GB2592390A

GB2592390A - Underwater measurement apparatus and method of monitoring a target portion of a water column

Info

Publication number: GB2592390A
Application number: GB2002715.7A
Authority: GB
Inventors: Louis Culverhouse Thomas
Original assignee: Sonardyne International Ltd
Current assignee: Sonardyne International Ltd
Priority date: 2020-02-26
Filing date: 2020-02-26
Publication date: 2021-09-01
Also published as: GB202002715D0

Abstract

A method of monitoring a target portion (306, fig 3) of a water column comprises an acoustic transducer (108, fig1) emitting 506 a first acoustic signal therefrom and receiving a second acoustic signal backscattered from the target portion. A processing resource (104, fig 1) calculates 508 a Doppler shift in respect of the received second signal and then calculates 510 a velocity therefrom in respect of the target portion. An inertial measurement unit (106, fig 1) measures 210 a velocity thereof, the inertial measurement unit travelling with the acoustic transducer. The processing resource then uses the velocity of the inertial measurement unit to compensate 512 the velocity measured acoustically in respect of the target portion.

Description

UNDERWATER MEASUREMENT APPARATUS AND METHOD OF

MONITORING A TARGET PORTION OF A WATER COLUMN

[0001] The present invention relates to an underwater acoustic apparatus of the type that, for example, comprises an acoustic transducer to receive an acoustic signal from a water column in order to perform, for example, current profiling. The present invention also relates to a method of monitoring a target portion of a water column, the method being of the type that, for example, comprises receiving an acoustic signal from the target portion of the water column.

[0002] A so-called current profiler finds numerous applications in the field of underwater acoustics. The current profiler, for example of the kind described in US 5,208,785, measures backscatter intensities and water velocity over various ranges of distance relative to the measurement instrument. Different current profilers, more accurately referred to as Acoustic Doppler Current Profilers (ADCPs), are designed to measure water currents over different ranges of distance.

[0003] In addition to stationary applications, it is known to provide a moving platform, for example a vessel, such as a ship, Autonomous Underwater Vehicle (AUV) or a Remotely Operated Vehicle (ROV), with an ADCP to acquire current profile measurements. Such deployments require the current profile velocities to be compensated for motion of the vessel in order to calculate absolute current velocities.

[0004] In order to provide such compensation, it is necessary to determine the velocity of the ADCP. So-called "bottom tracking", which is the measurement of the velocity of the ADCP over an underwater bottom surface, such as a seabed, is generally accepted as the ideal way to implement such compensation. In this respect, reflections of acoustic signals received from the bottom surface are used to measure the velocity of the ADCP across the bottom surface. Typically, the ADCP employs an acquisition schedule that interleaves emission of acoustic signals to make current profile measurements and measurements to track the seabed.

[0005] However, in some circumstances, measurement of bottom track velocity can be compromised. For example, a lack of "bottom lock" due to altitude, i.e. the distance along the acoustic axis of an acoustic transducer to the bottom surface is so large that either emitted signals attenuate before reaching the bottom surface or reflected acoustic signals are too weak to be detected due to attenuation over the distance. Sometimes, bottom lock is not lost completely but is unreliable owing to a partially reflective seabed not reflecting the emitted acoustic signal sufficiently at times. Also, measured vessel speeds derived from a bottom track velocity measurement can be systematically erroneous due to a moving seabed.

[0006] In such circumstances, if the bottom lock is compromised, insufficient velocity data is available to separate the velocity of the vessel from the velocities of the target portion of the water column. In this regard, it is known to acquire GNSS data to overcome the lack of, or compromised, bottom lock in order to determine the velocity of the vessel. However, such an approach requires the vessel to be on the water surface in order to receive GNSS signals, i.e. it is unsuitable for vessels that are immersed. Additionally, the GNSS data is only reliable in conditions where the GNSS signal is not interrupted by propagation obstacles and sources of interference. Furthermore, without suitably accurate compensation measures, surface water conditions need to be relatively calm, i.e. the vessel should not experience appreciable levels of heave, pitch and/or roll, otherwise the vessel velocity is calculated on the basis of insufficient data, thereby rendering the calculated vessel velocity an inaccurate compensation value.

[0007] The foregoing challenges thus at best diminish the usefulness of the water current velocity measurements when sections of data are invalid. At worst, they provide the user with misleading velocities when the vessel velocity cannot be adequately compensated.

[0008] Additionally, the ADCP also has a velocity ambiguity limit for a given desired measurement range. This is a limitation on the maximum velocity the ADCP can measure at the desired measurement range. Without the benefit of the bottom track velocity to separate the current profile velocity from the vessel velocity, the combined velocity measured by the ADCP can readily exceed the velocity ambiguity limit resulting in an inability to measure velocities at the desired measurement range. In such circumstances, measurement data is either lost as explained above or the vessel is constrained to travel below a velocity dictated by the velocity ambiguity limit. The limiting effect of the velocity ambiguity limit on the maximum velocity at which the vessel can travel during a survey is exacerbated as the depth at which current profiling takes place increases. In this regard, the maximum depth at which current profiles can be measured is inversely proportional to the bandwidth of an acoustic transmit pulse emitted by the ADCP, while the velocity ambiguity limit is proportional to the bandwidth of the same acoustic transmit pulse. Consequently, in order to increase the measurable depth, the bandwidth of the acoustic transmit pulse is decreased, which in turn impacts on the velocity ambiguity limit, which reduces.

[0009] US patent 9,500,484 describes subsea navigation using ADCP data in the absence of Global Navigation Satellite System (GNSS) data, but does not mitigate 15 the problems described above.

[0010] According to a first aspect of the present invention, there is provided a method of monitoring a target portion of a water column, the method comprising: an acoustic transducer emitting a first acoustic signal therefrom and receiving a second acoustic signal backscattered from the target portion; a processing resource calculating a Doppler shift in respect of the received second signal and calculating a velocity therefrom in respect of the target portion; an inertial measurement unit measuring a velocity thereof, the inertial measurement unit travelling with the acoustic transducer; and the processing resource using the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.

[0011] The processing resource may project the velocity of the inertial measurement unit to an acoustic axis of the acoustic transducer.

[0012] The processing resource may translate the velocity of the inertial measurement unit measured using a first mode of measurement to an expected 30 value of an acoustically measured velocity of the acoustic transducer measured using a second mode of measurement.

[0013] The first mode of measurement may be in respect of an inertial frame of reference and the second mode of measurement may be in respect of an acoustic frame of reference. The translation of the velocity may be projection of the velocity of the inertial measurement unit in the inertial frame of reference to the acoustic frame of reference.

[0014] The translation of the velocity may comprise calculating the expected value of the acoustically measured velocity in units of frequency.

[0015] The method may further comprise: pre-calibrating a geometric offset between the inertial measurement unit and the acoustic transducer.

[0016] The method may further comprise: using the pre-calibrated geometric offset to compensate the velocity measured acoustically in respect of the target portion.

[0017] The method may further comprise: the acoustic transducer receiving a backscattered bottom track acoustic signal; the processing resource using the backscattered bottom track signal received to calculate a bottom track velocity in respect of the acoustic transducer; and the processing resource using the backscattered bottom track velocity instead of the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.

[0018] The processing resource may determine accuracy of the bottom track velocity; and the processing resource may compensate the velocity measured acoustically in respect of the target portion using the bottom track velocity in response to a determination that the bottom track velocity is accurate.

[0019] The processing resource may determine the accuracy of the bottom track velocity by comparing a difference between the bottom track velocity and the velocity of the inertial measurement unit with respect to a predetermined velocity deviation threshold value.

[0020] The method may further comprise: compensating the velocity measured acoustically in respect of the target portion in order to determine a true water current speed.

[0021] The method may further comprise: providing a Doppler Velocity Log comprising the acoustic transducer.

[0022] According to a second aspect of the present invention, there is provided a method of monitoring underwater acoustically measured bottom track velocities, the method comprising: an acoustic transducer emitting a first acoustic signal therefrom and receiving a second acoustic signal backscattered from a bottom surface; a processing resource calculating a Doppler shift in respect of the received second signal and calculating an acoustically measured bottom track velocity therefrom; an inertial measurement unit measuring a velocity thereof, the inertial measurement unit travelling with the acoustic transducer; deriving an inertially measured expected bottom track velocity in respect of the acoustic transducer; and the processing resource evaluating the acoustic bottom track velocity and the inertially measured expected bottom track velocity with respect to a predetermined criterion.

[0023] The method may further comprise: comparing a difference between the measured bottom track velocity and the velocity of the inertial measurement unit 20 with a predetermined velocity deviation threshold value constituting the predetermined criterion.

[0024] The method may further comprise: generating a bottom track compromise alert signal in response to the predetermined criterion being met.

[0025] The method may further comprise: selecting the acoustically measured bottom track velocity or the inertially measured expected bottom track velocity in response to the evaluation thereof.

[0026] The method may further comprise providing a marine vessel and equipping the marine vessel with the acoustic transducer, the processing resource and the inertial measurement unit, and performing the method as set forth above.

[0027] The vessel may be a surface vessel or an underwater vessel.

[0028] According to a third aspect of the present invention, there is provided a method of detecting movement of an underwater bottom surface, the method comprising: an acoustic transducer emitting a first acoustic signal therefrom and receiving a second acoustic signal backscattered from the bottom surface; a processing resource calculating a Doppler shift in respect of the received second signal and calculating an acoustically measured bottom track velocity therefrom; an inertial measurement unit measuring a velocity thereof, the inertial measurement unit travelling with the acoustic transducer; deriving an inertially measured expected bottom track velocity in respect of the acoustic transducer; and the processing resource evaluating the acoustic bottom track velocity and the inertially measured expected bottom track velocity with respect to a predetermined criterion.

[0029] According to a fourth aspect of the invention, there is provided an underwater monitoring apparatus programmable to monitor a target portion of a water column, the apparatus comprising: an acoustic transducer configured to emit a first acoustic signal therefrom and to receive a second acoustic signal backscattered from the target portion; a processing resource configured to calculate a Doppler shift in respect of the second reflection signal and to calculate a velocity therefrom in respect of the target portion; an inertial measurement unit configured to measure a velocity thereof, the inertial measurement unit accompanying the acoustic transducer; wherein the processing resource is configured to use the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.

[0030] According to a fifth aspect of the present invention, there is provided an Acoustic Doppler Current Profiler apparatus comprising the apparatus as set forth above in relation to the third aspect of the invention.

[0031] It is thus possible to provide a method and apparatus that obviates or at least mitigates the complete loss or compromise of bottom track velocity measurements when measuring the velocity of a fluid, for example in relation to a target portion of a water column. Additionally, velocities measured using the inertial measurement unit can be measured more frequently than velocities measured acoustically, resulting in improved data accuracy: data is available substantially at the same time as the velocity of the target portion of the water column is being measured acoustically. Furthermore, the availability of vessel velocity data in respect of an entire survey duration obviates or at least mitigates the limitations imposed upon measuring current profiles from a moving vessel by the velocity ambiguity limit. Furthermore, the method and apparatus permit compensation for vessel movement when measuring water velocity in a target portion of a water column by ADCP, irrespective of whether measurement is taking place from a surface or underwater vessel. Also, such applications are supported by a single arrangement of an acoustic measurement instrument and an inertial measurement unit, which can be separately housed and thus located separately from each other but linked by a known offset, or collocated and housed as a single instrument.

[0032] At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0033] Figure 1 is a schematic diagram of a vessel comprising an underwater acoustic apparatus constituting an embodiment of the invention; [0034] Figure 2 is a flow diagram of a method of monitoring subsea acoustically measured bottom track velocities implemented by the apparatus of Figure 1 and constituting another embodiment of the invention; [0035] Figure 3 is a schematic diagram of the apparatus of Figure 1 measuring a velocity of a target portion of a water column when in range of a stable bottom surface in accordance with the method of Figure 5; [0036] Figure 4 is a graph comparing inertially measured velocities and acoustically measured velocities with time and changing bottom surface conditions; [0037] Figure 5 is a flow diagram of a method of measuring a target portion of a water column incorporating the method of Figure 2 and constituting a further embodiment of the invention; [0038] Figure 6 is a schematic diagram of the apparatus of Figure 1 measuring in accordance with the method of Figure 5 when out of range of a stable bottom surface; and [0039] Figure 7 is a schematic diagram of the apparatus of Figure 1 measuring 5 in accordance with the method of Figure 5 when in range of an unstable bottom surface.

[0040] Throughout the following description, identical reference numerals will be used to identify like parts.

[0041] Referring to Figure 1, a vessel 100, for example a marine vessel, such as an Autonomous Underwater Vehicle (AUV), is travelling through a body of water, for example the sea. The vessel 100 comprises an underwater apparatus capable of current profiling. The apparatus comprises an acoustic measurement unit 102 operably coupled to a processing resource 104, for example an application processor, such as a BCM2835 series application processor available from Broadcom Inc., the processing resource 104 being operably coupled to an inertial measurement unit 106. The vessel 100 also comprises, inter alia, a propulsion unit (not shown) and other features. However, for the sake of simplicity and clarity of description, these features will not be described in any further detail herein owing to these features not being central to understanding the examples set forth herein.

Indeed, the vessel 100 is simply a vehicle to transport the acoustic measurement unit 102, the processing resource 104 and the inertial measurement unit 106, and it should be appreciated that the AUV is simply one example of a typical vehicle that can be employed and other vessel types can be employed, for example a surface vessel or a Remotely Operated Vehicle (ROV).

[0042] In this example, the acoustic measurement unit 102 is an Acoustic Doppler Current Profiler (ADCP) adapted, for example by way of a firmware update, additionally to perform Doppler Velocity Log (DVL) measurements. Furthermore, in this example, the ADCP is provided with the inertial measurement unit 106 and the processing resource 104 in a single housing. In other examples, a DVL provided with an inertial measurement unit can be adapted by way of firmware update to perform additionally ADCP. A SPRINT-Nay 700 available from Sonardyne International Limited, UK, is one such instrument. The acoustic measurement unit 102 of the SPRINT-Nay 700 comprises a first acoustic transducer 108, a second acoustic transducer 110, a third acoustic transducer 112, and a fourth acoustic transducer 114. However, the examples set forth herein will be described in relation to one acoustic transducer, for example the first acoustic transducer 108, although the skilled person should appreciate that the examples described can be applied to two or more acoustic transducers, for example the first and second acoustic transducers 108, 110, the first, second and third acoustic transducers 108, 110, 112, or the first, second, third and fourth acoustic transducers 108, 110, 112, 114.

[0043] Referring to Figures 2 and 3, the vessel 100 is deployed (Step 200) and is travelling at a velocity having a velocity vector, U. The SPRINT-Nay 700 is placed in a measurement mode and begins operation. The acoustic measurement unit 102, in this example the first acoustic transducer 108 of the SPRINT-Nay 700, emits (Step 202) a bottom track acoustic signal 300 from the first acoustic transducer 108 thereof at a projection angle, 0, subtended between an acoustic axis of the first acoustic transducer 108 and the velocity vector, U, of the vessel 100. In this example, the bottom, for example the seabed 302 is within range of the first acoustic transducer 108 and stable. Consequently, a portion of the bottom track acoustic signal 300 is backscattered by the seabed 302 back to the acoustic transducer 108 of the SPRINT-Nay 700, which receives (Step 204) the portion of the bottom track acoustic signal that is backscattered by the seabed 302. The acoustic measurement unit 102 measures (Step 206) the Doppler shift associated with the portion of the bottom track acoustic signal backscattered and communicates the measured Doppler shift to the processing resource 104. The processing resource 104 then calculates (Step 208) a bottom track velocity having velocity vector, VA,BT, which is a calculated velocity of the vessel 100 using the bottom track acoustic signal 300. In this regard, the acoustically measured bottom track velocity, \An, is related to the velocity of the vessel 100, U, by the following expression: VA BT (1)

U COS 0

[0044] While the acoustic measurement unit 102 and the processing resource 104 are cooperating to calculate the bottom track velocity, VAST, the inertial measurement unit 106 of the SPRINT-Nay 700 measures (Step 210) the velocity, UiNs, of the vessel 100 mentioned above. In this regard, the inertial measurement unit 106 can make multiple estimates of the velocity, UiNs, of the vessel 100 in the time it takes the acoustic measurement unit 102 and the processing resource 104 to measure the bottom track velocity, VAST. The inertial measurement unit 106 communicates the measured velocity, UiNs, of the vessel 100 calculated thereby to the processing resource 104 and the processing resource 104 then projects (Step 212) the calculated velocity, UiNs, of the vessel 100 onto the acoustic axis of the first acoustic transducer 108 in order to obtain an inertially measured expected bottom track velocity, VINS,BT. In this regard, the inertially measured velocity, UINS, of the vessel 100 measured using a first mode of measurement, which in this example is an inertial measurement technique, is translated to the inertially measured expected bottom track velocity, ViNs.B-r, which is an expected value of the acoustically measured bottom track velocity, VAST, measured using a second mode of measurement, which in this example is an acoustic measurement technique. In this example, the inertially measured expected bottom track velocity, ViNs,BT, is expressed in units of frequency in order to match the natural units of measurement of the acoustically measured bottom track velocity, VAST. It should also be appreciated that, in this example, the first mode of measurement is in respect of an inertial reference frame of the inertial measurement unit 106 and the second mode of measurement is in respect of an acoustic reference frame of the first acoustic transducer 108.

[0045] The processing resource 104 then evaluates (Step 214) the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, VINS,I3T, in order to determine whether the acoustically measured velocity, WEIL is compromised, for example as a result of the bottom being out of range of the first acoustic transducer 108 or the bottom surface moving. In this regard, the processing resource 104 evaluates (Step 214) the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, VINS,IBT, with respect to a predetermined criterion, for example a threshold value. In this example, the processing resource 104 calculates a difference between the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, VINs,BT, and then compares the calculated difference with a predetermined velocity deviation threshold value, AV.

[0046] However, the skilled person should appreciate that the above technique involving the comparison of the difference between the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, VINs,B-r, with the predetermined velocity deviation threshold value, AV, is merely one simple example of a suitable technique. Other techniques can be employed to determine whether the acoustically measured velocity, VAST, is compromised, which take into account the respective measurement accuracies associated with the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, ViNs,e-r. For example, the difference between the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, ViNstT can be analysed to determine if the difference is statistically significant, statistically significant calculated differences being indicative of compromised bottom track.

[0047] Optionally, the above statistical analysis can be preceded by one or more of the following techniques to determine whether or not bottom track exists without reference to the inertially measured expected bottom track velocity, ViNs,B-r. Such techniques include analysis of the autocorrelation of the backscattered bottom track acoustic signal mentioned above, a low autocorrelation "score" being indicative of an absence of bottom track. Another technique comprises analysing velocity measurements in respect of more than one of the acoustic transducers 108, 110, 112, 114 in order to determine whether the respective velocities measured are consistent with each other. Additionally or alternatively, the signal-to-noise ratio of the bottom track acoustic signal can be analysed, a low value being indicative of an absence of bottom track.

[0048] Referring to Figure 4, when the acoustically measured bottom track velocity, VAST, is not compromised 400, the inertially measured expected bottom track velocity, ViNs jyr, does not differ greatly from the acoustically measured bottom track velocity, VAST, and so evaluation by comparing the acoustically measured bottom track velocity, VAST, and the inertially measured expected bottom track velocity, ViNst-r, enables compromise of the acoustically measured bottom track velocity, \kin, to be detected, for example by comparison of the difference therebetween and the predetermined velocity deviation threshold value, AV. In contrast, when the acoustically measured bottom track velocity, VA,B-r, is compromised 402, the inertially measured expected bottom track velocity, VINS,BT, deviates significantly from the acoustically measured bottom track velocity, VAST.

[0049] The above process (Steps 200 to 214) is continually repeated in order to detect promptly compromise of the acoustically measured bottom track velocity, VA.13-1.

[0050] Turning to Figure 5, the acoustic measurement unit 102 of the SPRINT-Nay 700 is configured to perform ADCP, which is the primary reason in this example for acoustically measuring the bottom track velocity, VAST. It should be appreciated though that, in this example, the bottom track velocity, Win, is also used to assist measurement of the velocity, U, of the vessel 100 inertially by the inertial measurement unit 106. As will be appreciated by the skilled person, when profiling water currents in a water column, for example in respect of a target portion 306 of the water column to be measured, using any suitable measurement technique it is necessary to compensate for the velocity, U, of the vessel 100, when measuring the velocity, W, in respect of the water in the target portion 306, because the velocity actually measured in respect of the target portion 306 is the combined velocities of the water in the target portion 306, W, and the velocity of the vessel 100, U, projected onto the acoustic axis of the first acoustic transducer 108, i.e. (W-FU) cos A. As such, the acoustically measured bottom track velocity, VAST, needs to be accurate to be able to remove the contribution of the velocity of the vessel 100 to the measured velocity, (W+U) cos 0, in respect of the target portion 306.

[0051] In this example, the acoustic measurement unit 102 multiplexes measurement and verification of the bottom track velocity, VAST, with the measurement of the velocity, (W+U) cos 0, in respect of the target portion 306, for example the measurement and verification of the bottom track velocity, VAST, is alternated with measurement of the velocity, (W+U) cos 0, in respect of the target portion 306, or more generally speaking the measurement and verification of the bottom track velocity, WEIL is alternated with current profiling measurements.

[0052] As such, following evaluation (Step 214) of the acoustically measured bottom track velocity, VAST, the processing resource 104 determines (Step 500) whether the acoustically measured bottom track velocity, VAST, is compromised and in the event that the acoustically measured bottom track velocity, VAST, is compromised, the processing resource 104 selects (Step 502) the inertially measured expected bottom track velocity, ViNs,B+, for subsequent use in calculating the true velocity, W, in respect of the target portion 306 by compensating for the velocity of the vessel 100, U, instead of using the compromised acoustically measured bottom track velocity, VAST. Otherwise, the acoustically measured bottom track velocity, VAST, is selected (Step 504) for subsequent use. In this regard, in some examples, the determination of the accuracy of the acoustically measured bottom track velocity, VAST, can yield an internal alert having a value to indicate selection of the inertially measured expected bottom track velocity, ViNs,B+, or the acoustically measured bottom track velocity, VA,B-r.

[0053] Thereafter, and referring briefly back to Figure 3, the first acoustic transducer 108 also emits (Step 506) a profiling acoustic signal 304 to ensonify the target portion 306 of the water column. The target portion 306 backscatters a portion of the emitted profiling acoustic signal 304 and the acoustic measurement unit 102 measures (Step 508) a Doppler shift associated therewith. The measured Doppler shift is communicated to the processing resource 104 and the processing resource 104 then calculates (Step 510) an uncompensated velocity, VADcp, in respect of the ensonified target portion 306, corresponding to the combined velocities of the water in the target potion 306 and the velocity of the vessel 100, (W+U) cos O. In this regard, the uncompensated velocity, VADep, has the following relationship with the velocity, U, of the vessel 100 and the true velocity, W, of the target portion 306 accounting for the velocity of the vessel 100: U-FW V_ ADCP (2)

COS B

[0054] The processing resource 104 then compensates (Step 512) the uncompensated velocity, VADcp, using the selected measure of the velocity of the vessel 100 in order to obtain the true water current speed in respect of the target portion 306. In this example, the selected measure of the velocity of the vessel 100 is the acoustically measured bottom track velocity, VA,BT, because the acoustically measured bottom track velocity, VABT, is not compromised owing to the seabed 302 being in range and stable. Consequently, the compensated velocity, W, in respect of the target portion 306 is calculated using equations (1) and (2) above: W= VADCP u VADCP-VA,BT cose COS [0055] The above process then continues (Steps 200 to 214 and 500 to 512) in 15 order to make further measurements of the target portion 306 of the water column or other target portions of the water column and the above example applies while the seabed 302 remains in range and stable.

[0056] Referring to Figure 6, in another example, the lock with the bottom, for example the seabed 302 is not possible, because the seabed 302 is out of range of the first acoustic transducer 108 of the SPRINT-Nay 700 in respect of DVL/bottom track measurements. As such, and referring to Figure 5, the acoustic measurement of the bottom track velocity, VA.BT, deviates considerably from the inertially measured expected bottom track velocity, ViNs jyr, and in this example the difference therebetween exceeds the predetermined velocity deviation threshold value, V. The processing resource 104 therefore determines (Step 500) that the acoustically measured bottom track velocity, VA.BT is compromised and selects the inertially measured expected bottom track velocity, ViNs,BT, instead of the acoustically measured bottom track velocity, \kin, for subsequent compensation (3) of the uncompensated velocity, VADep, in respect of the target portion 306 of the water column.

[0057] Thereafter, in accordance with the multiplexing scheme employed for current profiling, the first acoustic transducer 108 also em its (Step 506) the profiling acoustic signal 304 calculated to ensonify the target portion 306. The target portion 306 backscatters the portion of the emitted profiling acoustic signal 304 mentioned above in relation to the previous example and the acoustic measurement unit 102 measures (Step 508) the Doppler shift associated therewith. The measured Doppler shift is communicated to the processing resource 104 and the processing resource 104 then calculates (Step 510) the uncompensated velocity, VADcp, in respect of the ensonified target portion 306. The relationship between the uncompensated velocity, VADcp, the velocity, U, of the vessel 100 and the actual velocity, W, of the target portion 306 as set out in equation (2) above continues to apply.

[0058] However, the processing resource 104 then compensates (Step 512) the uncompensated velocity, \hoop, using the selected measure of the vessel velocity, which in this example is the inertially measured expected bottom track velocity, ViNs,g-r, because the acoustically measured bottom track velocity, VAST, is compromised owing to the seabed 302 being out of range of the first acoustic transducer 108. Consequently, the compensated velocity, W, in respect of the target portion 306 is calculated using equation (3) above, but using the inertially measured bottom track velocity, VINS,BT: w VADCP u VADCP-VINS,BT (4) cosO COS [0059] The above process then continues (Steps 200 to 214 and 500 to 512) in order to make further measurements of the target portion 306 of the water column or other target portions of the water column, the use of the inertially measured expected bottom track velocity, ViNs,BT, or the acoustically measured bottom track velocity, Win, to compensate the uncompensated velocity, VADcp, depending upon whether the acoustically measured bottom track velocity, VAST, is determined (Step 500) to be compromised.

[0060] Similarly, in another example (Figure 7), parts of the bottom, for example the seabed 302 are subject to movement. As such, though bottom-lock can be achieved, the acoustically measured bottom track velocity, VA,BT, does not reflect the true vessel speed, U. In this regard, if bottom track measurement is compromised as a result of movement of the seabed 302, for example at a seabed velocity, B, this is detected by the processing resource 104 by evaluating (Step 214) the acoustically measured bottom track velocity, \born with respect to the inertially measured expected bottom track velocity, ViNsirr, and determining (Step 500) whether the acoustically measured bottom track velocity, VA.BT, is compromised. Thereafter, either the acoustically measured bottom track velocity, VABT, or the inertially measured expected bottom track velocity, ViNs.B-r, is used to compensate the measured uncompensated velocity, VADcp, in respect of the target portion 306 of the water column (Steps 502 to 512) as described in the examples set forth above relating to bottom lock being achieved or unavailable.

[0061] Again, the above process continues (Steps 200 to 214 and 500 to 512) in order to make further measurements of the target portion 306 of the water column or other target portions of the water column, the use of the inertially measured expected bottom track velocity, ViNsirr, or the acoustically measured bottom track velocity, VA,BT, to compensate the uncompensated velocity, VADcp, depending upon whether the acoustically measured bottom track velocity, VA,E3T, is determined (Step 500) to be compromised.

[0062] For ADCP measurements, the depth of current profiling measurement is dependent upon the transmit bandwidth, B-rx, of the first acoustic transducer 108, but the transmit bandwidth requirement for depth of current profile measurement conflicts with the breadth of the measurable range of velocities that can be measured by the acoustic measurement unit 102.

[0063] In this respect, given the first acoustic transducer 108 has a slant range, s, subtending the projection angle, 8, the measurement depth, d, is given by: d = s sin 8 (5) [0064] The relationship between the slant range, s, and the transmission bandwidth, B-rx, of the first acoustic transducer 108 is: cxN (6)

S-

4xB-rx [0065] where c is the speed of sound in water, and N is the number of samples collected, for example 2500 samples.

[0066] Therefore, the measurement depth, d, relates to the transmission bandwidth, BT, of the first acoustic transducer 108 as follows: d = s sin 6 - GXN sine 4x BTx [0067] In contrast, the velocity ambiguity limit, ±vamb, which is the breadth of velocity measurements (expressed as a Doppler frequency shift), relates to the transmission bandwidth, B-N, as follows: Vamb 2xBT [0068] where BT is a bandwidth-time product, which is a constant, associated with transmission of a pulse of the profiling acoustic signal 304, and is typically a fixed integer value. Comparing equations (7) and (8), it can be seen that to increase depth of measurement, the transmission bandwidth, B-rx, needs to decrease, whilst to increase the range of measurable velocities, the transmission bandwidth, B+", needs to increase.

[0069] The ambiguity limit, ±vamb, expressed as a Doppler shift corresponds to a maximum actual measurable velocity, +vmax, and a minimum actual measurable velocity, -wain, which remain centred about a zero velocity. As a consequence of the location of the respective limits, when the vessel 100 and hence the SPRINT-Nay 700 move, the measurable velocity limits remain centred around zero, but are reached sooner owing to the acoustic measurement unit 102 measuring the combination of the water velocity and the velocity of the vessel 100, (W+U) cos 0, (7) _ BTx (8) in respect of the target portion 306. In this regard, compensation for the velocity of the vessel 100 effectively re-centres the otherwise uncompensated acoustically measured velocity, VADep, in respect of the target portion 306 about a zero velocity, thereby obviating the problem of premature attainment of one of the measurable velocity limits. When the inertially measured expected bottom track velocity, ViNss-r, is used alone or in combination with the acoustically measured bottom track velocity, VA,BT, it is usually possible to prevent the measurable velocity limits being prematurely reached through the uncompensated acoustically measured velocity, VADcp, comprising a component of the combination of the velocity of the target portion 306 and the velocity of the vessel 100, (W-FU) cos O. [0070] While specific examples of the invention have been described above, the skilled person will appreciate that many equivalent modifications and variations are possible. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0071] Although in the above examples, the inertial measurement unit 106 has been described as part of the SPRINT-Nay 700 instrument, the skilled person should appreciate that other instruments or instrument combinations can be employed. For example, the inertial measurement unit 106 need not be collocated with the acoustic measurement unit 102 and the inertial measurement unit 106 can be located remotely and/or separately from the acoustic measurement unit 102 provided a geometric offset between the acoustic measurement unit 102 and the inertial measurement unit 106 is known for the purpose of projecting the velocity, U, of the vessel 100 onto the acoustic axis of the first acoustic transducer 108 in order to obtain the inertially measured expected bottom track velocity, VINairn In this regard, the processing resource 104 is pre-calibrated with the geometric offset and is therefore used, in this example and other examples, to compensate the uncompensated velocity, VADcp, in respect of the target portion 306. Additionally or alternatively, the inertial measurement unit 106 need not comprise high precision ring laser gyroscopes and accelerometers as in the case of the SPRINT-Nay 700 and other alternative instruments can be employed to provide inertial measurement of the velocity of the vessel 100, for example micro-electromechanical systems (MEMS) gyroscopes and accelerometers.

[0072] More generally, although the above examples are described in the context of employing the SPRINT-Nay 700, the skilled person should appreciate that any underwater acoustic instrument capable of performing ADCP can be employed. In some examples, the functionality of one or more of the acoustic transducers of the acoustic instrument can be extended to permit performance of DVL measurements (bottom tracking). In this regard, and for the avoidance of doubt, it is envisaged that in some embodiments inertially measured vessel velocities can be used to provide compensation of ADCP measurements without the need to use acoustically measured bottom track measurements to provide compensation velocities, even when the acoustically measured bottom track velocities are considered uncompromised and therefore reliable. In other examples, it is envisaged that the inertially measured bottom track velocities are used to replace acoustically measured bottom track velocities for compensation purposes where the acoustically measured bottom track velocities are considered compromised.

[0073] In the above examples, the underwater current profiling apparatus described above is disposed in a vessel. As will be readily appreciated by the skilled person, the apparatus need not be provided as part of a vessel and can be supplied separately for installation in a vessel or attachment to a vessel that undertakes current profiling surveys while moving. Indeed, the measurement apparatus is nevertheless an underwater measurement apparatus, because the acoustic measurement component of the measurement apparatus is deployed underwater irrespective of type of vessel carrying the measurement apparatus.

[0074] Whilst the above examples refer to the velocity of the vessel 100, it should be appreciated that for the purposes of compensation of measurements, the measured velocity of the vessel should be mapped to the acoustic axis of an acoustic transducer taking into account a geometric offset between the acoustic axis of the acoustic transducer and the inertial measurement unit.

[0075] Although the technique to determine bottom track compromise is described herein in the context of current profile measurement, it should be appreciated that the same technique finds application in relation to moving bed detection.

[0076] As mentioned above, measurement of velocities using Doppler shifts are expressed in units of frequency. However, it should be appreciated that acoustically measured or expected acoustically measured velocities can be expressed and mathematically manipulated in other units, for example length and time, i.e. ms-1.

Claims

Claims 1. A method of monitoring a target portion of a water column, the method corn prising: an acoustic transducer emitting a first acoustic signal therefrom and receiving a second acoustic signal backscattered from the target portion; a processing resource calculating a Doppler shift in respect of the received second signal and calculating a velocity therefrom in respect of the target portion; an inertial measurement unit measuring a velocity thereof, the inertial measurement unit travelling with the acoustic transducer; and the processing resource using the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.
2. A method as claimed in Claim 1, wherein the processing resource projects the velocity of the inertial measurement unit to an acoustic axis of the acoustic transducer.
3. A method as claimed in Claim 1 or Claim 2, wherein the processing resource translates the velocity of the inertial measurement unit measured using a first mode 20 of measurement to an expected value of an acoustically measured velocity of the acoustic transducer measured using a second mode of measurement.
4. A method as claimed in any one of the preceding claims, further comprising: pre-calibrating a geometric offset between the inertial measurement unit and the acoustic transducer.
5. A method as claimed in Claim 4, further comprising: using the pre-calibrated geometric offset to compensate the velocity measured acoustically in respect of the target portion.
A method as claimed in any one of the preceding claims, further comprising: the acoustic transducer receiving a backscattered bottom track acoustic signal; the processing resource using the backscattered bottom track signal received to calculate a bottom track velocity in respect of the acoustic transducer; and the processing resource using the backscattered bottom track velocity instead of the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.
A method as claimed in Claim 6, wherein the processing resource determines accuracy of the bottom track velocity; and the processing resource compensates the velocity measured acoustically in respect of the target portion using the bottom track velocity in response to a determination that the bottom track velocity is accurate.
8. A method as claimed in Claim 7, wherein the processing resource determines the accuracy of the bottom track velocity by comparing a difference between the bottom track velocity and the velocity of the inertial measurement unit with respect to a predetermined velocity deviation threshold value.
9. A method as claimed in any one of the preceding claims, further comprising: compensating the velocity measured acoustically in respect of the target portion in order to determine a true water current speed.
10. A method as claimed in any one of the preceding claims, further comprising: providing a Doppler Velocity Log comprising the acoustic transducer.
11. A method of monitoring underwater acoustically measured bottom track velocities, the method comprising: an acoustic transducer emitting a first acoustic signal therefrom and receiving a second acoustic signal backscattered from a bottom surface; a processing resource calculating a Doppler shift in respect of the received second signal and calculating an acoustically measured bottom track velocity therefrom; an inertial measurement unit measuring a velocity thereof, the inertial measurement unit travelling with the acoustic transducer; deriving an inertially measured expected bottom track velocity in respect of the acoustic transducer; and the processing resource evaluating the acoustic bottom track velocity and the inertially measured expected bottom track velocity with respect to a predetermined criterion.
12. A method as claimed in Claim 11, further comprising: comparing a difference between the measured bottom track velocity and the velocity of the inertial measurement unit with a predetermined velocity deviation threshold value constituting the predetermined criterion.
13. A method as claimed in Claim 11 or Claim 12, further comprising: generating a bottom track compromise alert signal in response to the predetermined criterion being met.
14. A method as claimed in Claim 11 or Claim 12 or Claim 13, further comprising: selecting the acoustically measured bottom track velocity or the inertially measured expected bottom track velocity in response to the evaluation thereof.
15. An underwater monitoring apparatus programmable to monitor a target portion of a water column, the apparatus comprising: an acoustic transducer configured to emit a first acoustic signal therefrom and to receive a second acoustic signal backscattered from the target portion; a processing resource configured to calculate a Doppler shift in respect of the second reflection signal and to calculate a velocity therefrom in respect of the target portion; an inertial measurement unit configured to measure a velocity thereof, the inertial measurement unit accompanying the acoustic transducer; wherein the processing resource is configured to use the velocity of the inertial measurement unit to compensate the velocity measured acoustically in respect of the target portion.