GB2521681A - Underwater leak detection apparatus, underwater leak detection system and method of detecting an underwater leak of a fluid - Google Patents

Abstract

An underwater leak detection apparatus (100) comprises a housing (102) having a plane intersecting a pitch axis and a roll axis of the housing (102), a transducer (104) arranged to ensonify a volume of a first fluid, at least two sensor elements (108) substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements (108), and a processing resource (200) operably coupled to the at least two sensor elements (108), the processing resource (200) being arranged to support a correlator (304) and a leak detection module (310). The correlator (304) is arranged to receive the signal and to correlate the data in respect of the at least two sensor elements (108), thereby generating correlation output data. The leak detection module (310) is arranged to analyse the correlation output data in respect of the at least two sensor elements (108) and identify data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid. The leak detection module (310) is further arranged to generate an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.

Description

UNDERWATER LEAK DETECTION APPARATUS, UNDERWATER LEAK

DETECTION SYSTEM AND METHOD OF DETECTING AN UNDERWATER

LEAK OF A FLUID

[0001] The present invention relates to an underwater leak detection apparatus of the type that, for example, ensonifies a volume of a first fluid in order to detect a plume-like presence of a second fluid therein. The present invention also relates to an underwater leak detection system of the type that, for example, ensonifies a volume of a first fluid in order to detect a plume-like presence of a second fluid therein. The present invention also relates to a method of detecting an underwater leak of a fluid, the method being of the type that, for example, ensonifies a volume of a first fluid in order to detect a plume-like presence of a second fluid therein.

[0002] In the field of oilfield services, in particular underwater oilfields, it is important to detect leaks within an underwater oilfield facility, including from assets disposed on the seabed or elsewhere underwater within the facility, for example from seabed infrastructure, such as pipelines, manifolds, or even leaks directly from the seabed caused by natural seepage, or in the context of subsea endeavours in general as a result of man-made subsea storage activity.

[0003] One of the most pressing objectives in the oil and gas industry today is to provide the ability automatically to detect and localise small-to-medium sized hydrocarbon leaks over wide areas with alert times that are in the order of magnitude of minutes after leak initiation.

[0004] One known technique to detect leaks employs a so-called static' wide-area sensor, known as the Automatic Leak Detection Sonar (ALDS). The ALDS comprises a single sensor that provides automatic monitoring, and generates alerts and localises small leaks over millions of square metres centred on the position of the single sensor.

[0005] Multi-beam Echosounder (MBES) have also been used to infer the presence of leak flares' directly from the use of bathymetry. However, MBES suffers from relatively low spatial resolution compared with side-scan sonar imagery, but provides robust bathymetry. Detection of the presence of leaks from reflectivity imagery alone, using side-scan sonar, has also been employed, but this technique lacks height/bathymetry information. A further known technique is Phase Differencing Bathymetry Sonar (PDBS), sometimes referred to as interferometric sonar, but this technique lacks robust bathymetry for non-smooth surfaces, such as leak-like plumes.

[0006] Another objective is to reduce the size and increase the mobility of imaging sensors so that they can be carried by a Remotely Operated Vehicle (ROV) or an Autonomous Underwater Vehicle (AUV) for use during a survey or inspection phases of entire pipeline systems [0007] According to a first aspect of the present invention, there is provided an underwater leak detection apparatus, comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; a transducer arranged to ensonify a volume of a first fluid; at least two sensor elements substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements; a processing resource operably coupled to the at least two sensor elements, the processing resource being arranged to support a correlator and a leak detection module; wherein the correlator is arranged, when in use, to receive the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and the leak detection module is arranged, when in use, to analyse the correlation output data in respect of the at least two sensor elements and identify data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and the leak detection module is further arranged, when in use, to generate an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.

[0008] The data of the correlation output data indicative of the presence of the substantially elongate volume of the second fluid may correspond to a poor correlation.

[0009] The data in respect of the at least two sensor elements may correspond to receipt of a reflected sonar signal in respect of a reflection of the ensonified volume of the first fluid.

[0010] The substantially elongate volume of the second fluid may extend away from a background surface; the substantially elongate volume of the second fluid may be substantially upstanding with respect to the background surface. The background surface may be an underwater terrain. The underwater terrain may be a seabed.

[0011] The correlator may be arranged, when in use, to generate a cross-correlation coefficient in respect of the received signals of the at least two sensor elements. The cross-correlation coefficient may be a normalised cross-correlation coefficient. The normalised cross-correlation coefficient may be an averaged normalised cross-correlation coefficient.

[0012] The data indicative of the presence of the substantially elongate volume of the second fluid may constitute an acoustically anomalous volume that may be significantly different to an environment of the elongate volume of the second fluid.

[0013] The apparatus may further comprise: an elongate hydrophone element extending substantially in parallel with the roll axis of the housing; wherein the elongate hydrophone element may be arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.

[0014] The data of the another signal may be indicative of high reflectivity in respect of the substantially elongate volume of the second fluid.

[0015] The processing resource may be arranged to use the data received in respect of the elongate hydrophone element to determine confidence in the correlation output data indicative of the presence of the substantially elongate volume of a second fluid in the ensonified volume of the first fluid.

[0016] The apparatus may further comprise a source of geospatial data.

[0017] The processing resource may be arranged to enrich the data indicative of a presence of the substantially elongate volume of a second fluid with geospatial data.

[0018] The processing resource may be arranged to enrich the data in respect of the elongate hydrophone element with geospatial data. The geospatial data may be provided by the source of geospatial data.

[0019] The leak detection module may be arranged to analyse the geospatial data associated with the data indicative of a presence of the substantially elongate volume of a second fluid and the geospatial data associated with the data in respect of the elongate hydrophone element indicative of high reflectivity and identify a substantial correspondence of the geospatial data.

[0020] The at least two sensor elements may also be Bathymetry sensor elements.

[0021] According to a second aspect of the present invention, there is provided an underwater leak detection system, comprising: a top-side processing resource; an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; a transducer arranged to ensonify a volume of a first fluid; at least two sensor elements substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements; and a processing resource operably coupled to the at least two sensor elements; and a correlator arranged, when in use, to receive the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; wherein the top-side processing resource is arranged, when in use, to generate graphical output data representing the correlation output data.

[0022] The top-side processing resource may be arranged to represent graphically data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid.

[0023] The top-side processing resource may support a leak detection module arranged, when in use, to analyse the correlation output data in respect of the at least two sensor elements and identify graphically the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid.

[0024] The underwater leak detection apparatus may further comprise: an elongate hydrophone element extending substantially in parallel with the roll axis of the housing; wherein the elongate hydrophone element may be arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.

[0025] The leak detection module may be arranged to generate graphical output data in respect of the elongate hydrophone element and to incorporate the graphical output data in respect of the correlation output data into the graphical output data in respect of the elongate hydrophone element.

[0026] According to a third aspect of the present invention, there is provided a method of detecting an underwater leak of a fluid, the method comprising: providing an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; and at least two sensor elements substantially upstanding with respect to the plane; ensonifying a volume of a first fluid; generating respective signals comprising data in respect of the at least two sensor elements; receiving the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and analysing the correlation output data in respect of the at least two sensor elements; identifying data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and generating an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.

[0027] According to a fourth aspect of the present invention, there is provided a method of detecting an underwater leak of a fluid, the method comprising: providing an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; and at least two sensor elements substantially upstanding with respect to the plane; ensonifying a volume of a first fluid; generating respective signals comprising data in respect of the at least two sensor elements; receiving the signal; correlating the data in respect of the at least two sensor elements, thereby generating correlation output data; and generating graphical output data at a top-side representing the correlation output data.

[0028] It is thus possible to provide an underwater leak detection apparatus, an underwater leak detection system and a method of detecting an underwater leak of a fluid that provides survey data of high spatial resolution in respect of a terrain, but with definitive leak classification. Advantageously, the leak classification can be automated whilst still providing accurate results. Furthermore, the use of correlation coefficient data obviates or at least mitigates instances of so-called "false positives" when detecting leaks, which can occur when relying solely or substantially on reflectivity data to identify a leak. In addition to the detection of hydrocarbon leaks, this apparatus, system and method advantageously also find application in relation to leak monitoring in Carbon Capture and Storage (CCS) facilities of the near future, in particular in respect of marine CCS sites. Effective carbon capture and storage requires a zero tolerance policy for leakage at the storage sites, emphasising the need for precise measurement, monitoring and verification of the kind set forth herein.

[0029] At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is schematic diagram of an underwater leak detection apparatus implemented as an autonomous underwater vehicle, and constituting an embodiment of the invention; Figure 2 is a schematic diagram of functional elements of the underwater leak detection apparatus of Figure 1 in greater detail; Figure 3 is a block diagram of processing elements supported by a processing resource of the apparatus of Figure 2; Figure 4 is a schematic diagram of the autonomous underwater vehicle of Figure 1 having a substantially non-linear trajectory and surveying a part of a pipeline having a leak; Figure 5 is a flow diagram of operation of the apparatus of Figure 2 constituting another embodiment of the invention; Figure 6 is a schematic diagram of an underwater leak detection system constituting yet another embodiment of the invention; Figure 7 is a flow diagram of operation of the system of Figure 6 constituting a further embodiment of the invention; Figures 8 is a schematic diagram of side-scan imagery generated using data received from the autonomous underwater vehicle of Figure 1 and 6; and Figure 9 is a schematic diagram of correlation coefficient imagery generated using data received from the autonomous underwater vehicle of Figure 1 and 6.

[0030] Throughout the following description identical reference numerals will be used to identify like parts.

[0031] Referring to Figure 1, an underwater leak detection apparatus 100 comprises a housing 102, for example a waterproof housing, which houses a first acoustic transducer 104, a first longitudinal array of hydrophone elements 106 and a first upstanding array of hydrophone elements 108. In this example, the underwater leak detection apparatus 100 is an Autonomous Underwater Vehicle (AUV). However, the skilled person should appreciate that the housing 102 can be for any other suitable so-called "platform" for use underwater, for example an underwater Remotely Operated Vehicle (ROV) or a towed4ish platform.

[0032] In this example, the longitudinal hydrophone elements are provided as an array of such elements. The exact number of elements and/or dimensions thereof can be varied depending upon implementation requirements, for example at least one elongate hydrophone sensor element can be employed.

[0033] In relation to the first upstanding array of hydrophone elements 108, the first upstanding array of hydrophone elements 108 comprises at least two hydrophone elements. In addition to the first upstanding array of hydrophone elements 108 being used as set forth herein, this array can optionally be employed, in some examples, additionally to make Bathymetry measurements.

The exact number and dimensions of the sensor elements can also be varied depending upon implementation requirements. The first upstanding array of hydrophone elements 108 extends substantially upright with respect to the first longitudinal array of hydrophone elements 106. In some examples, a hydrophone element of the first longitudinal array of hydrophone elements 106 can be considered part of the at least two hydrophone elements of the first upstanding array of hydrophone elements 108.

[0034] It should be appreciated that the housing 102 of the underwater leak detection apparatus 100 has a body frame of reference that can be used to describe the relative positions of the housing 102, the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108. In this respect, the first longitudinal array of hydrophone elements 106 extends substantially in parallel with a roll axis of the housing 102. In the example of an elongate hydrophone element mentioned above, the dimensions of the elongate hydrophone element are such that the elongate hydrophone element can also extend substantially in parallel with the roll axis of the housing 102. A plane intersects the roll axis and a pitch axis of the housing 102 and the first upstanding array of hydrophone elements 108 extends away from the plane, for example substantially upstanding with respect to the plane.

[0035] In this example, the underwater leak detection apparatus 100 also comprises other components, for example batteries, a gyroscope, a propulsion system, a navigation system, fins, and a geolocation system. However, for the sake of clarity and conciseness of description, these parts (and other such parts) will not be described in further detail herein as they are not considered core to the

description of the following examples.

[0036] Turning to Figure 2, the underwater leak detection apparatus 100 further comprises a processing resource 200, for example one or more microprocessors, operably coupled to a memory and storage device 202, for example a digital memory and a hard disc drive. A transmitter unit 204 is also operably coupled to the processing resource 200. The transmitter unit 204 is coupled to the first acoustic transducer 104 constituting an acoustic signal projector array. The first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108 are also coupled to the processing resource 200. In this example, the first acoustic transducer 104, the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108 are disposed on a port side of the housing 102. The first acoustic transducer 104, the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108 are also replicated on a starboard side of the housing 102 by way of a second acoustic transducer 206, a second longitudinal array of hydrophone elements 208 and a second upstanding array of hydrophone elements 210, respectively. The second longitudinal array of hydrophone elements 208 and the second upstanding array of hydrophone elements 210 are operably coupled to the processing resource 200, and the second acoustic transducer 206 is operably coupled to the transmitter unit 204.

[0037] With reference to Figure 3, the underwater leak detection apparatus 100 will now be described in the context of the port side of the housing 102, namely in relation to the first acoustic transducer 104, the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108. However, it should be appreciated that the same structure applies in relation of the starboard side of the housing 102 in respect of the second acoustic transducer 206, the second longitudinal array of hydrophone elements 208 and the second upstanding array of hydrophone elements 210.

[0038] An Analogue-to-Digital Converter (ADC) and signal conditioning circuitry 300 is operably coupled to the first upstanding array of hydrophone elements 108 via a plurality of analogue signal channels 302, the smallest element of which is a single signal channel. The processing resource 200 supports a correlator 304 and the correlator 304 is operably coupled to the ADC and signal conditioning circuitry 300 via a plurality of data channels 305. In this example, each data channel corresponds to a respective analogue signal channel. The processing resource also supports a data combiner, buffer and stamping unit 306, which is operably coupled to the correlator 304. The processing resource 200 also supports a leak detection module 310, which is operably coupled to the data combiner, buffer and stamping unit 306. The underwater leak detection apparatus 100 also comprises a source of geospatial data 308, for example a navigation subsystem supported by the processing resource 200, which is operably coupled to the data combiner, buffer and stamping unit 306. However, it should be appreciated that the use of geospatial data in respect of the examples set forth herein is optional.

[0039] The leak detection module 310 comprises a low correlation decision unit 312 operably coupled to the data combiner, buffer and stamping unit 306 and a source of threshold data 314. The leak detection module 310 also comprises a comparison unit 316, an output of the low correlation decision unit 312 being coupled to a first input of the comparison unit 316. A second input of the comparison unit 316 is coupled to an output of a reflectivity decision unit 318. The reflectivity decision unit 318 is supported by the processing resource 200 and operably coupled to the first longitudinal array of hydrophone elements 106 via another ADC, signal conditioning circuitry and data processing logic (not shown in Figure 3) relating to reflectivity measurements.

[0040] The leak detection module 310 also comprises a match decision unit 320, an output of the comparison unit 316 being operably coupled to a first input of the match decision unit 320. It should be understood that use of the reflectivity decision unit 318 (and the comparison unit 316 and the match decision unit 320) is optional.

[0041] In operation (Figures 4 and 5), the underwater leak detection apparatus is powered-up and immersed in water, for example the sea. The underwater leak detection apparatus 100 is then remotely controlled to travel to an area of a seabed, constituting a terrain 400 to be surveyed. In this example, the terrain 400 relates to a part of an oilfield seabed infrastructure comprising a pipeline 402 having a leak 404. The leak 404 has a plume-like shape and constitutes a presence of a second fluid in a first fluid, the first fluid being seawater in this example. The leak 404 is substantially elongate in form and extends away from a background surface, for example an underwater terrain, such as a seabed, so as to be substantially upstanding with respect to the background surface. The substantially elongate volume of the second fluid constitutes an acoustically anomalous volume that is acoustically different to an environment of the elongate volume of the second fluid, for example has different acoustic properties to the environment of the elongate volume of the second fluid. The second fluid can be a liquid or a gaseous leak. The terrain 400 also comprises a number of terrain features 406, which are not of interest for the purposes of leak detection over the terrain 400.

[0042] Upon arrival of the underwater leak detection apparatus 100 at a boundary of the terrain 400 to be surveyed, the underwater leak detection apparatus 100 is instructed to enter into a survey mode, whereupon the transmitter unit 204 drives the first acoustic transducer 104, thereby causing the first acoustic transducer 104 to ensonify a volume of the sea, constituting the volume 408 of the first fluid mentioned above, as the underwater leak detection apparatus 100 travels over the terrain to be surveyed. The underwater leak detection apparatus 100 follows, in this example, a meandering path 401.

[0043] In relation to the ensonification, the transmitter unit 204 stores characteristics of an acoustic pulse to be transmitted, for example centre frequency, pulse duration, pulse type, pulse bandwidth and/or repetition rate. The acoustic pulse generated propagates through the seawater in a spatial region defined by the characteristics of the acoustic pulse and the geometry of the first acoustic transducer 104. Any reflections from within the ensonified volume of the first fluid constitute a reflected sonar signal, which is received (Step 500) by the first longitudinal array of hydrophone elements 106 and received (Step 502) by the first upstanding array of hydrophone elements 105.

[0044] In response to receipt of the reflected sonar signal, the first longitudinal array of hydrophone elements 106 translates the reflected sonar signal received into a plurality of analogue electrical signals, which are respectively digitised (Step 504) by the ADC associated with the first longitudinal array of hydrophone elements 106 and further processed by subjecting the outputs of the ADC to, for example, complex heterodyning, filtering and decimation in order to yield digital data. Similarly, the first upstanding array of hydrophone elements 108 receives the reflected sonar signal and translates the reflected sonar signal received into a plurality of analogue electrical signals, which are respectively digitised (Step 506) by the ADC 300 and further processed by subjecting the outputs of the ADC 300 to, for example, complex heterodyning, filtering and decimation in order to yield digital data.

[0045] At this stage, the data generated in respect of the reflected sonar signal received by the first longitudinal array of hydrophone elements 106 and/or the reflected sonar signal received by the first upstanding array of hydrophone elements 105 can be stored in the memory and storage device 202 for subsequent processing (optionally after enriching the data with geospatial data and/or time data), as will be described later herein in relation to another embodiment.

[0046] However, in this example, the digital data signals associated with the first longitudinal array of hydrophone elements 106 are then subjected to beam forming processing, for example mechanically focussed single-beam or multi-beam processing, or back-projection processing (Step 508). The processed data associated with the first longitudinal array of hydrophone elements 106 constitute reflectivity levels of backscattered acoustic signals and are processed (Step 512) by the reflectivity decision unit 318 in order to determine whether the reflectivity data corresponds to a leak. In this respect, the reflectivity data of interest can be data that is not indicative of a lack of a reflected sonar signal, for example reflectivity levels received that constitute high levels of reflectivity. To determine high reflectivity, a threshold value can be compared against.

[0047] The digital data signals associated with the first upstanding array of hydrophone elements 108 are subjected to cross-correlation (Step 510) by the correlator 304 so as to calculate an averaged normalised cross-correlation coefficient constituting output correlation data. In this respect, pairs of data signals are cross-correlated within a ping or frame to provide an estimate of the normalised cross-correlation coefficient. In this example, the values of the correlation output data range between 0 and 1, which is a feature of the type of normalised cross-correlation employed in this example. A value of zero indicates that the signals measured at the respective sensor elements of the first upstanding array of hydrophone elements 108 are spatially uncorrelated in an elevation plane defined by the first upstanding array of hydrophone elements 108 in respect of an intra-frame time/sample. In contrast: a value of unity indicates that the signals measured by the respective sensor elements of the first upstanding array of hydrophone elements 108 are identical subject to a time shift. An average of all the normalised cross-correlation data generated is created for each frame at each intra-frame time/sample. Once calculated, the averaged normalised cross-correlation coefficient is enriched with position data and/or time data obtained from the source of geospatial data 308, which generates longitude and latitude data, and applies position data and/or time data using any known suitable time-stamping technique. Likewise, the processed data relating to the first longitudinal array of hydrophone elements 106 mentioned above can also be enriched with position and/or time data, if desired. The position data can relate to orientation and/or spatial position of the underwater leak detection apparatus 100.

[0048] In order to determine automatically whether the averaged normalised cross-correlation coefficient represents poor or low correlation, the correlation output data is compared (Step 514) against a threshold value, for example 0.5, by the low correlation decision unit 312 in order to determine whether the correlation output data is sufficiently low to constitute identification of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid.

[0049] In order to increase confidence in the inference of the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid, the reflectivity data generated is used. In this regard, the output of the reflectivity decision unit 318 is used to determine if the inferred presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid corresponds to the reflected sonar signal received by the first longitudinal array of hydrophone elements 106 having a high reflectivity. To achieve this, the comparison unit 316 receives the output of the reflectivity decision unit 318 and compares (Step 516) the output with the indication of whether the correlation output data corresponds to a low correlation. Logic of the leak detection module 310, for example the match decision unit 320, determines (Step 518) whether a combination of low correlation and high reflectivity has been found. If the combination of low correlation and high reflectivity has been found, the match decision unit 320 generates (Step 520) alert data, constituting an alert, which is recorded by the memory and storage device 202 and/or communicated top-side in order to identify the detection of a leak. In this regard, the alert data can be enriched with the geospatial data associated with the correlation output data so that the position of the leak identified is known.

[0050] In another embodiment, as mentioned above, the data generated in respect of the reflected sonar signal received by the first longitudinal array of hydrophone elements 106 and/or the reflected sonar signal received by the first upstanding array of hydrophone elements 108 can be stored in the memory and storage device 202 for subsequent processing (optionally after enriching the data with geospatial data and/or time data).

[0051] In order to support an underwater leak detection system 600 (Figure 6) of this kind, the housing 102 of the underwater leak detection apparatus 100 comprises the ADC and signal conditioning circuitry 300 operably coupled to the first upstanding array of hydrophone elements 108 via a plurality of analogue signal channels 302, the smallest element of which is the single signal channel mentioned above. In this example, each data channel corresponds to a respective analogue signal channel.

[0052] The housing 102 of the underwater leak detection apparatus 100 also comprises another ADC and signal conditioning circuitry 604 operably coupled to the first longitudinal array of hydrophone elements 106 via a plurality of analogue signal channels 606, the smallest element of which is a single signal channel.

[0053] The underwater leak detection apparatus 100 additionally comprises a source of geospatial data 601 that is operably coupled to the processing resource 200, for example to a first stamping module 603 and a second stamping module 607 supported by the processing resource 200. The first stamping module 603 is operably coupled to the ADC 300 via a plurality of data channels 602, the first stamping module 603 also being operably coupled to the memory and data storage device 202 via another plurality of data channels 605. In this example, each data channel corresponds to a respective analogue signal channel. The second stamping module 607 is operably coupled to the ADC 604 via a plurality of data channels 608, the second stamping module 607 also being operably coupled to the memory and storage device 202 via another plurality of data channels 609.

In this example, each data channel corresponds to a respective analogue signal channel.

[0054] At the top-side 610, a top-side processing resource 612 is provided, which can be, for example, a computing apparatus, for example a Personal Computer (PC). In this example, the top-side processing resource 612 is operably coupled to or comprises a storage device 614, for example a hard disc drive. The top-side processing resource 612 supports an operating system for execution by functional hardware components of the top-side processing resource 612, which provides an environment in which application software can run. In this respect, the operating system serves to control the functional hardware components and resides between the application software and the functional hardware components.

[0055] A beam forming processing unit 616, supported by the top-side processing resource 612, is operably coupled to the storage device 614, the beam forming processing unit 616 also being coupled to an image processing engine 618 that is also supported by the top-side processing resource 612. The image processing engine 618 is operably coupled to an output device, for example a display device 620, such as a Liquid Crystal Display (LCD) device, which is coupled to the PC.

[0056] The top-side processing resource 612 supports the correlator 304 and the correlator 304 is operably coupled to the storage device 614 in order to receive a plurality of data channels 305. In this example, each data channel corresponds to a respective analogue signal channel. The top-side processing resource 612 also supports a data combiner and buffer 306, an input of which is operably coupled to the correlator 304. An output of the data combiner and buffer 306 is also coupled to the image processing engine 618. The top-side processing resource 612 also supports a leak detection module 310, which is operably coupled to the data combiner and buffer 306.

[0057] The leak detection module 310 comprises the low correlation decision unit 312 operably coupled to the output of the data combiner and buffer 306 and a source of threshold data 314. The leak detection module 310 also comprises the comparison unit 316, the output of the low correlation decision unit 312 being coupled to the first input of the comparison unit 316. The second input of the comparison unit 316 is coupled to the output of the reflectivity decision unit 318.

An input of the reflectivity decision unit 318 is coupled to the output of the beam forming processing unit 616.

[0058] The leak detection module 310 also comprises the match decision unit 320, the output of the comparison unit 316 being operably coupled to the first input of the match decision unit 320. The output of the match decision unit 320 is coupled to the image processing engine 618. It should be understood that the use of the leak detection module 310 and/or the reflectivity decision unit 318 is optional.

[0059] In operation (Figures 4 and 7), the underwater leak detection apparatus is powered-up and immersed in water, for example the sea. The underwater leak detection apparatus 100 is then remotely controlled to travel to an area of a seabed, constituting a terrain 400 to be surveyed. In this example, the terrain 400 relates to a part of an oilfield seabed infrastructure comprising a pipeline 402 having a leak 404. The leak 404 has a plume-like shape and constitutes a presence of a second fluid in a first fluid, the first fluid being seawater in this example. The leak 404 is substantially elongate in form and extends away from a background surface, for example an underwater terrain, such as a seabed, so as to be substantially upstanding with respect to the background surface. The substantially elongate volume of the second fluid constitutes an acoustically anomalous volume that is acoustically different to an environment of the elongate volume of the second fluid, for example has different acoustic properties to the environment of the elongate volume of the second fluid. The second fluid can be a liquid or a gaseous leak. The terrain 400 also comprises a number of terrain features 406, which are not of interest for the purposes of leak detection over the terrain 400.

[0060] Upon arrival of the underwater leak detection apparatus 100 at a boundary of the terrain 400 to be surveyed, the underwater leak detection apparatus 100 is instructed to enter into a survey mode, whereupon the transmitter unit 204 drives the first acoustic transducer 104, thereby causing the first acoustic transducer 104 to ensonify a volume of the sea, constituting the volume 408 of the first fluid mentioned above, as the underwater leak detection apparatus 100 travels over the terrain to be surveyed. The underwater leak detection apparatus 100 follows, in this example, a meandering path 401. In this respect, the transmitter unit 204 stores characteristics of an acoustic pulse to be transmitted, for example centre frequency, pulse duration, pulse type, pulse bandwidth and/or repetition rate. The acoustic pulse generated propagates through the seawater in a spatial region defined by the characteristics of the acoustic pulse and the geometry of the first acoustic transducer 104. Any reflections from within the ensonified volume of the first fluid constitute a reflected sonar signal, which is received (Step 500) by the first longitudinal array of hydrophone elements 106 and received (Step 502) by the first upstanding array of hydrophone elements 108.

[0061] In response to receipt of the reflected sonar signal, the first longitudinal array of hydrophone elements 106 translates the reflected sonar signal received into a plurality of analogue electrical signals, which are respectively digitised (Step 504) by the ADC associated with the first longitudinal array of hydrophone elements 106 and further processed by subjecting the outputs of the ADC to complex heterodyning, filtering and decimation in order to yield digital data.

Similarly, the first upstanding array of hydrophone elements 108 receives the reflected sonar signal and translates the reflected sonar signal received into a plurality of analogue electrical signal, which are respectively digitised (Step 506) by the ADC 300 and further processed by subjecting the outputs of the ADC 300 to complex heterodyning, filtering and decimation in order to yield digital data.

[0062] The processed data in respect of the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108 are then stored (Step 507) in the memory and storage device 202. Prior to storage, the processed data in respect of the first longitudinal array of hydrophone elements 106 and the first upstanding array of hydrophone elements 108 can be enriched by the processing resource 200 using longitude and latitude data and/or time data obtained from the source of geospatial data 601 so that the position of any leak subsequently identified is known. In this respect, the first stamping module 603 and/or the second stamping module 607 can enrich the processed data received in respect of the first longitudinal array of the hydrophone elements 106 and the first upstanding array of hydrophone elements 108 with position data and/or time data, for example by append said data to the processed data in accordance with a data structure definition. The position data can relate to orientation and/or spatial position of the underwater leak detection apparatus 100.

[0063] The data stored by the memory and storage device 202 can then be retrieved subsequently for post-processing. Similarly, in the case where the underwater leak detection apparatus 100 is a tethered Remotely Operated Vehicle (ROV) or towed platform, the data can be retrieved from the memory and storage device 202 via the tether.

[0064] In the present example, the data stored by the memory and storage device 202 is accessed for post-processing when the underwater leak detection apparatus 100 is retrieved from the water and the data stored by the memory and storage device 202 is transferred to the storage device 614 for use by the top-side processing resource 612.

[0065] At the top-side 610, the digital data associated with the first longitudinal array of hydrophone elements 106 is then retrieved from the storage device 614 and subjected to beam forming processing, for example mechanically focussed single-beam or multi-beam processing, or back-projection processing (Step 508) by the beam forming processing unit 616. The digital data associated with the first upstanding array of hydrophone elements 108 is also retrieved from the storage device 614 and subjected to cross-correlation (Step 510) by the correlator 304 so as to calculate an averaged normalised cross-correlation coefficient constituting output correlation data.

[0066] The processed digital data associated with the first longitudinal array of hydrophone elements 106 constitute reflectivity levels of backscattered acoustic signals. The image processing engine 618 receives the processed data in respect of the first longitudinal array of hydrophone elements 106 and generates (Step 700) a so-called waterfall image, which is built from layers respectively corresponding to each ping or frame representing a sonar ensonification and reception of reflected signals. The image builds from the top of the display device 620 downwards and then scrolls as the waterfall image continues to build. In this respect, waterfall imagery 800 (Figure 8) of reflectivity levels is generated. The data can be geocoded. One or more lines of the waterfall imagery displayed corresponds to the displacement of the underwater leak detection apparatus 100 in an "along-track" direction 802 in one dimension and the maximum range scale of ensonification in an "across-track" direction 804.

[0067] As mentioned above, the data associated with the first upstanding array of hydrophone elements 108 are, instead of being subjected to beam forming or back-projection processing, subjected to cross-correlation. In this respect, pairs of data signals are cross-correlated within the ping or frame to provide an estimate of the normalised cross-correlation coefficient. In this example, the values of the correlation output data range between 0 and 1, which is a feature of the type of normalised cross-correlation employed in this example. A value of zero indicates that the signals measured at the respective sensor elements of the first upstanding array of hydrophone elements 108 are spatially uncorrelated in an elevation plane defined by the first upstanding array of hydrophone elements 108 in respect of an intra-frame time/sample. In contrast, a value of unity indicates that the signals measured by the respective sensor elements of the first upstanding array of hydrophone elements 108 are identical subject to a time shift. An average of all the normalised cross-correlation data generated is created for each frame at each intra-frame time/sample.

[0068] The average normalised cross-correlation coefficient data generated by the correlator 304 is received by the combiner and buffer 306 in order to build data relating to the ping in respect of all combinations of elements from the array of sensor elements 108, which are then received by the image processing engine 618. The image processing engine 618 then uses the received data in respect of the first upstanding array of hydrophone elements 108 in order to generate (Step 702) waterfall imagery in respect of the first upstanding array of hydrophone elements 108 that is in precise spatial correspondence, for example in respect of along-track and across-track directions, with the imagery generated by the image processing engine 618 in respect of the first longitudinal array of hydrophone elements 106. This is achieved using the timestamp information used to enrich the average normalised cross-correlation coefficient data. The waterfall imagery in respect of the first upstanding array of hydrophone elements 108 is therefore graphically in registry with the waterfall imagery in respect of the first longitudinal array of hydrophone elements 106 (Figure 9). The waterfall imagery in respect of the first longitudinal array of hydrophone elements 106 comprises an indication of the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid. The data can also be geocoded. In another embodiment, the data in respect of the first upstanding array of hydrophone elements 108 can be incorporated into the waterfall imagery data associated with the first longitudinal array of hydrophone elements 106 in order to produce consolidated waterfall imagery.

[0069] However, in this example, the resulting imagery of Figures 8 and 9 are, as stated above, two co-registered waterfall images showing backscatter reflectivity levels, for each position of the underwater leak detection apparatus 100 in along- track 802 and across-track 804 directions, and average normalised cross-correlation coefficient values. The backscatter reflectivity imagery, sometimes referred to as side-scan imagery, contains imagery of many detailed features relating to, for example, the seabed terrain 400, but leaks may not be easily distinguishable from the side-scan imagery alone, for example the shape and/or intensity level, from other seabed features. In this example, the side-scan imagery reveals the pipeline 402 and the terrain features 406 as a meandering trace 806 and cluster traces 808 of representing high reflectivity. A high reflectivity zone 810 potentially identifies the leak 404.

[0070] The ability of an operator using the leak detection system 600 to identify leaks from side-scan imagery alone is hindered by the presence of non-leak features imaged by the side-scan sensor that only occur at the seabed and do not extent upwards into the water column. These seabed features are effectively localised at a particular elevation angle relative to sonar in the elevation plane for each intra-frame time/sample, for example the pipeline 402 and the terrain features 406 mentioned above being respectively displayed as the meandering trace 806 and the cluster traces 808. A scatter that is localised in this way tends to form a reflected signal that is highly correlated at the first upstanding array of hydrophone elements 105 corresponding to high average normalised cross-correlation coefficient values. However, as side-scan features, such as leaks, which naturally occur in a distributed manner within the water column] present themselves as far less correlated received signals, much lower average normalised cross-correlation coefficient values results in the region of the leak. In this respect, the pipeline 402 and the terrain features 406 are absent from the cross-correlation imagery of Figure 9, the pipeline 402 only being shown in broken lines in this example for the sake of providing a point of reference. A zone of low correlation 900 represents the leak 404, the correspondence between the zone of low correlation 900 in Figure 9 and the high reflectivity zone 810 in Figure 8 being indicative of the presence of the leak 404 with a greater degree of confidence than by inference from the side-scan imagery of Figure 8 alone.

[0071] Some of the functionality set forth above in relation to the previous embodiment can be employed in order to classify the leak 404. To this end, the processed data associated with the first longitudinal array of hydrophone elements 106, which constitutes reflectivity levels of backscattered acoustic signals, is then further processed in this example by the reflectivity decision unit 318 in order to determine whether the reflectivity data received correspond to a leak. In this respect, the reflectivity data of interest can be data that is not indicative of a lack of a reflected sonar signal, for example reflectivity levels received that constitute high levels of reflectivity. Moreover, the functionality of the reflectivity decision unit 318 is augmented in order to identify (Step 704) "plume-like" regions within the ensonified volume of the first fluid, for example using any suitable pattern matching algorithm.

[0072] In order to determine automatically whether the averaged normalised cross-correlation coefficient data represents poor or low correlation, the correlation output data is compared (Step 706) against a threshold value provided by the source of threshold data 314, for example about 0.5, by the low correlation decision unit 312 in order to determine whether the correlation output data is sufficiently low to constitute identification of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid. However, other thresholds can be employed, for example about 0.4 or about 0.3.

[0073] In order to increase confidence in the inference of the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid, the reflectivity data generated is used. In this regard, the output of the reflectivity decision unit 318 is used to determine if the inferred presence of the substantially elongate volume of the second fluid in the ensonified volume 408 of the first fluid corresponds to the reflected sonar signal received by the first longitudinal array of hydrophone elements 106 having a high reflectivity. To achieve this, the comparison unit 316 receives the output of the reflectivity decision unit 318 and compares (Step 708) the output with the indication of whether the correlation output data corresponds to a low correlation received from the low correlation decision unit 312. Logic of the leak detection module 310, for example the match decision unit 320, determines whether a combination of low correlation and high reflectivity (of plume-like structure) has been found. Where geospatial data is employed, a match represents a substantial correspondence of geospatial data associated with the reflectivity data with the geospatial data associated with the correlation data, for example within a few metres, such as less than 10 metres. If the combination of low correlation and high reflectivity has been found, the match decision unit 320 generates (Step 710) alert data, constituting an alert, which is received by the image processing engine 618. Optionally, but usefully, the match decision unit 320 can use the geospatial data with which the processed data associated with the first longitudinal array of hydrophone elements 106 has been enriched, and/or the processed data associated with the first upstanding array of hydrophone elements 108 has been enriched, in order to enrich the alert data so that the position of the leak identified is known.

[0074] The alert data received by the image processing engine 618 is used to identify graphically the presence, for example the location (Step 710), of the leak 404 in the waterfall imagery generated (Figure 9), for example by attributing a predetermined colour to the zone of low correlation 900.

[0075] In the above examples, reference has been made to the use of geospatial data. In this regard, the skilled person should appreciate that synchronism should be ensured between the clocks, or an offset known, associated with time stamping the acoustic data and time stamping the geospatial data.

[0076] The skilled person should appreciate that variations and modifications to the above examples are contemplated within the scope of the appended claims.

For example, the distribution of processing shared between the underwater leak detection apparatus 100 and the top-side processing resource 612 can be varied so that more pre-processing is performed by the underwater leak detection apparatus 100. In this respect, the correlation and beam-forming functionality can be performed by the underwater leak detection apparatus 100 instead of the top-side processing resource 612.

[0077] Alternative embodiments of the invention can be implemented as a computer program product for use with a computer system, the computer program product being, for example, a series of computer instructions stored on a tangible data recording medium, such as a diskette, CD-ROM, RaM, or fixed disk, or embodied in a computer data signal, the signal being transmitted over a tangible medium or a wireless medium, for example, microwave or infrared. The series of computer instructions can constitute all or part of the functionality described above, and can also be stored in any memory device, volatile or non-volatile, such as semiconductor, magnetic, optical or other memory device.

Claims (25)

1. Claims 1. An underwater leak detection apparatus, comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; a transducer arranged to ensonify a volume of a first fluid; at least two sensor elements substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements; a processing resource operably coupled to the at least two sensor elements, the processing resource being arranged to support a correlator and a leak detection module; wherein the correlator is arranged, when in use, to receive the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and the leak detection module is arranged, when in use, to analyse the correlation output data in respect of the at least two sensor elements and identify data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and the leak detection module is further arranged, when in use, to generate an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.
2. 2. An apparatus as claimed in Claim 1, wherein the data of the correlation output data indicative of the presence of the substantially elongate volume of the second fluid corresponds to a poor correlation.
3. 3. An apparatus as claimed in Claim 1 or Claim 2, wherein the data in respect of the at least two sensor elements corresponds to receipt of a reflected sonar signal in respect of a reflection of the ensonified volume of the first fluid.
4. 4. An apparatus as claimed in any one of Claim 1 or Claim 2 or Claim 3, wherein the substantially elongate volume of the second fluid extends away from a background surface, the substantially elongate volume of the second fluid being substantially upstanding with respect to the background surface.
5. 5. An apparatus as claimed in Claim 4, wherein the background surface is an underwater terrain.
6. 6. An apparatus as claimed in any one of the preceding claims, wherein the correlator is arranged, when in use, to generate a cross-correlation coefficient in respect of the received signals of the at least two sensor elements.
7. 7. An apparatus as claimed in Claim 6, wherein the cross-correlation coefficient is a normalised cross-correlation coefficient.
8. 8. An apparatus as claimed in any one of the preceding claims, wherein the data indicative of the presence of the substantially elongate volume of the second fluid constitutes an acoustically anomalous volume that is significantly different to an environment of the elongate volume of the second fluid.
9. 9. An apparatus as claimed in any one Claims 4 to 8, when dependent upon Claim 3, further comprising: an elongate hydrophone element extending substantially in parallel with the roll axis of the housing; wherein the elongate hydrophone element is arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.
10. 10. An apparatus as claimed in Claim 9, wherein the data of the another signal is indicative of high reflectivity in respect of the substantially elongate volume of the second fluid.
11. 11. An apparatus as claimed in Claim 9 or Claim 10, wherein the processing resource is arranged to use the data received in respect of the elongate hydrophone element to determine confidence in the correlation output data indicative of the presence of the substantially elongate volume of a second fluid in the ensonified volume of the first fluid.
12. 12. An apparatus as claimed in any one of the preceding claims, wherein the processing resource is arranged to enrich the data indicative of a presence of the substantially elongate volume of a second fluid with geospatial data.
13. 13. An apparatus as claimed in Claim 12, when dependent upon Claim 11, wherein the processing resource is arranged to enrich the data in respect of the elongate hydrophone element with geospatial data.
14. 14. An apparatus as claimed in Claim 13, wherein the leak detection module is arranged to analyse the geospatial data associated with the data indicative of a presence of the substantially elongate volume of a second fluid and the geospatial data associated with the data in respect of the elongate hydrophone element indicative of high reflectivity and identify a substantial correspondence of the geospatial data.
15. 15. An apparatus as claimed in any one of the preceding claims, wherein the at least two sensor elements are also Bathymetry sensor elements.
16. 16. An underwater leak detection system] comprising: a top-side processing resource; an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; a transducer arranged to ensonify a volume of a first fluid; at least two sensor elements substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements; and a processing resource operably coupled to the at least two sensor elements; and a correlator arranged, when in use, to receive the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; wherein the top-side processing resource is arranged, when in use, to generate graphical output data representing the correlation output data.
17. 17. A system as claimed in Claim 16, wherein the top-side processing resource is arranged to represent graphically data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid.
18. 18. A system as claimed in Claim 17, wherein the top-side processing resource supports a leak detection module arranged, when in use, to analyse the correlation output data in respect of the at least two sensor elements and identify graphically the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid.
19. 19. A system as claimed in Claim 17, wherein the underwater leak detection apparatus further comprises: an elongate hydrophone element extending substantially in parallel with the roll axis of the housing; wherein the elongate hydrophone element is arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.
20. 20. A system as claimed in Claim 19, wherein the leak detection module is arranged to generate graphical output data in respect of the elongate hydrophone element and to incorporate the graphical output data in respect of the correlation output data into the graphical output data in respect of the elongate hydrophone element.
21. 21. A method of detecting an underwater leak of a fluid, the method comprising: providing an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; and at least two sensor elements substantially upstanding with respect to the plane; ensonifying a volume of a first fluid; generating respective signals comprising data in respect of the at least two sensor elements; receiving the signal and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and analysing the correlation output data in respect of the at least two sensor elements; identifying data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and generating an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.
22. 22. A method of detecting an underwater leak of a fluid, the method comprising: providing an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; and at least two sensor elements substantially upstanding with respect to the plane; ensonifying a volume of a first fluid; generating respective signals comprising data in respect of the at least two sensor elements; receiving the signal; correlating the data in respect of the at least two sensor elements, thereby generating correlation output data; and generating graphical output data at a top-side representing the correlation output data.
23. 23. An underwater leak detection apparatus substantially as hereinbefore described with reference Figures ito 3, and/or 6.
24. 24. An underwater leak detection system substantially as hereinbefore described with reference to Figures 1, 2, 4 and/or 6.
25. 25. A method of detecting an underwater leak of a fluid substantially as hereinbefore described with reference to Figures 5 or 7.Amendments to the claims have been filed as follows Claims 1. An underwater leak detection apparatus, comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; a transducer arranged to ensonify a volume of a first fluid; at least two sensor elements substantially upstanding with respect to the plane and arranged to generate, when in use, respective signals comprising data in respect of the at least two sensor elements; a processing resource operably coupled to the at least two sensor elements, the processing resource being arranged to support a correlator and a leak detection module; wherein the correlator is arranged, when in use, to receive the respective signals and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and the leak detection module is arranged, when in use, to analyse a magnitude of the correlation output data in respect of the at least two sensor elements and N-identify data of the correlation output data indicative of a presence of a 0 substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and the leak detection module is further arranged, when in use, to generate an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid.2. An apparatus as claimed in Claim 1, wherein the data of the correlation output data indicative of the presence of the substantially elongate volume of the second fluid corresponds to a poor correlation.3. An apparatus as claimed in Claim 1 or Claim 2, wherein the data in respect of the at least two sensor elements corresponds to receipt of a reflected sonar signal in respect of a reflection of the ensonified volume of the first fluid.4. An apparatus as claimed in any one of Claim 1 or Claim 2 or Claim 3, wherein the substantially elongate volume of the second fluid extends away from a background surface, the substantially elongate volume of the second fluid being substantially upstanding with respect to the background surface.5. An apparatus as claimed in Claim 4, wherein the background surface is an underwater terrain.6. An apparatus as claimed in any one of the preceding claims, wherein the correlator is arranged, when in use, to generate a cross-correlation coefficient in respect of the received signals of the at least two sensor elements.7. An apparatus as claimed in Claim 6, wherein the cross-correlation coefficient is a normalised cross-correlation coefficient.8. An apparatus as claimed in any one of the preceding claims, wherein the substantially elongate volume of the second fluid constitutes an acoustically N-anomalous volume that is acoustically different to an environment of the elongate 0 volume of the second fluid.9. An apparatus as claimed in any one Claims 4 to 8, when dependent upon Claim 3, further comprising: an elongate hydrophone element extending substantially in parallel with the roll axis of the housing; wherein the elongate hydrophone element is arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.10. An apparatus as claimed in Claim 9, wherein the processing resource is arranged to support a reflectivity decision unit, the reflectivity decision unit using a threshold value to determine whether the data of the another signal is indicative of high reflectivity in respect of the substantially elongate volume of the second fluid.11. An apparatus as claimed in Claim 9 or Claim 10, wherein the processing resource is arranged to use the data received in respect of the elongate hydrophone element to determine confidence in the correlation output data indicative of the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid.12. An apparatus as claimed in any one of the preceding claims, wherein the processing resource is arranged to enrich the data indicative of a presence of the substantially elongate volume of the second fluid with geospatial data.13. An apparatus as claimed in Claim 12, when dependent upon Claim 11, wherein the processing resource is arranged to enrich the data in respect of the elongate hydrophone element with geospatial data. rN-14. An apparatus as claimed in Claim 13, wherein the leak detection module is 0 arranged to analyse the geospatial data associated with the data indicative of a presence of the substantially elongate volume of the second fluid and the geospatial data associated with the data in respect of the elongate hydrophone element indicative of high reflectivity and to identify whether the geospatial data associated with the data indicative of a presence of the substantially elongate volume of the second fluid spatially coincides substantially with the geospatial data associated with the data in respect of the elongate hydrophone element indicative of high reflectivity.15. An apparatus as claimed in any one of the preceding claims, wherein the at least two sensor elements are also Bathymetry sensor elements.16. An underwater leak detection system, comprising: the underwater leak detection apparatus as claimed in Claim 1; and a top-side processing resource arranged, when in use, to generate graphical output data representing the correlation output data.17. A system as claimed in Claim 16, wherein the top-side processing resource is arranged to represent graphically data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid.18. A system as claimed in Claim 17, wherein the top-side processing resource supports a leak detection module arranged, when in use, to analyse the correlation output data in respect of the at least two sensor elements and identify graphically the presence of the substantially elongate volume of the second fluid in the ensonified volume of the first fluid.19. A system as claimed in Claim 17, wherein the underwater leak detection 1-apparatus further comprises: an elongate hydrophone element extending substantially in parallel with the N-roll axis of the housing; wherein 0 the elongate hydrophone element is arranged to generate, when in use, another signal comprising data in respect of the elongate hydrophone element corresponding to receipt by the elongate hydrophone element of a reflected sonar signal in respect of a reflection in the ensonified volume of the first fluid.20. A system as claimed in Claim 19, wherein the leak detection module is arranged to generate graphical output data in respect of the elongate hydrophone element and to incorporate the graphical output data in respect of the correlation output data into the graphical output data in respect of the elongate hydrophone element.21. A method of detecting an underwater leak of a fluid, the method comprising: providing an underwater leak detection apparatus comprising: a housing having a plane intersecting a pitch axis and a roll axis of the housing; and at least two sensor elements substantially upstanding with respect to the plane; ensonifying a volume of a first fluid; generating respective signals comprising data in respect of the at least two sensor elements; receiving the respective signals and to correlate the data in respect of the at least two sensor elements, thereby generating correlation output data; and analysing a magnitude of the correlation output data in respect of the at least two sensor elements; identifying data of the correlation output data indicative of a presence of a substantially elongate volume of a second fluid in the ensonified volume of the first fluid; and generating an alert signal in response to identification of the data indicative of the presence of the substantially elongate volume of the second fluid. rN-22. A method of detecting an underwater leak of a fluid, the method comprising: 0 the method of detecting an underwater leak of a fluid as claimed in Claim 21;and generating graphical output data at a top-side representing the correlation output data.23. An underwater leak detection apparatus substantially as hereinbefore described with reference Figures 1 to 3, and/or 6.24. An underwater leak detection system substantially as hereinbefore described with reference to Figures 1, 2, 4 and/orG.25. A method of detecting an underwater leak of a fluid substantially as hereinbefore described with reference to Figures 5 or 7.