[0001] The present invention relates to a submersible sound speed measurement apparatus of the type that, for example, emits an acoustic signal through a medium to be measured and measures a time-of-flight of the acoustic signal through the medium. The present invention also relates to a method of submersibly measuring sound speed, the method being of the type that, for example, measures a time-of- flight of an acoustic signal through a medium to be measured.

[0002] In a marine environment, it is known to deploy so-called direct reading sound speed sensors, for example the Micro X SV sound velocity probe available from AML Oceanographic, Canada, or the UltraSV sound velocity sensor available from Valeport Limited, UK, in order measure the velocity of sound in the marine environment. In this regard, such sensors comprise a frame or housing having, for example a piezoelectric transducer positioned at a first end of the frame and a reflecting surface located at a second end of the frame opposite the piezoelectric transducer. The sensor is immersed in a liquid, for example seawater, and the piezoelectric transducer is controlled to emit an acoustic pulse. The acoustic pulse propagates from an active face of the piezoelectric transducer, through a matching layer disposed over the piezoelectric transducer, into the medium in which the sensor is immersed, in this example the seawater. The pulse is then reflected back towards the piezoelectric sensor by the reflecting surface. [0003] The reflecting surface is located at a predetermined distance from the from the piezoelectric transducer, the distance being known to a micron degree of accuracy by virtue of a calibration process involving immersing the sensor in distilled water at a precisely known temperature, known to a millikelvin (mK) level of accuracy. Upon arrival of the reflected pulse at the piezoelectric transducer, the acoustic energy is converted to electrical energy, resulting in an electrical signal being generated by the piezoelectric transducer. The electrical signal is digitally sampled and processed in order to determine the exact time-of-flight (signal phase) of the emitted and reflected acoustic pulse. With the a priori knowledge of the electronic time delays and distance between the piezoelectric transducer and the reflecting surface measured during the calibration process, i.e. the propagation distance, and a one-way travel time of the acoustic pulse (half the measured time- of-flight), it is possible to calculate the speed of sound in the immersion medium.

[0004] However, a drawback of the direct reading type of sound speed sensor is that it is extremely reliant upon a very precisely known and stable acoustic path length for all operating pressures and temperatures. Consequently, the piezoelectric transducer must be mounted very rigidly in order to ensure that it does not move between calibration, which for convenience is typically conducted at atmospheric pressure, and when the sensor is in use, which can be at pressures of several tens of megapascals (MPa). In this respect, the physical dimensions of the sensor are maintained as stable as possible so as to provide stability of sound speed in the materials of the piezoelectric transducer, thereby obviating changes in path length or at least rendering path length changes predictable.

[0005] According to a first aspect of the present invention, there is provided a submersible sound speed measurement apparatus comprising: a frame carrying an acoustic transducer and an acoustically reflecting surface respectively disposed at opposite ends thereof; a processing resource operably coupled to the acoustic transducer and arranged to measure a forward time-of-flight to the acoustically reflecting surface through a first medium and a reverse time-of-flight through a second medium; wherein the processing resource is also arranged to use the measured forward time-of-flight to calculate a sound speed value in respect of the first medium and to use the measured reverse time-of-flight to apply an offset to the calculation of the sound speed value.

[0006] The acoustically reflecting surface may comprise a first reflecting region at a first distance from the acoustic transducer and a second reflecting region at a second distance from the acoustic transducer.

[0007] The acoustic transducer may be coupled to the frame via the second medium. The second medium may be a compressible buffer material. [0008] A reverse direction speed of sound within the second medium may be known. [0009] The buffer material may be titanium putty or Tungsten loaded epoxy.

[0010] The frame may comprise a first end and a second end, and the acoustically reflecting surface may be disposed at the first end at a distance from an inwardly facing mounting surface of the second end; the distance may be known.

[0011] The processing resource may comprise a record of the distance between the acoustically reflecting surface and the inwardly facing mounting surface to a micron level of accuracy. [0012] The second medium may be disposed at the second end of the frame between the acoustic transducer and the inwardly facing mounting surface.

[0013] The processing resource may be configured to use the measured reverse time-of-flight to calculate the offset.

[0014] The offset may be calculated using the measured reverse time-of-flight and calibration data. The calibration data may comprise a temperature specific speed of sound in respect of the second medium. [0015] The offset may be a reverse direction distance from the acoustic transducer to the frame.

[0016] The processing resource may be arranged to calculate a distance from the acoustically reflecting surface to the acoustic transducer using the known distance between the acoustically reflecting surface and the inwardly facing mounting surface and the calculated offset. [0017] The speed of sound in respect of the first medium may be calculated using the calculated distance between the acoustically reflecting surface and the acoustic transducer and the measured forward time-of-flight. [0018] The apparatus may further comprise a temperature sensor operably coupled to the processing resource.

[0019] The offset may be calculated using the measured reverse time-of-flight and a calibrated speed of sound in respect of the second medium; the calibrated speed of sound may be calibrated for temperature.

[0020] The apparatus may further comprise a data store arranged to store calibration data; the calibration data may comprise a plurality of speeds of sound in the second medium and a plurality of temperatures respectfully corresponding to the plurality of speeds of sound.

[0021] The processing resource may be configured to measure a phase in respect of the reverse time-of-flight. [0022] The processing resource may be configured to use the calculated phase to calculate a phase centre with respect to the acoustic transducer.

[0023] The processing resource may be configured to calculate the offset using the measured reverse time-of-flight and the calculated phase centre.

[0024] According to a second aspect of the present invention, there is provided a method of submersibly measuring sound speed, the method comprising: immersing a sound speed apparatus in a first medium, the sound speed apparatus comprising a frame, and an acoustic transducer and an acoustically reflecting surface respectively disposed at opposite ends of the frame; measuring a forward time-of- flight through the first medium from the acoustic transducer to the acoustically reflecting surface; measuring a reverse time-of-flight from the acoustic transducer through a second medium disposed between the acoustic transducer and an end of the frame proximal to the acoustic transducer; calculating a sound speed value in respect of the first medium using the measured forward time-of-flight; and applying an offset to the calculation of the sound speed value using the measured reverse time-of-flight.

[0025] The offset may be a reverse distance from the acoustic transducer to the end of the frame proximal to the acoustic transducer. [0026] The method may further comprise: using the offset and a known length between the opposite ends of the frame to calculate a distance between the acoustically reflecting surface and the acoustic transceiver.

[0027] The method may further comprise: calculating the sound speed value using the measured forward time-of-flight and the distance between the acoustically reflecting surface and the acoustic transceiver.

[0028] The method may further comprise: calibrating the sound speed apparatus, calibration comprising: measuring the distance between the acoustic transducer and the end of the frame proximal to the acoustic transducer; immersing the sound speed apparatus to a predetermined depth in a calibration immersion medium of known composition, the measured distance being unaffected by the immersion at the predetermined depth; making a plurality of reverse time-of-flight measurements at a plurality of different temperatures, respectively; and calculating a plurality of sound speeds corresponding to the plurality of reverse time-of-flight measurements, respectively, using the measured distance.

[0029] The method may further comprise: measuring a temperature of the first medium; retrieving a stored sound speed corresponding to the measured temperature; and calculating the offset using the measured reverse time-of-flight and the retrieved sound speed. [0030] According to a third aspect of the present invention, there is provided a method of calibrating a sound speed measurement apparatus comprising a frame and an acoustic transducer and an acoustically reflecting surface respectively disposed at opposite ends of the frame, the method comprising: measuring a distance between the acoustic transducer and an end of the frame proximal to the acoustic transducer; immersing the sound speed measurement apparatus to a predetermined ambient pressure in a calibration immersion medium, the measured distance being unaffected by the immersion at the predetermined depth; making a plurality of reverse time-of-flight measurements at a plurality of different temperatures, respectively; and calculating a plurality of sound speeds corresponding to the plurality of reverse time-of-flight measurements, respectively, using the measured distance.

[0031] It is thus possible to provide an apparatus and method capable of measuring the speed of sound in an immersion medium with greater accuracy than known sensors. In this regard, the apparatus and method obviates or at least mitigates the effects of pressure due to immersion on position of a transducer within a frame and therefore the acoustic propagation distance of the acoustic pulse. [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:

Figure 1 is a schematic diagram of a submersible sound speed sensor apparatus constituting an embodiment of the invention;

Figure 2 is a flow diagram of a method of calibrating the sound speed sensor apparatus of Figure 1 constituting another embodiment of the invention;

Figure 3 is a schematic plot of waveforms present when the apparatus of Figure 1 is in operation; and Figure 4 is a flow diagram of a method of submersibly measuring sound speed constituting a further embodiment of the invention.

[0033] Throughout the following description identical reference numerals will be used to identify like parts.

[0034] Referring to Figure 1 , a submersible sound speed measurement apparatus 100 comprises a frame 102. The frame 102 comprises an end cap 104 disposed opposite a mounting member 106, for example a mounting plate. A set of separating rods 108, for example carbon fibre rods, extend between the end cap 104 and the mounting member 106. The set of separating rods 108 are fixed rigidly at a first end thereof to the end cap 104 and at a second end thereof to the mounting member 106. In another embodiment, the separating rods 108 can be formed from other materials, for example glasses or ceramics, which by their nature are isotropic and provide a predictable response to isotropic pressure, unlike carbon fibre composites.

[0035] A ceramic transducer, for example a piezoelectric transducer 110, is mounted on, for example fixed to, the mounting member 106 by a buffer material 112, for example a titanium putty or a tungsten loaded epoxy. In this example, the buffer material 112 is compressible. The buffer material 112 lies adjacent an inwardly facing mounting surface 116 of the mounting member 106 and is sandwiched between a rear surface 118 of the piezoelectric transducer 110 and the inwardly facing mounting surface 116. A polycarbonate encapsulation layer 120 overlies a forwardly facing active surface 122 of the piezoelectric transducer 110 and extends towards and abuts the inwardly facing mounting surface 116 so as to overlie lateral side surfaces of the piezoelectric transducer 110 and the buffer material 112. The encapsulation layer 120 serves, in this example, as an acoustic impedance matching medium. In another example, the encapsulation layer 120 can be formed from Zirconia.

[0036] An inwardly facing surface of the end cap 104 is, in this example an acoustically reflecting surface 124. A processing resource 126, for example a computing apparatus, such as a Personal Computer (PC), is operably coupled to the piezoelectric transducer 110 using a suitable interface 128. The processing resource 126 is also operably coupled to a storage medium 127 for storing calibration data. Additionally, a temperature sensor 130 is operably coupled to the buffer material 112, so as to be in thermal communication with the buffer material, and to the processing resource 126 using another suitable interface 130.

[0037] In some examples, the acoustically reflecting surface 124 can be configured to have more than one reflecting surface at respective different distances from the forwardly facing active surface 122, thereby enabling differences between multiple reflections from the different reflecting surfaces to be used to calculate sound velocity in respect of the medium, when immersed in the medium, between the reflecting surfaces. This serves to make the measurement of sound velocity independent of the forwardly facing active surface 122. In this regard, the acoustically reflecting surface 124 can comprise a first reflecting region (not shown) at a first distance from the acoustic transducer and a second reflecting region (not shown) at a second distance from the acoustic transducer.

[0038] The sensor apparatus 100 is immersible in a fluid, for example a liquid, such as seawater. However, before the sensor apparatus 100 can be used, the sensor apparatus 100 is calibrated. Calibration is performed in the following manner.

[0039] Referring to Figure 2, the sensor apparatus 100 is immersed (Step 200) in a liquid of known composition constituting a calibration immersion medium, for example pure water, at a predetermined starting temperature and a predetermined pressure, dictated by the environment in which the sensor apparatus 100 is to be used, for example typical temperatures and pressures of the ocean. It should be appreciated that the temperature can be negative. The processing resource 126 then triggers (Step 202) the emission of an acoustic waveform, for example an acoustic pulse, by the piezoelectric transducer 110. Turning to Figure 3, the emitted acoustic pulse 300 propagates in a forward direction 134 (Figure 1) towards the acoustically reflecting surface 124. The acoustic pulse 300 also propagates in a reverse direction 136 towards the mounting surface 116. [0040] Although not shown, the rear surface 118 of the piezoelectric transducer 100 comprises matching layers to reduce energy emitted rearwardly that subsequently would get reflected forward to the piezoelectric transducer 110, because such reflected energy can destructively interfere with a forwardly transmitted waveform. Furthermore, the buffer material 112 serves, in part, to absorb the rearwardly propagating energy. Nevertheless, some of the rearwardly propagating energy reaches the mounting surface 116 and is reflected thereby and received (Step 204) by the piezoelectric transducer 100 as a rearwardly reflected waveform, for example a rearwardly reflected pulse 302. The processing resource 126 receives a digitised and processed version of the rearwardly reflected pulse 302 and records a time and phase of arrival of the rearwardly reflected pulse. The acoustic energy propagating rearwardly and reflected by the mounting surface 116 has a reverse direction time-of-flight, T _r , associated therewith, which is calculated (Step 206) by the processing resource 126 using the time at which the acoustic pulse was triggered and the time of arrival of the rearwardly reflected pulse 302.

[0041] The reverse time-of-flight, T _r , is a two-way travel time and so the processing resource calculates a one-way travel time, which is half of the reverse time-of-flight, or takes this fact into account when calculating sound speed. The reverse direction distance, d _r , between the piezoelectric transducer 110 and the mounting surface 116 is accurately known to a micron level of accuracy. The reverse speed of sound, v _r , through the buffer material 112 can therefore be calculated (Step 208) as follows:

[0042] The processing resource 126 then stores the measured sound speed and temperature of the buffer material 112 and then determines (Step 210) whether the calibration process is complete or whether further measurements are required. In this respect, if the calibration process is complete, no further measurements are made. Otherwise, the temperature of the water in which the sensor apparatus 100 is immersed is changed (Step 212), for example increased (or decreased, depending upon the initial temperature of the water), and the apparatus is allowed to settle to reach thermal equilibrium with the environment in which it is immersed. The above steps (Steps 200 to 210) are then repeated in respect of the new temperature. The above process continues until the calibration of the sensor apparatus 100 is complete, i.e. sound speeds have been measured for all operating temperatures of interest. In this example, the temperatures are set in one-degree increments, the increments being to a millikelvin or microkelvin level of accuracy. However, other, more precise, levels of accuracy can be employed depending upon operational requirements.

[0043] Once calibrated, the sensor apparatus 100 is ready for use. In operation (Figure 4), the sound speed sensor apparatus 100 is immersed in a first medium, for example seawater at a desired depth and hence pressure. A force is therefore naturally applied to the piezoelectric transducer 110 and so movement of the piezoelectric transducer 110 relative to the acoustically reflecting surface 124 and the mounting surface 116 occurs. The processing resource 126 triggers (Step 400) emission of a waveform, for example an acoustic pulse 300 (Figure 3) into the first medium, which propagates in a forward direction towards the acoustically reflecting surface 124. However, as mentioned above, the acoustic pulse 300 also propagates rearwardly towards the mounting surface 116 and is reflected by the mounting surface 116 back towards the piezoelectric transducer 110, whereupon the acoustic energy of the reflected rearward pulse 302 is converted to electrical energy, digitised and processed for receipt (Step 402) by the processing resource 126. As in the case of the calibration process, the processing resource 126 records the time of receipt, t _b , of the reflected rearward pulse 302. The acoustic energy of the acoustic pulse 300 travelling in the forward direction is reflected by the acoustically reflecting surface 124, yielding a reflected acoustic pulse 304, and propagates back towards the piezoelectric transducer 110. A received reflected forward acoustic pulse 306, namely the acoustic energy of the reflected acoustic pulse 304, is received (Step 404) by the piezoelectric transducer 110, digitised and processed. The time of receipt, t _r , of the received reflected forward acoustic pulse 306 is recorded and the processing resource 126 calculates a forward time-of-f light, T _f , of the acoustic pulse generated by the piezoelectric transducer 110.

[0044] The processing resource 126 then, cooperating with the temperature sensor 130, measures (Step 406) the temperature of the buffer material 112. The processing resource 126 uses the temperature of the buffer material 112 when accessing the stored calibration data (Step 408) and determines a sound speed for the current temperature of the buffer material 112, to a sufficient level of accuracy, for example the closest degree Kelvin. Using the sound speed data and equation (1) above, in rearranged form, the reverse direction distance, d _r , or offset is calculated (Step 410), namely the distance between the piezoelectric transducer 110 and the mounting surface 116. To enhance accuracy of the measurement of the reverse time-of-flight, T _r , the processing resource 126 can, in some examples, measure the phase of the received rearwardly propagating acoustic signal. Optionally, the processing resource 126 can calculate a phase centre with respect to the piezoelectric transducer 110. The phase centre calculated can be used with the measured reverse time-of-flight, T _r , by the processing resource 126 to calculate the offset with improved accuracy. [0045] In a like manner to the computation of the calibration data, the processing resource 126 takes advantage of the relationship between speed, distance and time to calculate the speed of sound propagating through the first, immersion, medium: [0046] where V _f is the speed of sound through the first medium, which in this example is the immersion medium, d _f is the distance between the piezoelectric transducer 110 and the acoustically reflecting surface 124, and T _f is the two-way travel time of the acoustic pulse 300 in the forward direction. [0047] The distance, d _f , between the piezoelectric transducer 110 and the acoustically reflecting surface 124 can be calculated, because the overall distance between the acoustically reflecting surface 124 and the mounting surface 116, D, is known. As such, the offset, d _r , is subtracted from the overall distance, D, to yield (Step 412) the propagation distance, d _f , between the piezoelectric transducer 110 and the acoustically reflecting surface 124.

[0048] Using the calculated distance, d _f , between the piezoelectric transducer 110 and the acoustically reflecting surface 124, in conjunction with equation (2), the processing resource 126 calculates (Step 414) the speed of sound, Vf, in the first medium. Hence, it can be seen that the reverse time-of-flight, T _r , is used to apply the offset to the calculation of the speed of sound (through the first medium).

[0049] This process (Steps 400 to 414) can be repeated as often as is required to measure the speed of sound within the first medium. [0050] The skilled person should appreciate that the above-described implementations are merely examples of the various implementations that are conceivable within the scope of the appended claims. Indeed, the skilled person should appreciate that, for example, the end cap 104 can be formed from any suitable material in order to enhance acoustic reflectivity of the acoustically reflecting surface 124. Indeed, the material can be any suitable material for the environment in which measurement is to be made. As such, the material should not dissolve, corrode, erode and/or react with its environment and should ideally provide good acoustic impedance contrast to the fluid in which measurement is to be made, for example titanium or ceramics.

[0051] It should also be appreciated that whilst in the above examples, the distance between the acoustically reflecting surface and the inwardly facing mounting surface is calculated, it is also possible additionally or alternatively to calculate a calibration coefficient, for example speed per unit time. Also, in some examples it is desirable to account for processing-related latencies, for example electronic delays and processing delays. [0052] In some embodiments, it may be desirable to calculate a time offset as opposed to a distance offset and use the time offset in an analogous manner to the distance offset when determining the propagation speed.