System for measuring a time offset and method of measuring a time offset

1. A system for measuring a time offset of an acoustic sensor node relative to a reference time, the system comprising:

an acoustic communications unit arranged to transmit a first acoustic probe signal to the acoustic sensor node;

a processing resource operably coupled to the acoustic communications unit and arranged to record a transmission time of the first signal, the transmission time being relative to the reference time; wherein

the acoustic sensor node is arranged to receive the first signal and to transmit a second acoustic reply signal to the acoustic communications unit after a predetermined time delay, the second signal including time data indicative of a current internal time of the acoustic sensor node during the predetermined time delay; and

the acoustic communications unit is arranged to receive the second signal, the processing resource recording a receive time at which the second signal is received, the receive time also being relative to the reference time, and to calculate the combined time of flight of the first and second signals, and using the combined time of flight and the predetermined time delay to calculate the time offset between the current internal time of the sensor node and the reference time.

2. The system according to claim 1 , wherein the acoustic sensor node is arranged on the seabed and/or in the sea.

3. The system according to claim 1 , wherein the acoustic communications unit is coupled to or integrally formed with a surface vessel.

4. The system according to claim 1 , wherein the sensor node is arranged to record seismic sample data.

5. The system according to claim 1 , further comprising:

a vessel motion compensation unit arranged to compensate, when in use, for differences in the time of flight between the first signal and the second signal.

6. The system according to claim 5 , further comprising:

a location determination system; wherein

the vessel motion compensation unit is arranged to use the location determination system in order to determine a time compensation value associated with vessel movement during measurement for application to the time offset.

7. The system according to claim 5 , wherein the processing resource is arranged to calculate a first speed of the first signal with respect to a first vector between a vessel at a first location and the sensor node, a second speed of the second signal with respect to a second vector between the sensor node and the vessel at a second location.

8. The system according to claim 5 , wherein the motion compensation unit is arranged to cooperate with a GNSS location determination system and an Attitude and Heading Reference System.

9. The system according to claim 1 , wherein the acoustic communications unit is arranged to transmit a timing correction instruction to the acoustic sensor node after calculation of the time offset, the sensor node being arranged to use the time offset in order to update the internal time thereof to the reference time.

10. A subsea seismic survey system comprising:

a plurality of acoustic seismic sensor nodes;

a surface vessel; wherein

the surface vessel comprises an acoustic communications unit arranged to transmit a first acoustic probe signal to the plurality of acoustic sensor nodes and a processing resource operably coupled to the acoustic communications unit and arranged to record a transmission time of the first signal, the transmission time being relative to the reference time;

the plurality of acoustic sensor nodes are arranged to receive the first signal and to transmit a second acoustic reply signal to the acoustic communications unit after a predetermined time delay, the second signal including time data indicative of a current internal time of the acoustic sensor node during the predetermined time delay; and

the acoustic communications unit is arranged to receive the second signal, the processing resource recording a receive time at which the second signal is received, the receive time also being relative to the reference time, and to calculate the combined time of flight of the first and second signals, and using the combined time of flight and the predetermined time delay to calculate the time offset between the current internal time of the sensor node and the reference time.

11. A method of measuring a time offset of an acoustic sensor node relative to a reference time, the method comprising:

transmitting a first acoustic probe signal to the acoustic sensor node from an acoustic communications unit;

recording a transmission time of the first signal, the transmission time being relative to the reference time;

the acoustic sensor node receiving the first signal and transmitting a second acoustic reply signal to the acoustic communications unit after a predetermined time delay, the second signal including time data indicative of a current internal time of the acoustic sensor node; and

the acoustic communications unit receiving the second signal and recording a receive time at which the second signal is received, the receive time also being relative to the reference time, and calculating the combined time of flight of the first and second signals, and using the combined time of flight and the predetermined time delay to calculate the time offset between the current internal time of the sensor node and the reference time.

12. The method according to claim 11 , wherein the internal time of the acoustic sensor node is represented as a time value or as a count of the number of recorded seismic samples since a start of a seismic survey session.

13. The method according to claim 11 , wherein seismic samples data is recorded by the sensor node.

14. The method according to claim 11 , wherein a vessel motion compensation unit calculates a compensation value for a difference between the time of flight of the first signal and the time of flight of the second signal.

15. The method according to claim 14 , wherein the vessel motion compensation unit cooperates with a location determination system and calculates the time compensation value associated with vessel movement during measurement for application to the time offset.

16. The method according to claim 14 , further comprising:

calculating a first speed of the first signal with respect to a first vector between a vessel at a first location and the sensor node; and

calculating a second speed of the second signal with respect to a second vector between the sensor node and the vessel at a second location.

17. The method according to claim 11 , further comprising:

transmitting a timing correction instruction to the acoustic sensor node after calculation of the time offset in order to update the internal time of the acoustic node to the reference time.

18. The method according to claim 11 , wherein prior to the transmission of the first signal, an activation signal is sent to the sensor node from the acoustic communications unit to instruct the sensor node to power up or leave a standby state.

19. The method according to claim 11 , wherein after the sensor node has completed participation in a seismic survey, a deactivation signal is sent to the sensor node from the acoustic communications unit to instruct the sensor node to power down or enter a standby state.

20. The method of performing a subsea seismic survey, the method comprising:

measuring seismic information using a plurality of sensor nodes; and

determining the time offset for each sensor node using the method of measuring a time offset according to claim 11 .