Method for steering an autonomous underwater vehicle along a buried object in the seabed

Abstract

A method for steering an Autonomous Underwater Vehicle along an object buried below a seabed: the AUV being equipped with at least one acoustic transmitter for generating acoustic signal towards the buried object and the seabed; arranging a first sensor assembly flush with the AUV hull of the starboard side of the AUV for recording reflected acoustic signal from the buried object and the seabed, arranging a second sensor assembly flush with the AUV hull of the port side of the AUV for recording reflected acoustic signal from the buried object and the seabed.

Claims

1. A method for steering an Autonomous Underwater Vehicle (AUV) along a buried object below a seabed, the AUV being equipped with at least one acoustic transmitter for generating acoustic signal towards the buried object and the seabed; the method comprising: arranging a first sensor assembly flush with a starboard side of a hull of the AUV for recording reflected acoustic signals from the buried object and the seabed with the first sensor assembly comprising a first acoustic sensor; arranging a second sensor assembly flush with a port side of the hull of the AUV for recording the reflected acoustic signals from the buried object and the seabed with the second sensor assembly comprising a second acoustic sensor; generating an acoustic signal with a center frequency ranging between 1 kHz and 24kHz with the at least one acoustic transmitter; recording a reflected energy from the seabed and a reflected energy from the buried object with the first acoustic sensor and the second acoustic sensor; registering the reflected energy from the buried object in recorded reflected energy data provided by the first acoustic sensor and the second acoustic sensor, distinguishing the buried object from the seabed by a later arrival of the reflected energy from the seabed and the reflected energy from the buried object; determining which of the starboard side or the port side the buried object lies relative to the AUV by determining which of the first acoustic sensor or the second acoustic sensor recorded the reflected energy from the buried object first; and navigating the AUV towards the buried object based on which of the starboard side or the port side the buried object lies relative the AUV.

2. The method according to claim 1, wherein determining which of the starboard side or the port side the buried object lies relative to the AUV is based on a time difference between an arrival of the reflected energy from the buried object to the first sensor assembly and an arrival of the reflected energy from the buried object to the second sensor assembly.

3. The method according to claim 1, wherein determining which of the starboard side or the port side the buried object lies relative to the AUV is based on calculating a cross-correlation function between a first sensor assembly reflected energy signal and a second sensor assembly reflected energy signal.

4. The method according to claim 3, wherein determining which of the starboard side or the port side the buried object lies relative to the AUV is calculated using a two-way travel time of the reflected energy from the buried object at the first sensor assembly (t.sub.s) and the second sensor assembly (t.sub.p), given by: $t_s=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o-\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w}$ $t_p=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o+\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w}$ where v.sub.w denotes the an acoustic wave velocity, o denotes a distance between the AUV and the buried object measured perpendicular to a direction of the buried object, h is an altitude of the AUV above the seabed, and d is a burial depth of the buried object below the seabed, a is a distance of both the first acoustic sensor and the second acoustic sensor from a main axis of the AUV, and where k=1 when the buried object lies on the port side, and k=1 when the buried object lies on the starboard side.

5. The method according to claim 1, wherein the AUV is equipped with a processing unit for processing and analyzing the recorded reflected energy data.

6. The method according to claim 1, wherein the AUV is equipped with additional sensor assemblies arranged at locations different from those of the first sensor assembly and the second sensor assembly on the AUV.

7. The method according to claim 1, wherein the first sensor assembly and the second sensor assembly have a separation in a plane perpendicular to a vertical orientation of the AUV.

8. The method according to claim 1, wherein the first sensor assembly and the second sensor assembly have a separation in a plane perpendicular to a horizontal direction of the AUV.

9. The method according to claim 1, wherein the AUV is equipped one or more of the following: multibeam sensors, sidescan sonar sensors, magnetometers, and synthetic aperture sonar sensors.

10. The method according to claim 9, wherein determining which of the starboard side or the port side the buried object lies relative to the AUV is based on data combination of reflected energy data and data from one or more of the multibeam sensors, sidescan sonar sensors, magnetometers and synthetic aperture sonar sensors.

11. A computer program product comprising instructions to carry out processing and analysis of the recorded reflected energy data according to the method of claim 3.

12. A non-transitory computer readable storage medium comprising a computer program product according to claim 11.

13. The method according to claim 1, further comprising distinguishing the buried object from the seabed based on an amplitude difference between the reflected energy from the seabed and the reflected energy from the buried object.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

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

(2) FIG. 1 shows an AUV tracking a buried object in the seabed.

(3) FIG. 2 shows an AUV where the buried object is direct below the AUV.

(4) FIG. 3 shows an AUV 1 where a buried object is on the port side of the AUV.

(5) FIG. 4 shows an AUV 1 where a buried object is on the starboard side of the AUV.

(6) FIG. 5 shows a scenario where the buried object is positioned 0.5 m to the port side of the AUV.

DETAILED DESCRIPTION

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

(8) FIG. 1 and FIG. 2 show an Autonomous Underwater Vehicle 1 (AUV) tracking an object that is at least partly buried in seabed 8 seen in the plane spanned by the vertical axis and the elongated direction of the AUV 1. The AUV 1 is moving predominantly in the x-direction above the buried object 2. The AUV 1 is equipped with at least one acoustic transmitter 3 for generating acoustic signal 6 towards the buried object 2, a first sensor assembly 4 comprising one or more acoustic sensors for recording reflected energy 7 from the buried object 2 and the seabed 8, a second sensor assembly 5 comprising one or more acoustic sensors for recording reflected energy 7 from the buried object 2 and the seabed 8. The first sensor assembly 4 may be arranged in the proximity of the starboard side 100 of the AUV 1. The second sensor assembly may be arranged in the proximity of port side 101 of the AUV 1. The first sensor assembly, the second sensor assembly and the transmitter 3 may also be a separate unit that is mounted on the AUV. The buried object may be a pipeline for gas or liquid transportation, cables, installations or minerals that will reflect a seismic signal. The first and the second sensor assemblies may be separated apart a distance s in the directions perpendicular to the sail line direction, y-direction, they may also have a separation in the directions parallel to the sail line, x-direction. The first and the second sensor assemblies may have the same height above the seabed in y-direction or they may be mounted in different heights. The AUV may be equipped with additional sensor assemblies and may be arranged at different locations on the AUV.

(9) FIG. 3 shows the AUV 1 has an offset relative to the buried object 2 on its port side 101. The first sensor assembly 4 (starboard sensor) and the second sensor assembly 5 (port side sensor) may first register the seabed reflected energy 9 approximately at the same arrival time, but the later arrival of the reflected energy 7 from the buried object 2 may arrive at the second sensor assembly 5 before it arrives at the first sensor assembly 4. The difference in the arrival time of the recorded reflected energy from the buried object 2 at the first and the second sensor assembly determines whether the buried object is on the port 101 or starboard side 100 of the AUV 1.

(10) FIG. 4 shows a scenario where the buried object 2 is on the starboard side 100 of the AUV 1. For this scenario, the reflected energy 7 from the buried object 2 arrives at the first sensor assembly 4 on the starboard side 100 before the second sensor assembly 5 on the port side 101 of the AUV 1.

(11) FIG. 2 shows the scenario where the AUV 1 is approximately vertically above the buried object 2. In this scenario, the reflected energy from the buried object 2 arrives almost simultaneously.

(12) By recording the reflected acoustic energy from the buried object and determining the arrival time difference of the reflected energy in the first and the second sensor assembly, the AUV 1 may be steered towards the direction where the reflected energy from the buried object arrives first. Several techniques may be applied to detect the arrival time difference or which of the sensors receives the reflected energy from the object first. Time difference in maximum amplitude of the reflected signals may be utilized because the buried object 2 reflects a stronger signal than the seabed due to stronger impedance contrast between the buried object and the seabed compared to the contrast between water and sandy seabed. Another method may be to calculate the cross-correlation function between the two sensor signals, and use the time (time lag) where the cross-correlation function has its maximum value as the time difference. The difference in time or time lag may be utilized to determine if the AUV 1 needs to alter the sailing direction and how much it needs to in order to move along the buried object 2.

(13) The transmitter 3, the first sensor assembly 4 and the second sensor assembly 5 emit and record the reflected signal at a sufficiently high rate between signals to keep the AUV 1 on course. A processor inside the AUV 1 may detect the time difference or correlation lag and give the AUV 1 steering command on which direction to move, slightly more towards port side or towards starboard side, to enable the AUV to stay along (on top) the buried object while moving forward.

(14) The AUV 1 may also record reflections from other objects and geological anomalies and changes in layers. Hence, the analysis of the signal may use various techniques to identify the time difference for the reflected energy from the buried object 2. The methods used for example be a) time difference for reflected signal strength reaching a threshold, b) cross correlation of signals, c) various filtering and automated processing techniques, d) comparison analysis using machine learning algorithms, e) seismic real-time impedance analysis.

(15) To further enhance the probability of determining the precise offset of the buried object relative 2 to the AUV 1, it is possible to combine the data from the acoustic sensors with data from one or more multibeam sensors, side scan sonar sensors, magnetometers and synthetic aperture sonar sensors. The AUV may carry and records multiple sensor data, and multi-sensor decision approach may be to make decision on travel directions. The AUV may also be equipped with additional sensor assemblies and may be arranged in the proximity of port side 101 and starboard side 100 of the AUV 1.

(16) In one example, the transmitter 3 emits a signal with sufficient strength and frequency range that discontinuities such as layers and buried objects 0 m-40 m below the seabed are detectable from the first and the second sensor assemblies. For the application in this example, it is reflections in the closest vicinity of the seabed that is of interest. Assuming the AUV 1 is sailing at a height of 5-30 m above the seabed, with an acoustic wave velocity velocity of 1480 m/s, the two-way travel-time for the reflected signal from the seabed is in range of 6.76 ms-40 ms. The transmitter 3 may emit signals with a center frequency range between 1 kHz and 24 kHz, which means a signal period down to 0.04 ms.

(17) It is assumed that the signal travel-time difference can be resolved down to of the wavelength or the period. Hence, it is possible to detect changes in two-way travel-time down to at least 0.01 ms. The tracking of the buried object can be performed either within a narrow angle range from the vertical or within an angle range from one of the sides of the object.

(18) TABLE-US-00001 TABLE 1 Difference Arrival of Arrival of in buried Arrival reflected reflected object's Buried of energy from energy from arrival time object's Angle seabed buried buried between distance towards re- object object port and towards buried flected port starboard starboard port object energy sensor sensor sensor (m) (degrees) (ms) (ms) (ms) (ms) 0.5 2.6 13.5 14.87 14.90 0.03 1 5.2 13.5 14.90 14.96 0.06 2 10.3 13.5 15.05 15.17 0.12 3 15.3 13.5 15.32 15.90 0.18 4 20.0 13.5 15.70 15.94 0.23 5 24.4 13.5 16.19 16.47 0.28 6 28.6 13.5 16.77 17.10 0.32 7 32.5 13.5 17.44 17.80 0.36 8 36.0 13.5 18.18 18.58 0.40 9 39.3 13.5 18.99 19.42 0.43 10 42.3 13.5 19.86 20.32 0.45 11 45.0 13.5 20.78 21.26 0.48 12 47.5 13.5 21.75 22.25 0.50 13 49.8 13.5 22.76 23.27 0.52 14 51.8 13.5 23.80 24.33 0.53 15 53.7 13.5 24.86 25.41 0.54

(19) Table 1, above shows the difference in reflected energy travel-time recorded between the first sensor assembly 4 and the second sensor assembly 5 for the scenario where the buried object 2 is on the port side 101 of the AUV 1 as illustrated in 3. In this example, a flight height of 10m and a burial depth of 1m is used. The two sensors are here assumed to be mounted 0.5 meter apart in the direction perpendicular to the sail line direction and with the same height above the seabed. The numbers show that it is feasible to detect the rate of change in travel-time differences if the AUV 1 moves more than a 1 m further away from the buried object 2. The results are comparable for lower sailing altitudes. If the sensor separation s increases from the 0.5 m in this example, the travel-time difference will increase.

(20) In Table 1, Angles towards the buried object is calculated as the angle between the line from the AUV 1 and the vertical axis:

(21) $\begin{array}{cc}=\frac{180}{}\arctan \left(\frac{o}{h+d}\right),& \left(1\right)\end{array}$

(22) Where o denotes distance between AUV 1 and the buried object 2 in the distance perpendicular to the direction of the elongated buried object, h is the altitude of the AUV 1 above the seabed 8, and d is the burial depth of the buried object 2 below the seabed 8.

(23) The arrival of the seabed reflection t.sub.b is the two-way traveltime of an acoustic signal 9 in the water column between the AUV 1 to the nearest seabed 8 point and is calculated as:

(24) $\begin{array}{cc}{t}_b=2*\frac{h}{v_w},& \left(2\right)\end{array}$

(25) where v.sub.w, denotes the acoustic wave velocity. For a flat seabed 8, the recorded two-way traveltime for the seabed 8 reflected energy 9 will arrive simultaneously at both sensors 4, 5 independent upon the distance to the buried object 2. However, for the reflected energy 7 from the buried object 2, the difference in arrival times between the two sensor assemblies 4, 5 will be dependent upon the distance between the two sensor assemblies 4, 5 measured in the horizontal direction towards the nearest point on the buried object 2. Let us define an axis perpendicular to the main AUV axis and assume that both port and starboard sensors distance from the main axis is a.

(26) In Table 1, assuming that the acoustic wave velocity between the layers of the sensor assemblies and the buried is equal to the velocity in the water. Then the two-way traveltime t.sub.p for the reflected energy 7 recorded at second sensor assembly 5 on the port side 101 is:

(27) $\begin{array}{cc}{t}_p=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o+\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w},& \left(3\right)\end{array}$

(28) where k is 1 when the buried object 2 lays towards the port side 101, and k=1 when the buried object 2 lays on the starboard side 100.

(29) And for the first sensor assembly 4 on the starboard side 100, the two-way traveltime t.sub.s calculated as:

(30) $\begin{array}{cc}{t}_s=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o-\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w},& \left(4\right)\end{array}$

(31) The AUV 1 can perform its tasks without being vertically above the buried object. A deviation of 0.5 m from the buried object 2 towards port 101 or starboard 100 in according to the calculations in Table 1, shows a time difference of 0.06 ms, and for 1m deviation 0.11 ms. The first and the second sensors assemblies is configured to have a frequency and sampling rate that can detect such a time difference.

(32) In FIG. 5, the recorded data is modeled for the scenario where the buried object 2 is positioned 0.5 m to the port side 101 of the AUV 1. The theoretical time difference between the arrival time for the largest reflection which is from the buried object at the two sensor assemblies are only 0.028 ms. The recorded traces have signals with a frequency bandwidth of 5-20 kHz and the simulated sample rate is 0.05 ms. 20% bandlimited noise has been added to the traces. The noise alters the recorded data in a randomized way. However, by continuously recording and calculating the time difference several times, the average should converge towards the real difference. The AUV 1 may slowly correct its position in the crossline direction (y-direction) based on the average time difference observed in a sequence of recorded measurements for the first and the second sensor assemblies. The average time difference may be calculated by a given number of last recordings. Several methods for calculating the time difference exists and hence provide the AUV 1 with information for changing the course. In FIG. 5, one instance of recordings for the first and the second sensor assemblies is simulated. The sensors record samples of the pressure perturbations caused by the acoustic waves emitted and reflected from the seabed, from deeper reflectors, and from other objects. The time difference between the recorded times for the maximum amplitudes of the reflected energy for the two assemblies can be one method to determine if the object is to the port or to the starboard side of the AUV 1. If the port sensor assembly registers its maximum reflected amplitude earlier than the starboard assembly, it can be assumed that the object is at the port side. Similarly, if the starboard assembly reaches its maximum reflected amplitude before the port assembly, it can be assumed that the object is on the starboard side. By observing which assembly records its maximum reflected amplitude first several times in a sequence, it can be assumed that the object is on the same side as the assembly where the maximum amplitude were reached first most of the times. An alternative method would be to correlate the two traces from the two assemblies. The simulations using even the simplest method averaging over a few recordings could provide a good result. In the example in FIG. 5, the theoretical difference in time is less than one sample of the recorded signal. Still, it is possible to detect a difference in recorded time between the first sensor assembly and the second sensor assembly.

(33) In the above equations, the acoustic wave velocity is assumed to be constant. This is an approximation of the real case where the velocity will increase below the seabed. However, this approximation is sufficient to exemplify the method, and does not impact the method itself.

(34) The first aspect of this disclosure shows a method for steering an Autonomous Underwater Vehicle AUV 1 along an object 2 that is at least partly buried in seabed 8: the AUV 1 being equipped with at least one acoustic transmitter 3 for generating acoustic signal 6 towards the buried object 2 and the seabed 8; a first sensor assembly 4 comprising one or more acoustic sensors for recording reflected acoustic signal 7,9 from the buried object 2 and the seabed 8, the first sensor assembly 4 being arranged in the proximity of the starboard side 100 of the AUV 1; a second sensor assembly 5 comprising one or more acoustic sensors for recording reflected acoustic signal 7,9 from the buried object 2 and the seabed 8, the second sensor assembly 5 being arranged in the proximity of the port side 101 of the AUV 1; the method comprising: generating acoustic signal 6 with the at least one acoustic transmitter 3; recording reflected energy from the seabed 8 and the buried object 2 with the first and second sensors 4,5; identifying reflected energy 7 from the buried object 2 in the recorded reflected energy data provided by the first and the second sensors; determining an offset of the buried object 2 relative to the AUV 1 from the reflected energy data; and steering the AUV 1 along the buried object 2 based on the offset.

(35) Two-way travel time of the reflected energy (7) from the buried object (2) at the first t.sub.s and the second t.sub.p sensor assemblies are given:

(36) $t_s=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o-\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w}$$t_p=\frac{\sqrt{o^2+{\left(h+d\right)}^2}}{v_w}+\frac{\sqrt{{\left(o+\mathrm{ka}\right)}^2+{\left(h+d\right)}^2}}{v_w}$

(37) where v.sub.w, denotes the acoustic wave velocity, o denotes distance between AUV 1 and the buried object 2 in the distance perpendicular to the direction of the elongated buried object, h is the altitude of the AUV 1 above the seabed 8, and d is the burial depth of the buried object 2 below the seabed 8, angle towards the buried object (2), where the k is given 1 when the buried object 2 lays towards the port side 101, and k=1 when the buried object 2 lays on the starboard side 100.

(38) The second aspect of this disclosure shows a computer program product comprising instructions adapted to carry out the method of the first aspect.

(39) The third aspect of this disclosure shows a computer readable storage medium comprising a computer program product according to the second aspect.

(40) The person skilled in the art realizes that the present disclosure is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. For example, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.