WO2018178913A1

WO2018178913A1 - Dual accelerometer vector sensor

Info

Publication number: WO2018178913A1
Application number: PCT/IB2018/052167
Authority: WO
Inventors: Sérgio JESUS; Paulo FELISBERTO
Original assignee: Universidade Do Algarve
Priority date: 2017-03-31
Filing date: 2018-03-29
Publication date: 2018-10-04
Also published as: PT110003A

Abstract

The present invention relates to a dual a ccelerometer vector sensor (DAVS) device. Said device comprises two tri-axial accelerometers and one hydrophone moulded in one unit. Such device is useful to measure and improve the efficiency of methodologies used to perform geophysical acoustic surveys at sea. The DAVS has the potential to contribute to this aim in various ways, for example, owing to its spatial filtering capability, it may reduce the amount of post processing by discriminating the bottom from the surface reflections. Also its compact size allows easier integration with AUVs and hence facilitates the vehicle manoeuvrability compared to the classical towed arrays. Therefore, another object of the present invention refers to an Autonomous Underwater Vehicles (AUVs) comprising the DAVS device of the present invention for acquiring and monitoring underwater seismic data. The present invention is thus useful for acquiring and monitoring seismic data and belongs to the technical field of seismology, seismic prospecting, detecting and monitoring.

Description

Dual Accelerometer Vector Sensor

Technical domain of the invention

The present invention relates to a dual accelerometer vector sensor (DAVS) device. The device comprises at least two tri-axial accelerometer s and one hydrophone moulded in one unit and is useful for acquiring and monitoring seismic data.

Another aspect of the present invention relates to a method for acquiring and monitoring seismic data.

The DAVS device of the present invention can be integrated in an Autonomous Underwater Vehicles (AUVs) and therefore, another object of the present invention refers to such vehicles (AUVs) comprising the device of the present invention useful for acquiring and monitoring seismic data, in particular underwater seismic data.

Thus, another aspect of the present invention relates to a method for acquiring and monitoring seismic data, in particular underwater seismic data.

The present invention belongs to the technical field of seismology, seismic prospecting, detecting and monitoring . Background of the invention

Acoustic vector sensors are relatively compact sensors with spatial filtering capabilities. They measure acoustic pressure and particle velocity and usually combine these two quantities in an intensity estimation, which results in an inherently directional beam .

The particle velocity can be measured directly or as a derived value from acceleration or pressure differential, see for example [1], for the underlying theory.

Applications of vector sensors include target tracking [2,3], detection and estimation of Direction Of Arrival (DOA) of sound sources [4-6], underwater communication [7,8] and geo-acoustic inversion [9,10] .

An important area of application for vector sensors is geo-acoustic surveys, where traditionally they are deployed on the earth surface or laid with cables on the seafloor. Owing to their directionality, they can distinguish between vertical and horizontal earth motions and hence they are used to record multicomponent seismic data .

In sea surveys, in particular water-bottom cables with such sensors have been used for the attenuation of water- column reverberations [11] . In recent years vector sensors have been used on towed streamers for the elimination of surface reflections (ghosts) [12], however details of these developments have limited publicity as they may contain commercially sensitive information. Another advancement for marine geo-acoustic surveys is the replacement of the ship-towed streamers with sensors either towed or carried by Autonomous Underwater Vehicles (AUV) .

Document US6474254 relates to an autonomous underwater vehicle (AUV) comprising a cable section and a sensor, a storage reel in the AUV for deploying the assembly from the AUV to an ocean floor and for retrieving the assembly from the ocean floor; a controller/processor for performing a diagnostic on the sensor before deployment of the assembly; and a robot arm for replacing inoperable sensors before manipulating the assembly for at least one of deployment and retrieval. The AUV and cable/sensors are neutrally buoyant to reduce the size of the vehicle and eliminate the need for an apparatus which adjusts the dynamic buoyancy of the AUV for changes in buoyancy caused by dispensing negative or positively buoyant cables and sensors. The AUV herein disclosed attaches the neutrally buoyant cable to the ocean floor by means of a fastener thereby actively coupling the sensors to the ocean floor .

The major disadvantage of said system is having to place the sensor on the sea floor. This demands time and because of its limited range of operation, the device must be removed and replaced each time.

In contrast, the AUV of the present invention carries the sensor displacing it along the selected area. This not only saves time but also optimizes the data acquisition. Document US2015/0219776 discloses a fibre optic hydrophone having the optical fibre coiled around the circumference of a mandrel wherein the longitudinal stiffness of the strain is at least one half the circumferential stiffness and at most 50 times the circumferential stiffness. However, due to their longitudinal strains said hydrophone present low sensitivity in particular when subject to compression by pressure.

This device does not allow the direct acquisition of any directional information of the acoustic field (sensor vector) as the one of the present invention.

Document US7646670 relates to an autonomous node seismic recording device having an integrated modular design and one or more features that assist coupling of the unit to the sea floor in order to improve the vector fidelity of seismic signal measurement, comprising an integrated modular housing, a base plate forming the bottom of said housing, a battery, a recording unit having a signal recording unit and vector sensor unit .

This device is placed directly on the sea bottom and therefore needs to be moved on a case-by-case basis to other zones. It does not use vehicles (AUVs) to move.

The aim of the present invention is to overcome the above mentioned problems by expanding the functionalities of the current cooperative marine robotic systems, in order to enable deployment of an optimal distributed acoustic array for geophysical surveying in a set-up composed of a ship towing a source and a DAVS vector-sensor receiving array carried by an AUV. The DAVS has the potential to contribute to this aim in various ways, for example, owing to its spatial filtering capability, it may reduce the amount of post processing by discriminating the bottom from the surface reflections .

Also its compact size allows easier integration with AUVs and hence facilitates the vehicle manoeuvrability compared to the classical towed arrays.

Summary of the invention

The present invention related to a dual a ccelerometer vector sensor (DAVS) useful for acquiring and monitoring seismic data according to claim 1.

Due to its two tri-axial accelerometer s and one hydrophone moulded in one unit the device is able to form acoustic arrays and thus to record mult icomponent seismic data.

Therefore, another aspect of the present invention relates to a method for acquiring and monitoring seismic data by using the device of the present invention according to claim 7.

This method is more reliable and more sensitive than the ones of the state of the art due to the sensitivity of the 3 sensors and their assembly allowing to obtain independent measurements. The DAVS device of the present invention can be integrated in an Autonomous Underwater Vehicles (AUVs) and therefore, another object of the present invention refers to such vehicles (AUVs) comprising the device of the present invention useful for acquiring and monitoring underwater seismic data, according to claim 9.

Thus, another aspect of the present invention relates to a method for acquiring and monitoring underwater seismic data, according to claim 10.

When using an AUV, according to the present invention the method for acquisition and monitoring underwater seismic data is possible to obtain a more precise estimation of the sound azimuth direction of the sound source even when using a simple algorithm. When using a higher resolution algorithm is possible to correct the azimuth angle for the AUV motion and thus to minimize interaction between sensors obtaining better and more precise seismic data.

Description of the figures

Fig. 1. shows a DAVS according to one of the embodiments of the invention:

Fig. 1(a) is an image of said DAVS, with the acoustic sensing part (black nose) and the container (white tube) .

Fig. 1(b) is an exploded view of DAVS in 3D solid modelling, showing the Delrin container (half tube), the acoustically sensing part constituted by the two accelerometers (numbered blocks), the hydrophone (cylinder), the threaded caps, the electronics and the battery pack. The figure shows the coordinate convention of the device, which coincides with the three sensing dimensions of the accelerometers .

Fig. 2. shows an overview of a preferred embodiment of the acquisition system of DAVS of the present invention having a digital platform for the acoustic sensors and a non-acoustic motion sensor.

Herein it is possible to observe that the hydrophone and the two accelerometer signals are acquired on the multichannel acquisition board, 7 channels in total.

The micro-controller receives data from multi-channel acquisition board and stores it in the SD card. Non- acoustic sensors give the roll, pitch and heading of the device. There are two amplification stages. Power can be obtained from internal batteries or from external source through a power cable .

Fig. 3 shows DAVS orientation at the starting position

(0°) for directivity measurements; left top view and right side view of the device's coordinate system. The X direction is pointing to the bottom of the tank.

Fig. 4. Shows the directivity beam patterns of the hydrophone and accelerometer 49 and accelerometer 50 components at 2 kHz (a) Polar plots of the hydrophone beam patterns (solid line) and the two accelerometers in the x-direction (dotted and dashed line, accelerometer 49 and 50 respectively), (b) and (c) in the y and z direction respectively of the two accelerometer , solid line for accelerometer 49 and dashed line for accelerometer 50. Fig. 5. First sailed track of the AUV towards the source (black asterisk) . The black dot indicates the starting point and the arrow the end point for the estimation. The illustration on the top right corner of the figure shows the sensor coordinate system with respect to the track.

Fig. 6. Second sailed track of the AUV towards the source (black asterisk) . The black dot indicates the starting point and the arrow the end point for the estimation. The orientation of the device relative to the track is the same with the one shown in Figure 4.

Fig. 7. Estimation of sound azimuth direction of the sound relative to AUV sailing from the track, shown in Figure 7, as computed from the lower (dashed light curve) and the upper (dashed curve) accelerometer . The solid line gives the source azimuth estimate as derived from the AUV positional sensors starting from the point of the track indicated with the dot in Figure 5.

Fig. 8. Arrival patterns in the interval of time of the straight line of AUV ' s trajectory, Fig. 6, considering pressure only a) and combination of particle velocity and particle velocity difference b) .

Description of a preferred embodiment of the invention

The present invention related to a dual a ccelerometer vector sensor (DAVS) device useful for acquiring and monitoring seismic data. 1. A dual a ccelerometer vector sensor (DAVS) device

The device comprises at least two tri-axial accelerometer s and one hydrophone moulded in one unit and is useful for acquiring and monitoring seismic data.

At least one hydrophone is made preferably of a PZT piezoelectric material.

The acoustically sensing part of the DAVS device is, preferably made of a polyurethane material for lightness.

The sensors can be moulded together with a threaded cap, which can be screwed to a container, which contains the electronics . The container can be closed with another threaded cap and may comprise a battery pack.

The acquisition system of DAVS is a digital platform for the acoustic sensors and a non-acoustic motion sensor, the system overview is shown in Figure 1. Its electronic components comprise a micro-controller, an analogue multi-channel simultaneous acquisition system, non-acoustic positioning sensors for pitch, roll and heading, power management and an external communications port .

In a preferred embodiment, the DAVS digital platform comprises a data storage unit, for example a removable flash device.

The device can operate autonomously on batteries and storing data in a microSD card. Alternatively or additionally, it can be powered externally to a 24 V DC power supply, streaming data via Ethernet .

Data related to the devices and methods of the present invention were collected and analysed.

Sensitivity of the DAVS was measured using the method 'calibration by comparison', as described in [14] in an anechoic chamber having two carriage systems for positioning the acoustic devices to avoid any external interference affecting the sensor elements of the device.

The source was placed in front of the DAVS at 2 m distance and was emitting tone bursts of 20 cycles; both devices were submerged at a depth of 2.5 m, striving to achieve free-field conditions.

The DAVS hydrophone sensitivity was measured -196 ± 2 dB re V/pPa in the frequency range mentioned above. To estimate the acceleration sensitivity, first the equivalent pressure sensitivity (M_P) was estimated from the reference hydrophone and then using the formula (1), it was converted to acceleration sensitivity (see [14]), where p, c and ω are the water density, sound speed and the acoustic frequency respectively.

The acceleration sensitivity of 24±1 mV/m/s² was found for both accelerometers at 2 kHz . Directivity measurements were carried out in the anechoic chamber by mounting the DAVS device vertically on a rotating table with the orientation (Fig. 3) . At the angle of 0° of the directivity polar plots, the source was insonifing the accelerometer 50 first (Fig. 3) .

It was observed that the hydrophone is to be within 3.5 dB omnidirectional. The accelerometers have beam patterns close to a figure of eight shape on the plane of insonification (y and z directions), whilst is ominidirectional in plane perpendicular to insonification (x-direction) .

Theoretically it was expected the hydrophone to be omnidirectional and the accelerometer s to have a beam patterns with DI equal to 48 dB and 3 dB beam width of 90° .

2. Method, for acquiring and monitoring seismic data

The hydrophone and the two accelerometer signals are acquired on the multi-channel acquisition board.

The micro-controller receives data from multi-channel acquisition board and stores it in the SD card and/or transmit it by streaming.

Non acoustic sensors give the roll, pitch and heading of the device. Power can be obtained from internal batteries or from external source through a power cable.

There are two amplification stages: (i) at the front end of the analogue signal there is a 6 dB gain pre-amplifier with one pole high pass filter at 120 Hz for attenuation of low frequency vibrations originating from device motion. This first stage pre-amplifier has been designed to allow for a maximum input voltage of 10 Vpp, which for a hydrophone sensing element with a sensitivity of -195 dB re V/μ Pa permits a maximum input SPL of 206 dB re μ Pa;

This is followed by (ii) a PGA with variable gain, which allows the user to select the second stage amplification according to the application. The SNR of the analogue to digital converter is 106 dB when operating at a sampling rate of 10547 Hz and can go up to 110 dB when operating at a sampling rate of 52734 Hz. Using a 24 bit sigma— delta analogue to digital converter (ADC) the flat passband frequency response for this acquisition system is 4.8 kHz and 23.9 kHz for the sampling rates of 10547 Hz and 52734 Hz respectively.

In the preferred frequency range of interest, the hydrophone sensitivity is within 1 dB of its design value and omnidirectional within 3.5 dB . The accelerometer sensitivity at 2 kHz is 24 mV/m/s ² and the spatial response features a figure-of-eight beam pattern.

3. Integration of a DAVS in an AUV

The DAVS device was tested on an AUV in order to assess the device performance and evaluate its capabilities for estimating sub-bottom properties. The signal processing scheme, which was applied to the data is based on intensity measurements, as described. The DAVS device was tested on an AUV, under conditions where the waters were protected with a sluice from current and rough sea conditions to evaluate the ability of the DAVS to estimate the azimuthal direction of incoming sound waves when it is in motion.

When DAVS is mounted on an AUV may carry a GPS antenna.

AUV's trajectory, can be analysed by an immersed sound source, such as an underwater speaker, for bottom characterization. Trajectories are referenced relative to the position of the sound source (0,0) on the experimental X — Y plane parallel to the sea floor and the DAVS accelerometers ' orientation relatively to the source position.

Figures 4 and 5 show the trajectories, which were analysed in the scope of the present invention. The trajectories are referenced relative to the position of the sound source (0,0) on the experimental X — Y plane parallel to the sea floor.

A first trajectory was analysed (Fig. 5) where the dot and the arrow indicate the beginning and the end of the track of the acoustic data presented here. The dot indicates starting point for the azimuthal calculation using the AUV positional information from non—acoustic sensors, as an independent check for the estimates obtained with the DAVS.

A second track was also analysed (Fig. 6), where the black dot indicates the beginning of the track. The sound source can be considered omnidirectional for the frequencies used. In these scenarios the azimuthal position of the source relative to DAVS can be approximated from the instantaneous angle between the tangent of the trajectory and the trajectory curve itself using the motional sensor information and GPS.

The sound source was emitting chirp signals from 1 kHz to 2 kHz at a regular period of time, approximately 0.1 seconds. The signals were sampled at 10547 Hz. DAVS sensor x - y plane was parallel to the experiment X — Y plane with the positive z direction pointing upwards and the positive x in the direction of sailing, according to the right hand coordinate convention shown in the top right of Figure 5.

The signals were filtered in the frequency range of the chirp signal content and the estimators were computed in the time domain with an unweighted moving average filter.

The pressure and all velocity components were filtered using a band pass filter and a mat ched—filter with the emitted signal.

The azimuth estimate from the non-acoustic data was obtained from the positional information of the AUV, as mentioned before.

To compare with the acoustic estimates, the non-acoustic azimuth estimates were smoothed using a third order Savit zky-Golay filter. As shown in Figure 7 two acoustic azimuth estimates (dashed curves) are superimposed with the estimated angle from the AUV to the sound source as derived from the AUV track and motion sensors (solid curve) . The estimates are computed by combining the hydrophone signals with each accelerometer ' s signals.

For this track, the two estimates show the same trend and follow the AUV motion features as appear from the positional sensors, so that the change in direction is well detected.

The three sensors can be moulded in the same encapsulation material and they are sufficiently acoustically decoupled to give a pair of independent measurements .

In this way, even a relatively simple algorithm was able to obtain an azimuth angle estimation of the sound source .

When a correction of the azimuth angle estimates for the AUV motion are applied better results are obtained.

The application of a higher resolution algorithm for direction finding as well as methods to minimise interaction between sensors also contribute for an increased sensitivity of the devices herein described and therefore also for the reliability of the methods for acquiring and monitoring seismic data. 4. Bottom characterization

As mentioned before, the main advantage of the dual accelerometer configuration of DAVS system of the present invention is to improve the bottom characterization, where the combination of particle velocity with the particle velocity gradient allows us to filter out undesirable direct and surface-reflected paths, improving the bottom-reflected paths.

The dual accelerometer configuration on DAVS system improves bottom—reflected paths by the use of particle velocity difference. Considering part of the AUV's trajectory, in this case the straight line of Fig. 6, comparisons between arrivals patterns achieved from the pressure only and from the combination of particle velocity and particle velocity difference were obtained based on formula (2) :

Where

V(C0)is the particle velocity average added with the particle velocity difference;

are the z-axis particle velocity

components of both accelerometers ;

D is the spacing between the accelerometers (in our case 44 mm) ;

k is the wave number. Fig. 8 presents the arrival patterns achieved from DAVS outputs by the pressure only a) and by the combination of particle velocity and

difference b) .

The advantage of using the combination of

velocity and particle velocity difference is presented in plot b) , where, compared with pressure only (a) , the direct path and surface reflected path are attenuated, improving the bottom reflected paths .

Results for bottom characterization show the advantages of using the combination of particle velocity with particle velocity difference when it is compared with the pressure only. This combination attenuates the direct and surface—reflected path and enhances bottom—reflected paths, useful for monitoring seismic data, in particular underwater seismic data.

Examples

Example 1. A DAVS device

A DAVS device was built predominantly with off—the— self components, with the intention of being a low cost sensor deployable independently from different platforms. To be flexible in use, the device can be powered autonomously for 20 hours and can record and store data up to 128 (GB) .

Moreover, the application of direction finding algorithms with vector sensor reguires knowledge of the sensor orientation relative to the ambient environment, for this reason the device is equipped with motion sensors.

In addition knowledge of the individual sensor acoustic response within the overall construction is required in order to combine results from different sensors .

The device acoustic sensors is one build in-house end- cupped cylindrical hydrophone made of PZT piezoelectric material and two tri-axial accelerometer s acquired from PCB Piezotronics, model number 356A17.

The acquisition system of DAVS is a digital platform for the acoustic sensors and a non-acoustic motion sensor, the system overview is shown in Figure 1. Its electronic components include a micro-controller, an analogue multi-channel simultaneous acquisition system, data storage on a removable flash device, real time clock, non-acoustic positioning sensors for pitch, roll and heading, power management and an external communications port.

Table 1 gives an overview of the DAVS system design characteristics for a specific embodiment of the invention .

This device can operate autonomously on batteries for 20 hours and storing data in a microSD card and/or it can be powered externally to a 24 V DC power supply, streaming data via Ethernet .

Example 2. DAVS sensitivity measurements

Prior to applying a direction finding algorithm to the signals received by the DAVS device, it was necessary to check the performance of the sensing elements in the moulded unit .

The hydrophone sensitivity was measured using the method 'calibration by comparison', as described in [13] . A Reson hydrophone TC4033, served as the reference hydrophone for these measurements.

The DAVS hydrophone sensitivity was measured -196 ± 2 dB re V/pPa in the frequency range under consideration. To estimate the acceleration sensitivity, first the equivalent pressure sensitivity (M_P) was estimated from the Reson hydrophone and then using the relation 1, it was converted to acceleration sensitivity (see [14] ) , where p, c and ω are the water density, sound speed and the acoustic frequency respectively.

Example 3. DAVS integration with an AUV

The DAVS device was tested on an AUV in order to assess the device performance and evaluate the tolerance of the azimuth finding algorithm to the above mentioned deviations. The signal processing scheme, which was applied to the data is based on intensity measurements.

The DAVS device was tested on an AUV, in conditions of protected waters with a sluice from current and rough sea conditions. The objective was to evaluate the ability of the DAVS to estimate the azimuthal direction of incoming sound waves when it is in motion. The AUV was sailing on the surface and was carrying a GPS antenna, the depth of the DAVS during the experiment was approximately 0.5 m.

The AUV was sailing on pre—programmed tracks with a nominal speed of 0.26 m/s relative to an immersed sound source (Lubell KK916C underwater speaker), which was deployed by a rope at approximately mid-water, 1.5 meters depth. Figures 4 and 5 show the trajectories, which are examined in this paper. The trajectories are referenced relative to the position of the sound source (0,0) on the experimental X — Y plane parallel to the sea floor.

The sound source was emitting chirp signals from 1 kHz to 2 kHz every 0.396 s. The signals were sampled at 10547 Hz. DAVS sensor x - y plane was parallel to the experiment X — Y plane with the positive z direction pointing upwards and the positive x in the direction of sailing, according to the right hand coordinate convention shown in the top right of Figure 4.

The DAVS was positioned on the AUV such that the two accelerometer s were aligned with the vertical, i.e. the device z axis, with the accelerometer 50 been at the top of the accelerometer 49. In this set up, two estimates were obtained from the two sets of x and y velocity components (ux(t) and u_y(t) respectively) derived from the accelerometer signals and the hydrophone pressure output using formula (2) .

Figure 6 shows the results from the dataset of the first track (Figure 4) . Two acoustic azimuth estimates (dashed curves) are superimposed with the estimated angle from the AUV to the sound source as derived from the AUV track and motion sensors (solid curve) . One estimate is computed by combining the upper accelerometer and the hydrophone signals (dashed curve) and the other estimate is obtained by combining the lower accelerometer with the same hydrophone signals (dashed light curve) . Bibliography

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Claims

1. A dual a ccelerometer vector sensor (DAVS) device for acquiring and monitoring seismic data comprising at least 2 tri-axial accelerometer s and 1 hydrophone, and a digital platform moulded in one unit, said platform comprising at least a micro-controller, an analogue multi-channel simultaneous acquisition system and a non-acoustic positioning sensors for pitch, roll and heading.

2. A DAVS device according to claim 1 wherein the digital platform comprises a data storage device.

A DAVS device according to any of the claims 1 or 2 wherein the DAVS device is able to store data and/or to stream data via Ethernet .

4. A DAVS device according to any of the claims 1 to 3 wherein the DAVS device operates autonomously on batteries .

A DAVS device according to any of the claims 1 to 4 wherein the DAVS device is powered externally to power supply .

6. A DAVS device according to any of the claims 1 to 5, wherein the signals produced by the tri-axial accelerometer s and the hydrophone are acquired by a multi-channel acquisition board, being passed to a micro-controller and stores in a storage device, thus allowing to retrieve and define directivity patterns.

7. A method for acquiring and monitoring seismic data comprising the use of a dual a ccelerometer vector sensor (DAVS) device as described in any of the claims 1 to 6 to produce signals that are acquired by a multi-channel acquisition board that are further passed to a micro-controller and stored in a storage device or transmitted via Ethernet to a processor, which is able to process said data, comprising the following steps :

a) geo—localization of the device platform;

b) adjust reference position data using internal non—acoustic sensor information (pitch, roll, heading) ;

c) combining pressure with particle or with particle velocity difference

8. A method according to claim 7, wherein 2 amplification stages are performed: (i) the first, at the front end of the analogue signal for signal conditioning and attenuation of low frequency vibrations originating from device motion, and (ii) a second stage amplification according to the intended application selected according to a PGA with variable gain.

9. An Autonomous Underwater Vehicle (AUVs) for acquiring and monitoring seismic data comprising at least a dual a ccelerometer vector sensor (DAVS) as described in any of the claims 1 to 6.

10. A method for acquiring and monitoring underwater seismic data wherein an AUV as described in claim 9 is used defining trajectories through a combination of particle velocity with the particle velocity gradient thus allowing to collect data relative to underwater bottom-reflected paths.