the other electrode in the second group.

[0028] The longest distance between paired electrodes is found when selecting one electrode from the first group and one electrode from the second group.

[0029] According to possible embodiments, the electrode pairs in the longitudinal direction are paired such that the distance between the electrode in the first group and the electrode in the second group is further increased by an orbital location difference outside the underwater vehicle between the two electrode positions.

[0030] When configuring a longitudinal directed pair to span over an orbital portion of the outer hull of the underwater vehicle, AUV, the symmetry of the loop the pair is a member of may be secured, as well as the maximum volume that is sought after.

[0031] According to possible embodiments, the underwater vehicle is one of an AUV, ROV, or UID.

[0032] According to a second aspect there is provided a method for passive measurement of an electric field in a fluid, the method being characterized by comprising the steps: providing an underwater vehicle, AUV, installing four or more electrodes on the underwater vehicle, mapping the exact installation positions of the electrodes, pairing the electrodes in electrode pairs wherein the pairs are set up to measure orthogonal electric field components in three dimensions, and when the underwater vehicle is operating in a fluid: measuring orthogonal electric field components in three dimensions by the electrode pairs, and reconstructing the electric field potential in the underwater vehicle coordinate system.

[0033] The implementation allows for a flexible installation process where the electrodes pairs are not required to be aligned with the AUV reference frame, but rather is installed at optimal locations relative the form of, and location of other installations in, the underwater vehicle, AUV.

[0034] According to possible embodiments, the mapping comprises defining offset coordinates of the electrodes relative the underwater vehicle.

[0035] Using the exact installation positions of the electrodes on the AUV, highly accurate measurements of electric field potential in the AUV coordinate system can be reconstructed using a minimization method as discussed below.

[0036] According to possible embodiments, the pairing of the electrodes further comprises configuring the electrode pairs to form one or more loops with maximum physical electrode spread.

[0037] According to possible embodiments, the method further comprises arranging the electrodes in a first group in the front, and a second group in the back, of the underwater vehicle, and configuring the electrode pair loops in a substantially symmetrical configuration, within the loop and / or between the loops.

[0038] According to possible embodiments, the method further comprises installing of the electrodes slightly recessed with the surface of the underwater vehicle.

[0039] According to possible embodiments, the method further comprises when the numbers of electrodes is 8, arranging the electrodes in two loops of 4 electrodes each.

[0040] According to possible embodiments, the method comprises: separating the electrodes at its furthest possible extent, longitudinal, horizontal and vertical.

[0041] These features of the arrangement of pairs, groups, and loops provides for a setup having very low vulnerability to faults in the electrode setup, whether it is caused by a faulty electrode, a wiring error, a controller fault, non-predicted noise peak or other. To a large extent, the setup may regenerate or mitigate missing or faulty electrode readings, and provide a very robust system.

[0042] According to some possible embodiments, the method comprises: pairing the electrodes such that any first electrode of an electrode pair within the first or within the second group are chosen to be paired with a second electrode being arranged with the longest orbital length outside the underwater vehicle from the first electrode.

[0043] According to possible embodiments, the method comprises: pairing the electrodes such that any first electrode of an electrode pair in the longitudinal direction are chosen to be paired with a second electrode having one electrode in the first group, and the other electrode in the second group.

[0044] According to possible embodiments, the method comprises: pairing the electrode pairs in the longitudinal direction such that the distance between the electrode in the first group and the electrode in the second group is further increased by an orbital location difference outside the underwater vehicle between the two electrode positions.

[0045] The advantage with maximizing the spread, is that the volume that the electrodes embrace will be larger, and electric field being self-induced by the underwater vehicle, AUV, will have reduced impact on the measured fields.

[0046] According to possible embodiments, the fluid is seawater.

[0047] According to possible embodiments, the method comprises: providing a field perturbation model of the underwater vehicle.

[0048] The field perturbation model plays a role when processing the measurements, and is used to clean up the measurements such that a correct measurement of the electric field of the environment can be produced.

[0049] According to possible embodiments, the method comprises: reconstructing any erroneous electrode pair measurement from 3 other electrode pair measurements within a measurement loop.

[0050] This is possible based on the fact that the electrodes in either loop forms a voltage loop, which according to Kirkhoff's law will ideally sum to zero. According to some possible embodiments, the method comprises: when there are 8 electrodes, arranging the electrodes according to the following matrix: wherein the “front" and “aft" subscripts refer to front of and back positions respectively, the “port" and “stb" subscripts refer to port and starboard positions respectively, and the “up" and “down" subscripts refer to positions above or below the centre of mass of the underwater vehicle.

[0051] According to possible embodiments, the method comprises: forming a measurement matrix according to measurement loops in the following matrix:

[0052] According to possible embodiments, the method comprises: confirming a valid measurement by comparing the measured and analysed results from the two measurement loops.

[0053] Compared with prior art having to perform several measurement passes above an object to qualify its readings, the system and method of present disclosure provides for a much larger confidence level in the measurements, advocating for fewer redundancy passes over an object / area of interest.

[0054] Having better control of the electrodes and corrective measures available, the speed of the underwater vehicles may be higher than in comparable measuring methods. Effects and features of the second aspect are to a large extent analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.

[0055] The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure.

[0056] Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

[0057] Terminology-

[0058] The term "maximum volume spread" when used when choosing the spread of electrode pairs and loops is to be understood as a best practical selection seeking a maximal possible volume spread taking into account possible trade-offs related the need to avoid short distances, such as when defining the pair inside one of the groups, fore or aft, where it is desirable to avoid, if possible, choosing electrodes being arranged next to each other, and also convenient to avoid defining orthogonal or parallel to the 3D electric field component planes.

[0059] The term "underwater vehicle" is to be understood as any type of underwater vehicle suitable for hosting electrodes in the task of passive measurement of an electric field in the water. The term shall include but not be limited to Autonomous Underwater Vehicles, AUVs, Remotely Operated Vehicles, ROVs, Underwater Intervention Drone, UID. The term "passive measurement" is used to emphasize that the electrodes measure electric ambient field potentials by only sensing the ambient fields. The electrodes themselves never impose any signal to the environment.

[0060] The term "substantially symmetrical" or "symmetrical" shall be understood to define any type of similar forms, also rotational substantial symmetrical or rotational symmetrical forms shall be encompassed.

[0061] Brief of the

[0062] The above objects, as well as additional objects, features and advantages of the present disclosure, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings.

[0063] Figure 1 shows a side view of an AUV having the electrodes of present disclosure installed.

[0064] Figure 2 shows a front view of the an AUV having the electrodes of present disclosure installed.

[0065] Figure 3 shows a schematic layout of the first (front) and second (aft) group of electrodes connected to the controller.

[0066] Figure 4A shows a transparent side view of an AUV having the electrodes of present disclosure installed, wherein a first loop of electrode pairs are shown.

[0067] Figure 4B shows a transparent top view of an AUV having the electrodes of present disclosure installed, wherein the first loop of electrode pairs are shown.

[0068] Figure 4C shows a transparent side view of an AUV having the electrodes of present disclosure installed, wherein a second loop of electrode pairs are shown.

[0069] Figure 4D shows a transparent top view of an AUV having the electrodes of present disclosure installed, wherein the second loop of electrode pairs are shown.

[0070] Figure 5 shows one embodiment of a schematic overview of a rolled out exploded layout of the electrodes and the first and second loops. Figure 6 shows an embodiment of an electrode arranged slightly recessed of the outer surface of the body of the underwater vehicle.

[0071] Figure 7 illustrates a typical module layout of the controller.

[0072] Figure 8 shows a simplified overview of a flow chart for the data processing.

[0073] Figure 9 shows an example of a voltage field map around an AUV.

[0074] Detailed description

[0075] The present disclosure will now be described with reference to the accompanying drawings, in which preferred example embodiments of the disclosure are shown. The disclosure may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person.

[0076] Using underwater vehicles for passive measurement of the electric field from for example a Cathodic Protection system on seafloor constructions such as pipelines is a highly efficient method of maintenance surveying. For this method to be feasible though, the electric field needs to be measured using low noise electronics and a purpose built sensor system based on electrode pairs positioned along the body of an AUV. These measurements need to be referenced to the underwater vehicle since the position and orientation of the underwater vehicle are important parameters in the inversion calculation for the outgoing currents from the above mentioned constructions.

[0077] In prior art there is a big problem that erroneous measurements of electric field made in sub-sea operations will lead to low quality surveys. Low quality survey results produce low quality data interpretations. Often multiple passes are made over the same area to mitigate the risk of poor survey data. This is not an absolute fix, since the error may repeat itself, or may be related to instrument failure / deterioration. It is also a costly operation to do multiple passes over an area to survey, but it is an even larger cost tied to the process of rerunning the survey. Present disclosure provides a solution to the above discussed problems and risks. It is a goal to provide a method and system that is capable to detect and reduce and / or mitigate any type of erroneous measurement, such that the quality of the survey will improve, and the need for multiple passes over area to be surveyed or survey reruns are reduced or avoided. It is further provided a method for improving the quality of the measurements, and mapping of the electric field in global coordinates.

[0078] To characterize the electric field in three dimensions with a fixed electrode configuration, three pairs of electrodes arranged so that they contain orthogonal electric field components is required. This means a minimum of four electrodes. To further reduce the effect of mounting the electrodes on a non-conductive body that will distort the electrical field a substantially symmetrical or symmetrical electrode configuration is suggested.

[0079] The symmetry simplifies the post processing of the measurements, but in embodiments where the symmetry is not achieved, the post processing of the measured data may take this into account when comparing the measured electric field components.

[0080] Figure 1 shows a side view of an underwater vehicle of an underwater vehicle, typically an AUV, ROV or UID, in which electrodes are arranged in the hull of the underwater vehicle. The example embodiment in this disclosure implements 8 electrodes. Using 8 sensors grouped in 8 electrode pair arranged in two symmetrical measurement loops has advantages related to being an overdetermined system, and the two loop measurements may be compared and used as redundancy loops for each other. . A configuration carrying only 4 or more electrodes grouped in 4 electrode pair configurations and one measurement loop may achieve adequate results.

[0081] Operation activities concerned by navigation and driving thrust of the underwater vehicle 100 is considered part of the prior art, and not discussed in present disclosure. These tasks are considered obvious for the skilled person, and is hereby included to enable the navigation of the underwater vehicle 100. Likewise is the powering of, and, the electronics used for collecting the electrode 20 measured data.

[0082] In an advantageous embodiment the electrodes are arranged on an underwater vehicle having a rounded cross sectional shape and a longitudinal extending body as illustrated in the figures.

[0083] The measurement system of present disclosure is built upon two independent measurement loops, each able to reconstruct the external 3D electrical field from its measurements. The finesse in this setup is that it provides an overdetermined system from which it is possible to assess the uncertainties within the measurement system itself. An assessment that would have been impossible should it not have been overdetermined. Furthermore the system provides built in redundancy which provides resilience to failures on single measurement channels. The system further provides the ability to assess what the missing measurement should have been from the three remaining measurements in a loop.

[0084] The first aspect of this disclosure shows an electrode system for passive measurement of electric potential field in seawater characterized by comprising: an underwater vehicle 100, four or more electrodes 20 arranged on the underwater vehicle 100, wherein the exact installation positions of the electrodes 20 are arranged to be paired i electrode pairs 21,22 and the pairs are set up to measure orthogonal electric field components in all three dimensions, and a controller 150 provided with means to reconstruct the electric field potential in the underwater vehicle 100 coordinate system. At least 3 pairs are necessary to define a 3D volume for measuring electric field components in three dimensions.

[0085] The electrodes are installed in a symmetrical pattern to the furthest possible extent. If possible, in an embodiment where 8 electrodes are installed, 4 in the front and 4 in the back, the four electrodes at the front or back should be separated maximally in their cross-sectional plane (in respect to the central axis of the underwater vehicle).

[0086] The electrodes can be placed in optimal locations avoiding conflicting placement with existing equipment due to a compensation model provided to calibrate the actual real position of the electrodes relative the underwater vehicle coordinate system. Any variations necessary to implement is then compensated using for example an exact CAD data model or equivalent from the design phase. The system can then be calibrated using exact real positions of the electrodes.

[0087] Modelling of the field perturbation caused by the underwater vehicle itself is advantageous to achieve maximum accuracy. A perturbation model is periodically measured and generated to be used in the processing of data from, and the validation of, the electrode measurements. Since the body of the underwater vehicle often is of a non-conducting material, the background noise of the underwater vehicle itself may impact the readings in a negative way. These noises are advantageously measured / estimated and subtracted from the measurements.

[0088] The controller is powered and set up to control the electrodes, and record and process the measurements.

[0089] The system further include arranging the electrode pairs 21,22 to form one or more loops 1-2-3-4, 5-6-7-8 with maximum physical electrode spread as exemplified in figure 4A- 4D. The electrodes 20 are arranged in a first group in the front, and a second group in the back of the underwater vehicle, and the electrode pairs 21,22 are configured in loops 1-2-3-4, 5-6-7-8 in a substantially symmetrical configuration, within the loop 1-2-3-4, 5-6-7-8 and / or between the loops 1-2-3-4, 5-6-7-8.

[0090] The loop is arranged to span as much as possible of the underwater vehicle with maximum volume spread, wherein the symmetry between multiple loops preferably is maintained, as the loop configurations are exemplified in one embodiment as described in figure 4A - 4D.

[0091] Choosing the electrode pairs and loops for maximum volume spread is a trade-off equation. It is important to choose the longest, outside the hull, distance between the two electrodes. This must be traded with the need to avoid short distances, such as when defining the pair inside one of the groups, fore or aft, it is important to avoid, if possible, choosing electrodes being arranged next to each other. Such as for example electrode a and electrode h. Electrode a can in the illustrated embodiment then only be paired with electrode d. This sets some limits to what electrodes can be paired across the groups, and in the embodiments shown in the figures the pairing comprising electrodes from each group spans TT / 2 of the hull in the orbital direction. The total loop form also matters, and as seen in the figure the chosen pair combination provides two loops having almost identical form and volume. The loops should also advantageously be chosen not to define orthogonal or parallel to 3D electric field component planes, since this will not be ideal when assuming the electrode pairs in each loop shall measure electric field components from all planes.

[0092] Figure 5 showing the electrode arrangement in a spread / rolled out fashion where the cylinder representing the hull of the underwater vehicle has been rolled out to “a sheet" and highlights one embodiment of the loop configuration in a way where the volume spread is more visible. The point "e" is superimposed above point "d" to illustrate that the real path from "f" to "e" is the shortest route only spanning TT / 2 of the orbital distance to define the shortest path between the two electrodes.

[0093] It is possible to configure the loops 1-2-3-4, 5-6-7-8 differently, but at the risk of not getting a proper spread and volume embraced by the loop 1-2-3-4, 5-6-7-8.

[0094] The underwater vehicle 100 hull is advantageously in a non-conductive material.

[0095] The electrodes 20 are arranged slightly recessed a distance r into the outer surface of the body of the underwater vehicle 100, as seen in figure 6. This in order to avoid the turbulence and noise arising from such turbulence in the close vicinity of the outer surface of the hull. The figure illustrates how the electrode in one embodiment is “hidden" into the hull body. The figure show the electrode seen from 3 angles, a front image, an oblique side view and a substantially side view. Other implementations may be facilitated to bring the electrode slightly recessed with the hull surface of the underwater vehicle. The slightly recessed arrangement of the electrodes further enhance the robustness towards instability due to drag, vulnerability to swirl effects and collision with objects, all which will represent potential reasons for disturbing the measurements. The recess may be covered by a permeable cloth 29, a gel, or similar to protect the electrode and to maintain a plane surface in line with the outer surface of the underwater vehicle 100.

[0096] In one possible embodiment the numbers of electrodes 20 are 8, and the electrodes 20 are arranged in two loops 1-2-3-4, 5-6-7-8 of 4 electrodes 20 each.

[0097] Two loops when arranged symmetrical to each other, may be compared and used to mitigate any deficiencies in the electrodes and / or measured fields. There are many potential fault sources in a system like this. An electrode may be faulty or stop working due to age or damage, or even faults in cable wiring to the electrodes may give rise to faulty electrode readings / measurements, and the way the setup of the electrode pairs, providing measuring around a complete loop, are made may be used to detect such fault. Summing the voltage contributions along the lop should ideally result in a zero residual. Deviations from may be related to noise contributions. These faults and noise contributions can be compared between the loops and suppressed by either removing a single measurement from the measurement matrix, or just letting the overdetermined system reduce its contributions by the minimization that happens when the matrix equation below is solved. Since the electrode pairs are set up to measure electric field components in three dimensions, then any variances that stands out from the other measurements is likely to represent a faulty measurement. Faults may also be caused by peaks of background noise either induced by the underwater vehicle itself or by other external sources.

[0098] Although an overdetermined measurement is not strictly needed (from an theoretic point of view), it is a major advantage from a practical point where it is not uncommon that a single measurement may be subject to noise of unknown origin. With the redundancy in the system it is possible to either remove this noise contribution, or even better, get an independent measurement to relate to the noisy measurement to assess the magnitude of the external noise. As is exemplified by the figures 4A-D and figure 5 the electrodes 20 are separated at its furthest possible extent, longitudinal, horizontal and vertical. This way it is possible to maximize the path length between the electrodes in a pair.

[0099] The electrodes may be paired such that any first electrode 20 of an electrode pair 21 within the first or within the second group are chosen to be paired with a second electrode 20 being arranged with the longest possible orbital length outside the underwater vehicle 100 from the first electrode 20. The electrode pair defined within the first or within the second group are arranged within the group in a plane angled relative the longitudinal direction 160, survey line, preferably close to or substantially perpendicular to the longitudinal direction 160 of the underwater vehicle. The electrodes in each pair are chosen to have the longest possible distance between themselves, and at the same time not being arranged at the same horizontal level, thus each measurement covers at least electrical field components from two dimensions. In some embodiments where there are 4 electrodes or more in each group it is possible to define a path that spans n of the outer orbital circle, and for example at an angle a between electrodes defining a 45°° diagonal path towards the horizontal plane.

[0100] The electrodes may be paired such that any first electrode 20 of an electrode pair 22 in the longitudinal direction are chosen to be paired with a second electrode 20 having one electrode 20 in first group, and one electrode 20 in the second group. As for the shorter pairs within the groups, the longer distance pair configurations are chosen to maximize the span of the loop they are part of, to enable measurement of electric field components in three dimensions, but within the possible paths available to provide loops that are symmetrical in form. This is achieved by the electrode pairs 22 in the longitudinal direction being paired such that the distance between the electrode in the first group and the electrode in the second group is further increased by an orbital location difference outside the underwater vehicle 100 between the two electrode 20 positions.

[0101] The orbital shift is advantageous since the measurement is performed along the curvature of the underwater vehicle resulting in a cross line amplification due to the higher measured field along the surface, and the isolating effect of the AUV, resulting in a "longer" electrode pair distance , typically for the electrode pairs between the electrodes within the same group. The cross field amplification appears due to a "large" cylindrical isolated object in a conductive environment, the AUV in the seawater, causing field crowding on the outside of the hull, and the sensors arranged on the AUV hull will measure a higher voltage in the water than would have been measured without the AUV present. Figure 9 exemplifies this with a voltage field map 50 herein the lighter colored field identifies the higher voltage. The upper portion of the figure shows the AUV 100. The lower portion of the figure shows a section of the field only and the separation line 51 illustrates for example the 1.0 V line as it would have appeared if no AUV was present, and the actual separation line 51' illustrates the same IV level when the presence of an AUV causes the field crowding. The orbital shift of the pairs arranged in the longitudinal direction will have some improved cross field amplification as discussed above, although not at the same extent.

[0102] As discussed above, the underwater vehicle may be one of an AUV, ROV, or UID, but the measurement system and concept may be adapted to be used with other carriers or moving or non-moving installations.

[0103] The second aspect of this disclosure shows a method for passive measurement of an electric field in a fluid, the method being characterized by comprising the steps: providing an underwater vehicle 100, installing four or more electrodes 20 on the underwater vehicle 100, mapping the exact installation positions of the electrodes 20, pairing the electrodes 20 in electrode pairs 21,22 wherein the pairs are set up to measure orthogonal electric field components in three dimensions, and when the underwater vehicle 100 is operating in a fluid: measuring orthogonal electric field components in three dimensions by the electrode pairs 21,22, and reconstructing the electric field potential in the underwater vehicle 100 coordinate system.

[0104] The mapping comprises defining offset coordinates of the electrodes 20 relative the underwater vehicle 100. Further compensation must be made in regards of parameters such as tilt, roll and pitch of the underwater vehicle 100. One could say that the task is to measure / compose the electric field in the global coordinates.

[0105] The pairing of the electrodes 20 further comprises: configuring the electrode pairs 21,22 to form one or more loops 1-2-3-4, 5-6-7-8 with maximum physical electrode spread.

[0106] Further arranging the electrodes 20 in a first group in the front, and a second group in the back, of the underwater vehicle, and configuring the electrode pair 22,22 loops 1-2-3-4, 5- 6-7-8 in a substantially symmetrical configuration, within the loop 1-2-3-4, 5-6-7-8 and / or between the loops l-2-3-4,5-6-7-8 is done to optimize fault correction.

[0107] The method further comprises the installing of the electrodes 20 slightly recessed with the surface of the underwater vehicle 100, to minimize instability in the positioning and make the electrodes less vulnerable to external impact. In many embodiments the fluid is seawater.

[0108] The method further comprises to establish, by in-situ measurements or by estimation, providing a field perturbation model of the underwater vehicle 100, to be used in the analysis process when signal measurements are analyzed.

[0109] When a measurement is deemed to be erroneous, the method may comprise reconstructing any of the erroneous electrode pair 21,22 measurement from 3 other electrode pair measurements 21,22 within a measurement loop l-2-3-4,5-6-7-8.

[0110] This is possible based on the fact that the electrodes in either loop forms a voltage loop, which according to Kirkhoff's law will ideally sum to zero. There will be some deviations, and no measurements are ideal, but the residual may be estimated due to known noise components. This may then be taken into account when assessing the resulting voltage loop.

[0111] Finding the electrical field may then, in the embodiment when there are 8 electrodes 20, be found by arranging the electrodes 20 according to the following matrix: wherein the “front" and “aft" subscripts refer to front of and back positions respectively, the “port" and “stb" subscripts refer to port and starboard positions respectively, and the “up" and “down" subscripts refer to positions above or below the centre of mass of the underwater vehicle.

[0112] Then following by forming a measurement matrix according to measurement loops 1- 2-3-4, 5-6-7-8 in the following matrix: Ea- Eb~

[0113] Eb~ Ec

[0114] Ec~ Ed

[0115] Ed- Ea

[0116] M~ Ee- Ef

[0117] Ef ~Eg

[0118] Eg - Eh

[0119] -Eh- Ee.

[0120] This solution effectively forms two measurement loops where any electrode pair can be reconstructed from three other pairs in the corresponding loop. This will give a good opportunity for a quality control of the data. At any point in time, the sum of all components should be no larger than the system noise (and the electrode pair offsets).

[0121] The measurement matrix can then be abbreviated as M, the electrical field as x and the measured values of the above constructed electrode pairs as b, the system can be formulated as:

[0122] M • x = b

[0123] The solution to the above system may then be formulated as: x = (MT• M1■ MTb

[0124] The configuration proposed above, provides a measurement system that is overdetermined and the solution will be a least squares solution further improving the noise characteristics. The overdetermined measurement system will inevitably result in a non-zero residual when solving this matrix equation. This residual is an effective means of tracking the quality of the solution and a good way to flag if something suddenly happened to the system.

[0125] As the electrode positions are fixed, the matrix inversion can be pre-calculated and the remaining processing is a 3x8 matrix multiplication with an 8x1 vector.

[0126] Thus, the method comprises the feature of confirming a valid measurement by comparing the measured and analysed results from the two measurement loops 1-2-3-4, 5-6- 7-8. As illustrated in figure 5, an advantageous pairing of electrodes is shown wherein the loops have comparable form and volume span.

[0127] Figure 7 illustrates a typical module layout of the controller, comprising at least one of a Front end being connected to the electrodes via a high impedance interface. The Front end comprising an analogue to digital signal converter, converting all the electrode lines continuously. Sample rate in exemplified in the figure and shown as 2kSPS (kilo Samples Per Second), but the invention according to present disclosure is not limited to this. Further components may be a Controller timing module for providing sample timing and stamping of data samples. A Host interface comprising on-board processing and power supply, enabling transfer of data, processed and / or raw, via a communication line, here exemplified as an Ethernet communication line 170 is comprised. The communication line is not restricted to Ethernet only and any communication protocol, wired or wireless may be comprised. The communication line may be connected to further on-board processing means, and / or remote processing means, such as a cloud 180 based back-office server system.

[0128] Figure 8 shows a simplified overview of a flow chart for the data processing, identifying a low pass filter at the input for filtering out irrelevant frequencies. Typical detected frequencies of interest lies in the 0-lHz spectra for subsea surveys performed with a low speed AUV. Typical AUV speed is 3-4 knots. Dependent on the environment and survey task other LP (Low Pass) or BP (Band Pass) filter segments may be used. Anything above such frequencies tends to be noise. The signal may further be processed to individual components in a decomposition stage Le. going from 8 measurements to 3 orthogonal field components . Finally it may be advantageous to Down Sample the data to reduce the output data rate to save storage space and optimize later processing. It may be advantageous to let the Decomp and Down sample modules switch place and do the Down Sample before the Decomp, to for example save processing time and power.

[0129] The person skilled in the art realizes that the present disclosure is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims.

[0130] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.

Claims

AMENDED CLAIMS received by the International Bureau on 10 July 2024 (10.07.2024)1. An electrode system for passive measurement of an electric potential field in seawater: an underwater vehicle (100), four or more electrodes (20) arranged on the underwater vehicle (100), wherein the electrode system is c h a r a c t e r i z e d b y : the exact installation positions of the electrodes (20) are arranged to be paired in electrode pairs (21, 22) and the pairs are set up to measure orthogonal electric field components in three dimensions, wherein: the electrodes (20) are arranged in a first group (a, e, d, h) in the front, and a second group (b, f, c, g) in the back of the underwater vehicle, the electrodes of each group being arranged with a spread above and below the centre of mass of the underwater vehicle (100) and on starboard respective port side, the electrode pairs (21, 22) are arranged to form one or more loops (1-2-3-4, 5-6-7-8) with maximum physical electrode spread, wherein the setup of the electrode pairs provides measuring around a complete loop, and a controller (150) provided with means to reconstruct the electric field potential in the underwater vehicle (100) coordinate system and to reconstruct any erroneous electrode pair (21, 22) measurement from 3 other electrode pair measurements (21, 22) within any one of the one or more measurement loop (1-2-3-4, 5-6-7-8).

2. The system according to claim 1, wherein the electrode pairs (21, 22) are configured in loops (1-2-3-4, 5-6-7-8) in a substantially symmetrical configuration, within the loop (1-2-3-4, 5-6-7-8) and / or between the loops (1-2-3-4, 5-6- 7-8).

3. The system according to any of the previous claims, wherein the underwater vehicle (100) is a non-conductive underwater vehicle.

4. The system according to any of the previous claims, wherein the electrodes (20) are arranged slightly recessed (r) with the surface of the underwater vehicle (100).

5. The system according to any of the previous claims, wherein the numbers of electrodes (20) are 8, and25AMENDED SHEET (ARTICLE 19)the electrodes (20) are arranged in two loops (1-2-3-4, 5-6-7-8) of 4 electrodes (20) each.

6. The system according to any of the previous claims, wherein the electrodes (20) are separated at its furthest possible extent, longitudinal, horizontal and vertical.

7. The system according to claim 6, wherein the electrodes are paired such that any first electrode (20) of an electrode pair (21) within the first or within the second group are chosen to be paired with a second electrode (20) being arranged with the longest orbital length outside the underwater vehicle (100) from the first electrode (20).

8. The system according to claim 6 or 7, wherein the electrodes are paired such that any first electrode (20) of an electrode pair (22) in the longitudinal direction are chosen to be paired with a second electrode (20) having one electrode (20) in the first group, and the other electrode(20) in the second group.

9. The system according to claim 8, wherein the electrode pairs (22) in the longitudinal direction are paired such that the distance between the electrode in the first group and the electrode in the second group is further increased by an orbital location difference outside the underwater vehicle (100) between the two electrode (20) positions.10.The system according to any of the previous claims, wherein the underwater vehicle is one of an AUV, ROV, or UID.

11. A method for passive measurement of electric field in a fluid, the method being comprising the steps of: providing an underwater vehicle (100), and the method being c h a r a c t e r i z e d b y : arranging the electrodes (20) , in a first group (a, e, d, h) in the front, and a second group (b, f, c, g) in the back of the underwater vehicle, , arranging the electrodes of each group with a spread above and below the centre of mass of the underwater vehicle (100) and on starboard respective port side,26AMENDED SHEET (ARTICLE 19)installing four or more electrodes (20) in the underwater vehicle (100) and configuring the electrode pairs (21,22) to form one or more loops (1-2-3-4, 5-6-7-8) with maximum physical electrode spread wherein the setup of the electrode pairs provides measuring around a complete loop, mapping the exact installation positions of the electrodes (20), pairing the electrodes (20) in electrode pairs (21, 22) wherein the pairs are set up to measure orthogonal electric field components in three dimensions, and when the underwater vehicle (100) is operating in a fluid: measuring orthogonal electric field components in three dimensions by the electrode pairs (21, 22), reconstructing the electric field potential in the underwater vehicle (100) coordinate system, and reconstructing any erroneous electrode pair (21, 22) measurement from 3 other electrode pair measurements (21, 22) within any of the one or more measurement loop (1-2-3-4, 5-6-7-8).12.The method according to claim 11, wherein the mapping comprising defining offset coordinates of the electrodes (20) relative the underwater vehicle (100).13.The method according to any of the previous claims 11 - 12, further comprising: arranging the electrodes (20) in a first group in the front, and a second group in the back, of the underwater vehicle, and configuring the electrode pair (22,22) loops (1-2-3-4, 5-6-7-8) in a substantially symmetrical configuration, within the loop (1-2-3-4, 5-6-7-8) and / or between the loops (1-2-3-4, 5-6-7-8).14.The method according to any of the previous claims 11 - 13, further comprising: installing of the electrodes (20) slightly recessed (r) with the surface of the underwater vehicle (100).15.The method according to any of the previous claims 11 - 14, wherein when the numbers of electrodes (20) are 8: arranging the electrodes (20) in two loops (1-2-3-4, 5-6-7-8) of 4 electrodes (20) each.16.The method according to any of the previous claims 11 - 15, further comprising:27AMENDED SHEET (ARTICLE 19)separating the electrodes (20) at its furthest possible extent, longitudinal, horizontal and vertical.17.The method according to claim 16, further comprising: pairing the electrodes such that any first electrode (20) of an electrode pair (21) within the first or within the second group are chosen to be paired with a second electrode (20) being arranged with the longest orbital length outside the underwater vehicle (100) from the first electrode (20).18.The method according to claim 16 or 17, further comprising: pairing the electrodes such that any first electrode (20) of an electrode pair (22) in the longitudinal direction are chosen to be paired with a second electrode (20) having one electrode (20) in the first group, and the other electrode(20) in the second group.19.The method according to claim 18, further comprising: pairing the electrode pairs (22) in the longitudinal direction such that the distance between the electrode in the first group and the electrode in the second group is further increased by an orbital location difference outside the underwater vehicle (100) between the two electrode (20) positions.20.The method according to any of the previous claims 11 - 19, wherein the fluid is seawater.21.The method according to any of the previous claims 11 - 20, further comprising: providing a field perturbation model of the underwater vehicle (100).22.The method according to any of the previous claims 11 - 21, further comprising: when there are 8 electrodes (20): arranging the electrodes (20) according to the following matrix:28AMENDED SHEET (ARTICLE 19)wherein the "front" and "aft" subscripts refer to front of and back positions respectively, the"port" and "stb" subscripts refer to port and starboard positions respectively, and the "up" and "down" subscripts refer to positions above or below the center of mass of the underwater vehicle. 23.The method according to claim 22, further comprising: forming a measurement matrix according to measurement loops (1-2-3-4, 5-6-7-8) in the following matrix:

24. The method according to claim 22, further comprising: confirming a valid measurement by comparing the measured and analysed results from the two measurement loops (1-2-3-4, 5-6-7-8).29AMENDED SHEET (ARTICLE 19)

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